Power Quality Control of Smart Hybrid AC/DC Microgrids: An Overview
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paper_title: Assessing the Collective Harmonic Impact of Modern Residential Loads—Part I: Methodology

paper_content:

The proliferation of power-electronic-based residential loads has resulted in significant harmonic distortion in the voltages and currents of residential distribution systems. There is an urgent need for techniques that can determine the collective harmonic impact of these modern residential loads. These techniques can be used, for example, to predict the harmonic effects of mass adoption of compact fluorescent lights. In response to the need, this paper proposes a bottom-up, probabilistic harmonic assessment technique for residential feeders. The method models the random harmonic injections of residential loads by simulating their random operating states. This is performed by determining the switching-on probability of a residential load based on the load research results. The result is a randomly varying harmonic equivalent circuit representing a residential house. By combining multiple residential houses supplied with a service transformer, a probabilistic model for service transformers is also derived. Measurement results have confirmed the validity of the proposed technique. The proposed model is ideally suited for studying the consequences of consumer behavior or regulatory policy changes.
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paper_title: Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles

paper_content:

This paper reviews the current status and implementation of battery chargers, charging power levels, and infrastructure for plug-in electric vehicles and hybrids. Charger systems are categorized into off-board and on-board types with unidirectional or bidirectional power flow. Unidirectional charging limits hardware requirements and simplifies interconnection issues. Bidirectional charging supports battery energy injection back to the grid. Typical on-board chargers restrict power because of weight, space, and cost constraints. They can be integrated with the electric drive to avoid these problems. The availability of charging infrastructure reduces on-board energy storage requirements and costs. On-board charger systems can be conductive or inductive. An off-board charger can be designed for high charging rates and is less constrained by size and weight. Level 1 (convenience), Level 2 (primary), and Level 3 (fast) power levels are discussed. Future aspects such as roadbed charging are presented. Various power level chargers and infrastructure configurations are presented, compared, and evaluated based on amount of power, charging time and location, cost, equipment, and other factors.
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paper_title: A Modular Multiport Power Electronic Transformer With Integrated Split Battery Energy Storage for Versatile Ultrafast EV Charging Stations

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This paper proposes a power converter architecture for the implementation of an ultrafast charging station for electric vehicles (EVs). The versatile converter topology is based on the concept of the power electronic transformer. For the direct transformerless coupling to the medium-voltage grid, a cascaded H-bridge (CHB) converter is utilized. On the level of each submodule, integrated split battery energy storage elements play the role of power buffers, reducing thus the influence of the charging station on the distribution grid. The power interface between the stationary split storage stage and the EV batteries is performed through the use of parallel-connected dual-half-bridge dc/dc converters, shifting the isolation requirements to the medium-frequency range. By choosing several different submodule configurations for the parallel connection, a multiport output concept is achieved, implying the ability to charge several EVs simultaneously without the use of additional high-power chargers. All possible charging station operating modes among with the designed necessary control functions are analyzed. The state-of-charge self-balancing mode of the delta-connected CHB converter is also introduced. Finally, the development of a downscaled laboratory prototype is described, and preliminary experimental results are provided.
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paper_title: Power Quality Issues in Railway Electrification: A Comprehensive Perspective

paper_content:

Since the beginning of railway electrification, power quality has been a main problem in railway networks because of their special characteristics. Many ways of power quality improvement have been investigated and applied to ac and dc traction systems through railway electrification history. This paper presents a perspective on power quality issues through railway electrification development and investigates the necessity of power quality and system requirements for appropriate power quality. Compensation strategies are classified and compared. This aims to provide a comprehensive perspective of the power quality issue in railway power/distribution networks for researchers and engineers working on railway electrification. Less than 100 research articles are referenced for researchers to obtain good background information.
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paper_title: Overview of Power Management Strategies of Hybrid AC/DC Microgrid

paper_content:

Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid ac/dc systems with dc and ac sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid ac/dc microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures (ac-coupled, dc-coupled, and ac–dc-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.
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paper_title: The Impact of Charging Plug-In Hybrid Electric Vehicles on a Residential Distribution Grid

paper_content:

Alternative vehicles, such as plug-in hybrid electric vehicles, are becoming more popular. The batteries of these plug-in hybrid electric vehicles are to be charged at home from a standard outlet or on a corporate car park. These extra electrical loads have an impact on the distribution grid which is analyzed in terms of power losses and voltage deviations. Without coordination of the charging, the vehicles are charged instantaneously when they are plugged in or after a fixed start delay. This uncoordinated power consumption on a local scale can lead to grid problems. Therefore, coordinated charging is proposed to minimize the power losses and to maximize the main grid load factor. The optimal charging profile of the plug-in hybrid electric vehicles is computed by minimizing the power losses. As the exact forecasting of household loads is not possible, stochastic programming is introduced. Two main techniques are analyzed: quadratic and dynamic programming.
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paper_title: Evaluation of power-quality field measurements of an electric bus charging station using remote monitoring systems

paper_content:

Vehicle electrification has become an important measure for greenhouse gas reduction in many countries worldwide. Regardless of what type of electric vehicles (EVs), the on-board battery must be charged via a charger or charging station. In the future, when large numbers of EV chargers or charging stations are connected to distribution systems, there will be a degree of influence on the system operation and power quality. This paper aims for presenting the methodology and results for evaluation of power-quality field measurements of an electric bus charging station in Taiwan by using remote monitoring systems. The charging station under study has 11 charging poles, including 2 fast charging and 9 slow charging poles. The power quality issues including electrical power measurements, voltage flickers, harmonic distortion, and three-phase unbalance are discussed. Power quality measurements of the high-voltage customer closing to the station are also collected and evaluated. Test results show that with the proposed method the power quality of electric bus charging stations can be effectively monitored and evaluated. The results of this study can provide utilities with useful information regarding grid planning against significant electric bus penetration.
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paper_title: Monitoring and analysis of power quality in electric vehicle charging stations

paper_content:

Electric vehicle (EV) charging may lead to power quality problems for chargers work as electric-electronic converters. It is necessary to get to know the influence to the power system caused by chargers before EVs operating in scale. Based on monitoring data, power quality problems in EV charging stations are presented and analyzed in this paper. Harmonic current emissions of different kinds of chargers are investigated first. Then harmonic current saturation is detected and discussed. In the next section, power quality of bus voltage and the influencing factors are analyzed . At the end of this paper, suggestions and recommendations are given to improve the power quality of EV charging stations.
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paper_title: STATIC VAR COMPENSATION FOR PHASE BALANCING AND POWER FACTOR IMPROVEMENT OF SINGLE PHASE TRAIN LOADS

paper_content:

ABSTRACT The components of an ideal reactive power static VAr compensator (SVC) are determined for balancing a single-phase electrified train allover its traction cycle. In the same time, the load power factor is corrected to unity. Method of voltage regulation and balancing is explained and the control system which can perform all these functions is given. Energy loss saving due to this application is evaluated and the improvement is highlighted
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paper_title: Power Quality Issues in Railway Electrification: A Comprehensive Perspective

paper_content:

Since the beginning of railway electrification, power quality has been a main problem in railway networks because of their special characteristics. Many ways of power quality improvement have been investigated and applied to ac and dc traction systems through railway electrification history. This paper presents a perspective on power quality issues through railway electrification development and investigates the necessity of power quality and system requirements for appropriate power quality. Compensation strategies are classified and compared. This aims to provide a comprehensive perspective of the power quality issue in railway power/distribution networks for researchers and engineers working on railway electrification. Less than 100 research articles are referenced for researchers to obtain good background information.
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paper_title: Traction system unbalance problem-analysis methodologies

paper_content:

This paper presents a novel methodology using a utility's existing software for the accurate prediction of total unbalance (UB) impact due to ac traction loads with emphasis on the estimation of negative sequence (N-Seq) current injection into utility generators. Traditionally, due to the requirement of complex 3/spl Phi/ power-flow simulation, UB analysis has been limited to the evaluation of UB voltage at the point of common coupling (PCC) without addressing whether supply of this N-Seq current would exceed the remaining margin of each utility generator. By using the salient feature that a utility's N-Seq network is basically "sourceless" as such for any N-Seq current injection at a specific node, the "contribution ratio" of each utility generator for that N-Seq current injection is fixed irrespective of the magnitude of that injection, this paper provides a solution to this important problem by using a utility's existing fault analysis software.
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paper_title: Potential Power Quality Benefits of Electric Vehicles

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Electric vehicles (EVs) are likely to have a continued presence in the light-vehicle market in the next few decades. As a result, EV charging will put an extra burden on the distribution grid and adjustments need to be made in some cases. On the other hand, EVs have the potential to support the grid as well. This paper presents a single-phase bidirectional charger topology which pairs up a photovoltaic (PV) source with an EV charger resulting in production cost reduction. The presented topology is then used for vehicle-to-grid (V2G) services. The main focus of this paper is on power quality services which only slightly discharge the battery. Among these services, it studies the possibility of local reactive injection of EVs connected to the grid through a single-phase charger to compensate for voltage drops caused by motor startup or inductive loads. It also studies the possibility of active power injection of EVs for short time periods during PV transients in cloudy weather to keep the system stable. It also studies the potential of EVs to help during low voltage ride-through of the PV sources. The studies are performed using Simulink simulations and a real-time implementation in Real Time Digital Simulator (RTDS). The results demonstrate the effectiveness of power quality V2G services with small wear on the EV battery.
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paper_title: Power quality investigation of a typical Telecom data centre electrical network

paper_content:

In this paper a power quality investigation of the Libyan Telecom Technology (LTT) headquarter and data center electrical distribution network is presented. A full description of the center single-phase and three-phase load types are illustrated. Loads individual current harmonic distortion theoretical levels are explained. Accumulated current harmonic components at each connection point are also predicted. Estimation process involves an optimistic and pessimistic approach of the excepted harmonics due to the mix of the various data center loads at each busbar. A real measurement results of the current and voltage Total Harmonic Distortion (THD) and line power factor are presented, compared and assessed according to the IEEE519-1992 harmonic control recommendations. The paper proposes some practical procedures and precautions to be applied in order to improve the overall power quality of the data center that could minimize the related risk and distortion issues.
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paper_title: A novel active power quality compensator topology for electrified railway

paper_content:

To improve the power quality of traction power system, a novel active power quality compensator (APQC) and a new compensating currents detection method are proposed. The APQC consists of a three-phase voltage source converter and a Scott transformer. The Scott transformer, taken as an isolation transformer, not only connects the three-phase converter to the traction power system, but also converts the traction power system to a nearly balanced three-phase power system. Therefore, a general three-phase converter could be used in APQC. Regarding the traction substation as a compensating object, the power quality of a traction substation can be improved integrally. Simulation and prototype experimental results show that the proposed APQC is able to compensate reactive power, harmonic, and negative-sequence currents in two feeders of a traction substation.
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paper_title: Assessing the Collective Harmonic Impact of Modern Residential Loads—Part I: Methodology

paper_content:

The proliferation of power-electronic-based residential loads has resulted in significant harmonic distortion in the voltages and currents of residential distribution systems. There is an urgent need for techniques that can determine the collective harmonic impact of these modern residential loads. These techniques can be used, for example, to predict the harmonic effects of mass adoption of compact fluorescent lights. In response to the need, this paper proposes a bottom-up, probabilistic harmonic assessment technique for residential feeders. The method models the random harmonic injections of residential loads by simulating their random operating states. This is performed by determining the switching-on probability of a residential load based on the load research results. The result is a randomly varying harmonic equivalent circuit representing a residential house. By combining multiple residential houses supplied with a service transformer, a probabilistic model for service transformers is also derived. Measurement results have confirmed the validity of the proposed technique. The proposed model is ideally suited for studying the consequences of consumer behavior or regulatory policy changes.
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paper_title: Autonomous Control of Interlinking Converter With Energy Storage in Hybrid AC–DC Microgrid

paper_content:

The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence.
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paper_title: Synchronverters: Inverters That Mimic Synchronous Generators

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In this paper, the idea of operating an inverter to mimic a synchronous generator (SG) is motivated and developed. We call the inverters that are operated in this way synchronverters. Using synchronverters, the well-established theory/algorithms used to control SGs can still be used in power systems where a significant proportion of the generating capacity is inverter-based. We describe the dynamics, implementation, and operation of synchronverters. The real and reactive power delivered by synchronverters connected in parallel and operated as generators can be automatically shared using the well-known frequency- and voltage-drooping mechanisms. Synchronverters can be easily operated also in island mode, and hence, they provide an ideal solution for microgrids or smart grids. Both simulation and experimental results are given to verify the idea.
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paper_title: Overview of control, integration and energy management of microgrids

paper_content:

Microgrids are being developed as a building block for future smart grid system. Key issues for the control and operation of microgrid include integration technologies and energy management schemes. This paper presents an overview of grid integration and energy management strategies of microgrids. It covers a review of power electronics interface topologies for different types of distributed generation (DG) units in a microgrid, a discussion of energy management strategies, as well as the DG interfacing converter control schemes. Considering the intermittent nature of many renewable energy based DG units, the ancillary services of DGs using the available interfacing converter rating are also discussed in the paper.
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paper_title: Self-Synchronized Synchronverters: Inverters Without a Dedicated Synchronization Unit

paper_content:

A synchronverter is an inverter that mimics synchronous generators, which offers a mechanism for power systems to control grid-connected renewable energy and facilitates smart grid integration. Similar to other grid-connected inverters, it needs a dedicated synchronization unit, e.g., a phase-locked loop (PLL), to provide the phase, frequency, and amplitude of the grid voltage as references. In this paper, a radical step is taken to improve the synchronverter as a self-synchronized synchronverter by removing the dedicated synchronization unit. It can automatically synchronize itself with the grid before connection and track the grid frequency after connection. This considerably improves the performance, reduces the complexity, and computational burden of the controller. All the functions of the original synchronverter, such as frequency and voltage regulation, real power, and reactive power control, are maintained. Both simulation and experimental results are presented to validate the control strategy. Experimental results have shown that the proposed control strategy can improve the performance of frequency tracking by more than 65%, the performance of real power control by 83%, and the performance of reactive power control by about 70%.
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paper_title: Voltage-Sourced Converters in Power Systems: Modeling, Control, and Applications

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Presents Fundamentals of Modeling, Analysis, and Control of Electric Power Converters for Power System ApplicationsElectronic (static) power conversion has gained widespread acceptance in power systems applications; electronic power converters are increasingly employed for power conversion and conditioning, compensation, and active filtering. This book presents the fundamentals for analysis and control of a specific class of high-power electronic convertersвЂ”the three-phase voltage-sourced converter (VSC). Voltage-Sourced Converters in Power Systems provides a necessary and unprecedented link between the principles of operation and the applications of voltage-sourced converters. The book:Describes various functions that the VSC can perform in electric power systemsCovers a wide range of applications of the VSC in electric power systemsвЂ”including wind power conversion systemsAdopts a systematic approach to the modeling and control design problemsIllustrates the control design procedures and expected performance based on a comprehensive set of examples and digital computer time-domain simulation studiesThis comprehensive text presents effective techniques for mathematical modeling and control design, and helps readers understand the procedures and analysis steps. Detailed simulation case studies are included to highlight the salient points and verify the designs.VoltageSourced Converters in Power Systems is an ideal reference for senior undergraduate and graduate students in power engineering programs, practicing engineers who deal with grid integration and operation of distributed energy resource units, design engineers, and researchers in the area of electric power generation, transmission, distribution, and utilization.
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paper_title: Autonomous operation of hybrid microgrid with AC and DC sub-grids

paper_content:

This paper investigates on the active and reactive power sharing of an autonomous hybrid microgrid. Unlike existing microgrids which are purely ac, the hybrid microgrid studied here comprises dc and ac sub-grids, interconnected by power electronic interfaces. The main challenge here is to manage the power flow among all the sources distributed throughout the two types of sub-grids, which certainly is tougher than previous efforts developed for only either ac or dc microgrid. This wider scope of control has not yet been investigated, and would certainly rely on the coordinated operation of dc sources, ac sources and interlinking converters. Suitable control and normalization schemes are therefore developed for controlling them with results presented for showing the overall performance of the hybrid microgrid.
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paper_title: Hybrid ac/dc microgrids—Part II: Review and classification of control strategies

paper_content:

Microgrids are envisioned as one of the most suitable alternatives for the integration of distributed generation units in the utility grid, as they efficiently combine generation, energy storage and loads in the same distribution network. In this context, hybrid ac/dc microgrids are arising as an interesting approach as they combine the advantages of ac and dc networks and do not require excessive modifications in the distribution network. However, they require more complex control strategies as they need to control the ac and dc networks and the interface power converter simultaneously. This paper identifies and analyses the control strategies that can be implemented in hybrid microgrids for an adequate power management in grid-tied and islanded modes of operation. The review is focused on hierarchical controls as they are the most extended approach in the literature. A classification has been elaborated, which covers the three main levels of hierarchical control strategies (primary, secondary and tertiary). Each of the levels has been independently studied in order to provide a comprehensive analysis of the alternatives found in the literature. The future trends related to this topic show that a higher research effort is required regarding the control of the interface device and the ancillary services that the management strategy must provide—e.g. blackstart, transition between islanded and grid-connected modes of operation, interconnection of microgrids, etc.
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paper_title: Application of SVC and single-phase shunt capacitor to improve voltage profiles and reduce losses of unbalanced multiphase smart grid with PEV charging stations

paper_content:

In smart grids, the conventional approach of locating compensation devices based on the forecasted daily load curves is not realistic as the locations, times and durations of some loads such as plug-in electric vehicles (PEVs) and smart appliances are randomly changing during the 24 hour period. This paper proposes a new approach to improve the performance of unbalanced multiphase distribution systems consisting of single-, two- and three-phase networks with PEV charging stations. The approach is designated to perform online VRI ranking, place SVCs and single-phase capacitors at the weakest three-phase and single-phase buses, respectively; and then switch these devices in and out of the service according to the lowest voltage ranking index (VRI) values in order to improve voltage profiles and reduce total system losses. Simulation results are performed and compared for an unbalanced multiphase 13 node test feeder with PEV charging stations using DIgSILENT PowerFactory software.
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paper_title: Distribution grid impact of Plug-In Electric Vehicles charging at fast charging stations using stochastic charging model

paper_content:

Charging PEVs (Plug-In Electric Vehicles) at public fast charging station can improve the public acceptance and increase their penetration level by solving problems related to vehicles' battery. However, the price for the impact of fast charging stations on the distribution grid has to be dealt with. The main purpose of this paper is to investigate the impacts of fast charging stations on a distribution grid using a stochastic fast charging model and to present the charging model with some of its results. The model is used to investigate the impacts on distribution transformer loading and system bus voltage profiles of the test distribution grid. Stochastic and deterministic modelling approaches are also compared. It is concluded that fast charging stations affect transformer loading and system bus voltage profiles. Hence, necessary measures such as using local energy storage and voltage conditioning devices, such as SVC (Static Var Compensator), have to be used at the charging station to handle the problems. It is also illustrated that stochastic modelling approach can produce a more sound and realistic results than deterministic approach.
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paper_title: Grid-Interfacing Converter Systems With Enhanced Voltage Quality for Microgrid Application—Concept and Implementation

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Grid-interfacing converter systems with enhanced voltage quality are proposed for microgrid applications in this paper. By adapting the conventional series-parallel structure, a group of grid-interfacing system topologies are proposed for the purpose of interfacing local generation/microgrid to the grid, or interconnecting microgrids. The functionality of the proposed systems is also reconfigured in order to ease the control design and to improve overall system performance, differing from existing series-parallel structure-based systems. Experiments with a concrete laboratory system are given to detail the proposed concepts and to demonstrate the practical implementations. Two three-phase four-leg inverters, together with dc microsources and nonlinear loads, are employed to construct a general series-parallel grid-interfacing system. Through the introduction of multilevel control objectives, it is illustrated that the proposed system could ride through voltage disturbances and continue the power transfer between the local generation and the grid, while a high-quality voltage is maintained for the local loads. The system also shows the possibility to achieve auxiliary functions such as voltage unbalance correction and harmonic current compensation. The main design aspects of the controllers are specified, and the entire system is effectively validated on a laboratory setup.
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paper_title: A DSP Based Optimal Algorithm for Shunt Active Filter Under Nonsinusoidal Supply and Unbalanced Load Conditions

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The use of power electronic circuits in a wide range of applications has resulted in distorted current waveforms in the power system. This results in nonsinusoidal voltage drops across the transformers and transmission line impedances, resulting in a nonsinusoidal voltage supply at the point of common coupling. Asymmetrical distribution of large, 1-phi loads further complicates the issue by causing imbalance in the line currents of the 3-phi system. Not only should this imbalance be compensated, other parameters, such as the current total harmonic distortion and the power factor should also be maintained as per the norms. However, the requirements of harmonic free current waveforms and good power factor, under nonsinusoidal voltage conditions, are contradictory to each other. Under these conditions, an optimum performance is the best one can achieve. This paper proposes a new scheme for balancing the currents and obtaining the best compromise between the power factor and current distortion under nonsinusoidal voltage conditions. The technique does not use p-q theory and does not require sequence transformation for power calculations, even though the current is unbalanced. All the details of this work are presented
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paper_title: Advanced Control Architectures for Intelligent Microgrids—Part II: Power Quality, Energy Storage, and AC/DC Microgrids

paper_content:

This paper summarizes the main problems and solutions of power quality in microgrids, distributed-energy-storage systems, and ac/dc hybrid microgrids. First, the power quality enhancement of grid-interactive microgrids is presented. Then, the cooperative control for enhance voltage harmonics and unbalances in microgrids is reviewed. Afterward, the use of static synchronous compensator (STATCOM) in grid-connected microgrids is introduced in order to improve voltage sags/swells and unbalances. Finally, the coordinated control of distributed storage systems and ac/dc hybrid microgrids is explained.
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paper_title: Power Quality Problems Compensation With Universal Power Quality Conditioning System

paper_content:

The aim of this paper is to present a universal power quality conditioning system (UPQS) named after unified power quality conditioner, which is extended by adding a shunt active filter at the load side. Its main purpose is to compensate for both supply voltage and load current imperfections, such as: sags, swells, interruptions, imbalance, flicker, harmonics, reactive currents and current unbalance. Such system has better harmonic compensation characteristic, since there are no problems associated with passive filter operation. Converter and control analysis is presented, together with results showing the UPQS modes of operation
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paper_title: Voltage Support Control Strategies for Static Synchronous Compensators Under Unbalanced Voltage Sags

paper_content:

Static synchronous compensators have been broadly employed for the provision of electrical ac network services, which include voltage regulation, network balance, and stability improvement. Several studies of such compensators have also been conducted to improve the ac network operation during unbalanced voltage sags. This paper presents a complete control scheme intended for synchronous compensators operating under these abnormal network conditions. In particular, this control scheme introduces two contributions: a novel reactive current reference generator and a new voltage support control loop. The current reference generator has as a main feature the capacity to supply the required reactive current even when the voltage drops in amplitude during the voltage sag. Thus, a safe system operation is easily guaranteed by fixing the limit required current to the maximum rated current. The voltage control loop is able to implement several control strategies by setting two voltage set points. In this paper, three voltage support control strategies are proposed, and their advantages and limitations are discussed in detail. The two theoretical contributions of this paper have been validated by experimental results. Certainly, the topic of voltage support is open for further research, and the control scheme proposed in this paper can be viewed as an interesting configuration to devise other control strategies in future works.
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paper_title: An Implementation of an Adaptive Control Algorithm for a Three-Phase Shunt Active Filter

paper_content:

This paper deals with the hardware implementation of a shunt active filter (SAF) for compensation of reactive power, unbalanced loading, and harmonic currents. SAF is controlled using an adaptive-linear-element (Adaline)-based current estimator to maintain sinusoidal and unity-power-factor source currents. Three-phase load currents are sensed, and using least mean square (LMS) algorithm-based Adaline, online calculation of weights is performed and these weights are multiplied by the unit vector templates, which give the fundamental-frequency real component of load currents. The dc bus voltage of voltage source converter (VSC) working as a SAF is maintained at constant value using a proportional-integral controller. The switching of VSC is performed using hysteresis-based pulsewidth-modulation indirect-current-control scheme, which controls the source currents to follow the derived reference source currents. The practical implementation of the SAF is realized using dSPACE DS1104 R&D controller having TMS320F240 as a slave DSP. The MATLAB-based simulation results and implementation results are presented to demonstrate the effectiveness of the SAF with Adaline-based control for load compensation.
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paper_title: Autonomous Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

paper_content:

Recently, there has been an increasing interest in using distributed generators (DGs) not only to inject power into the grid but also to enhance the power quality. In this paper, a stationary-frame control method for voltage unbalance compensation in an islanded microgrid is proposed. This method is based on the proper control of DGs interface converters. The DGs are properly controlled to autonomously compensate for voltage unbalance while sharing the compensation effort and also active and reactive powers. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage and current controllers, and an unbalance compensator. The design approach of the control system is discussed in detail, and simulation and experimental results are presented. The results demonstrate the effectiveness of the proposed method in the compensation of voltage unbalance.
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paper_title: Active and Reactive Power Strategies With Peak Current Limitation for Distributed Generation Inverters During Unbalanced Grid Faults

paper_content:

Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.
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paper_title: A low voltage ride-through technique for grid-connected converters of distributed energy resources

paper_content:

With more and more distributed energy resources (DERs) being installed, the utility requires these generation systems and their interface converters to remain grid connected during voltage sags to ensure the operating stability of the ac power system. These low-voltage ride-through (LVRT) requirements also suggest that the DER generation system injects real power and reactive power to support grid voltages. In this paper, a positive- and negative-sequence current injection method is proposed to meet the LVRT requirement. The proposed method imposes a predefined ampere constraint in its current injection to reduce the risk of overcurrent during the LVRT operation. Its operation principle and control method are explained and analyzed. Experimental results are presented to validate the effectiveness of the proposed method. Comparisons of the proposed method and other LVRT techniques are also presented.
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paper_title: Control Strategies of Three-Phase Distributed Generation Inverters for Grid Unbalanced Voltage Compensation

paper_content:

The high penetration level of power electronics interfaced distributed generation (DG) systems creates great ancillary services potential through the DG interfacing converters, such as the grid unbalanced voltage compensation. However, the unbalanced voltage compensation may cause adverse effects on the DGs' operation, such as output active power oscillation and dc-link voltage variations. Moreover, since the compensation is realized through the available rating of DGs' interfacing converters, it is equally important to consider the effectiveness of control strategy for unbalanced voltage compensation. Considering these challenging issues, two grid unbalanced voltage compensation strategies for three-phase power electronics interfaced DG systems are proposed in this paper. Especially, the first control strategy aims at minimizing the DG's active power oscillation and reducing the adverse effects of unbalanced voltage compensation on DG's operation. The second control strategy focuses on the effectiveness of unbalanced voltage compensation by controlling DG's negative sequence current to be inphase with the grid negative sequence current. Performances of the two proposed control strategies under different grid conditions and DG operating conditions are studied, and recommendations for appropriate control strategy utilization under various conditions are provided. Finally, validity of the proposed strategies is verified by both simulations and experimental results.
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paper_title: Control Strategies Based on Symmetrical Components for Grid-Connected Converters Under Voltage Dips

paper_content:

Low-voltage ride-through (LVRT) requirements demand wind-power plants to remain connected to the network in presence of grid-voltage dips. Most dips present positive-, negative-, and zero-sequence components. Hence, regulators based on symmetrical components are well suited to control grid-connected converters. A neutral-point-clamped topology has been considered as an active front end of a distributed power-generation system, following the trend of increasing power and voltage levels in wind-power systems. Three different current controllers based on symmetrical components and linear quadratic regulator have been considered. The performance of each controller is evaluated on LVRT requirement fulfillment, grid-current balancing, maximum grid-current value control, and oscillating power flow. Simulation and experimental results show that all three controllers meet LVRT requirements, although different system performance is found for each control approach. Therefore, controller selection depends on the system constraints and the type of preferred performance features.
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paper_title: Current control method for distributed generation power generation plants under grid fault conditions

paper_content:

The operation of distributed power generation systems under grid fault conditions is a key issue for the massive integration of renewable energy systems. Several studies have been conducted to improve the response of such distributed generation systems under voltage dips. In spite of being less severe, unbalanced fault cause a great number of disconnections, as it may produce the injection of unbalanced currents that can easily conduct to the tripping of the converter. In this paper a current control strategy for power converters able to deal with these grid conditions, without exceeding the current ratings of the converter is introduced. Moreover, a novel flexible algorithm has been proposed in order to regulate easily the injection of positive and negative currents for general purpose applications.
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paper_title: Active and Reactive Power Strategies With Peak Current Limitation for Distributed Generation Inverters During Unbalanced Grid Faults

paper_content:

Distributed generation inverters have become a key element to improve grid efficiency and reliability, particularly during grid faults. Under these severe perturbations, inverter-based power sources should accomplish low-voltage ride-through requirements in order to keep feeding the grid and support the grid voltage. Also, rated current can be required to better utilize reactive power provisions. This paper presents a reference generator capable to accomplish these two objectives: to keep the injected currents within safety values and to compute the power references for a better utilization of the inverter power capacity. The reference generator is fully flexible since positive and negative active and reactive powers can be simultaneously injected to improve ride-through services. Selected experimental results are reported to evaluate the performance of the proposed reference generator under different control strategies.
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paper_title: New Optimized PWM VSC Control Structures and Strategies Under Unbalanced Voltage Transients

paper_content:

Control structures and strategies have a critical influence on a power electronic converter's behavior during disturbances. Most of the previous works in the field of pulsewidth modulation voltage-source converter (VSC) operation under unbalanced conditions propose dual vector controllers with dc bus voltage optimization strategies, which have been proven to be well adapted for rectifier applications. In this paper, two major contributions are made. On the one hand, a new optimized operation strategy based on exchanged power maximization is proposed for vector control structures, which permits the extension of optimized operation to other VSC applications (e.g., flexible alternating current transmission system and distributed generation interfaces). On the other hand, a scalar control structure is proposed based on resonant controllers, together with three adapted optimized operation strategies, namely: 1) dc bus voltage optimization; 2) power exchange maximization; and 3) a hybrid strategy (an intermediary mode between the other two strategies). Their main advantage is their simplicity and the lack of real-time symmetrical component extraction techniques. This scalar controller and the proposed optimized operation strategies are compared to conventional vector controllers. It is proven through experimental analysis that the proposed scalar controller offers very good performances with simpler structures and bigger flexibility in terms of operation modes.
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paper_title: Pliant Active and Reactive Power Control for Grid-Interactive Converters Under Unbalanced Voltage Dips

paper_content:

During voltage dips continuous power delivery from distributed generation systems to the grid is desirable for the purpose of grid support. In order to facilitate the control of inverter-based distributed power generation adapted to the expected change of grid requirements, generalized power control strategies based on symmetric-sequence components are proposed in this paper, aiming to manipulate the delivered instantaneous power under unbalanced voltage dips. It is shown that active and reactive power can be independently controlled with two individually adaptable parameters. By changing these parameters, the relative amplitudes of oscillating power can be smoothly regulated, as well as the peak values of three-phase grid currents. As a result, the power control of grid-interactive inverters becomes quite flexible and adaptable to various grid requirements or design constraints. Furthermore, two strategies for simultaneous active and reactive power control are proposed that preserve flexible controllability; an application example is given to illustrate the simplicity and adaptability of the proposed strategies for online optimization control. Finally, experimental results are provided that verify the proposed power control.
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paper_title: Control Strategies of Three-Phase Distributed Generation Inverters for Grid Unbalanced Voltage Compensation

paper_content:

The high penetration level of power electronics interfaced distributed generation (DG) systems creates great ancillary services potential through the DG interfacing converters, such as the grid unbalanced voltage compensation. However, the unbalanced voltage compensation may cause adverse effects on the DGs' operation, such as output active power oscillation and dc-link voltage variations. Moreover, since the compensation is realized through the available rating of DGs' interfacing converters, it is equally important to consider the effectiveness of control strategy for unbalanced voltage compensation. Considering these challenging issues, two grid unbalanced voltage compensation strategies for three-phase power electronics interfaced DG systems are proposed in this paper. Especially, the first control strategy aims at minimizing the DG's active power oscillation and reducing the adverse effects of unbalanced voltage compensation on DG's operation. The second control strategy focuses on the effectiveness of unbalanced voltage compensation by controlling DG's negative sequence current to be inphase with the grid negative sequence current. Performances of the two proposed control strategies under different grid conditions and DG operating conditions are studied, and recommendations for appropriate control strategy utilization under various conditions are provided. Finally, validity of the proposed strategies is verified by both simulations and experimental results.
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paper_title: Control Strategies of Three-Phase Distributed Generation Inverters for Grid Unbalanced Voltage Compensation

paper_content:

The high penetration level of power electronics interfaced distributed generation (DG) systems creates great ancillary services potential through the DG interfacing converters, such as the grid unbalanced voltage compensation. However, the unbalanced voltage compensation may cause adverse effects on the DGs' operation, such as output active power oscillation and dc-link voltage variations. Moreover, since the compensation is realized through the available rating of DGs' interfacing converters, it is equally important to consider the effectiveness of control strategy for unbalanced voltage compensation. Considering these challenging issues, two grid unbalanced voltage compensation strategies for three-phase power electronics interfaced DG systems are proposed in this paper. Especially, the first control strategy aims at minimizing the DG's active power oscillation and reducing the adverse effects of unbalanced voltage compensation on DG's operation. The second control strategy focuses on the effectiveness of unbalanced voltage compensation by controlling DG's negative sequence current to be inphase with the grid negative sequence current. Performances of the two proposed control strategies under different grid conditions and DG operating conditions are studied, and recommendations for appropriate control strategy utilization under various conditions are provided. Finally, validity of the proposed strategies is verified by both simulations and experimental results.
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paper_title: Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

paper_content:

The concept of microgrid hierarchical control is presented recently. In this paper, a hierarchical scheme is proposed which includes primary and secondary control levels. The primary level comprises distributed generators (DGs) local controllers. The local controllers mainly consist of power, voltage and current controllers, and virtual impedance control loop. The central secondary controller is designed to manage the compensation of voltage unbalance at the point of common coupling (PCC) in an islanded microgrid. Unbalance compensation is achieved by sending proper control signals to the DGs local controllers. The design procedure of the control system is discussed in detail and the simulation results are presented. The results show the effectiveness of the proposed control structure in compensating the voltage unbalance.
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paper_title: A Cooperative Imbalance Compensation Method for Distributed-Generation Interface Converters

paper_content:

Distributed generation has attracted great attention in recent years, thanks to the progress in new generation technologies and advanced power electronics. The Micro-grid has been a successful example by integrating various generation sources with the existing power distribution network through power electronics converters. The Micro-grid converters are required to operate in a de-centralized fashion, and the power- frequency droop control and reactive power-voltage droop control have been adopted for this purpose. In this paper, a droop control technique for imbalance compensation within the three- phase AC distributed generation system is proposed for Micro- grid converters. The proposed technique allows even sharing of imbalance current among various converters, can be seamlessly integrated with existing active power-frequency droop control and reactive power-voltage droop control to improve the power quality within the distributed power system. Its control functionalities are discussed, simulation and laboratory test results are also presented to validate the proposed method.
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paper_title: Autonomous Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

paper_content:

Recently, there has been an increasing interest in using distributed generators (DGs) not only to inject power into the grid but also to enhance the power quality. In this paper, a stationary-frame control method for voltage unbalance compensation in an islanded microgrid is proposed. This method is based on the proper control of DGs interface converters. The DGs are properly controlled to autonomously compensate for voltage unbalance while sharing the compensation effort and also active and reactive powers. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage and current controllers, and an unbalance compensator. The design approach of the control system is discussed in detail, and simulation and experimental results are presented. The results demonstrate the effectiveness of the proposed method in the compensation of voltage unbalance.
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paper_title: Parallel Three-Phase Interfacing Converters Operation Under Unbalanced Voltage in Hybrid AC/DC Microgrid

paper_content:

Parallel interfacing converters (IFCs) with the same dc link are commonly used to handle higher power and currents. In hybrid AC/DC microgrids, they are used to connect ac and dc subsystems, called interlinking IFCs. However, researches on parallel IFCs under unbalanced voltage are quite limited. Considering the adverse effects of unbalanced voltage on a single IFC operation (such as output power oscillations, dc link ripples, and output current increase) and the possibility of aggregation of these effects for the parallel IFCs with common dc link, a novel control strategy for parallel IFCs with various power factors is proposed in this paper. The proposed control strategy cancels out active power oscillation of parallel IFCs, which results in oscillation-free dc link/subsystem voltage. In this paper, a thorough study on the peak current of individual and parallel IFCs is conducted, and their relationship with active power oscillation mitigation is analyzed. Based on analysis, it is proven that under zero active power oscillation, the collective peak current of parallel IFCs is constant under fixed average active and reactive powers. The proposed control strategy keeps the individual IFCs’ peak currents in the same phase with collective peak current of parallel IFCs, and thus ensures reduced peak current for redundant IFC.
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paper_title: Active Power Oscillation Cancelation With Peak Current Sharing in Parallel Interfacing Converters Under Unbalanced Voltage

paper_content:

In a hybrid ac/dc grid, the ac-side unbalance voltage introduces adverse effects on dc grids and interfacing converters (IFC) (active power oscillations, dc-link voltage oscillations, and IFC peak current increase at the same average power production). For parallel IFCs, these adverse effects can be easily aggregated. In this paper, two new control strategies are proposed for parallel IFCs to improve both ac and dc subgrids power quality. The proposed control strategies focus on canceling active power oscillations of parallel IFCs that provide oscillation-free dc link and sharing collective peak current of parallel IFCs. In the first proposed control strategy, IFCs’ power coefficients are controlled by solving a set of nonlinear equations, and this method is called as coefficient-based strategy, whereas, in the second proposed control strategy, peak currents of IFCs are controlled directly through the derived relationship of IFCs' peak currents under zero power oscillation in this paper, and this method is named as peak current based strategy. This peak current based strategy features much simplified calculation and could be easy to implement. To achieve control objectives, thorough study on parallel IFCs' peak currents is conducted. Based on the study, it is shown that the collective peak current of all IFCs is a constant under zero total active power oscillation, and therefore, keeping all IFCs' peak currents in the same phase and in-phase with collective peak current optimizes the utilization range of parallel IFCs. The numerical examples and experimental results are provided to verify the validity of the proposed control strategies under different operating conditions.
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paper_title: Parallel Operation of Bidirectional Interfacing Converters in a Hybrid AC/DC Microgrid Under Unbalanced Grid Voltage Conditions

paper_content:

Today, interests on hybrid ac/dc microgrids, which contain the advantages of both ac and dc microgrids, are growing rapidly. In the hybrid ac/dc microgrid, the parallel-operated ac/dc bidirectional interfacing converters (IFCs) are increasingly used for large capacity renewable energy sources or as the interlinking converters between the ac and dc subsystems. When unbalanced grid faults occur, the active power transferred by the parallel-operated IFCs must be kept constant and oscillation-free to stabilize the dc bus voltage. However, under conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs is affected due to the converters' current rating limitations. Moreover, unbalanced voltage adverse effects on IFCs (such as output power oscillations, dc-link ripples, and output current enhancement) could be amplified by the number of parallel converters. Therefore, this paper investigates parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposes a novel control strategy to enhance the active power transfer capability with zero active power oscillation. The proposed control strategy employs a new current sharing method which introduces adjustable current reference coefficients for parallel IFCs. In the proposed control strategy, only one IFC, named as redundant IFC, needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power of parallel IFCs. Simulation and experimental results verify the feasibility and effectiveness of the proposed control strategy.
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paper_title: Design and analysis of active power control strategies for distributed generation inverters under unbalanced grid faults

paper_content:

Distributed power generation systems are expected to deliver active power into the grid and support it without interruption during unbalanced grid faults. Aiming to provide grid-interfacing inverters the flexibility to adapt to the coming change of grid requirements, an optimised active power control strategy is proposed to operate under grid faults. Specifically, through an adjustable parameter it is possible to change the relative amplitudes of oscillating active and reactive power smoothly, while simultaneously eliminating the second-order active or reactive power ripple at the two extremes of the parameter range. The steering possibility of the proposed strategy enables distributed generation inverters to be optimally designed from the perspectives of both the power-electronic converters and the power system. The proposed strategy is proved through simulation and further validated by experimental results.
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paper_title: Active Power Oscillation Cancelation With Peak Current Sharing in Parallel Interfacing Converters Under Unbalanced Voltage

paper_content:

In a hybrid ac/dc grid, the ac-side unbalance voltage introduces adverse effects on dc grids and interfacing converters (IFC) (active power oscillations, dc-link voltage oscillations, and IFC peak current increase at the same average power production). For parallel IFCs, these adverse effects can be easily aggregated. In this paper, two new control strategies are proposed for parallel IFCs to improve both ac and dc subgrids power quality. The proposed control strategies focus on canceling active power oscillations of parallel IFCs that provide oscillation-free dc link and sharing collective peak current of parallel IFCs. In the first proposed control strategy, IFCs’ power coefficients are controlled by solving a set of nonlinear equations, and this method is called as coefficient-based strategy, whereas, in the second proposed control strategy, peak currents of IFCs are controlled directly through the derived relationship of IFCs' peak currents under zero power oscillation in this paper, and this method is named as peak current based strategy. This peak current based strategy features much simplified calculation and could be easy to implement. To achieve control objectives, thorough study on parallel IFCs' peak currents is conducted. Based on the study, it is shown that the collective peak current of all IFCs is a constant under zero total active power oscillation, and therefore, keeping all IFCs' peak currents in the same phase and in-phase with collective peak current optimizes the utilization range of parallel IFCs. The numerical examples and experimental results are provided to verify the validity of the proposed control strategies under different operating conditions.
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paper_title: Parallel Three-Phase Interfacing Converters Operation Under Unbalanced Voltage in Hybrid AC/DC Microgrid

paper_content:

Parallel interfacing converters (IFCs) with the same dc link are commonly used to handle higher power and currents. In hybrid AC/DC microgrids, they are used to connect ac and dc subsystems, called interlinking IFCs. However, researches on parallel IFCs under unbalanced voltage are quite limited. Considering the adverse effects of unbalanced voltage on a single IFC operation (such as output power oscillations, dc link ripples, and output current increase) and the possibility of aggregation of these effects for the parallel IFCs with common dc link, a novel control strategy for parallel IFCs with various power factors is proposed in this paper. The proposed control strategy cancels out active power oscillation of parallel IFCs, which results in oscillation-free dc link/subsystem voltage. In this paper, a thorough study on the peak current of individual and parallel IFCs is conducted, and their relationship with active power oscillation mitigation is analyzed. Based on analysis, it is proven that under zero active power oscillation, the collective peak current of parallel IFCs is constant under fixed average active and reactive powers. The proposed control strategy keeps the individual IFCs’ peak currents in the same phase with collective peak current of parallel IFCs, and thus ensures reduced peak current for redundant IFC.
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paper_title: Active Power Oscillation Cancelation With Peak Current Sharing in Parallel Interfacing Converters Under Unbalanced Voltage

paper_content:

In a hybrid ac/dc grid, the ac-side unbalance voltage introduces adverse effects on dc grids and interfacing converters (IFC) (active power oscillations, dc-link voltage oscillations, and IFC peak current increase at the same average power production). For parallel IFCs, these adverse effects can be easily aggregated. In this paper, two new control strategies are proposed for parallel IFCs to improve both ac and dc subgrids power quality. The proposed control strategies focus on canceling active power oscillations of parallel IFCs that provide oscillation-free dc link and sharing collective peak current of parallel IFCs. In the first proposed control strategy, IFCs’ power coefficients are controlled by solving a set of nonlinear equations, and this method is called as coefficient-based strategy, whereas, in the second proposed control strategy, peak currents of IFCs are controlled directly through the derived relationship of IFCs' peak currents under zero power oscillation in this paper, and this method is named as peak current based strategy. This peak current based strategy features much simplified calculation and could be easy to implement. To achieve control objectives, thorough study on parallel IFCs' peak currents is conducted. Based on the study, it is shown that the collective peak current of all IFCs is a constant under zero total active power oscillation, and therefore, keeping all IFCs' peak currents in the same phase and in-phase with collective peak current optimizes the utilization range of parallel IFCs. The numerical examples and experimental results are provided to verify the validity of the proposed control strategies under different operating conditions.
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paper_title: Flexible Unbalanced Compensation of Three-Phase Distribution System Using Single-Phase Distributed Generation Inverters

paper_content:

In this paper, flexible compensation of negative and zero sequences current in three-phase distribution power system by using single-phase distributed generations (DGs) smart inverters is proposed. In the proposed control strategy, power factors of the DGs are controlled without modifying the active powers production. The instantaneous power analysis from single-phase perspective is developed and used to achieve the objective function for negative and zero sequences current compensation. In the proposed strategy, voltage regulation of three phases and available reactive powers of the single-phase DGs are considered as constraints of compensation problem. The developed objective function subjected to the constraints is minimized on-line by using Karush–Kuhn–Tucker optimization method. After determining the required reactive power in each phase, this power demand is shared among DGs in that phase based on their available power ratings. In this paper, comprehensive studies on the effects of voltages regulation and DGs available reactive power ratings on the proposed control strategy are conducted. An IEEE 13-node test system with seven integrated single-phase DGs has been adopted for case studies.
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paper_title: Current unbalance reduction in three-phase systems using single phase PHEV chargers

paper_content:

A method to reduce the current three phase unbalance using single phase Plug-in Hybrid Electric Vehicle (PHEV) chargers is presented in this paper. A reduction in the voltage unbalance has been achieved as a consequence of the mitigation of the current unbalance. The method performs the reactive power management on the PHEV charger in order to reduce the unbalance factor without affecting the battery charging time or charging power. The method works under restrictions to preserve the voltage quality. A simulation on Matlab-Simulink® of a three phase housing group with power profiles, initial states of charge, charging start time and PHEV category randomly selected is presented. A reduction of 50 % at some times in the day is illustrated.
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paper_title: A network voltage control strategy for LV inverter interfaced users

paper_content:

Low voltage distribution networks are characterized by an ever growing diffusion of single and three phase distributed generators whose unregulated operation may deplete the power quality level. It is thus of interest to investigate how such active elements could be managed in order to optimize the system operation in a smart grid scenario. In this paper local control strategies of single-phase and three-phase inverter-interfaced generation units for voltage magnitude regulation and current unbalance compensation respectively are presented. Simulation results show the positive effects that this type of distributed compensation may have on the power quality of low voltage distribution systems. (5 pages)
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paper_title: Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles

paper_content:

This paper reviews the current status and implementation of battery chargers, charging power levels, and infrastructure for plug-in electric vehicles and hybrids. Charger systems are categorized into off-board and on-board types with unidirectional or bidirectional power flow. Unidirectional charging limits hardware requirements and simplifies interconnection issues. Bidirectional charging supports battery energy injection back to the grid. Typical on-board chargers restrict power because of weight, space, and cost constraints. They can be integrated with the electric drive to avoid these problems. The availability of charging infrastructure reduces on-board energy storage requirements and costs. On-board charger systems can be conductive or inductive. An off-board charger can be designed for high charging rates and is less constrained by size and weight. Level 1 (convenience), Level 2 (primary), and Level 3 (fast) power levels are discussed. Future aspects such as roadbed charging are presented. Various power level chargers and infrastructure configurations are presented, compared, and evaluated based on amount of power, charging time and location, cost, equipment, and other factors.
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paper_title: Flexible Unbalanced Compensation of Three-Phase Distribution System Using Single-Phase Distributed Generation Inverters

paper_content:

In this paper, flexible compensation of negative and zero sequences current in three-phase distribution power system by using single-phase distributed generations (DGs) smart inverters is proposed. In the proposed control strategy, power factors of the DGs are controlled without modifying the active powers production. The instantaneous power analysis from single-phase perspective is developed and used to achieve the objective function for negative and zero sequences current compensation. In the proposed strategy, voltage regulation of three phases and available reactive powers of the single-phase DGs are considered as constraints of compensation problem. The developed objective function subjected to the constraints is minimized on-line by using Karush–Kuhn–Tucker optimization method. After determining the required reactive power in each phase, this power demand is shared among DGs in that phase based on their available power ratings. In this paper, comprehensive studies on the effects of voltages regulation and DGs available reactive power ratings on the proposed control strategy are conducted. An IEEE 13-node test system with seven integrated single-phase DGs has been adopted for case studies.
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paper_title: Use of single-phase inverter-interfaced DGs for power quality improvement in LV networks

paper_content:

The growing diffusion of single-phase distributed generators in LV distribution networks, already stressed by unbalanced loadings, is likely to give rise to further power quality problems, mainly voltage magnitude and unbalance issues. This paper first focuses on some recent normative decisions regarding the reactive power management for LV active users, then investigates the effects of a stronger participation of the latter to the network voltage regulation, enabled by a suitable local control strategy based on these standards. Simulations on a case study LV network are conducted for a 24 h time window, using a Backward-Forward sweep solution method.
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paper_title: Voltage unbalance compensation in LV networks with inverter interfaced distributed energy resources

paper_content:

Low voltage distribution networks are characterized by an ever growing diffusion of single and three phase distributed generators whose unregulated operation may deplete the power quality level. It is thus of interest to investigate how such active elements could be managed in order to optimize the system operation in a smart grid scenario.
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paper_title: Harmonic Iron Loss Analysis of Electrical Machines for High-Speed Operation Considering Driving Condition

paper_content:

In this paper, we proposed new harmonic iron loss calculation methods for electrical machines. The new methods based on the concept of variable loss coefficients due to their flux density and frequency estimate harmonic iron loss in the time domain and the frequency domain. In order to verify the performance of the proposed method, we calculated the iron loss with conventional methods. Then, the estimated iron losses were compared with the experimental data. It was clarified that the proposed methods are more effective than the conventional methods at high-speed operation. Based on these newly proposed methods, we analyzed the iron loss of the machine for electric vehicle (EV) according to its driving condition. From the analysis results, it was shown that the harmonic iron losses of stator are larger than before at field-weakening region. In addition, it was revealed that rotor iron loss mainly induced by stator slot ripples does not show strong dependence on the current angle and only varied primarily according to the speed.
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paper_title: Autonomous control of inverter-interfaced Distributed Generation units for harmonic current filtering and resonance damping in an islanded microgrid

paper_content:

Harmonic current filtering and resonance damping have become important concerns in the operation and control of the islanded microgrids. To address these challenges, this paper proposes a control method for the inverter-interfaced distributed generation (DG) units to autonomously share the harmonic currents and resonance damping burdens. The approach employs a load compensator based on the decomposition of output current, in addition to the outer droop-based power controller and the inner voltage and current controllers. The load compensator consists of a virtual-fundamental-impedance loop for the enhanced reactive power sharing and a variable-harmonic-impedance loop which allows to counteract the harmonic voltage drops across the grid-side inductance of the DG inverter and also to dampen out harmonic resonance propagation in the microgrid. Finally, the laboratory tests on a three-phase islanded microgrid setup are carried out to validate the performance of the proposed control scheme.
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paper_title: Assessing the Collective Harmonic Impact of Modern Residential Loads—Part I: Methodology

paper_content:

The proliferation of power-electronic-based residential loads has resulted in significant harmonic distortion in the voltages and currents of residential distribution systems. There is an urgent need for techniques that can determine the collective harmonic impact of these modern residential loads. These techniques can be used, for example, to predict the harmonic effects of mass adoption of compact fluorescent lights. In response to the need, this paper proposes a bottom-up, probabilistic harmonic assessment technique for residential feeders. The method models the random harmonic injections of residential loads by simulating their random operating states. This is performed by determining the switching-on probability of a residential load based on the load research results. The result is a randomly varying harmonic equivalent circuit representing a residential house. By combining multiple residential houses supplied with a service transformer, a probabilistic model for service transformers is also derived. Measurement results have confirmed the validity of the proposed technique. The proposed model is ideally suited for studying the consequences of consumer behavior or regulatory policy changes.
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paper_title: A Negative Feedback Repetitive Control Scheme for Harmonic Compensation

paper_content:

In this letter, a different feedback structure of the repetitive control that apparently is more appropriate for applications in power electronics is proposed. Moreover, a simple analog-circuit implementation is proposed which is suitable for high-frequency power electronics applica- tions, where digital control is unpractical due to cost and performance of available DSPs and microcontrollers.
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paper_title: Mitigation of Grid-Current Distortion for LCL-Filtered Voltage-Source Inverter With Inverter-Current Feedback Control

paper_content:

LCL filters feature low inductance; thus, the injected grid current from an LCL -filtered voltage-source inverter can be easily distorted by grid-voltage harmonics. This problem is especially tough for the control system with inverter-side current feedback (ICF), since the grid-current harmonics can freely flow into the filter capacitor. In this case, because of the loss of harmonic information, traditional harmonic controllers fail to mitigate the grid-current distortion. Although this problem may be avoided using the grid-voltage feedforward scheme, the required differentiators may cause the noise amplification. In light of the above issue, this paper develops a simple method for the ICF control system to mitigate the grid-current harmonics without extra sensors. In the proposed method, resonant harmonic controllers and an additional compensation loop are adopted at the same time. The potential instability introduced by the compensation loop can be avoided through a special design of the compensation position. Finally, the effectiveness of the proposed method for harmonic rejection is verified by detailed experimental results.
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paper_title: Frequency Adaptive Selective Harmonic Control for Grid-Connected Inverters

paper_content:

In this paper, a frequency adaptive selective harmonic control (FA-SHC) scheme is proposed. The FA-SHC method is developed from a hybrid SHC scheme based on the internal model principle, which can be designed for grid-connected inverters to optimally mitigate feed-in current harmonics. The hybrid SHC scheme consists of multiple parallel recursive (nk ± m)-order (k = 0,1,2,. . ., and m ≤ n/2) harmonic control modules with independent control gains, which can be optimally weighted in accordance with the harmonic distribution. The hybrid SHC, thus, offers an optimal tradeoff among cost, complexity, and also performance in terms of high accuracy, fast response, easy implementation, and compatible design. The analysis and synthesis of the hybrid SHC are addressed. More important, in order to deal with the harmonics in the presence of grid frequency variations, the hybrid SHC is transformed into the FA-SHC, being the proposed fractional-order controller, when it is implemented with a fixed sampling rate. The FA-SHC is implemented by substituting the fractional-order elements with the Lagrange-polynomial-based interpolation filters. The proposed FA-SHC scheme provides fast on-line computation and frequency adaptability to compensate harmonics in grid-connected applications, where the grid frequency is usually varying within a certain range (e.g., 50 ± 0.5 Hz). Experimental tests have demonstrated the effectiveness of the proposed FA-SHC scheme in terms of accurate frequency adaptability and also fast transient response.
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paper_title: Deadbeat Weighted Average Current Control With Corrective Feed-Forward Compensation for Microgrid Converters With Nonstandard LCL Filter

paper_content:

Microgrid converters are required to have the capability of both grid-tied mode and islanding mode operation. For this dual-mode operation, large shunt capacitors are often used in the interfacing converter output LCL filter, as it can help to stabilize supply voltage and to reduce switching ripple pollutions to sensitive loads during autonomous islanding operation. At the same time, this modification causes a few challenges, including the low-frequency harmonic distortions, the steady-state tracking errors and the slow dynamic response, to the line current regulation during grid-tied operation. To overcome these drawbacks, a modified weighted average current controller is developed. First, to realize a fast line current response, a deadbeat control of weighted average current is developed based on a reduced-order virtual filter plant. Second, a grid voltage feed-forward term is added to the weighted average current reference to mitigate the steady-state line current tracking errors. Note that this compensation term is directly added to the current reference, thus, it is very well decoupled from the closed-loop current regulator. In addition, it can be seen that the low-order line current harmonics caused by grid voltage distortion is inherently compensated by this proposed corrective feed-forward control.
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paper_title: A Proportional ${\bm +}$ Multiresonant Controller for Three-Phase Four-Wire High-Frequency Link Inverter

paper_content:

A new solution for unbalanced and nonlinear loads in terms of power circuit topology and controller structure is proposed in this paper. A three-phase four-wire high-frequency ac-link inverter is adopted to cater to such loads. Use of high-frequency transformer results in compact and light-weight systems. The fourth wire is taken out from the midpoint of the isolation transformer in order to avoid the necessity of an extra leg. This makes the converter suitable for unbalanced loads and eliminates the requirements of bulky capacitor in half-bridge inverter. The closed-loop control is carried out in stationary reference frame using proportional + multiresonant controller (three separate resonant controller for fundamental, fifth and seventh harmonic components). The limitations on improving steady-state response of harmonic resonance controllers is investigated and mitigated using a lead-lag compensator. The proposed voltage controller is used along with an inner current loop to ensure excellent performance of the power converter. Simulation studies and experimental results with 1 kVA prototype under nonlinear and unbalanced loading conditions validate the proposed scheme.
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paper_title: A Single Synchronous Frame Hybrid (SSFH) Multifrequency Controller for Power Active Filters

paper_content:

Conventional integration-based controllers, such as the multisynchronous PI and the multiresonant controllers, are well adapted for multifrequency-current-control applications. The first controller involves multiple reference frames, while the second one operates in a static frame using multiple resonant regulators. This paper presents a hybrid type of controller, called a single synchronous frame hybrid (SSFH) controller, which combines both features: It operates in an SSF mixing conventional PI and resonant controllers. A detailed design criterion for the SSFH controller is presented based on a frequency-response approach. Digital-implementation aspects (such as computation delays) and the phase margin of the system are taken into consideration during the design process. The SSFH and the multiresonant controllers are compared considering various criteria such as the computational load and the performances in terms of transient and steady-state response. It is concluded that the SSFH controller is a very interesting and execution time-saving structure for heavily distorted multifrequency applications, which is especially adapted for balanced or slightly unbalanced cases
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paper_title: Optimal Selective Harmonic Control for Power Harmonics Mitigation

paper_content:

This paper proposes an internal model principle (IMP)-based optimal selective harmonic controller (SHC) for power converters to mitigate power harmonics. According to the harmonics distribution caused by power converters, a universal recursive SHC module is developed to deal with a featured group of power harmonics. The proposed optimal SHC (OSHC) is of hybrid structure: All recursive SHC modules with weighted gains are connected in parallel. It bridges the real “ $nk\pm m$ order RC ” and the complex “parallel structure RC”. Compared with other IMP-based control solutions, it offers an optimal tradeoff among the cost, complexity, and performance: high accuracy, fast transient response, easy implementation, cost effective, and also easy to design. The analysis and synthesis of the OSHC system are addressed. The proposed SHC offers power converters a tailor-made optimal control solution for compensating selected harmonic frequencies. Application examples of grid-connected inverters confirm the effectiveness of the proposed control scheme.
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paper_title: Repetitive-based control for selective harmonic compensation in active power filters

paper_content:

This paper proposes a repetitive-based controller for active power filters, which compensates selected current harmonics produced by distorting loads. The approach is based on the measurement of line currents and performs the compensation of selected harmonics using a closed-loop repetitive-based control scheme based on a finite-impulse response digital filter. Compared to conventional solutions based on stationary-frame current control, this approach allows full compensation of selected frequencies, even if the active filter has limited bandwidth. Compared to synchronous-frame harmonic regulations on line currents, the complexity of the proposed algorithm is independent of the number of compensated harmonics. Moreover, it is more appropriate for digital signal processor implementation and less sensitive to rounding and quantization errors when finite word length or fixed-point implementation is considered. Experimental results on a 5-kVA prototype confirm the theoretical expectations.
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paper_title: An Improved Deadbeat Control for a Three-Phase Three-Line Active Power Filter With Current-Tracking Error Compensation

paper_content:

An active power filter (APF) plays an important role in compensating the harmonic component of a nonlinear load current. In this paper, deadbeat control (DBC), which has high control performance and fast dynamic response, is used as a current controller for an APF. An improved DBC (Imp_DBC) is introduced to suppress the adverse effect of the current sampling error and to enhance the interference rejection capability of the control system. The characteristics of the conventional DBC and Imp_DBC are analyzed using transfer functions in the discrete-time domain. The robustness of the system is improved with the inductance variation. Specifically, aiming at the dramatic change of the harmonic component of the nonlinear load current, a current-tracking error compensation method is proposed. The phenomena of the upper co-nvex and lower concave in grid current are eliminated. Finally, the experimental setup of three-phase three-line APF is built, and the effectiveness of this method is verified by experiments.
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paper_title: Stationary frame generalized integrators for current control of active power filters with zero steady state error for current harmonics of concern under unbalanced and distorted operation conditions

paper_content:

The paper proposes several concepts of integrators for sinusoidal signals. Parallel and series associations of the basic PI units using the stationary frame generalized integrators are used for current control of active power filters. Zero steady state error for the concerned current harmonics are realized, with reduced computation, under unbalanced utility or load conditions. Designing of the PI constants, digital realization of the generalized integrators, as well as compensation of the computation delay etc. are studied. Extensive test results from a 10 kW active power filter prototype are demonstrated.
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paper_title: Dead-Beat Current Controller for Voltage-Source Converters With Improved Large-Signal Response

paper_content:

A digital dead-beat current controller for voltage source converters is presented in this paper. The control structure is specified in a digital hardware description language, synthesized, and deployed on a field-programmable gate array chip. By updating, with negligible computation delay, the duty cycle twice in a switching period, the reference current error is nulled in half a modulation period, so that the controller's small-signal bandwidth is maximized. In addition, due to a simple transient detection circuit, the large-signal response delay is reduced to a small fraction of the modulation period, which is determined by the chosen current signal oversampling rate. The controller can effectively support different voltage-source inverter applications, such as active filters, uninterruptible power supplies, microgrid distributed energy resource controllers, and dc–dc converter applications, including interface converters for renewable energy sources, laboratory battery chargers, and electronic welding machines.
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paper_title: Current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

paper_content:

In this paper, a current control scheme, based on proportional-integral regulators using sinusoidal signal integrators (SSIs), is proposed for shunt type power conditioners. The aim is to simplify the implementation of SSI-based current harmonic compensation for industrial implementations where strict limitations on the harmonic distortion of the mains' currents are required. To compensate current harmonics, the SSIs are implemented to operate both on positive and negative sequence signals. One regulator, for the fundamental current component, is implemented in the stationary reference frame. The other regulators, for the current harmonics, are all implemented in a synchronous reference frame rotating at the fundamental frequency. This allows the simultaneous compensation of two current harmonics with just one regulator, yielding a significant reduction of the computational effort compared with other current control methods employing sinusoidal signal integrators implemented in stationary reference frame. A simple and robust voltage filter is also proposed by the authors to obtain a smooth and accurate position estimation of the voltage vector at the point of common coupling (PCC) under distorted mains' voltages. The whole control algorithm has been implemented on a 16-b, fixed-point digital signal processor (DSP) platform controlling a 20-kVA power conditioner prototype. The experimental results presented in this paper for inductive and capacitive loads show the validity of the proposed solutions.
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paper_title: High-Performance Digital Resonant Controllers Implemented With Two Integrators

paper_content:

Resonant controllers are one of the highest performance alternatives for ac current/voltage control. The implementations based on two integrators are widely employed to achieve frequency adaptation without substantial computational burden. However, the discretization of these schemes causes a significant error both in the resonant frequency and in the phase lead provided by the delay compensation. Therefore, perfect tracking is not assured, and stability may be compromised. This paper proposes solutions for both problems without adding a significant resource consumption by correction of the roots placement. A simple expression to calculate the target leading angle, in delay compensation schemes, is also proposed to improve stability margins by means of a better accuracy than previous approaches. Experimental results obtained with a laboratory prototype corroborate the theoretical analysis and the improvement achieved by the proposed discrete-time implementations.
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paper_title: Multirate Fractional-Order Repetitive Control of Shunt Active Power Filter Suitable for Microgrid Applications

paper_content:

This paper presents a multirate fractional-order repetitive control (MRFORC) of three-phase shunt active power filter (APF) suitable for microgrid applications. The proposed APF control scheme includes an inner proportional–integral control loop with a sampling rate identical to switching frequency and an external plug-in repetitive control (RC) loop with a reduced sampling rate. The MRFORC loop is implemented in the dq-frame with interleaving for the reduction of computational burden in each control cycle. Moreover, in order to deal with the harmonics in the presence of wide grid frequency variations, FORC, which replaces the fractional-order elements with the Lagrange-interpolation-based finite impulse response filter, is adopted instead of the conventional RC. The synthesis, stability analysis, and parameter tuning criteria of the MRFORC system are derived in detail. A step-by-step design example of the proposed controller is also given in this paper. Finally, experiments are performed to validate the feasibility and effectiveness of the proposed scheme.
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paper_title: An Advanced SVPWM-Based Predictive Current Controller for Three-Phase Inverters in Distributed Generation Systems

paper_content:

Space vector pulsewidth modulation (SVPWM) has been widely applied in the current control of three-phase voltage source inverters (VSIs). However, for grid-connected VSIs in distributed generation (DG) systems, the performance of current controllers based on SVPWM is compromised by the grid harmonics and the control delay due to computation and sampling. In this paper, an advanced SVPWM-based predictive current controller is proposed and studied. The controller mimics deadbeat control in the synchronous d-q reference frame, and is very simple and robust to implement. With the necessary grid voltage detection in DG systems for protection, grid harmonics disturbance is effectively suppressed through feedforward compensation. Based on a dual-timer sampling scheme, the control delay compensation becomes more simple yet effective. The comprehensive analysis on the proposed predictive current control system is provided. The simulation and experimental test results show that the proposed current controller has an excellent steady-state response as well as an extremely fast dynamic response.
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paper_title: Mitigation of Lower Order Harmonics in a Grid-Connected Single-Phase PV Inverter

paper_content:

In this paper, a simple single-phase grid-connected photovoltaic (PV) inverter topology consisting of a boost section, a low-voltage single-phase inverter with an inductive filter, and a step-up transformer interfacing the grid is considered. Ideally, this topology will not inject any lower order harmonics into the grid due to high-frequency pulse width modulation operation. However, the nonideal factors in the system such as core saturation-induced distorted magnetizing current of the transformer and the dead time of the inverter, etc., contribute to a significant amount of lower order harmonics in the grid current. A novel design of inverter current control that mitigates lower order harmonics is presented in this paper. An adaptive harmonic compensation technique and its design are proposed for the lower order harmonic compensation. In addition, a proportional-resonant-integral (PRI) controller and its design are also proposed. This controller eliminates the dc component in the control system, which introduces even harmonics in the grid current in the topology considered. The dynamics of the system due to the interaction between the PRI controller and the adaptive compensation scheme is also analyzed. The complete design has been validated with experimental results and good agreement with theoretical analysis of the overall system is observed.
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paper_title: Effects of Discretization Methods on the Performance of Resonant Controllers

paper_content:

Resonant controllers have gained significant importance in recent years in multiple applications. Because of their high selectivity, their performance is very dependent on the accuracy of the resonant frequency. An exhaustive study about different discrete-time implementations is contributed in this paper. Some methods, such as the popular ones based on two integrators, cause that the resonant peaks differ from expected. Such inaccuracies result in significant loss of performance, especially for tracking high-frequency signals, since infinite gain at the expected frequency is not achieved, and therefore, zero steady-state error is not assured. Other discretization techniques are demonstrated to be more reliable. The effect on zeros is also analyzed, establishing the influence of each method on the stability. Finally, the study is extended to the discretization of the schemes with delay compensation, which is also proved to be of great importance in relation with their performance. A single-phase active power filter laboratory prototype has been implemented and tested. Experimental results provide a real-time comparison among discretization strategies, which validate the theoretical analysis. The optimum discrete-time implementation alternatives are assessed and summarized.
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paper_title: A Modified Stationary Reference Frame-Based Predictive Current Control With Zero Steady-State Error for LCL Coupled Inverter-Based Distributed Generation Systems

paper_content:

This paper proposes a modified stationary reference frame-based predictive current controller with zero steady-state error for LCL coupled inverter-based distribution generation systems. Analytical expressions for the predictive current control error sources are investigated. The stability margin for the predictive current gain is derived. Online adaptive control for the grid voltage is embedded in the control system to eliminate steady-state real and reactive power errors. The grid voltage estimation expressions for sensorless operation are employed to enhance system robustness and performance. Finally, simulated and practical results verify the derived expressions and the proposed control system performance.
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paper_title: Mitigation of DC and Harmonic Currents Generated by Voltage Measurement Errors and Grid Voltage Distortions in Transformerless Grid-Connected Inverters

paper_content:

The dc offset and scaling errors in the voltage measurements cause the injection of undesired dc and harmonic currents into the three-phase output currents of the grid-connected inverter. This study proposes an enhanced current control scheme to eliminate the dc and harmonic currents caused by voltage distortion and voltage measurement error in three-phase grid-connected inverters. The proposed current controller is designed in the synchronous ( d-q ) reference frame and composed of a proportional resonant (PR) controller and a repetitive controller (RC). The role of the RC is to regulate the grid current follow the reference value as well as compensate all $6{\rm{n}}\omega _{{\rm{s}}}$ and $(6{\rm{n}}\pm 2)\omega_{{\rm{s}}}\,(n= 1,\,2,\,3,\ldots)$ harmonic components caused by distorted grid voltage and scaling errors. Meanwhile, the PR controller helps to compensate the dc current generated by dc offset measurement errors to guarantee that the three-phase grid currents are balanced and sinusoidal with extremely low dc component. Since the RC employed in the proposed current controller reduces to time delay by four times compared to the conventional RC, the proposed control system ensures good steady-state performance of the grid current without deteriorating its fast dynamic response. The effectiveness of the suggested solution is verified by various experimental tests.
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paper_title: Odd-harmonic digital repetitive control of a single-phase current active filter

paper_content:

Shunt active power filters have been proved as useful elements to correct distorted currents caused by nonlinear loads in power distribution systems. This work presents an all-digital approach, based on the repetitive control technique, for their control. In particular, a special digital repetitive plug-in controller for odd-harmonic discrete-time periodic references and disturbances is used. This approach does not introduce high gain at those frequencies for which it is not needed, and thus it improves robustness. Additionally, the necessary data memory capacity is lower than in traditional repetitive controllers. The design is performed for the particular case of single-phase shunt active filter with a full-bridge boost topology. Several experimental results are also presented to show the good behavior of the closed-loop system.
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paper_title: Control Design Guidelines for Single-Phase Grid-Connected Photovoltaic Inverters With Damped Resonant Harmonic Compensators

paper_content:

The injection of low-harmonic current to the electrical grid is nowadays regulated by international standards. In order to accomplish these standards, selective harmonic compensation is carried out in grid-connected photovoltaic inverters by means of resonant harmonic compensators. This paper gives a systematic design procedure for selecting the gains and parameters of these harmonic compensators. Other factors considered in the design process include frequency deviation, grid synchronization, and transient response. Both a design example meeting the requirements of grid interconnection and selected experimental results from a digital-signal-processor-based laboratory prototype are also reported.
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paper_title: Active Harmonic Filtering Using Current-Controlled, Grid-Connected DG Units With Closed-Loop Power Control

paper_content:

The increasing application of nonlinear loads may cause distribution system power quality issues. In order to utilize distributed generation (DG) unit interfacing converters to actively compensate harmonics, this paper proposes an enhanced current control approach, which seamlessly integrates system harmonic mitigation capabilities with the primary DG power generation function. As the proposed current controller has two well-decoupled control branches to independently control fundamental and harmonic DG currents, local nonlinear load harmonic current detection and distribution system harmonic voltage detection are not necessary for the proposed harmonic compensation method. Moreover, a closed-loop power control scheme is employed to directly derive the fundamental current reference without using any phase-locked loops (PLL). The proposed power control scheme effectively eliminates the impacts of steady-state fundamental current tracking errors in the DG units. Thus, an accurate power control is realized even when the harmonic compensation functions are activated. In addition, this paper also briefly discusses the performance of the proposed method when DG unit is connected to a grid with frequency deviation. Simulated and experimental results from a single-phase DG unit validate the correctness of the proposed methods.
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paper_title: Control strategy and site selection of a shunt active filter for damping of harmonic propagation in power distribution systems

paper_content:

This paper deals with a shunt active filter which will be installed by an electric utility, putting much emphasis on the control strategy and the best point of installation of the shunt active filter on a feeder in a power distribution system. The objective of the shunt active filter is to damp harmonic propagation, which results from harmonic resonance between many capacitors for power factor improvement and line inductors in the feeder, rather than to minimize voltage distortion throughout the feeder. Harmonic mitigation is a welcome "by-product" of the shunt active filter, which comes from damping of harmonic propagation. This paper concludes that the shunt active filter based on detection of voltage at the point of installation is superior in stability to others, and that the best site selection is not the beginning terminal but the end terminal of the primary line in the feeder. Computer simulation is performed to verify the validity and effectiveness of the shunt active filter by means of an analog circuit simulator, which is characterized by installing it on a feeder of a radial distribution system in a residential area.
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paper_title: Direct Grid Current Control of LCL-Filtered Grid-Connected Inverter Mitigating Grid Voltage Disturbance

paper_content:

A direct grid current control strategy for LCL-filtered grid-connected inverters is proposed in this paper. The conventional current control strategies are analyzed and compared, and then the necessity of direct grid current control is presented to mitigate the grid voltage disturbance. In the proposed control strategy, the virtual resistance based on the capacitance current is used to realize active damping, zero compensation is brought in to enhance the stability, and the proportional resonant (PR) controller under two-phase static coordinate is designed to track the ac reference current as well as to avoid the strong coupling brought by the coordinate transformation. Under the distortion grid voltage, the PR plus harmonic compensator (PR+HC) structure is adopted to restrain the distortion of the grid current. Finally, the proposed control strategy is verified by the experimental results.
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paper_title: Frequency Response Analysis of Current Controllers for Selective Harmonic Compensation in Active Power Filters

paper_content:

This paper compares four current control structures for selective harmonic compensation in active power filters. All controllers under scrutiny perform the harmonic compensation by using arrays of resonant controllers, one for the fundamental and one for each harmonic of interest, in order to achieve zero phase shift and unity gain in the closed-loop transfer function for selected harmonics. The complete current controller is the superposition of all individual harmonic controllers and may be implemented in various reference frames. The analysis is focused on the comparison of harmonic and total closed-loop transfer functions for each controller. Analytical similarities and differences between schemes in terms of frequency response characteristics are emphasized. It is concluded that three of them have identical harmonic behavior despite the fact that their implementation is significantly different. It emerges that the fourth one has superior behavior and robustness and can stably work at higher frequencies than the others. Theoretical findings and analysis are supported by comparative experimental results on a 7-kVA laboratory setup. The highest harmonic frequency that can be stably compensated with each control method has been determined, indicating significant differences in the control performance.
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paper_title: An Enhanced Grid Current Compensator for Grid-Connected Distributed Generation Under Nonlinear Loads and Grid Voltage Distortions

paper_content:

This paper introduces an advanced current control strategy for grid-connected operations of distributed generation (DG), which supports the DG to transfer a sinusoidal current into the utility grid despite the distorted grid voltage and nonlinear local load conditions. The proposed current controller is designed in the synchronous reference frame and composed of a proportional-integral (PI) controller and a repetitive controller (RC). An RC serves as a bank of resonant controllers, which can compensate a large number of harmonic components with a simple delay function. Hence, the control strategy can be greatly simplified. In addition, the proposed control method does not require the local load current measurement or harmonic analysis of the grid voltage. Therefore, the proposed control method can be easily adopted into the traditional DG control system without installation of extra hardware. Despite the reduced number of sensors, the grid current quality is significantly improved compared with the traditional methods with the PI controller. The operation principle of the proposed control method is analyzed in detail, and its effectiveness is validated through simulated and experimental results.
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paper_title: Current Harmonics From Single-Phase Grid-Connected Inverters—Examination and Suppression

paper_content:

Environmental conditions and operational modes may significantly impact the distortion level of the injected current from single-phase grid-connected inverter systems, such as photovoltaic (PV) inverters, which may operate in cloudy days with a maximum power point tracking, in a nonunity power factor, or in the low-voltage ride-through mode with reactive current injection. In this paper, the mechanism of the harmonic current injection from grid-connected single-phase inverter systems is thus explored, and the analysis is conducted on single-phase PV systems. In particular, the analysis is focused on the impacts of the power factor and the feed-in grid current level on the quality of the feed-in grid current from single-phase inverters. As a consequence, an internal model principle-based high-performance current control solution is tailor-made and developed for single-phase grid-connected systems to produce high-quality currents in different operation conditions, where a design procedure is also provided. The developed current controller in this paper can achieve a minimum steady-state error while maintaining a relatively fast transient response, and also being feasible in other single-phase applications as a promising harmonic mitigation solution. Experiments on single-phase grid-connected systems have verified the correctness of the relevant analysis and also the effectiveness of the tailor-made control solution in terms of good harmonic mitigation.
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paper_title: Active Harmonic Filtering Using Current-Controlled, Grid-Connected DG Units With Closed-Loop Power Control

paper_content:

The increasing application of nonlinear loads may cause distribution system power quality issues. In order to utilize distributed generation (DG) unit interfacing converters to actively compensate harmonics, this paper proposes an enhanced current control approach, which seamlessly integrates system harmonic mitigation capabilities with the primary DG power generation function. As the proposed current controller has two well-decoupled control branches to independently control fundamental and harmonic DG currents, local nonlinear load harmonic current detection and distribution system harmonic voltage detection are not necessary for the proposed harmonic compensation method. Moreover, a closed-loop power control scheme is employed to directly derive the fundamental current reference without using any phase-locked loops (PLL). The proposed power control scheme effectively eliminates the impacts of steady-state fundamental current tracking errors in the DG units. Thus, an accurate power control is realized even when the harmonic compensation functions are activated. In addition, this paper also briefly discusses the performance of the proposed method when DG unit is connected to a grid with frequency deviation. Simulated and experimental results from a single-phase DG unit validate the correctness of the proposed methods.
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paper_title: Harmonic and Negative-Sequence Current Control in an Islanded Multi-Bus MV Microgrid

paper_content:

This paper presents a multifunctional control strategy for the autonomous operation of a multi-bus medium voltage (MV) microgrid under nonlinear and unbalanced load conditions. The main objective is to effectively compensate the harmonic currents of nonlinear loads using electronically-coupled distributed generation (DG) units. The proposed control strategy consists of a multi proportional resonant controller (MPRC) with adjustable resonance frequency and a harmonic impedance controller (HIC). The MPRC and HIC are, respectively, proposed to regulate the microgrid voltage and to share the harmonic currents of the nonlinear loads among the DG units. A conventional droop scheme is also employed to share the average powers of the loads between the DG units. Moreover, a virtual negative-sequence impedance control loop is augmented to the proposed control system to compensate the negative-sequence currents of the unbalanced loads. The proposed strategy minimizes the flow of harmonic and negative-sequence currents in the MV lines of the microgrid by local compensation of harmonic and negative-sequence currents of the feeders. The performance of the proposed control scheme is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.
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paper_title: A Uniform Control Strategy for the Interlinking Converter in Hierarchical Controlled Hybrid AC/DC Microgrids

paper_content:

This paper presents a uniform control strategy for the bidirectional ac/dc interlinking converter (BIC) in hierarchical controlled hybrid microgrid. The uniform control strategy unifies different control structures for multifunctional BIC in terms of power management, ac and dc voltage support. With the unified control structure, various triggering mechanisms for BIC mode-switch are not compulsory in response to the relevant scenario changes in the power network. Therefore, the negative consequences, such as unsmooth transition and system collapse due to the inaccurate or slower mode switch, can be avoided. Moreover, the uniform control strategy is also applicable for the hierarchical controlled hybrid microgrid, whose control architecture is typically comprised of the centralized and decentralized levels. This enables the communication fault ride-through capability and the impact from the communication failures can thus be mitigated. To sum up the above two features, the proposed control strategy is able to provide a smooth operation for the BIC without control mode switch in the event of unintentional scenario changes in power or communication network. To verify the validity of the proposed uniform control strategy, the controller hardware-in-the-loop experimental results have been provided in this paper.
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paper_title: Virtual synchronous machine

paper_content:

Demands in the area of electrical energy generation and distribution, as a result of energy policies, are leading to far reaching changes in the structure of the energy supply, which is characterised, on the one hand, by the substitution of conventional power stations by renewable energy generation, a decision which has already been made, and, on the other hand, by the changeover from centralised to decentralised energy generation. From an electrical engineering point of view, a new situation will arise for consumers concerning security of supply and power quality, which calls for further technical measures by the grid operators to ensure that the increasingly stringent supply criteria can be met. This article describes a new power electronics based approach which allows a grid compatible integration of predominantly renewable electricity generators even in weak grids making them appear to be electromechanical synchronous machines. As a consequence, all the proven properties of this type of machine which have so far defined the grid continue to do so, even when integrating photovoltaic or wind energy. These properties include, for instance, interaction between grid and generator as in a remote power dispatch, reaction to transients as well as the full electrical effects of a rotating mass. In addition, this new development can be operated in such a way that it provides primary reserve allowing, from a grid point of view, electricity generators such as wind and PV to be regarded as conventional power stations.
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paper_title: Equivalence of Virtual Synchronous Machines and Frequency-Droops for Converter-Based MicroGrids

paper_content:

Over the last decade, frequency-droop-based control schemes have become the preferred solution in microgrids dominated by power electronic converters. More recently, the concept of virtual synchronous machines (VSMs) has emerged as an effective method for adding virtual inertia to the power system through the control of power electronic converters. These two approaches have been developed in two separate contexts, but present strong similarities. In fact, they are equivalent under certain conditions, as demonstrated in this letter. Analysis of this equivalence provides additional physics-based insight into the tuning and operation of both types of controllers.
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paper_title: Voltage unbalance and harmonics compensation for islanded microgrid inverters

paper_content:

Voltage source inverters (VSIs) are usually used for all kinds of distributed generation interfaces in a microgrid. It is the microgrid's superiority to power the local loads continuously when the utility fails. When in islanded mode, the voltage and frequency of the microgrid are determined by the VSIs; therefore the power quality can be deteriorated under unbalanced and non-linear loads. A voltage unbalance and harmonics compensation strategy for the VSIs in islanded microgrid is proposed in this study. This method is implemented in a single synchronous reference frame (SRF) and is responsible for both the voltage unbalance and harmonic compensation. Furthermore, the virtual impedance loop is modified to improve the compensation effect. The impedance model of the VSI is built to explain the compensation ability of the proposed strategy. The whole control system mainly includes power droop controllers, a modified virtual impedance loop and inner SRF-based voltage unbalance and harmonics compensators. The proposed strategy is demonstrated in detail and validated with simulations and experiments.
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paper_title: Mitigation of Harmonics in Grid-Connected and Islanded Microgrids Via Virtual Admittances and Impedances

paper_content:

Optimization of the islanded and grid-connected operation of microgrids is important to achieve a high degree of reliability. In this paper, the authors consider the effect of current harmonics in single phase microgrids during both modes of operation. A detailed analysis of the effect of the output impedance of the considered primary control loops on the harmonic output of the considered voltage source inverters is initially carried out. A virtual admittance loop is proposed to attenuate the current harmonic output in grid-connected operation that is generated due to the grid voltage distortion present at the point of common coupling (PCC) and due to local non-linear loads. This paper also considers the harmonic current sharing and resulting voltage harmonics at the PCC during islanded operation of the microgrid. A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC. Experimental results are given to validate the operation of the proposed algorithms.
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paper_title: Secondary Control for Voltage Quality Enhancement in Microgrids

paper_content:

In this paper, a hierarchical control scheme is proposed for enhancement of sensitive load bus (SLB) voltage quality in microgrids. The control structure consists of primary and secondary levels. The primary control level comprises distributed generators (DGs) local controllers. Each of these controllers includes a selective virtual impedance loop which is considered to improve sharing of fundamental and harmonic components of load current among the DG units. The sharing improvement is provided at the expense of increasing voltage unbalance and harmonic distortion. Thus, the secondary control level is applied to manage the compensation of SLB voltage unbalance and harmonics by sending proper control signals to the primary level. DGs compensation efforts are controlled locally at the primary level. The system design procedure for selecting proper control parameters is discussed. Simulation results are provided in order to demonstrate the effectiveness of the proposed control scheme.
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paper_title: A Flexible Harmonic Control Approach Through Voltage-Controlled DG–Grid Interfacing Converters

paper_content:

The increased penetration of nonlinear loads and power-electronics-based distributed generation (DG) systems may introduce power quality issues to the distribution power system. However, if controlled and regulated properly, the DG-grid interfacing converters are able to improve the distribution system efficiency and power quality. This paper focuses on the distribution system harmonic control through the DG-grid interfacing converters. Two alternative DG systems, namely, current-controlled DG and voltage-controlled DG, are considered. While most of the previous works on harmonic compensation are based on the current-controlled method, a novel harmonic control scheme using a voltage-controlled method is developed in this paper. The voltage-controlled method is more flexible and has similar compensation performance compared to the conventional current-controlled method. In addition, by avoiding the implementation of a harmonic current tracking loop, the proposed voltage-based method can be seamlessly incorporated into a voltage-controlled DG unit, which is important to provide direct voltage and frequency support in a microgrid. Moreover, the possible harmonic circulating current among multiple DG systems is also investigated in this paper. Simulations and experimental results from a three-phase 5-kVA laboratory DG prototype are provided to validate the discussion.
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paper_title: Active Power Quality Improvement Strategy for Grid-Connected Microgrid Based on Hierarchical Control

paper_content:

When connected to a distorted grid utility, droop-controlled grid-connected microgrids (DCGC-MGs) exhibit low equivalent impedance. The harmonic and unbalanced voltage at the point of common coupling (PCC) deteriorates the power quality of the grid-connected current (GCC) of DCGC-MG. This paper proposes an active, unbalanced, and harmonic GCC suppression strategy based on hierarchical theory. The voltage error between the bus of the DCGC-MG and the grid’s PCC was transformed to the dq frame. On the basis of the grid, an additional compensator, which consists of multiple resonant voltage regulators, was then added to the original secondary control to generate the negative fundamental and unbalanced harmonic voltage reference. Proportional integral and multiple resonant controllers were adopted as voltage controller at the original primary level to improve the voltage tracking performance of the inverter. Consequently, the voltage difference between the PCC and the system bus decreased. In addition, we established a system model for parameter margin and stability analyses. Finally, the simulation and experiment results from a scaled-down laboratory prototype were presented to verify the validity of the proposed control strategy.
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paper_title: Distributed Active Synchronization Strategy for Microgrid Seamless Reconnection to the Grid Under Unbalance and Harmonic Distortion

paper_content:

Microgrids can operate in both grid-connected and islanded modes. In order to seamlessly transfer from islanded to grid-connected modes, it is necessary to synchronize microgrid voltage and frequency, and phase to the main grid. However, since the microgrid is often based on power electronic converters, the synchronization process is quite different compared with the quasi-synchronism control in conventional power systems. First, in order to address this concern, the microgrid synchronization criteria are derived. Based on these criteria, a novel distributed active synchronization strategy is proposed, which takes into account not only the fundamental component, but also positive and negative sequences of the harmonic components. This way, a seamless reconnection to the main grid can be performed. The proposed method is implemented in the secondary control level of a hierarchical control structure. Real-time hardware-in-the-loop results show the feasibility of the proposed technique.
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paper_title: A harmonic compensation approach for interlinking voltage source converters in hybrid AC-DC microgrids with low switching frequency

paper_content:

In recent years, the hybrid AC-DC microgrid has been well accepted as it combines the advantages of both AC and DC systems. As the microgrid contains both DC sub-grids and AC sub-grids, interlinking DC-AC converters are essential. Meanwhile, considering the nonlinear AC loads may deteriorate the voltage quality of the AC bus, embedding an ancillary harmonic compensation function to the interlinking converters is promising. However, the conventional harmonic control methods used for active power filters (APFs) may not be suitable for the interlinking converters due to the main purpose of it is to exchange real and reactive power between the DC and AC sub-grids. The switching frequency is preferred to be lower than the APFs when the capacity of the microgrid is large. At low switching frequency, harmonic compensation performance or even the system stability may be affected. In this paper, a harmonic compensation approach suitable for hybrid AC-DC interlinking converters at low switching frequency is proposed. Through feeding the PWM reference signal with the harmonic compensation component directly to avoid the multi-loop control path of the fundamental component, the proposed method can achieve the effective harmonics compensation without being limited by the closed-loop control bandwidth. The proposed method, modeling approaches, stability analysis, as well as detailed virtual impedance design are presented. Experimental verification is also provided.
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paper_title: Injected Grid Current Quality Improvement for a Voltage-Controlled Grid-Connected Inverter

paper_content:

The injected grid current of the voltage-controlled grid-connected inverter has serious distortion due to the low frequency harmonic components in the output voltage, which is resulted by the dead time of the drive signals of the power switches, and the distortion cannot be eliminated even when an output voltage closed loop is incorporated. This paper introduces a virtual series impedance to increase the output impedance of the grid-connected inverter only at the dominated lower harmonic frequencies; thus, the injected grid current quality can be improved and the dynamic performance of the grid-connected inverter is not deteriorated. Experimental results from a 6-kW single-phase grid-connected inverter confirm the effectiveness of the proposed method.
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paper_title: A Flexible Harmonic Control Approach Through Voltage-Controlled DG–Grid Interfacing Converters

paper_content:

The increased penetration of nonlinear loads and power-electronics-based distributed generation (DG) systems may introduce power quality issues to the distribution power system. However, if controlled and regulated properly, the DG-grid interfacing converters are able to improve the distribution system efficiency and power quality. This paper focuses on the distribution system harmonic control through the DG-grid interfacing converters. Two alternative DG systems, namely, current-controlled DG and voltage-controlled DG, are considered. While most of the previous works on harmonic compensation are based on the current-controlled method, a novel harmonic control scheme using a voltage-controlled method is developed in this paper. The voltage-controlled method is more flexible and has similar compensation performance compared to the conventional current-controlled method. In addition, by avoiding the implementation of a harmonic current tracking loop, the proposed voltage-based method can be seamlessly incorporated into a voltage-controlled DG unit, which is important to provide direct voltage and frequency support in a microgrid. Moreover, the possible harmonic circulating current among multiple DG systems is also investigated in this paper. Simulations and experimental results from a three-phase 5-kVA laboratory DG prototype are provided to validate the discussion.
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paper_title: A Voltage Feedback Based Harmonic Compensation Strategy for Current-Controlled Converters

paper_content:

Harmonics have been considered as one of the major issues in future power grids. With the increasing demand in advanced control functions, power electronic converter interfaced distributed generators (DGs) are expected to perform harmonic compensation when necessary. It has been demonstrated in a number of studies that DG converters operating in voltage-controlled mode can be easily configured to realize voltage harmonic compensation utilizing naturally embedded voltage control loop. While for DGs operating in current-controlled mode (CCM), such function was rarely studied. Considering that CCM is commonly used in renewable energy based generators and energy storage systems, it has certain significance to achieve the same function with CCM converters. Aiming at such objectives, this paper proposes a voltage detection based harmonic compensator (HC) for CCM converters. The novelty and main advantages of the proposed method include the following. It realizes seamless interface of HC with inner fundamental current control loop. Compared with a conventional active power filtering method, it does not require remote load harmonic current measurement since it is local voltage detection based. Compared with a conventional voltage detection based method, it offers better performance because of directly harmonic voltage regulation. Experimental results are presented to demonstrate the effectiveness of the method.
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paper_title: Hybrid Voltage and Current Control Approach for DG-Grid Interfacing Converters With LCL filters

paper_content:

This paper presents a hybrid voltage and current control method to improve the performance of interfacing converters in distributed generation (DG) units. In general, current-controlled methods have been widely adopted in grid-connected converters nowadays. Nevertheless, in an islanded system, the voltage control of DG units is desired to provide direct voltage support to the loads. Due to the absence of closed-loop line current controller, the voltage control scheme can hardly regulate the DG unit's line current harmonics. Furthermore, if not addressed properly, the transfer between the grid-connected operation and autonomous islanding operation will introduce nontrivial transient currents. To overcome the drawbacks of voltage- and current-controlled DG units, this paper develops a hybrid voltage and current control method (HCM). The proposed method allows the coordinated closed-loop control of the DG unit fundamental voltage and line harmonic currents. With the HCM, local harmonic loads of the DG unit can even be compensated without using harmonic current extraction. In addition, the HCM guarantees smooth transition during the grid-connected/islanding operation mode transfer. Simulated and experimental results are provided to verify the feasibility of the proposed approach.
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paper_title: Full Feedforward of Grid Voltage for Grid-Connected Inverter With LCL Filter to Suppress Current Distortion Due to Grid Voltage Harmonics

paper_content:

The grid-connected inverter with an LCL filter has the ability of attenuating the high-frequency current harmonics. However, the current distortion caused by harmonics in the grid voltage is difficult to be eliminated. Increasing the loop gain can reduce the current distortion, but this approach is compromised by the system stability requirement. Without increasing the loop gain, applying feedforward of the grid voltage can suppress the effect of grid voltage harmonics. This paper proposes the feedforward function of the grid voltage for the grid-connected inverter with an LCL filter. Specifically, the proposed feedforward function involves proportional, derivative, and second derivative of the grid voltage, and can be simplified according to the dominant harmonics in the grid voltage. The proposed feedforward scheme can effectively suppress the current distortion arising from the grid voltage harmonics, and the steady-state error of the injected current can be substantially reduced even if a conventional proportional and integral regulator is applied. A 6-kW experimental prototype has been tested to verify the effectiveness of the proposed feedforward scheme.
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paper_title: A novel DSP-based current-controlled PWM strategy for single phase grid connected inverters

paper_content:

The three major approaches for current-regulated inverters are ramp comparison, hysteresis control, and predictive current control. From these three, predictive current control offers the potential for achieving more precise current control with minimum distortion and harmonic noise. But, the predictive method is difficult to implement and needs more computational burden. The traditional predictive current controller has a poor performance under component parameter variations, and is less robust to filters inductance mismatch. A robust predictive current control strategy was proposed and studied. Both simulations and experimental results show that the proposed method is much more robust to parameter mismatch than the traditional one.
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paper_title: Impedance Shaping of the Grid-Connected Inverter with LCL Filter to Improve Its Adaptability to the Weak Grid Condition

paper_content:

The current-controlled grid-connected inverter with LCL filter is widely used in the distributed generation system (DGS), due to its fast dynamic response and better power quality features. However, with the increase of power injected into the grid, control performances of the inverter will be significantly influenced by the nonideal grid conditions. Specifically, the possible wide variation of the grid impedance challenges the system stability. Meanwhile, background harmonics of the grid can greatly distort the injected current. Therefore, the control of the inverter should be designed with strong stability-robustness and high harmonic-rejection-ability, both of which correlate closely with the inverter output impedance. However, it is difficult to shape the output impedance into the one with a desirable characteristic simply by adjusting the current loop gain. In this paper, an impedance shaping method is proposed with virtual impedances, and the current control loop can be designed independently. The implementation and parameter design of the virtual impedances are studied under the practical considerations. With this proposed method, the grid-connected inverter can work stably over a wide range of the typical inductive-resistive grid impedance and exhibit strong rejection ability of grid-voltage harmonics. Experimental results from a 6-kW single-phase grid-connected inverter confirm the effectiveness of the proposed method.
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paper_title: Hybrid AC/DC System Harmonics Control Through Grid Interfacing Converters With Low Switching Frequency

paper_content:

The nonlinear loads in distribution grid may generate harmful low-order harmonics, polluting the grid and deteriorating the voltage quality particularly when the grid is weak. The grid interfacing converters in the distribution system, such as the distributed generation (DG) interfacing converters or hybrid ac/dc grid interlinking converters, can participate in distribution grid harmonic control. In this paper, two virtual-impedance-based harmonics control methods are developed for grid interfacing voltage-source inverters (VSIs) to improve the power quality of the distribution grid. As the control parameters are designed based on the virtual impedance theory, clear physical meanings are given to explain their impacts on the system. Also, the proposed methods do not rely on the closed loop feedback control and, therefore, are very suitable for VSIs with low switching frequency (such as those for high-power DGs or interlinking converters for hybrid ac/dc systems), whose closed-loop control feedback bandwidth may be limited for harmonic regulation. The influence of system delay and feedback control loop is considered and modeled in the design procedure; thus more accurate virtual impedance control is achieved for low-switching-frequency VSIs. Moreover, a comprehensive comparison of the two methods, including stability and harmonic compensation performance, are given. Their effectiveness is verified by experiment results.
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paper_title: A Real-Time Computation Method With Dual Sampling Mode to Improve the Current Control Performance of the $LCL$-Type Grid-Connected Inverter

paper_content:

Due to the higher attenuation of switching frequency current harmonics, the $LCL$ filter has been widely used in grid-connected inverters. To deal with the resonance of the $LCL$ filter, the capacitor current is usually fed back to damp the resonance actively. However, the computation and pulsewidth modulation (PWM) delays in the digital control system have a significant influence on the active damping method, resulting in poor system robustness. Meanwhile, these delays also reduce the control bandwidth greatly and thus impose a severe limitation on the low-frequency gains. In this paper, a real-time computation method with dual sampling mode is proposed to remove the computation delay from the inner active damping loop and the outer grid-current control loop simultaneously; thus, the system robustness and the control performance can be greatly improved. Moreover, the time duration between the sampling instant and the switching transition of the inverter bridge is extended by the proposed method, which effectively prevents the switching noise distorting the sampled signals. Therefore, the noise immunity of the inverter is also improved greatly. Experimental results from a 6-kW $LCL$ -type single-phase grid-connected inverter confirm the theoretical expectations and the effectiveness of the proposed method.
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paper_title: Study of a Current Control Strategy Based on Multisampling for High-Power Grid-Connected Inverters With an LCL filter

paper_content:

The high-power grid-connected inverters with LCL filters have been widely used. Current control plays a key role in the grid-connected inverter control system. To cope with the inherent resonance of the LCL filter, active damping (AD) methods are usually employed. However, the AD performance is impaired by control delays which are introduced in the digital control implementation process. Besides, the lag phase due to control delays limits the system bandwidth and stability margin. The effects of control delays are more noteworthy in the high-power grid-connected inverter due to its low switching frequency. This paper investigates the current control strategy based on multisampling for the high-power grid-connected inverter with the LCL filter. First, the multisampled AD scheme is studied, which can reduce control delays effectively and improve AD performance. Besides, the multisampled control without additional damping whether passive or active is researched. Through the inverter-side current feedback, the system can realize single-current-loop control based on multisampling. Thus, the control system is simplified and it can be stable, and achieve better dynamic performance. Finally, experimental results show that the proposed control schemes are effective.
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paper_title: DC Microgrid Technology: System Architectures, AC Grid Interfaces, Grounding Schemes, Power Quality, Communication Networks, Applications, and Standardizations Aspects

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To meet the fast-growing energy demand and, at the same time, tackle environmental concerns resulting from conventional energy sources, renewable energy sources are getting integrated in power networks to ensure reliable and affordable energy for the public and industrial sectors. However, the integration of renewable energy in the ageing electrical grids can result in new risks/challenges, such as security of supply, base load energy capacity, seasonal effects, and so on. Recent research and development in microgrids have proved that microgrids, which are fueled by renewable energy sources and managed by smart grids (use of smart sensors and smart energy management system), can offer higher reliability and more efficient energy systems in a cost-effective manner. Further improvement in the reliability and efficiency of electrical grids can be achieved by utilizing dc distribution in microgrid systems. DC microgrid is an attractive technology in the modern electrical grid system because of its natural interface with renewable energy sources, electric loads, and energy storage systems. In the recent past, an increase in research work has been observed in the area of dc microgrid, which brings this technology closer to practical implementation. This paper presents the state-of-the-art dc microgrid technology that covers ac interfaces, architectures, possible grounding schemes, power quality issues, and communication systems. The advantages of dc grids can be harvested in many applications to improve their reliability and efficiency. This paper also discusses benefits and challenges of using dc grid systems in several applications. This paper highlights the urgent need of standardizations for dc microgrid technology and presents recent updates in this area.
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paper_title: Grid Converters for Photovoltaic and Wind Power Systems

paper_content:

About the Authors. Preface. Acknowledgements. 1 Introduction. 1.1 Wind Power Development. 1.2 Photovoltaic Power Development. 1.3 The Grid Converter The Key Element in Grid Integration of WT and PV Systems. 2 Photovoltaic Inverter Structures. 2.1 Introduction. 2.2 Inverter Structures Derived from H-Bridge Topology. 2.3 Inverter Structures Derived from NPC Topology. 2.4 Typical PV Inverter Structures. 2.5 Three-Phase PV Inverters. 2.6 Control Structures. 2.7 Conclusions and Future Trends. 3 Grid Requirements for PV. 3.1 Introduction. 3.2 International Regulations. 3.3 Response to Abnormal Grid Conditions. 3.4 Power Quality. 3.5 Anti-islanding Requirements. 3.6 Summary. 4 Grid Synchronization in Single-Phase Power Converters. 4.1 Introduction. 4.2 Grid Synchronization Techniques for Single-Phase Systems. 4.3 Phase Detection Based on In-Quadrature Signals. 4.4 Some PLLs Based on In-Quadrature Signal Generation. 4.5 Some PLLs Based on Adaptive Filtering. 4.6 The SOGI Frequency-Locked Loop. 4.7 Summary. 5 Islanding Detection. 5.1 Introduction. 5.2 Nondetection Zone. 5.3 Overview of Islanding Detection Methods. 5.4 Passive Islanding Detection Methods. 5.5 Active Islanding Detection Methods. 5.6 Summary. 6 Grid Converter Structures forWind Turbine Systems. 6.1 Introduction. 6.2 WTS Power Configurations. 6.3 Grid Power Converter Topologies. 6.4 WTS Control. 6.5 Summary. 7 Grid Requirements for WT Systems. 7.1 Introduction. 7.2 Grid Code Evolution. 7.3 Frequency and Voltage Deviation under Normal Operation. 7.4 Active Power Control in Normal Operation. 7.5 Reactive Power Control in Normal Operation. 7.6 Behaviour under Grid Disturbances. 7.7 Discussion of Harmonization of Grid Codes. 7.8 Future Trends. 7.9 Summary. 8 Grid Synchronization in Three-Phase Power Converters. 8.1 Introduction. 8.2 The Three-Phase Voltage Vector under Grid Faults. 8.3 The Synchronous Reference Frame PLL under Unbalanced and Distorted Grid Conditions. 8.4 The Decoupled Double Synchronous Reference Frame PLL (DDSRF-PLL). 8.5 The Double Second-Order Generalized Integrator FLL (DSOGI-FLL). 8.6 Summary. 9 Grid Converter Control for WTS. 9.1 Introduction. 9.2 Model of the Converter. 9.3 AC Voltage and DC Voltage Control. 9.4 Voltage Oriented Control and Direct Power Control. 9.5 Stand-alone, Micro-grid, Droop Control and Grid Supporting. 9.6 Summary. 10 Control of Grid Converters under Grid Faults. 10.1 Introduction. 10.2 Overview of Control Techniques for Grid-Connected Converters under Unbalanced Grid Voltage Conditions. 10.3 Control Structures for Unbalanced Current Injection. 10.4 Power Control under Unbalanced Grid Conditions. 10.5 Flexible Power Control with Current Limitation. 10.6 Summary. 11 Grid Filter Design. 11.1 Introduction. 11.2 Filter Topologies. 11.3 Design Considerations. 11.4 Practical Examples of LCL Filters and Grid Interactions. 11.5 Resonance Problem and Damping Solutions. 11.6 Nonlinear Behaviour of the Filter. 11.7 Summary. 12 Grid Current Control. 12.1 Introduction. 12.2 Current Harmonic Requirements. 12.3 Linear Current Control with Separated Modulation. 12.4 Modulation Techniques. 12.5 Operating Limits of the Current-Controlled Converter. 12.6 Practical Example. 12.7 Summary. Appendix A Space Vector Transformations of Three-Phase Systems. A.1 Introduction. A.2 Symmetrical Components in the Frequency Domain. A.3 Symmetrical Components in the Time Domain. A.4 Components 0 on the Stationary Reference Frame. A.5 Components dq0 on the Synchronous Reference Frame. Appendix B Instantaneous Power Theories. B.1 Introduction. B.2 Origin of Power Definitions at the Time Domain for Single-Phase Systems. B.3 Origin of Active Currents in Multiphase Systems. B.4 Instantaneous Calculation of Power Currents in Multiphase Systems. B.5 The p-q Theory. B.6 Generalization of the p-q Theory to Arbitrary Multiphase Systems. B.7 The Modified p-q Theory. B.8 Generalized Instantaneous Reactive Power Theory for Three-Phase Power Systems. B.9 Summary. Appendix C Resonant Controller. C.1 Introduction. C.2 Internal Model Principle. C.3 Equivalence of the PI Controller in the dq Frame and the P+Resonant Controller in the Frame. Index.
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paper_title: A new technique to reject DC-link voltage ripple for inverters operating on programmed PWM waveforms

paper_content:

A new proposal to reject DC-link voltage ripple in inverters operating on programmed PWM (pulse width modulation) waveforms is examined in detail. It is demonstrated how continuous elimination of harmonics is achieved at the inverter output while simultaneously rejecting the DC-link voltage ripple. Thus, with the proposed technique, high-quality voltage is guaranteed at the inverter output terminals even with a substantial low-frequency voltage ripple on the DC-link. A thorough modeling of this technique, along with the tradeoffs involved in acquiring the immunity to DC-link ripple, is illustrated in detail. Potential applications of the proposed technique are in fixed and variable frequency inverters for power supplies and AC motor drives which experience voltage ripple in the DC-link, such as when fed from a weak AC-system which is frequently unbalanced. A design procedure and the digital implementation of the proposed technique are described. Selected results are verified experimentally on a laboratory inverter.<<ETX>>
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paper_title: Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults

paper_content:

The increasing penetration of distributed power generation into the power system leads to a continuous evolution of grid interconnection requirements. In particular, active power control will play an important role both during grid faults (low-voltage ride-through capability and controlled current injection) and in normal conditions (reserve function and frequency regulation). The aim of this paper is to propose a flexible active power control based on a fast current controller and a reconfigurable reference current selector. Several strategies to select the current reference are studied and compared using experimental results that are obtained during an unsymmetrical voltage fault. The results of the analysis allow selection of the best reference current in every condition. The proposed methods facilitate multiple choices for fault ride through by simply changing the reference selection criteria.
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paper_title: Topology Study of Photovoltaic Interface for Maximum Power Point Tracking

paper_content:

This paper looks at the performance of photovoltaic modules in nonideal conditions and proposes topologies to minimize the degradation of performance caused by these conditions. It is found that the peak power point of a module is significantly decreased due to only the slightest shading of the module, and that this effect is propagated through other nonshaded modules connected in series with the shaded one. Based on this result, two topologies for parallel module connections have been outlined. In addition, dc/dc converter technologies, which are necessary to the design, are compared by way of their dynamic models, frequency characteristics, and component cost. Out of this comparison, a recommendation has been made
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paper_title: Plug-and-Play Voltage Ripple Mitigator for DC Links in Hybrid AC–DC Power Grids With Local Bus-Voltage Control

paper_content:

In this paper, a straightforward plug-and-play voltage ripple mitigator (RM) is proposed. Unlike existing voltage ripple reduction methods, the proposed device can be attached to the dc link of a hybrid ac–dc power system without modifying the host system itself. In particular, a local bus-voltage control scheme is employed to achieve the plug-and-play operation. With the requirement of the dc-link voltage measurement only, this device can be operated as a standalone module. Theoretical analysis and experimental work on a boost-type power factor correction rectification system have been successfully performed to validate the effectiveness of the ripple-mitigating function, the hot-swap operation and the nonintrusive property of the RM. Practical results obtained from a 110-W miniature hybrid ac–dc power system comprising an ac/dc converter and two resistive loads are included to demonstrate some of the functions of the RM.
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paper_title: Optimization of perturb and observe maximum power point tracking method

paper_content:

Maximum power point tracking (MPPT) techniques are used in photovoltaic (PV) systems to maximize the PV array output power by tracking continuously the maximum power point (MPP) which depends on panels temperature and on irradiance conditions. The issue of MPPT has been addressed in different ways in the literature but, especially for low-cost implementations, the perturb and observe (PO moreover, it is well known that the P&O algorithm can be confused during those time intervals characterized by rapidly changing atmospheric conditions. In this paper it is shown that, in order to limit the negative effects associated to the above drawbacks, the P&O MPPT parameters must be customized to the dynamic behavior of the specific converter adopted. A theoretical analysis allowing the optimal choice of such parameters is also carried out. Results of experimental measurements are in agreement with the predictions of theoretical analysis.
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paper_title: Active Suppression of Selected DC Bus Harmonics for Dual Active Bridge DC–DC Converters

paper_content:

AC coupled dual active bridge (DAB) dc–dc converters typically use phase shifted square wave (PSSW) modulation to manage the power flow between two dc sources. With this scheme, the current flowing between the converter bridges injects high-magnitude current harmonics into each dc port at multiples of the primary switching frequency, which can excite resonances in the $LC$ circuits created by parasitic second-order impedances such as wiring inductances in the dc-link connections. This can cause substantial dc bus voltage and current oscillations, particularly with the higher switching frequencies that are used with wide bandgap devices, leading to excessive electromagnetic interference, significant filter stress, and eventual component operational failure. Conventionally, a relatively large dc bus filter capacitor (or inductor) helps to suppress these dc bus harmonic dynamics. However, the use of adaptive three-level modulation for a single phase DAB provides a much greater solution space to achieve a desired power transfer condition, with the three PSSW control angles that are available. This paper now explores the additional use of these angles to selectively suppress particular dc bus current harmonics across the entire operating range of the converter and thus allow the size of the DAB dc bus bridge capacitors to be minimized. This new active harmonic suppression (AHS) strategy is validated by theory, simulation, and matching experimental results.
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paper_title: Frequency-Adaptive Filtering of Low-Frequency Harmonic Current in Fuel Cell Power Conditioning Systems

paper_content:

Based on one of the most researched multiinput dc-dc converter topologies for renewable energy systems, the multi-input dual-active bridge (DAB) dc-dc converter, the effectiveness of harmonic current absorption by the energy storage branch in fuel cell power conditioning systems is critically evaluated. The closed-loop output impedances of the converter under single-voltage-loop and dual-loop controls are derived and compared. It is shown that both control strategies can effectively reduce the converter's closed-loop output impedance, thus favoring the flow of harmonic current and prevent it from being drawn from either the fuel cell branch or the dc-link capacitor. However, as shown by experimental results, the use of conventional PI control alone still produces noticeable voltage ripple on the dc voltage bus due to harmonic current being drawn from the dc-link capacitor. Proportional-resonant control is proposed to effectively compensate for the dc-bus voltage variation by generating an extremely low impedance path for harmonic current flow at specific frequency. An analog-based frequency tracking circuit is further proposed to adjust the resonant frequency for compensating the effect of harmonic frequency variation.
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paper_title: Parallel Three-Phase Interfacing Converters Operation Under Unbalanced Voltage in Hybrid AC/DC Microgrid

paper_content:

Parallel interfacing converters (IFCs) with the same dc link are commonly used to handle higher power and currents. In hybrid AC/DC microgrids, they are used to connect ac and dc subsystems, called interlinking IFCs. However, researches on parallel IFCs under unbalanced voltage are quite limited. Considering the adverse effects of unbalanced voltage on a single IFC operation (such as output power oscillations, dc link ripples, and output current increase) and the possibility of aggregation of these effects for the parallel IFCs with common dc link, a novel control strategy for parallel IFCs with various power factors is proposed in this paper. The proposed control strategy cancels out active power oscillation of parallel IFCs, which results in oscillation-free dc link/subsystem voltage. In this paper, a thorough study on the peak current of individual and parallel IFCs is conducted, and their relationship with active power oscillation mitigation is analyzed. Based on analysis, it is proven that under zero active power oscillation, the collective peak current of parallel IFCs is constant under fixed average active and reactive powers. The proposed control strategy keeps the individual IFCs’ peak currents in the same phase with collective peak current of parallel IFCs, and thus ensures reduced peak current for redundant IFC.
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paper_title: Active Power Oscillation Cancelation With Peak Current Sharing in Parallel Interfacing Converters Under Unbalanced Voltage

paper_content:

In a hybrid ac/dc grid, the ac-side unbalance voltage introduces adverse effects on dc grids and interfacing converters (IFC) (active power oscillations, dc-link voltage oscillations, and IFC peak current increase at the same average power production). For parallel IFCs, these adverse effects can be easily aggregated. In this paper, two new control strategies are proposed for parallel IFCs to improve both ac and dc subgrids power quality. The proposed control strategies focus on canceling active power oscillations of parallel IFCs that provide oscillation-free dc link and sharing collective peak current of parallel IFCs. In the first proposed control strategy, IFCs’ power coefficients are controlled by solving a set of nonlinear equations, and this method is called as coefficient-based strategy, whereas, in the second proposed control strategy, peak currents of IFCs are controlled directly through the derived relationship of IFCs' peak currents under zero power oscillation in this paper, and this method is named as peak current based strategy. This peak current based strategy features much simplified calculation and could be easy to implement. To achieve control objectives, thorough study on parallel IFCs' peak currents is conducted. Based on the study, it is shown that the collective peak current of all IFCs is a constant under zero total active power oscillation, and therefore, keeping all IFCs' peak currents in the same phase and in-phase with collective peak current optimizes the utilization range of parallel IFCs. The numerical examples and experimental results are provided to verify the validity of the proposed control strategies under different operating conditions.
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paper_title: Closed-Form Solution of Time-Varying Model and Its Applications for Output Current Harmonics in Two-Stage PV Inverter

paper_content:

The single-phase photovoltaic (PV) inverter needs significant capacitance to buffer the double-line frequency power pulsation at ac port. The two-stage inverter allows the designer to choose the dc-link voltage and the capacitor size flexibly. With the reduced capacitance, the lifetime of the dc-link capacitor can be prolonged by replacing the electrolytic capacitors with film capacitors. However, the capacitance deduction results in high double-line frequency voltage ripple on dc-link, which increases a series of odd harmonics in the output current. This paper hence analyzes the harmonics caused by the voltage ripple in an inverter with feedback control. The inverter is modeled as a time-varying system by considering the dc-link voltage ripple. A closed-form solution is derived to calculate the amplitude of the ripple-caused harmonics. This analysis helps the designer to understand the effect of the dc-link voltage ripple on current harmonics, evaluate effectiveness of existing approaches, and stimulate new ideas and solutions. The study also derived the theoretical limit to select dc-link capacitance and sampling rate of current reference without violating the grid-tied regulations in power quality. The analysis is verified both by simulation and experimental evaluation.
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paper_title: Power Control of Grid-Connected Converters Under Unbalanced Voltage Conditions

paper_content:

This paper presents a grid current control technique on the power converter level. The submitted analysis deals with a control method of positive and negative current sequences in a grid-connected converter during voltage sags. The control method should assure the proper exchange of active and reactive powers without power ripple, the mitigation of harmonic distortion of grid currents, and should also enable the control of grid current peaks. A simple expression is determined, which allows estimation of maximum active and reactive powers exchanged between the grid and the converter without exceeding the admissible converter current. Some simulation and experimental results are reported to validate the theoretically predicted performance of the control strategy.
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paper_title: Improved Residential Distribution System Harmonic Compensation Scheme Using Power Electronics Interfaced DGs

paper_content:

Increased non-linear residential loads in today’s distribution system is a concern due to the harmonics related power quality issues. The situation gets worsened by the harmonic resonance introduced by the installation of power factor correction (PFC) capacitor banks in the distribution network. At the same time, more and more renewable energy-based distributed generation (DG) units are being installed in the residential area. These DG systems can be used as an effective way to mitigate the harmonic related power quality problems introduced by the nonlinear residential loads. In literature, very limited work has been done to identify harmonic compensation priorities that should be assigned to different DGs operating at different locations of the distribution system for improved compensation performance. This issue is addressed in this paper. A selective harmonic compensation scheme based on modal analysis is developed to assign compensation priorities on DGs operating at different distribution system nodes for improved compensation performance. A modeled residential distribution system containing distribution components such as distribution line, PFC capacitors, transformers, and household appliances along with DG units is used to verify the improvement of compensation performance. Experimental verification of the proposed method is also provided.
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paper_title: A New Virtual Harmonic Impedance Scheme for Harmonic Power Sharing in an Islanded Microgrid

paper_content:

This paper proposes a control strategy for harmonic power sharing in an islanded microgrid, without affecting the voltage quality at the load bus. The harmonic power distribution in islanded microgrid depends on the line impedance. The proposed control strategy employs negative virtual harmonic impedance to compensate the effect of line impedance on harmonic power delivery. The influence of mismatched line impedances on nonlinear load distribution is minimized by virtually varying the effective line impedance at harmonic frequencies. The fundamental active and reactive power sharing is implemented using droop technique and the proposed method operates independently, without affecting the linear load sharing. Furthermore, a new droop method, named as Z_h-H droop, is also proposed to estimate the virtual harmonic impedance value adaptively. The proposed concept of negative virtual harmonic impedance and Z_h-H droop is validated using simulation and DSP based experimental studies.
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paper_title: Consensus-Based Distributed Control for Accurate Reactive, Harmonic, and Imbalance Power Sharing in Microgrids

paper_content:

This paper investigates the issue of accurate reactive, harmonic, and imbalance power sharing in a microgrid. Harmonic and imbalance droop controllers are developed to proportionally share the harmonic power and the imbalance power among distributed generation (DG) units and improve the voltage quality at the point of common coupling (PCC). Further, a distributed consensus protocol is developed to adaptively regulate the virtual impedance at fundamental frequency and selected harmonic frequencies. Additionally, a dynamic consensus based method is adopted to restore the voltage to their average voltage. With the proposed methods, the microgrid system reliability and flexibility can be enhanced and the knowledge of the line impedance is not required. And the reactive, harmonic, and imbalance power can be proportionally shared among the DG units. Moreover, the quality of the voltage at PCC can be greatly improved. Simulation and experimental results are presented to demonstrate the proposed method.
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paper_title: Sharing of nonlinear load in parallel connected three-phase converters

paper_content:

In this paper, a new control method is presented which enables equal sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters. The paper focuses on solving the problem that arises when two converters with harmonic compensation are connected in parallel. Without the new solution, they are normally not able to distinguish the harmonic currents that flow to the load and harmonic currents that circulate between the converters. Analysis and experimental results on two 90-kVA 400-Hz converters in parallel are presented. The results show that both linear and nonlinear loads can be shared equally by the proposed concept.
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paper_title: An Enhanced Islanding Microgrid Reactive Power, Imbalance Power, and Harmonic Power Sharing Scheme

paper_content:

To address inaccurate power sharing problems in autonomous islanding microgrids, an enhanced droop control method through online virtual impedance adjustment is proposed. First, a term associated with DG reactive power, imbalance power, or harmonic power is added to the conventional real power-frequency droop control. The transient real power variations caused by this term are captured to realize DG series virtual impedance tuning. With the regulation of DG virtual impedance at fundamental positive sequence, fundamental negative sequence, and harmonic frequencies, an accurate power sharing can be realized at the steady state. In order to activate the compensation scheme in multiple DG units in a synchronized manner, a low-bandwidth communication bus is adopted to send the compensation command from a microgrid central controller to DG unit local controllers, without involving any information from DG unit local controllers. The feasibility of the proposed method is verified by simulated and experimental results from a low-power three-phase microgrid prototype.
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paper_title: Harmonic Droop Controller to Reduce the Voltage Harmonics of Inverters

paper_content:

In this paper, the load and/or grid connected to an inverter is modeled as the combination of voltage sources and current sources at harmonic frequencies. As a result, the system can be analyzed at each individual frequency, which avoids the difficulty in defining the reactive power for a system with different frequencies because it is now defined at each individual frequency. Moreover, a droop control strategy is developed for systems delivering power to a constant current source, instead of a constant voltage source. This is then applied to develop a harmonic droop controller so that the right amount of harmonic voltage is added to the inverter reference voltage to compensate the harmonic voltage dropped on the output impedance due to the harmonic current. This forces the output voltage at the individual harmonic frequency to be close to zero and improves the total harmonic distortion (THD) of the output voltage considerably. Both simulation and experimental results are provided to demonstrate that the proposed strategy can significantly improve the voltage THD.
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paper_title: Implementation and performance of automatic gain adjustment in a shunt-active filter for harmonic damping throughout a power distribution system

paper_content:

This paper discusses automatic gain adjustment in a fully-digital-controlled shunt active filter. This is the first step in cooperative control of the multiple active filters based on voltage detection for harmonic damping throughout power distribution systems. In general, the active filter should be equipped with an optimal control gain corresponding to the characteristic impedance of a distribution line. However, it is difficult to know circuit parameters of a real distribution line having various shunt capacitors and loads. Therefore, a main purpose of the gain adjustment is to make the active filter damp out harmonic propagation without considering the circuit parameters. In addition, the gain adjustment can reduce the compensating currents and losses in the active filter, and moreover it can avoid over-damping performance. Experiment results obtained from a 200-V, 20-kW laboratory system verify the effectiveness of the active filter equipped with automatic gain adjustment.
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paper_title: Improved Residential Distribution System Harmonic Compensation Scheme Using Power Electronics Interfaced DGs

paper_content:

Increased non-linear residential loads in today’s distribution system is a concern due to the harmonics related power quality issues. The situation gets worsened by the harmonic resonance introduced by the installation of power factor correction (PFC) capacitor banks in the distribution network. At the same time, more and more renewable energy-based distributed generation (DG) units are being installed in the residential area. These DG systems can be used as an effective way to mitigate the harmonic related power quality problems introduced by the nonlinear residential loads. In literature, very limited work has been done to identify harmonic compensation priorities that should be assigned to different DGs operating at different locations of the distribution system for improved compensation performance. This issue is addressed in this paper. A selective harmonic compensation scheme based on modal analysis is developed to assign compensation priorities on DGs operating at different distribution system nodes for improved compensation performance. A modeled residential distribution system containing distribution components such as distribution line, PFC capacitors, transformers, and household appliances along with DG units is used to verify the improvement of compensation performance. Experimental verification of the proposed method is also provided.
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paper_title: Autonomous control of inverter-interfaced Distributed Generation units for harmonic current filtering and resonance damping in an islanded microgrid

paper_content:

Harmonic current filtering and resonance damping have become important concerns in the operation and control of the islanded microgrids. To address these challenges, this paper proposes a control method for the inverter-interfaced distributed generation (DG) units to autonomously share the harmonic currents and resonance damping burdens. The approach employs a load compensator based on the decomposition of output current, in addition to the outer droop-based power controller and the inner voltage and current controllers. The load compensator consists of a virtual-fundamental-impedance loop for the enhanced reactive power sharing and a variable-harmonic-impedance loop which allows to counteract the harmonic voltage drops across the grid-side inductance of the DG inverter and also to dampen out harmonic resonance propagation in the microgrid. Finally, the laboratory tests on a three-phase islanded microgrid setup are carried out to validate the performance of the proposed control scheme.
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paper_title: Secondary Control for Voltage Quality Enhancement in Microgrids

paper_content:

In this paper, a hierarchical control scheme is proposed for enhancement of sensitive load bus (SLB) voltage quality in microgrids. The control structure consists of primary and secondary levels. The primary control level comprises distributed generators (DGs) local controllers. Each of these controllers includes a selective virtual impedance loop which is considered to improve sharing of fundamental and harmonic components of load current among the DG units. The sharing improvement is provided at the expense of increasing voltage unbalance and harmonic distortion. Thus, the secondary control level is applied to manage the compensation of SLB voltage unbalance and harmonics by sending proper control signals to the primary level. DGs compensation efforts are controlled locally at the primary level. The system design procedure for selecting proper control parameters is discussed. Simulation results are provided in order to demonstrate the effectiveness of the proposed control scheme.
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Title: Power Quality Control of Smart Hybrid AC/DC Microgrids: An Overview

Section 1: INTRODUCTION
Description 1: Write an introduction to smart hybrid AC/DC microgrids, emphasizing their emerging role in next-generation power systems, power quality challenges, and opportunities for control using power electronic converters.

Section 2: HYBRID AC/DC MICROGRIDS AND POWER QUALITY ISSUES
Description 2: Provide an overview of the different structures of hybrid AC/DC microgrids and discuss various power quality issues, highlighting real-world examples.

Section 3: PRIMARY CONTROL OF INTERFACING CONVERTERS
Description 3: Discuss the primary control strategies of interfacing converters, including current control method (CCM) and voltage control method (VCM), with examples.

Section 4: SUPERVISORY CONTROL OF INTERFACING CONVERTERS
Description 4: Explain the supervisory control mechanisms for interfacing converters in smart hybrid microgrids, including centralized, distributed, and master-slave structures.

Section 5: UNBALANCE COMPENSATION IN HYBRID AC/DC MICROGRIDS THROUGH SMART INTERFACING CONVERTERS
Description 5: Review the strategies for compensating unbalanced conditions in hybrid AC/DC microgrids using smart interfacing converters, focusing on both three-phase and single-phase interfacing.

Section 6: HARMONIC COMPENSATION IN HYBRID AC/DC MICROGRIDS THROUGH INTERFACING CONVERTERS
Description 6: Discuss harmonic compensation in AC and DC subgrids of hybrid microgrids using interfacing converters, covering control strategies suitable for different harmonics.

Section 7: CONTROL OF LOW-SWITCHING IFCS FOR HARMONIC COMPENSATION IN AC SUBGRID
Description 7: Detail control strategies for low-switching frequency interfacing converters (IFCs) used in AC subgrids for harmonic compensation, highlighting feed-forward strategies and new sampling methods.

Section 8: CONTROL OF IFCS FOR HARMONIC COMPENSATION IN DC SUBGRID
Description 8: Describe the strategies for harmonic compensation in DC subgrids using interfacing converters, addressing challenges in eliminating specific harmonics.

Section 9: COORDINATED CONTROL OF MULTIPLE IFCS FOR HARMONICS COMPENSATION
Description 9: Explain the coordinated control approaches for multiple IFCs to share harmonic compensation efforts in hybrid AC/DC microgrids, covering autonomous and supervisory control methods.

Section 10: SUMMARY AND CONCLUSIONS
Description 10: Summarize the reviewed power quality control strategies in smart hybrid AC/DC microgrids and discuss future trends and expected developments in this field.