Small-signal stability analysis for the multi-terminal VSC MVDC distribution network; a review
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paper_title: VSC-Based MVDC Railway Electrification System

paper_content:

This paper proposes a new railway electrification system in which the voltage-source converter (VSC) becomes the basic building block. This will allow existing railways, comprising several ac and dc subsystems, to be transformed into simpler medium-voltage dc (MVDC) multiterminal power systems feeding mobile loads. Moreover, the VSC-based unified scheme will substantially facilitate the connectivity among otherwise heterogeneous railway systems, while the integration of distributed generation and storage is achieved in a straightforward fashion. In addition to the general MVDC architecture, details are provided about the dc catenary layout, dual-voltage locomotive configurations, and dc-dc links between urban and long-distance railways. The need for a supervisory control system, and its role in coordinating local VSC controllers, so that the resulting power flows are optimized while the catenary voltage is kept within limits, are discussed. The proposed railway electrification paradigm is compared with the standard 25-kV, ac electrification system by means of a real case study.
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paper_title: Constant power loads in More Electric Vehicles - an overview

paper_content:

Power electronic converters are playing an ever increasingly important role in today's electric vehicles. These regulated power electronic converters act as Constant Power Loads (CPLs) to the converters driving them. A CPL has a negative input impedance which results in a destabilising effect on closed-loop converters driving them. The use of power electronics in More Electric Vehicles (MEVs) is discussed, along with the CPLs they present and how they are modeled. Negative impedance instability is then discussed and the stability of various DC-DC converter topologies when loaded by CPLs is analyzed in both voltage-mode and current-mode control operating in Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). Finally, the currently proposed control strategies that can stabilize a power electronic converter are discussed.
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paper_title: POWER ELECTRONICS FOR DISTRIBUTED ENERGY SYSTEMS AND TRANSMISSION AND DISTRIBUTION APPLICATIONS

paper_content:

Power electronics can provide utilities the ability to more effectively deliver power to their customers while providing increased reliability to the bulk power system. In general, power electronics is the process of using semiconductor switching devices to control and convert electrical power flow from one form to another to meet a specific need. These conversion techniques have revolutionized modern life by streamlining manufacturing processes, increasing product efficiencies, and increasing the quality of life by enhancing many modern conveniences such as computers, and they can help to improve the delivery of reliable power from utilities. This report summarizes the technical challenges associated with utilizing power electronics devices across the entire spectrum from applications to manufacturing and materials development, and it provides recommendations for research and development (R&D) needs for power electronics systems in which the U.S. Department of Energy (DOE) could make a substantial impact toward improving the reliability of the bulk power system.
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paper_title: A case for medium voltage DC for distribution circuit applications

paper_content:

In this survey paper, applications of medium voltage direct current (MVDC) to distribution substations are discussed and compared to conventional ac for power conversion from transmission voltage levels to distribution voltage levels. Technical changes in the substation due to the dc equipment, efficiency of the substation, and protective measures are major aspects to consider in the MVDC concept development.
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paper_title: Impact and opportunities of medium-voltage DC grids in urban railway systems

paper_content:

Due to the increasing number of power-electronic components connected to the grid, medium-voltage dc (MVDC) grids are a promising alternative to established ac distribution grids. Within this work, the advantages and additional flexibility of substations for (urban) railways connected to future MVDC distribution grids are investigated. Possible topologies are presented and the efficiencies are compared.
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paper_title: VSC HVDC transmission corridor: An option for PV power injection and AC network stability support

paper_content:

The pressure of increasing power demand and supply inequality is forcing utilities to interconnect AC systems to meet demands. High Voltage Direct Current (HVDC) schemes are becoming a more attractive solution as they have been used extensively in interconnected power systems worldwide. This paper investigates the role of voltage source converter (VSC) based HVDC transmission corridor for PV power injection and for AC network stability support. Overview of Namibia Caprivi-link interconnector a case study, potential of very large scale PV in Namibia and prospects of PV power injection on the DC-link is presented. The system is modelled simulated in Matlab/Simulink. Critical contingencies such as sudden island conditions, three-phase to ground fault are simulated with and without PV penetration. Results show the stability support on the AC side networks by PV power injection on the dc-link.
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paper_title: Design and simulation of a DC electric vehicle charging station connected to a MVDC infrastructure

paper_content:

Medium Voltage DC (MVDC) infrastructure serves as a platform for interconnecting renewable electric power generation, including wind and solar. Abundant loads such as industrial facilities, data centers, and electric vehicle charging stations (EVCS) can also be powered using MVDC technology. MVDC networks are expected to improve efficiency by serving as an additional layer between the transmission and distribution level voltages for which generation sources and loads could directly connect with smaller rated power conversion equipment. This paper investigates an EVCS powered by a MVDC bus. A bidirectional DC-DC converter with appropriate controls serves as the interface between the EVCS and MVDC bus. Two scenarios are investigated for testing and comparing EVCS operation: 1) EVCS power supplied by the interconnected MVDC model and 2) EVCS power supplied by an equivalent voltage source. Comparisons between both are discussed. The CCM/DCM buck mode operation of the bidirectional DC-DC converter is explored as well as the system isolation benefits that come with its use.
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paper_title: Medium Voltage DC Network Modeling and Analysis with Preliminary Studies for Optimized Converter Configuration Through PSCAD Simulation Environment

paper_content:

With the advancement of high capacity power electronics technologies, most notably in high voltage direct current (HVDC) applications, the concept of developing and implementing future transmission networks through a DC backbone presents a realistic and advantageous option over traditional AC approaches. Currently, most consumer electrical equipment requires DC power to function, thus requiring an AC/DC conversion. New forms of distributed generation, such as solar photovoltaic power, produce a direct DC output. Establishing an accessible and direct supply of DC power to serve such resources and loads creates the potential to mitigate losses experienced in the AC/DC conversion process, reduce overall electrical system infrastructure, and lessen the amount of power generated from power plants, as well as other advantages. For the reasons listed, medium voltage DC (MVDC) networks represent a promising, initial platform for interconnecting relatively low voltage generation resources such as photovoltaic panels, serving loads, and supplying other equipment on a common DC bus bar. Future industrial parks, ship power systems, hybrid plug-in vehicles, and energy storage systems are all avenues for future implementation of the concept. This thesis introduces an initial design and simulation model of the MVDC network concept containing renewable generation, power electronic converters, and induction machine loads. Each of the equipment models are developed and modeled in PSCAD and validated analytically. The models of the represented system equipment and components are individually presented and accompanied with their simulated results to demonstrate the validity of the overall model. Finally, the equipment models are assembled together into a meshed system to perform traditional preliminary studies on the overall power system including wind speed adjustments, load energizing, and fault-clearing analysis in order to evaluate aspects of various operational phenomena such as potential overvoltages, system stability issues, and other unexpected occurrences.
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paper_title: The potential of distributed generation to provide ancillary services

paper_content:

The growing concerns regarding electric power quality and availability have led to the installation of more and more distributed generation. In parallel and in the context of an accelerating trend towards deregulation of the electric industry, the unbundling of services, many grouped under ancillary services, should create a market for some of these services. This paper discusses the potential of distributed generation (DG) to provide some of these services. In particular, DG can serve locally as the equivalent of a spinning reserve and voltage support of the AC bus. The main types of distributed generation with emphasis on the power electronic interface and the configurations appropriate to provide ancillary services are reviewed. The flexibility and features provided by the power electronic interface are illustrated. In addition to control of the real power, other functions can be incorporated into the design of the interface to provide services, such as reactive power, and resources associated with power quality. These include voltage sag compensation and harmonic filtering. The implications on the design of the power converter interface are discussed.
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paper_title: Wind Farm Grid Integration Using VSC Based HVDC Transmission - An Overview

paper_content:

The paper gives an overview of HVAC and HVDC connection of wind farm to the grid, with an emphasis on voltage source converter (VSC)-based HVDC for large wind farms requiring long distance cable connection. Flexible control capabilities of a VSC-based HVDC system enables smooth integration of wind farm into the power grid network while meeting the grid code requirements (GCR). Operation of a wind farm with VSC-based HVDC connection is described.
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paper_title: DC distribution for industrial systems: opportunities and challenges

paper_content:

This paper investigates the opportunities and chal- lenges associated with adopting a dc distribution scheme for indus- trial power systems. A prototype dc distribution system has been simulated to investigate the issues. One of the issues focused is the interaction between power converters that are used to convert ac to dc and dc to ac. Another challenging issue investigated is the system grounding. These issues become challenging mainly due to the neu- tral voltage shift associated with the power converters. The paper shows that converter interactions can be minimized with proper filtering and control on the converters. The paper also proposes a grounding scheme and shows that this scheme provides an ef- fective solution by keeping the neutral voltages low under normal conditions and by limiting the fault currents during fault condi- tions. With these features, dc distribution provides very reliable and high-quality power.
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paper_title: Voltage transient propagation in AC and DC datacenter distribution architectures

paper_content:

Global industries, governments, organizations, and institutions rely on the operation of datacenters in order to successfully meet their day-to-day objectives. The increased reliance on datacenters combined with the growth of the industry sector has led to more careful considerations for datacenter design. Stakeholders in the datacenter industry have begun to consider alternative electrical distribution systems that provide greater reliability and operating efficiency, as well as lower capital costs and ease-of-installation. Among the many alternatives proposed, facility-level DC distribution has emerged as the option providing the highest operating efficiency.
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paper_title: MVDC marine electrical distribution: Are we ready?

paper_content:

Marine high-power on-board electrical systems are predominantly utilizing three-phase medium voltage alternating current (MVAC) distribution. Depending on the size and purpose of the ship, on-board electrical systems provide supply to loads in excess of 60MW. Increasingly, medium voltage direct current (MVDC) distribution systems are being considered as an alternative. An increase in the fuel efficiency of the prime movers, the removal of bulky low frequency transformer and the easier integration of different storage technologies are especially attractive. This paper discusses the opportunities but also the technical difficulties associated with the transition from an MVAC to an MVDC system, using an existing LNG tanker MVAC on-board distribution system, as example.
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paper_title: Medium-voltage DC distribution grids in urban areas

paper_content:

The potential of medium-voltage dc (MVDC) grids in various applications has been under investigation by different researchers. In several works, the advantages of implementing MVDC technology into collector grids of wind and photovoltaic (PV) parks are pointed out. The main driver is that the grid-side inverter of the distributed generators as well as lossy grid filters and bulky 50 Hz transformer become obsolete. However, MVDC grids are a promising alternative to established ac grids not only for power generation. Within this work, the advantages regarding system efficiency and investment costs of using MVDC for the distribution of electrical energy is presented. Conventional low-voltage ac-grids and possible medium-voltage ac grids are also analyzed to compare the different implementation approaches.
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paper_title: Constant power loads and negative impedance instability in automotive systems: definition, modeling, stability, and control of power electronic converters and motor drives

paper_content:

Power electronic converters and electric motor drives are being put into use at an increasingly rapid rate in advanced automobiles. However, the new advanced automotive electrical systems employ multivoltage level hybrid ac and dc as well as electromechanical systems that have unique characteristics, dynamics, and stability problems that are not well understood due to the nonlinearity and time dependency of converters and because of their constant power characteristics. The purpose of this paper is to present an assessment of the negative impedance instability concept of the constant power loads (CPLs) in automotive power systems. The main focus of this paper is to analyze and propose design criteria of controllers for automotive converters/systems operating with CPLs. The proposed method is to devise a new comprehensive approach to the applications of power electronic converters and motor drives in advanced automotive systems. Sliding-mode and feedback linearization techniques along with large-signal phase plane analysis are presented as methods to analyze, control, and stabilize automotive converters/systems with CPLs
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paper_title: Optimum design of medium-voltage DC collector grids depending on the offshore-wind-park power

paper_content:

DC collector grids within offshore wind park offer advantages regarding efficiency and investment costs. Dual-active bridge (DAB) dc-dc converter systems can be applied for stepping-up the dc output-voltage of wind turbines (WT). The increase of the voltage level reduces the effort for cabling and increases the system efficiency. Within this paper, the advantage of DAB converter systems used in offshore medium-voltage dc (MVDC) grids is presented. Also, the optimum design of wind park clusters regarding the wake effect and grid losses is investigated to maximize the energy yield.
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paper_title: Issues of Connecting Wind Farms into Power Systems

paper_content:

Wind power industry is developing rapidly, more and more wind farms are being connected into power systems. Integration of large scale wind farms into power systems presents some challenges that must be addressed, such as system operation and control, system stability, and power quality. This paper describes modern wind power systems, presents requirements of wind turbine connection and discusses the possible control methods for wind turbines to meet the specifications
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paper_title: Impact of DFIG Based Offshore Wind Farms Connected Through VSC-HVDC Link on Power System Stability

paper_content:

With the increased levels of offshore wind power penetration into power systems, the impact of offshore wind power on stability of power systems require more investigation. In this paper, the effects of a large scale doubly fed induction generator (DFIG) based offshore wind farm (OWF) on power system stability are examined. The OWF is connected to the main onshore grid through a voltage source converter (VSC) based high voltage direct current (HVDC) link. A large scale DFIG based OWF is connected to the New England 10-machine 39-bus test system through a VSC-HVDC. One of the synchronous generators in the test system is replaced by an OWF with an equivalent generated power. As the voltage source converter can control the active and reactive power independently, the use of the onshore side converter to control its terminal voltage is investigated. The behaviour of the test system is evaluated under both small and large grid disturbances in both cases with and without the offshore wind farm.
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paper_title: Design aspects of a medium-voltage direct current (MVDC) grid for a university campus

paper_content:

Today's power systems use alternating current (ac) for transmission and distribution of electrical energy, although the first grids were based on direct current (dc). Due to the absence of appropriate equipment to change voltage levels dc technology did not become widely accepted and was finally ruled out by the more efficient ac infrastructure. However, as a result of considerable technical progress, high-voltage direct current (HVDC) transmission has found its way back into power systems. At lower voltage and power levels medium-voltage dc (MVDC) distribution has been proposed for offshore wind farms and industrial applications. This paper describes the design of an MVDC grid for the interconnection of high-power test benches at a university campus. Voltage control within the dc grid as well as the behavior in different fault scenarios is analyzed using numerical simulations. To assess the environmental impact of the grid the magnetic flux density emitted by the dc cable lines is calculated.
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paper_title: Ship to Grid: Medium-Voltage DC Concepts in Theory and Practice

paper_content:

Corporate research centers, universities, power equipment vendors, end users, and other market participants around the world are beginning to explore and consider the use of dc in future transmission and distribution system applications. Recent developments and trends in electric power consumption indicate an increasing use of dc-based power and constant power loads. In addition, growth in renewable energy resources requires dc interfaces for optimal integration. A strong case is being made for intermeshed ac and dc networks, with new concepts emerging at the medium-voltage (MV) level for MV dc infrastructure developments.
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paper_title: Power System Stability and Control

paper_content:

Part I: Characteristics of Modern Power Systems. Introduction to the Power System Stability Problem. Part II: Synchronous Machine Theory and Modelling. Synchronous Machine Parameters. Synchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems. High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems, High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Oscillations. Mid-Term and Long-Term Stability. Methods of Improving System Stability.
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paper_title: Power electronic technologies for flexible DC distribution grids

paper_content:

Market liberalization has significantly changed the energy supply system in Europe, i.e. from a top-down centralized power generation system towards a more decentralized system. In addition, partially due to incentive programs, vast amounts of renewable power generator systems (mostly wind and PV) have been installed. More flexible grid structures are needed to cope with this new landscape of distributed generation. This paper explores the role of state-of-the-art power electronics to realize the required infrastructure. The potential efficiency gains and cost savings that can be realized by using DC-to-DC converters in electronic substations are presented in detail.
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paper_title: Modeling, analysis, and validation of a preliminary design for a 20 kV medium voltage DC substation

paper_content:

With the advancement of high capacity power electronics technologies, most notably in HVDC applications, the concept of developing and implementing future transmission subsystems through a DC backbone presents a realistic and advantageous option over traditional AC approaches. Currently, electrical equipment or devices requiring DC power to function, whether loads or resources, necessitate AC/DC conversion technologies. Having an accessible and direct supply of DC power to serve such loads and resources creates the potential to mitigate losses experienced in the AC/DC conversion process, reduce overall electrical system infrastructure, and lessen the amount of power generated from power plants, as well as other advantages. This paper introduces an initial design and simulation model of a medium voltage DC (MVDC) substation concept containing renewable generation, power electronic converters, and induction machine loads. Each of the components is developed and modeled in PSCAD and validated analytically. The models of the represented system equipment and components are individually presented and accompanied with their simulated results to demonstrate the validity of the overall model. Future work will build upon the model to develop additional loads and resources, control strategies for optimized integration, and more detailed component models.
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paper_title: MVDC - The New Technology for Distribution Networks

paper_content:

MVDC is starting to be considered as an option for enhancing transfer capacity and providing increased power quality at distribution networks. There is a term starting to be used of “soft open-point” which can provide controlled power transfer between two 11kV or 33kV distribution groups, without affecting short-circuit levels, voltage differences, loop flows or limitations due to phase-angle differences. The 4-quadrant converters can provide reactive power support and voltage control at each end of the link and multi-terminal is also feasible. There are future technology opportunities including enhancement of existing corridors through the conversion of existing AC lines to DC. This paper provides a technology overview as well as information on recently deployed projects ranging from linking of oil and gas platforms, through to an urban infeed. It will summarise the benefits of MVDC and the applications where it may provide a competitive or preferential alternative solution to conventional technology.
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paper_title: A review of design criteria for low voltage DC distribution stability

paper_content:

The performance advantages for Low Voltage Direct Current electrical distribution is becoming clearer however, the commercial opportunities, design processes and standardisation is currently lacking. This paper presents an overview of the current development status of LVDC distribution and reviews the modelling and stability criteria available to designers of DC distribution systems for land, aerospace and marine power system applications.
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paper_title: Understanding of tuning techniques of converter controllers for VSC-HVDC

paper_content:

A mathematical model of a voltage source converter is presented in the synchronous reference frame for investigating VSC-HVDC for transferring wind power through a long distance. This model is used to analyze voltage and current control loops for the VSC and study their dynamics. Vector control is used for decoupled control of active and reactive power and the transfer functions are derived for the control loops. In investigating the operating conditions for HVDC systems, the tuning of controllers is one of the critical stages of the design of control loops. Three tuning techniques are discussed in the paper and analytical expressions are derived for calculating the parameters of the current and voltage controllers. The tuning criteria are discussed and simulations are used to test the performance of such tuning techniques.
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paper_title: Admittance space stability analysis of power electronic systems

paper_content:

Power electronics based power distribution systems (PEDSs) are becoming increasingly common, particularly in marine and aerospace applications. Stability analysis of this class of systems is crucial due to the potential for negative impedance instability. Existing techniques of stability analysis introduce artificial conservativeness, are sensitive to component grouping, and at the same time do not explicitly address uncertainties and variations in operating point. A new stability criterion, which reduces artificial conservativeness and is also insensitive to component grouping is described. In addition, a means of readily establishing design specifications from an arbitrary stability criterion which specifically includes a provision to incorporate uncertainty, parameter variation, and nonlinearities is set forth. The method is presented in the context of a hardware test system and is experimentally validated.
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paper_title: A review of design criteria for low voltage DC distribution stability

paper_content:

The performance advantages for Low Voltage Direct Current electrical distribution is becoming clearer however, the commercial opportunities, design processes and standardisation is currently lacking. This paper presents an overview of the current development status of LVDC distribution and reviews the modelling and stability criteria available to designers of DC distribution systems for land, aerospace and marine power system applications.
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paper_title: Null double injection and the extra element theorem

paper_content:

The extra element theorem (EET) states that any transfer function of a linear system can be expressed in terms of its value when a given 'extra' element is absent, and a correction factor involving the extra element and two driving-point impedances are seen by the element. In the present work, the EET is derived and applied to several examples in a manner that has been developed and refined in the classroom over a number of years. The concept of null double injection is introduced first, because it is the key to making easy the calculation of the two driving-point impedances needed for the EET correction factor. The EET for series and parallel elements is then considered, and attention is also given to the EET as an analysis tool, to the symmetry of the two forms of the EET, and to return ratios and sensitivity. >
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paper_title: Comprehensive Review of Stability Criteria for DC Power Distribution Systems

paper_content:

Power-electronics-based dc power distribution systems, consisting of several interconnected feedback-controlled switching converters, suffer from potential degradation of stability and dynamic performance caused by negative incremental impedances due to the presence of constant power loads. For this reason, the stability analysis of these systems is a significant design consideration. This paper reviews all the major stability criteria for dc distribution systems that have been developed so far: the Middlebrook Criterion, the Gain Margin and Phase Margin Criterion, the Opposing Argument Criterion, the Energy Source Analysis Consortium (ESAC) Criterion, and the Three-Step Impedance Criterion. In particular, the paper discusses, for each criterion, the artificial conservativeness characteristics in the design of dc distribution systems, and the formulation of design specifications that ensure system stability. Moreover, the Passivity-Based Stability Criterion is discussed, which has been recently proposed as an alternative stability criterion. While all prior stability criteria are based on forbidden regions for the polar plot of the so-called minor loop gain, which is an impedance ratio, the proposed criterion is based on imposing passivity of the overall bus impedance. A meaningful simulation example is presented to illustrate the main characteristics of the reviewed stability criteria.
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paper_title: Three-dimensional stability analysis of DC power electronics based systems

paper_content:

Power electronics based power distribution systems are becoming increasingly common, particularly in marine and aerospace applications. Stability analysis of this class of systems is crucial due to the potential for negative impedance instability. Existing techniques of stability analysis introduce artificial conservativeness, are sensitive to component grouping, and at the same time do not explicitly address uncertainties and variations in operating point. Recently, a new stability criterion which reduces artificial conservativeness and is also insensitive to component grouping has been set forth along with a means of readily establishing design specifications from an arbitrary stability criterion which specifically includes a provision to incorporate uncertainty, parameter variation, and nonlinearities. Therein, the method is used to develop a load admittance constraint based on a generalized source impedance. In this paper, that work is further explained and the converse problem, that of generating a constraint on the source impedance from the load admittance, is also illustrated.
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paper_title: Comprehensive review of stability criteria for DC distribution systems

paper_content:

Power-electronics-based DC power distribution systems, consisting of several interconnected feedback-controlled switching converters, suffer from potential degradation of stability induced by negative incremental impedances due to the presence of constant power loads. For this reason, the stability analysis of these systems is a significant design consideration. This paper reviews all the major stability criteria for DC distribution systems that have been developed so far: the Middlebrook Criterion, the Gain Margin and Phase Margin Criterion, the Opposing Argument Criterion, and the Energy Source Analysis Consortium Criterion. In particular, the paper discusses, for each criterion, the artificial conservativeness characteristics in the design of DC distribution systems, and the formulation of design specifications so that the system would be stable. Finally, the Passivity-Based Stability Criterion is discussed, which has been recently proposed to reduce design conservativeness and improve design-oriented characteristics. While all prior stability criteria are based on forbidden regions for the polar plot of the so-called minor loop gain which is an impedance ratio, the proposed criterion is based on imposing passivity of the overall bus impedance. However, stability is clearly an insufficient requirement for DC power distribution systems; performance is also a requirement. A simulation example is presented to illustrate that the Passivity-Based Stability Criterion guarantees both stability and performance.
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paper_title: Design and simulation of a DC electric vehicle charging station connected to a MVDC infrastructure

paper_content:

Medium Voltage DC (MVDC) infrastructure serves as a platform for interconnecting renewable electric power generation, including wind and solar. Abundant loads such as industrial facilities, data centers, and electric vehicle charging stations (EVCS) can also be powered using MVDC technology. MVDC networks are expected to improve efficiency by serving as an additional layer between the transmission and distribution level voltages for which generation sources and loads could directly connect with smaller rated power conversion equipment. This paper investigates an EVCS powered by a MVDC bus. A bidirectional DC-DC converter with appropriate controls serves as the interface between the EVCS and MVDC bus. Two scenarios are investigated for testing and comparing EVCS operation: 1) EVCS power supplied by the interconnected MVDC model and 2) EVCS power supplied by an equivalent voltage source. Comparisons between both are discussed. The CCM/DCM buck mode operation of the bidirectional DC-DC converter is explored as well as the system isolation benefits that come with its use.
---
paper_title: Apparent impedance analysis: A new method for power system stability analysis

paper_content:

In this paper a new method for power system stability analysis is introduced. The method is based on injection of a small voltage or current in an arbitrary point of the system. The apparent impedance is defined as the ratio between the voltage and current in the injection point. It is shown that the apparent impedance can be used to estimate the eigenvalues of the system that are observable from the injection point. The eigenvalues are obtained by applying the Vector Fitting algorithm to the measured set of apparent impedances. The proposed method holds some advantages over the well established impedance-based analysis method: It is no longer needed to estimate the source and load impedance equivalents separately, and it is not necessary to make any assumption regarding where the source and load are located. This reduces the required measurements and data processing. Furthermore, the stability analysis is global in the sense that the resulting stability margin does not depend on the injection point location. Finally, the method is well suited for real-time implementation due to low computational requirements. The method is outlined for DC-systems, while further work will extend the theory to cover single-phase and three-phase AC systems.
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paper_title: DC distribution for industrial systems: opportunities and challenges

paper_content:

This paper investigates the opportunities and chal- lenges associated with adopting a dc distribution scheme for indus- trial power systems. A prototype dc distribution system has been simulated to investigate the issues. One of the issues focused is the interaction between power converters that are used to convert ac to dc and dc to ac. Another challenging issue investigated is the system grounding. These issues become challenging mainly due to the neu- tral voltage shift associated with the power converters. The paper shows that converter interactions can be minimized with proper filtering and control on the converters. The paper also proposes a grounding scheme and shows that this scheme provides an ef- fective solution by keeping the neutral voltages low under normal conditions and by limiting the fault currents during fault condi- tions. With these features, dc distribution provides very reliable and high-quality power.
---
paper_title: Small-Signal Stability Assessment of Power Electronics Based Power Systems: A Discussion of Impedance- and Eigenvalue-Based Methods

paper_content:

This paper investigates the small-signal stability of power electronics-based power systems in frequency domain. A comparison between the impedance-based and the eigenvalue-based stability analysis methods is presented. A relation between the characteristics equation of the eigenvalues and poles and zeros of the minor-loop gain from the impedance-based analysis have been derived analytically. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of the minor-loop gain indicates that the system can exhibit harmonic oscillations. A weakness of the impedance method is the limited observability of certain states given its dependence on the definition of local source-load subsystems, which makes it necessary to investigate the stability at different subsystems. To address this limitation, the paper discusses critical locations where the application of the method can reveal the impact of a passive component or a controller gain on the stability. On the other hand, the eigenvalue-based method, being global, can determine the stability of the entire system; however, it cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching. To generalize the observations, the two methods have been applied to dc-dc converters. To illustrate the difference and the relation between the two-methods, the two stability analysis methods are then applied to a two-terminal VSC-based HVdc system as an example of power electronics-based power systems, and the theoretical analysis has been further validated by simulation and experiments.
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paper_title: Impedance-Based Local Stability Criterion for DC Distributed Power Systems

paper_content:

This paper addresses the stability issue of dc distributed power systems (DPS). Impedance-based methods are effective for stability assessment of voltage-source systems and current-source systems. However, these methods may not be suitable for applications involving variation of practical parameters, loading conditions, system's structures, and operating modes. Thus, for systems that do not resemble simple voltage-source systems or current-source systems, stability assessment is much less readily performed. This paper proposes an impedance-based criterion for stability assessment of dc DPS. We first classify any converter in a dc DPS as either a bus voltage controlled converter (BVCC) or a bus current controlled converter (BCCC). As a result, a dc DPS can be represented in a general form regardless of its structure and operating mode. Then, the minor loop gain of the standard dc DPS is derived precisely using a two-port small signal model. Application of the Nyquist criterion on the derived minor loop gain gives the stability requirement for the dc DPS. This proposed criterion is applicable to dc DPSs, regardless of the control method and the connection configuration. Finally, a 480 W photovoltaic (PV) system with battery energy storage and a 200 W dc DPS, in which the source converter employs a droop control, are fabricated to validate the effectiveness of the proposed criterion.
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paper_title: Power electronic technologies for flexible DC distribution grids

paper_content:

Market liberalization has significantly changed the energy supply system in Europe, i.e. from a top-down centralized power generation system towards a more decentralized system. In addition, partially due to incentive programs, vast amounts of renewable power generator systems (mostly wind and PV) have been installed. More flexible grid structures are needed to cope with this new landscape of distributed generation. This paper explores the role of state-of-the-art power electronics to realize the required infrastructure. The potential efficiency gains and cost savings that can be realized by using DC-to-DC converters in electronic substations are presented in detail.
---
paper_title: Apparent Impedance Analysis: A Small-Signal Method for Stability Analysis of Power Electronic-Based Systems

paper_content:

In this paper, a new method for power system stability analysis is introduced. The method is based on injection of a small voltage or current in an arbitrary point of a power system. The apparent impedance is then defined as the ratio between the voltage and current at the injection point. It is shown that the apparent impedance can be used to estimate the eigenvalues of the system that are observable from the injection point. The eigenvalues are obtained by applying system identification techniques to the measured set of apparent impedances. The method is similar to the well-established impedance-based stability analysis based on source and load impedance models. However, while the source/load impedance ratio is viewed as the minor-loop gain, the apparent impedance can be viewed as a closed-loop transfer function. It can also be expressed as the parallel connection of the source and load impedance. It is shown, in this paper, how the system eigenvalues can be extracted based on a set of apparent impedance values. The apparent impedance holds, therefore, complementary information compared with the existing impedance-based stability analysis. The method can also be used as a tool to validate analytically derived state-space models. In this paper, the method is presented as a simulation tool, while further work will extend it to include experimental setups. Two case studies are presented to illustrate the method: 1) a dc case with a buck converter feeding a constant power load and 2) a three-phase grid-connected voltage source converter with a current controller and a phase lock loop. The estimated (apparent) eigenvalues of the studied systems are equal to those obtained from the analytic state-space model.
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paper_title: Stability analysis of an all-electric ship MVDC Power Distribution System using a novel Passivity-Based Stability Criterion

paper_content:

The present paper describes a recently proposed Passivity-Based Stability Criterion (PBSC) and demonstrates its usefulness and applicability to the stability analysis of the MVDC Power Distribution System for the All-Electric Ship. The criterion is based on imposing passivity of the overall DC bus impedance. If passivity of the bus impedance is ensured, stability is guaranteed as well. The PBSC, in contrast with existing stability criteria, such as the Middlebrook criterion and its extensions, which are based on the minor loop gain concept, i.e. an impedance ratio at a given interface, offers several advantages: reduction of artificial design conservativeness, insensitivity to component grouping, applicability to multi-converter systems and to systems in which the power flow direction changes, for example as a result of system reconfiguration. Moreover, the criterion can be used in conjunction with an active method to improve system stability, called the Positive Feed-Forward (PFF) control, for the design of virtual damping networks. By designing the virtual impedance so that the bus impedance passivity condition is met, the approach results in greatly improved stability and damping of transients on the DC bus voltage. Simulation validation is performed using a switching-model DC power distribution system.
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paper_title: A review of design criteria for low voltage DC distribution stability

paper_content:

The performance advantages for Low Voltage Direct Current electrical distribution is becoming clearer however, the commercial opportunities, design processes and standardisation is currently lacking. This paper presents an overview of the current development status of LVDC distribution and reviews the modelling and stability criteria available to designers of DC distribution systems for land, aerospace and marine power system applications.
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paper_title: Comprehensive Review of Stability Criteria for DC Power Distribution Systems

paper_content:

Power-electronics-based dc power distribution systems, consisting of several interconnected feedback-controlled switching converters, suffer from potential degradation of stability and dynamic performance caused by negative incremental impedances due to the presence of constant power loads. For this reason, the stability analysis of these systems is a significant design consideration. This paper reviews all the major stability criteria for dc distribution systems that have been developed so far: the Middlebrook Criterion, the Gain Margin and Phase Margin Criterion, the Opposing Argument Criterion, the Energy Source Analysis Consortium (ESAC) Criterion, and the Three-Step Impedance Criterion. In particular, the paper discusses, for each criterion, the artificial conservativeness characteristics in the design of dc distribution systems, and the formulation of design specifications that ensure system stability. Moreover, the Passivity-Based Stability Criterion is discussed, which has been recently proposed as an alternative stability criterion. While all prior stability criteria are based on forbidden regions for the polar plot of the so-called minor loop gain, which is an impedance ratio, the proposed criterion is based on imposing passivity of the overall bus impedance. A meaningful simulation example is presented to illustrate the main characteristics of the reviewed stability criteria.
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paper_title: Comprehensive review of stability criteria for DC distribution systems

paper_content:

Power-electronics-based DC power distribution systems, consisting of several interconnected feedback-controlled switching converters, suffer from potential degradation of stability induced by negative incremental impedances due to the presence of constant power loads. For this reason, the stability analysis of these systems is a significant design consideration. This paper reviews all the major stability criteria for DC distribution systems that have been developed so far: the Middlebrook Criterion, the Gain Margin and Phase Margin Criterion, the Opposing Argument Criterion, and the Energy Source Analysis Consortium Criterion. In particular, the paper discusses, for each criterion, the artificial conservativeness characteristics in the design of DC distribution systems, and the formulation of design specifications so that the system would be stable. Finally, the Passivity-Based Stability Criterion is discussed, which has been recently proposed to reduce design conservativeness and improve design-oriented characteristics. While all prior stability criteria are based on forbidden regions for the polar plot of the so-called minor loop gain which is an impedance ratio, the proposed criterion is based on imposing passivity of the overall bus impedance. However, stability is clearly an insufficient requirement for DC power distribution systems; performance is also a requirement. A simulation example is presented to illustrate that the Passivity-Based Stability Criterion guarantees both stability and performance.
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paper_title: Small-Signal Stability Assessment of Power Electronics Based Power Systems: A Discussion of Impedance- and Eigenvalue-Based Methods

paper_content:

This paper investigates the small-signal stability of power electronics-based power systems in frequency domain. A comparison between the impedance-based and the eigenvalue-based stability analysis methods is presented. A relation between the characteristics equation of the eigenvalues and poles and zeros of the minor-loop gain from the impedance-based analysis have been derived analytically. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of the minor-loop gain indicates that the system can exhibit harmonic oscillations. A weakness of the impedance method is the limited observability of certain states given its dependence on the definition of local source-load subsystems, which makes it necessary to investigate the stability at different subsystems. To address this limitation, the paper discusses critical locations where the application of the method can reveal the impact of a passive component or a controller gain on the stability. On the other hand, the eigenvalue-based method, being global, can determine the stability of the entire system; however, it cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching. To generalize the observations, the two methods have been applied to dc-dc converters. To illustrate the difference and the relation between the two-methods, the two stability analysis methods are then applied to a two-terminal VSC-based HVdc system as an example of power electronics-based power systems, and the theoretical analysis has been further validated by simulation and experiments.
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paper_title: Impedance based stability analysis of VSC-based HVDC system

paper_content:

With increasing activities in development of Voltage Source Converter (VSC) based High Voltage dc (HVDC) transmission system, it is necessary to pre-asses their impact on system stability before connecting to the main ac grid. Existing approaches to study such instability are mainly based on converter control models and do not include the effects of grid impedance. This paper presents the stability analysis of VSC- based HVDC system which includes controller dynamics and also main ac grid impedance. It is shown that the system will remain stable as longs as the ratio of dc system impedance and HVDC converter dc input impedance satisfies the Nyquist stability criterion. An analytical linearized model of a point-to-point connection HVDC system is developed to demonstrate the application of this method and the result is compared with nonlinear model developed in matlab/simulink association with simpower system.
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paper_title: Passivity-Based Stability Assessment of Grid-Connected VSCs—An Overview

paper_content:

The interconnection stability of a grid-connected voltage-source converter (VSC) can be assessed by the passivity properties of the VSC input admittance. If critical grid resonances fall within regions where the input admittance acts passively, i.e., has nonnegative real part, then their destabilization is generally prevented. This paper presents an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived. Design recommendations for minimizing the negative-real-part region are given as well.
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paper_title: A novel Passivity-Based Stability Criterion (PBSC) for switching converter DC distribution systems

paper_content:

A novel Passivity-Based Stability Criterion (PBSC) is proposed for the stability analysis and design of DC power distribution systems. The proposed criterion is based on imposing passivity of the overall bus impedance. If passivity of the overall DC bus impedance is ensured, stability is guaranteed as well. The PBSC reduces artificial design conservativeness and sensitivity to component grouping typical of existing stability criteria, such as the Middlebrook criterion and its extensions. Moreover, the criterion is easily applicable to multi-converter systems and to systems in which the power flow direction changes, for example as a result of system reconfiguration. Moreover, the criterion can be used for the design of active damping networks for DC power distribution systems. The approach results in greatly improved stability and damping of transients on the DC bus voltage. Experimental validation is performed using a hardware test-bed that emulates a DC power distribution system.
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paper_title: Small signal stability analysis of a shipboard MVDC power system

paper_content:

Recent developments in high power rated Voltage Source Converters (VSCs) have resulted in their successful application in Multi-Terminal HVDC (MTDC) transmission systems and also have potential in the Medium Voltage DC (MVDC) distribution systems. A multi-zonal MVDC architecture has been identified by the US Navy as one of the possible architectures for the shipboard power distribution. Selection of a particular architecture for the shipboard power system will require extensive studies to be carried out with respect to various protection, control and stability issues. This paper presents the findings of small signal stability studies carried out on a zonal MVAC as well as a MVDC architecture for the ship board power distribution system.
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paper_title: Eigenvalue Sensitivity Analysis for Dynamic Power System

paper_content:

Eigenvalue sensitivity analysis has been an effective tool for power system controller design. However, research on eigenvalue sensitivity with respect to system operating parameters is still limited. This paper presents new results of an eigenvalue sensitivity analysis with respect to operating parameters, preceded with a comprehensive review on eigenvalue sensitivity analysis and applications during the last few decades. The method is based on explicit expression of the derivatives of augmented system matrix with respect to system operating parameters. IEEE 5-machine 14-bus system is used to demonstrate the effectiveness of the method. The eigenvalue sensitivity analysis provides useful information for power system planning and control.
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paper_title: Small-Signal Stability Analysis of Offshore AC Network Having Multiple VSC-HVDC Systems

paper_content:

This paper presents a methodology to perform small-signal analysis of an offshore ac network, which is formed by interconnecting several offshore wind power plants. The offshore ac network is connected with different onshore ac grids using point-to-point voltage-source-converter high-voltage direct current (VSC-HVDC) transmission links. In such a network, each offshore VSC-HVDC converter operates in grid-forming mode. In this paper, the offshore VSC grid-forming control is enhanced by using a frequency and voltage droop scheme in order to establish a coordinated grid control among the offshore converters. A small-signal model of the offshore ac network is developed that includes the high-voltage alternating current cables' model, the converters' current, and voltage-control model, the frequency droop scheme, and the voltage droop scheme. Based on this model, an eigenvalue analysis is performed in order to study the influence of the frequency and voltage droop gains on overall offshore ac network stability. Finally, theoretical analysis is validated by performing nonlinear dynamic simulation.
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paper_title: Small-Signal Stability Analysis of Multi-Terminal VSC-Based DC Transmission Systems

paper_content:

A model suitable for small-signal stability analysis and control design of multi-terminal dc networks is presented. A generic test network that combines conventional synchronous and offshore wind generation connected to shore via a dc network is used to illustrate the design of enhanced voltage source converter (VSC) controllers. The impact of VSC control parameters on network stability is discussed and the overall network dynamic performance assessed in the event of small and large perturbations. Time-domain simulations conducted in Matlab/Simulink are used to validate the operational limits of the VSC controllers obtained from the small-signal stability analysis.
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paper_title: A case for medium voltage DC for distribution circuit applications

paper_content:

In this survey paper, applications of medium voltage direct current (MVDC) to distribution substations are discussed and compared to conventional ac for power conversion from transmission voltage levels to distribution voltage levels. Technical changes in the substation due to the dc equipment, efficiency of the substation, and protective measures are major aspects to consider in the MVDC concept development.
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paper_title: Small-signal stability study of the Cigré DC grid test system with analysis of participation factors and parameter sensitivity of oscillatory modes

paper_content:

This paper presents a detailed case study of small-signal stability analysis for the Cigre DC-grid test system. The presented investigations are intended for identifying critical modes of interaction between different parts of the electrical system and the controllers in multi-terminal HVDC (MTDC) schemes. Oscillatory and critical modes of the system are investigated by modal analysis, including participation factor analysis and studies of the parameters sensitivity. The potentially unstable modes are investigated by calculating the participation factors of the system, which identify the influencing components. The sensitivity of the eigenvalues to the control parameters is also presented in order to reach an improved control design for MTDC with sufficient damping and enhanced dynamic response. Time-domain simulation results are further presented in order to verify the transient performance of the control system.
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paper_title: Wind Farm Grid Integration Using VSC Based HVDC Transmission - An Overview

paper_content:

The paper gives an overview of HVAC and HVDC connection of wind farm to the grid, with an emphasis on voltage source converter (VSC)-based HVDC for large wind farms requiring long distance cable connection. Flexible control capabilities of a VSC-based HVDC system enables smooth integration of wind farm into the power grid network while meeting the grid code requirements (GCR). Operation of a wind farm with VSC-based HVDC connection is described.
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paper_title: Small-Signal Stability Assessment of Power Electronics Based Power Systems: A Discussion of Impedance- and Eigenvalue-Based Methods

paper_content:

This paper investigates the small-signal stability of power electronics-based power systems in frequency domain. A comparison between the impedance-based and the eigenvalue-based stability analysis methods is presented. A relation between the characteristics equation of the eigenvalues and poles and zeros of the minor-loop gain from the impedance-based analysis have been derived analytically. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of the minor-loop gain indicates that the system can exhibit harmonic oscillations. A weakness of the impedance method is the limited observability of certain states given its dependence on the definition of local source-load subsystems, which makes it necessary to investigate the stability at different subsystems. To address this limitation, the paper discusses critical locations where the application of the method can reveal the impact of a passive component or a controller gain on the stability. On the other hand, the eigenvalue-based method, being global, can determine the stability of the entire system; however, it cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching. To generalize the observations, the two methods have been applied to dc-dc converters. To illustrate the difference and the relation between the two-methods, the two stability analysis methods are then applied to a two-terminal VSC-based HVdc system as an example of power electronics-based power systems, and the theoretical analysis has been further validated by simulation and experiments.
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paper_title: A review on the small signal stability of microgrid

paper_content:

Small signal stability is the key issue in the operation and control of a microgrid. For the small signal stability of an inverter-based microgrid, existing analysis methods mainly contain eigenvalue analysis using state-space models, impedance analysis based on impedance models and other nonlinear analysis methods. This paper reviews these analytical methods as well as their respective advantages and disadvantages. Finally, for the improvement of the small signal stability, this paper summarizes the research situation in terms of the optimization of controller parameters, the improvement of droop control and the optimized design of hierarchical control strategy.
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paper_title: Power System Stability and Control

paper_content:

Part I: Characteristics of Modern Power Systems. Introduction to the Power System Stability Problem. Part II: Synchronous Machine Theory and Modelling. Synchronous Machine Parameters. Synchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems. High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems, High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Oscillations. Mid-Term and Long-Term Stability. Methods of Improving System Stability.
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paper_title: Stability Analysis of Aircraft Power Systems Based on a Unified Large Signal Model

paper_content:

Complex power electronic conversion devices, most of which have high transmission performance, are important power conversion units in modern aircraft power systems. However, these devices can also affect the stability of the aircraft power system more and more prominent due to their dynamic and nonlinear characteristics. To analyze the stability of aircraft power systems in a simple, accurate and comprehensive way, this paper develops a unified large signal model of aircraft power systems. In this paper, first the Lyapunov linearization method and the mixed potential theory are employed to analyze small signal and large signal stability, respectively, and then a unified stability criterion is proposed to estimate small and large signal stability problems. Simulation results show that the unified large signal model of aircraft power systems presented in this paper can be used to analyze the stability problem of aircraft power systems in an accurate and comprehensive way. Furthermore, with simplicity, universality and structural uniformity, the unified large signal model lays a good foundation for the optimal design of aircraft power systems.
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paper_title: VSC HVDC transmission corridor: An option for PV power injection and AC network stability support

paper_content:

The pressure of increasing power demand and supply inequality is forcing utilities to interconnect AC systems to meet demands. High Voltage Direct Current (HVDC) schemes are becoming a more attractive solution as they have been used extensively in interconnected power systems worldwide. This paper investigates the role of voltage source converter (VSC) based HVDC transmission corridor for PV power injection and for AC network stability support. Overview of Namibia Caprivi-link interconnector a case study, potential of very large scale PV in Namibia and prospects of PV power injection on the DC-link is presented. The system is modelled simulated in Matlab/Simulink. Critical contingencies such as sudden island conditions, three-phase to ground fault are simulated with and without PV penetration. Results show the stability support on the AC side networks by PV power injection on the dc-link.
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paper_title: Stability Criterion for Cascaded System With Constant Power Load

paper_content:

With the development of renewable energy, dc distribution power system (DPS) becomes more and more attractive. The stability of whole system is still a big concern though every single converter is well designed based on the stand-alone operation with sufficient stability. Since the cascaded connection of power converters is one of the most dominant connection forms in the dc DPS, the stability analysis of the cascaded system is very important to ensure stability of the whole system. Based on the Lyapunov linearization method and Brayton–Moser's mixed potential theory, stability of equilibrium point and an estimation of the region of attraction are investigated for cascaded system in the dc DPS. Based on the analysis, stability prediction criteria for cascaded system under small-signal and large-signal disturbances are obtained. The two criteria are simple and straightforward, which can be unified to get a general stability criterion to predict system's stability under both small-signal and large-signal transient disturbances. The relationship between system parameters and the stability is presented and discussed in the paper. Therefore, instead of trial and error, the proposed criterion can predict and guarantee the stability operation of cascaded system during the design process, and it is also helpful to select matched power converters in system level design. The simulation and experimental results verify the effectiveness of the proposed criterion.
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paper_title: Issues of Connecting Wind Farms into Power Systems

paper_content:

Wind power industry is developing rapidly, more and more wind farms are being connected into power systems. Integration of large scale wind farms into power systems presents some challenges that must be addressed, such as system operation and control, system stability, and power quality. This paper describes modern wind power systems, presents requirements of wind turbine connection and discusses the possible control methods for wind turbines to meet the specifications
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paper_title: Power System Stability and Control

paper_content:

Part I: Characteristics of Modern Power Systems. Introduction to the Power System Stability Problem. Part II: Synchronous Machine Theory and Modelling. Synchronous Machine Parameters. Synchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems. High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems, High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Oscillations. Mid-Term and Long-Term Stability. Methods of Improving System Stability.
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paper_title: Modeling, analysis, and validation of a preliminary design for a 20 kV medium voltage DC substation

paper_content:

With the advancement of high capacity power electronics technologies, most notably in HVDC applications, the concept of developing and implementing future transmission subsystems through a DC backbone presents a realistic and advantageous option over traditional AC approaches. Currently, electrical equipment or devices requiring DC power to function, whether loads or resources, necessitate AC/DC conversion technologies. Having an accessible and direct supply of DC power to serve such loads and resources creates the potential to mitigate losses experienced in the AC/DC conversion process, reduce overall electrical system infrastructure, and lessen the amount of power generated from power plants, as well as other advantages. This paper introduces an initial design and simulation model of a medium voltage DC (MVDC) substation concept containing renewable generation, power electronic converters, and induction machine loads. Each of the components is developed and modeled in PSCAD and validated analytically. The models of the represented system equipment and components are individually presented and accompanied with their simulated results to demonstrate the validity of the overall model. Future work will build upon the model to develop additional loads and resources, control strategies for optimized integration, and more detailed component models.
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paper_title: MATCONT: A MATLAB package for numerical bifurcation analysis of ODEs

paper_content:

MATCONT is a graphical MATLAB software package for the interactive numerical study of dynamical systems. It allows one to compute curves of equilibria, limit points, Hopf points, limit cycles, period doubling bifurcation points of limit cycles, and fold bifurcation points of limit cycles. All curves are computed by the same function that implements a prediction-correction continuation algorithm based on the Moore-Penrose matrix pseudo-inverse. The continuation of bifurcation points of equilibria and limit cycles is based on bordering methods and minimally extended systems. Hence no additional unknowns such as singular vectors and eigenvectors are used and no artificial sparsity in the systems is created. The sparsity of the discretized systems for the computation of limit cycles and their bifurcation points is exploited by using the standard Matlab sparse matrix methods. The MATLAB environment makes the standard MATLAB Ordinary Differential Equations (ODE) Suite interactively available and provides computational and visualization tools; it also eliminates the compilation stage and so makes installation straightforward. Compared to other packages such as AUTO and CONTENT, adding a new type of curves is easy in the MATLAB environment. We illustrate this by a detailed description of the limit point curve type.
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paper_title: Bifurcation theory and its application to nonlinear dynamical phenomena in an electrical power system

paper_content:

A tutorial introduction in bifurcation theory is given, and the applicability of this theory to study nonlinear dynamical phenomena in a power system network is explored. The predicted behavior is verified through time simulation. Systematic application of the theory revealed the existence of stable and unstable periodic solutions as well as voltage collapse. A particular response depends on the value of the parameter under consideration. It is shown that voltage collapse is a subset of the overall bifurcation phenomena that a system may experience under the influence of system parameters. A low-dimensional center manifold reduction is applied to capture the relevant dynamics involved in the voltage collapse process. The need for the consideration of nonlinearity, especially when the system is highly stressed, is emphasized. >
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paper_title: Controlling chaos and bifurcations of SMIB power system experiencing SSR phenomenon using SSSC

paper_content:

Abstract This paper presents the effect of Static Synchronous Series Compensation (SSSC) on the bifurcations of heavily loaded Single Machine Infinite Bus (SMIB) power system experiencing Subsynchronous Resonance (SSR). In SSR phenomenon, the series compensation increases the power transfer capability of the transmission line. However, Hopf bifurcation is depicted at certain compensation levels. The system then routes to chaos via torus breakdown scenario in case of conventional compensation (variable series capacitor) scheme. In this study, the effect of replacing the conventional compensation with SSSC is highlighted. Varying the SSSC controller reference voltage changes the compensation degree. The results show that the operating point of the system never loses stability at any realistic compensation degree in case of SSSC which means that all bifurcations of the system have been eliminated. Time domain simulations coincide with the results of the bifurcation analysis. The robustness of the SSSC compensation scheme and its controller is verified by subjecting a single-phase to ground fault at the end of the transmission line. The results are compared with the case of conventional compensation. Additionally, the effect of the SSSC controller gain on the location of the Hopf bifurcation is addressed.
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paper_title: Dynamic Stability Analysis of Synchronverter-Dominated Microgrid Based on Bifurcation Theory

paper_content:

In the traditional power grid, inverter-inter-faced distributed energy resources are widely used in microgrids such as conventional synchronous generators (SGs) in the traditional power grid. They are expected to decrease the rotational inertia and spinning reserve capacity that are specific to SGs, while synchronverters can compensate for the loss. Thus, an increasing number of interfaced inverters may act as synchronverters in a microgrid. The dynamic stability of the synchronverter-dominated microgrid must thus be carefully evaluated to ensure the reliable operation of this type of system. This paper presents a nonlinear model of a synchronverter-dominated microgrid. The eigenvalues of the system state matrix, along with the participation factors, are analyzed to determine the predominant parameters affecting the stability. Bifurcation theory is then used to predict and describe the unstable phenomenon as the system parameters fluctuate; the effect of these parameters on the system stability is then examined. A simulation on MATLAB/Simulink and experiments were performed and results validated the presented analysis.
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paper_title: Discrete-Time Tool for Stability Analysis of DC Power Electronics-Based Cascaded Systems

paper_content:

DC distribution power systems are vulnerable to instability because of the destabilizing effect of converter-controlled constant power loads (CPLs) and input filters. Standard stability analysis tools based on averaging linearization techniques can be used only when the switching frequency of the converter is significantly higher than the cutoff frequency of the filter. However, dc distribution systems with a reduced size filter, and consequently a high cutoff frequency, are common in transportation applications. Conventional methods fail to detect instabilities in the system because they do not take into account the switching effect. To overcome this drawback, this paper proposes a discrete-time method to analyze the stability of dc distribution systems. This model is applied here to a dc power system with a CPL. The switching effects and the nonlinearities of the system model are taken into account with a simple discretization approach. The proposed method is able to predict the dynamic properties of the system, such as slow-scale and fast-scale instabilities. An active stabilizer is also included in the system model in order to extend the stability margin of the system. Finally, these observations are validated experimentally on a laboratory test bench.
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paper_title: Discrete-Time Modeling, Stability Analysis, and Active Stabilization of DC Distribution Systems With Multiple Constant Power Loads

paper_content:

This paper presents the stability analysis of dc distributed power systems with multiple converter-controlled loads. The load converters are tightly controlled, behaving as constant power loads with low-damped $LC\;$ filters. The dynamic behavior of the system in high frequency range is often not studied with the classical tools based on conventional averaging techniques. However, dc power systems with the reduced size filter, and consequently, the high resonant frequency, are widely used in transportation applications. In this paper, the stability analysis of the system is established based on a discrete-time model of the system, taking into account the switching frequency and intrinsic nonlinearities of the system model. The impacts of the filter parameters and interactions among the constant power loads are investigated with the proposed discrete-time method. Moreover, an active stabilizer is developed and included in the dynamic model of the system, in order to extend the stability margin. The theoretical observations are then validated experimentally on a laboratory hardware prototype.
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paper_title: An application of chaos and bifurcation in nonlinear dynamical power systems

paper_content:

In this article, a nonlinear dynamical phenomena leading to bifurcation and chaos in power systems is explored using a sample power system. After giving an introduction to nonlinear dynamical power systems in section II a basic knowledge to nonlinear dynamics and chaos theory is given. Section III deals with bifurcation theory. In section IV a dynamical power system model has examined. In section V the theories are applied to a sample power system example and various bifurcation and chaotic phenomena are examined. The predicted behavior is verified by simulations using MATLAB tools.
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paper_title: A review on the small signal stability of microgrid

paper_content:

Small signal stability is the key issue in the operation and control of a microgrid. For the small signal stability of an inverter-based microgrid, existing analysis methods mainly contain eigenvalue analysis using state-space models, impedance analysis based on impedance models and other nonlinear analysis methods. This paper reviews these analytical methods as well as their respective advantages and disadvantages. Finally, for the improvement of the small signal stability, this paper summarizes the research situation in terms of the optimization of controller parameters, the improvement of droop control and the optimized design of hierarchical control strategy.
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paper_title: Design aspects of a medium-voltage direct current (MVDC) grid for a university campus

paper_content:

Today's power systems use alternating current (ac) for transmission and distribution of electrical energy, although the first grids were based on direct current (dc). Due to the absence of appropriate equipment to change voltage levels dc technology did not become widely accepted and was finally ruled out by the more efficient ac infrastructure. However, as a result of considerable technical progress, high-voltage direct current (HVDC) transmission has found its way back into power systems. At lower voltage and power levels medium-voltage dc (MVDC) distribution has been proposed for offshore wind farms and industrial applications. This paper describes the design of an MVDC grid for the interconnection of high-power test benches at a university campus. Voltage control within the dc grid as well as the behavior in different fault scenarios is analyzed using numerical simulations. To assess the environmental impact of the grid the magnetic flux density emitted by the dc cable lines is calculated.
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Title: Small-signal stability analysis for the multi-terminal VSC MVDC distribution network; a review

Section 1: Introduction
Description 1: Provide an overview of the motivation for the study, the significance of MVDC technology, and its benefits and challenges.

Section 2: Classical control-based stability analysis
Description 2: Discuss various classical control techniques such as Routh-Hurwitz criterion, root-locus plot, and frequency-response methods used for small-signal stability analysis of the MVDC system.

Section 3: Impedance-based stability analysis
Description 3: Explain the impedance-based method where the system is analyzed using source and load impedances, including Middlebrook criterion and its extensions.

Section 4: Passivity-based stability criteria
Description 4: Describe the passivity-based method that imposes a passivity condition on the overall bus impedance to ensure system stability.

Section 5: State-space modelling and eigenvalue analysis
Description 5: Examine the state-space modelling and eigenvalue analysis method that provides a global stability assessment by analyzing the system's eigenvalues.

Section 6: Lyapunov linearisation method
Description 6: Review the Lyapunov linearisation method, evaluating system stability using the roots of its state-equation and applying Lyapunov's criteria.

Section 7: Bifurcation theory
Description 7: Detail the use of bifurcation theory to study the stability of non-linear dynamic systems, focusing on changes in system behavior due to parameter variations.

Section 8: Conclusion
Description 8: Summarize the key findings of the review, compare the strengths and weaknesses of the stability methods, and suggest suitable approaches for MVDC distribution networks.