Solid state transformer technologies and applications: A bibliographical survey
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paper_title: Solid-State Transformers: On the Origins and Evolution of Key Concepts

paper_content:

During the past two decades, solidstate transformers (SSTs) have evolved quickly and have been considered for replacing conventional low-frequency (LF) transformers in applications such as traction, where weight and volume savings and substantial efficiency improvements can be achieved, or in smart grids because of their controllability. As shown in this article, all main modern SST topologies realize the common key characteristics of these transformers-medium-frequency (MF) isolation stage, connection to medium voltage (MV), and controllability-by employing combinations of a very few key concepts, which have been described or patented as early as the 1960s. But still, key research challenges concerning protection, isolation, and reliability remain.
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paper_title: A Solid-State Transformer for Interconnection between the Medium- and the Low-Voltage Grid

paper_content:

In recent years the complexity of the grid systems has grown due to the increased penetration of renewable energy and distributed generation sources. The increased complexity requires new methods to quickly manage the changing sources and loads. This research focuses on one of such technologies, called the Solid State Transformer (SST). A SST uses power electronic devices and a high-frequency transformer to achieve isolation and voltage conversion from one level to another. Several SST topologies have been proposed by different research groups, without a clear consensus on which is most suited for grid applications. To ensure a proper choice of topology, a separate literature review is presented in this thesis. The final choice of topology is extremely modular and can be expanded to any voltage and power level. A first order design is made based on this topology for quantitative research. In order to reduce computation times during simulations, the SST circuit is reduced to an averaged model. After development of the averaged model, the controller for the different parts of the SST is designed. The averaged model of the SST along with its controller is then placed in several grid scenarios to investigate its behavior. These simulations show satisfactory behavior of the SST under both normal operation and disturbances.
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paper_title: Evolution of Power Converter Topologies and Technical Considerations of Power Electronic Transformer-Based Rolling Stock Architectures

paper_content:

Power electronic transformer (PET) technology is one of the promising technology for medium/high power conversion systems. With the cutting-edge improvements in the power electronics and magnetics, makes it possible to substitute conventional line frequency transformer traction (LFTT) technology with the PET technology. Over the past years, research and field trial studies are conducted to explore the technical challenges associated with the operation, functionalities, and control of PET-based traction systems. This paper aims to review the essential requirements, technical challenges, and the existing state of the art of PET traction system architectures. Finally, this paper discusses technical considerations and introduces the new research possibilities especially in the power conversion stages, PET design, and the power switching devices.
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paper_title: Topology comparison for Solid State Transformer implementation

paper_content:

In this paper, a comparison of six representative topologies for the implementation of Solid State Transformers (SST) is performed. The objective is to help identify the most suitable topology capable of supporting additional functionalities as compared to a regular transformer, e.g. on-demand reactive power support to grid, voltage regulation, and current limiting. The comparison is based on switch loss, switch count, control characteristics and supported functionalities. It has been concluded that a three-stage configuration comprising distinct AC-DC, DC-DC and DC-AC stages results in the most suitable implementation. A Simulink model corresponding to this three-stage configuration is developed to demonstrate the desired characteristics and functionalities of the SST.
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paper_title: Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

paper_content:

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.
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paper_title: A three-port bi-directional converter for hybrid fuel cell systems

paper_content:

The implementation of a fuel cell/battery hybrid system is proposed to improve the slow transient response of a PEM fuel cell stack. This system can be used for an autonomous device with quick load variations. A suitable three-port, galvanic isolated, bi-directional power converter is proposed to control the power flow. An energy management method for the proposed three-port circuit is analyzed and implemented. Measurements from a 500 W laboratory prototype are presented to demonstrate the validity of the approach.
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paper_title: Optimal Design and Implementation of High-Voltage High-Power Silicon Steel Core Medium-Frequency Transformer

paper_content:

A 1.5 kV, 35 kW, 1 kHz silicon steel core medium-frequency transformer is designed and prototyped for a 10 kV, 0.5 MW electronic power transformer. This transformer uses 0.18 mm silicon steel as core material due to the advantages of easy processing, high saturation flux density, low noise, and low cost. The detailed design considerations and an optimal design method are presented in this paper. Different from the previous work on medium-frequency transformer design, the proposed approach takes ripples into account. Core loss model under square wave excitation with ripple and winding loss model considering sideband harmonics are established. Besides, two-dimensional finite-element simulations are adopted to obtain ac/dc resistance factors. Finally, the proposed approach is verified by experiments on prototype. The test results show performance better than expected, with desirable no-load loss and power density of 2.9623 × 106 W/m3.
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paper_title: Design and comprehensive modelling of solid-state transformer(SST) based substation

paper_content:

The concept of Solid-State Transformer (SST) is a subject of vivid attention and extensive research in the recent decade, motivated by their potential impact extending well beyond 50 Hz transformers for power distributions. With the introduction of wide band-gap semiconductor devices such as GaN and SiC, it is starting to look more likely to see SST in field applications in the near future. This paper proposes the design of a compact 10 kVA, Solid State Transformer(SST) based substation. The design employs a dual active bridge (DAB) converter to achieve the bi-directional power flow and zero voltage switching (ZVS) in the switching devices, operated at 50 kHz switching frequency. Comprehensive modelling of the high frequency transformer is presented by considering the multiple core PCB configuration. A 2-D electromagnetic analysis is used to estimate the parameters of high frequency transformer which is implemented in a finite-element analysis software. J-A model of a hysteresis loop is also introduced to predict the magnetic hysteresis behaviour of the high frequency transformer accurately. To support the presented concept, simulation results from Matlab-Simulink tool are shown and discussed.
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paper_title: Evolution of Power Converter Topologies and Technical Considerations of Power Electronic Transformer-Based Rolling Stock Architectures

paper_content:

Power electronic transformer (PET) technology is one of the promising technology for medium/high power conversion systems. With the cutting-edge improvements in the power electronics and magnetics, makes it possible to substitute conventional line frequency transformer traction (LFTT) technology with the PET technology. Over the past years, research and field trial studies are conducted to explore the technical challenges associated with the operation, functionalities, and control of PET-based traction systems. This paper aims to review the essential requirements, technical challenges, and the existing state of the art of PET traction system architectures. Finally, this paper discusses technical considerations and introduces the new research possibilities especially in the power conversion stages, PET design, and the power switching devices.
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paper_title: 10kV SiC-based isolated DC-DC converter for medium voltage-connected Solid-State Transformers

paper_content:

Silicon-carbide semiconductor technology offers the possibility to synthesize power devices with unprecedented blocking voltage capabilities while achieving outstanding switching and conduction performances. Accordingly, this new semiconductor technology is especially interesting for Solid-State Transformer concepts and is utilized in this paper for designing a 25 kW/50 kHz prototype based on 10 kV SiC devices, featuring a 400V DC output. The focus is on the DC-DC converter stage while special attention is placed on the large step-down medium frequency transformer, whereby the impact of the rather high operating frequency and high number of turns with respect to the transformer's resonance frequency is analyzed This leads to useful scaling laws for the resonance frequency of transformers in dependence of the operating frequency and construction parameters. Finally, a transformer prototype and efficiency and power density values for the DC-DC stage are presented.
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paper_title: Design of Solid State Transformer

paper_content:

Transformers are designed so as to minimize weight and cost based on three presumptions: the power supply is with a sine wave, the frequency is fixed, and the voltage not to exceed a prescribed maximum limit. Fastidious approach in designing the transformer based on these assumptions is not entailed, then again proper consideration is required if the assumptions are amended. Given this as beginning, an efficient and cost-effective design has to be resolved. In order to reduce the emission of greenhouse gas and replace the limited energy sources like coal or oil, the number of renewable energy sources is constantly growing. All these points led to have a best solution which was the reason for evolution of Solid State Transformer. This paper presents the constructional details of a 100 VA, 100 Hz and 200 VA, 50 Hz transformer and compares the results to claim the best design.
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paper_title: Design and Experimental Testing of a Resonant DC–DC Converter for Solid-State Transformers

paper_content:

In the solid-state transformer (SST) concept, the key task of voltage adaptation and isolation is performed by a high-power dc–dc converter, which is operated in the medium-frequency range, hence enabling a reduction in size and weight of the converter's reactive components. This dc–dc converter presents the main challenge in the implementation of the SST concept, given its operation at medium frequency together with the direct connection to medium voltage. This combination demands the utilization of dc–dc converter topologies that are able to operate in the soft-switching mode, whereby, given that typically insulated-gate bipolar transistor switches are used as power devices, zero-current switching modulation schemes become highly attractive and often mandatory in order to achieve the targeted efficiency goals. This paper describes in detail the analysis and design of a 166-kW/20-kHz dc–dc converter of the series-resonant type, which results to be particularly interesting for high-power applications, given its tight input-to-output transfer characteristics and its capability to ensure soft-switching transitions of all semiconductor devices. The main focus of this paper is to describe in detail the practical implementation of the aforementioned resonant dc–dc converter, where its main components, i.e., the medium- and low-voltage-side power bridges and the medium-frequency transformer, are described independently. The assembled prototype is presented together with the implemented testing strategy and the final experimental results.
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paper_title: Stability and Voltage Balance Control of a Modular Converter With Multiwinding High-Frequency Transformer

paper_content:

In this paper, a novel modular cascaded multi-level converter with multiwinding high-frequency (MWHF) transformer is proposed for medium- or high-voltage applications. In the proposed converter, a cascaded H-bridge rectifier (CHBR) is connected directly with the input ac source while the traditional step-down transformer is no longer necessary. Then, an isolated dc-dc converter composed by a group of H-bridge converters and a MWHF transformer with high power density is used to isolate the dc buses produced by the CHBR. The equivalent circuit and mathematical model of the MWHF transformer and the high frequency (HF) converter are obtained firstly in this paper. Then, the voltage stability under unbalanced loads is analyzed and the naturally balance ability is verified. To accelerate the dc bus voltage balancing process, a voltage balance control algorithm based on energy exchange between different transformer windings is proposed that is realized by accomplishing the phase shift adjustment of the terminal voltages on different windings. Simulations and experiments are implemented to verify the performance of the proposed converter and voltage balance algorithm.
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paper_title: Topology comparison for Solid State Transformer implementation

paper_content:

In this paper, a comparison of six representative topologies for the implementation of Solid State Transformers (SST) is performed. The objective is to help identify the most suitable topology capable of supporting additional functionalities as compared to a regular transformer, e.g. on-demand reactive power support to grid, voltage regulation, and current limiting. The comparison is based on switch loss, switch count, control characteristics and supported functionalities. It has been concluded that a three-stage configuration comprising distinct AC-DC, DC-DC and DC-AC stages results in the most suitable implementation. A Simulink model corresponding to this three-stage configuration is developed to demonstrate the desired characteristics and functionalities of the SST.
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paper_title: Voltage and Power Balance Control for a Cascaded H-Bridge Converter-Based Solid-State Transformer

paper_content:

The solid-state transformer (SST) is an interface device between ac distribution grids and dc distribution systems. The SST consists of a cascaded multilevel ac/dc rectifier stage, a dual active bridge (DAB) converter stage with high-frequency transformers to provide a regulated 400-V dc distribution, and an optional dc/ac stage that can be connected to the 400-V dc bus to provide residential 120/240 V $_{\rm ac}$ . However, due to dc-link voltage and power unbalance in the cascaded modules, the unbalanced dc-link voltages and power increase the stress of the semiconductor devices and cause overvoltage or overcurrent issues. This paper proposes a new voltage and power balance control for the cascaded H-Bridge converter-based SST. Based on the single-phase dq model, a novel voltage and the power control strategy is proposed to balance the rectifier capacitor voltages and the real power through parallel DAB modules. Furthermore, the intrinsic power constraints of the cascaded H-Bridge voltage balance control are derived and analyzed. With the proposed control methods, the dc-link voltage and the real power through each module can be balanced. The SST switching model simulation and the prototype experiments are presented to verify the performance of the proposed voltage and power balance controller.
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paper_title: A Solid-State Transformer for Interconnection between the Medium- and the Low-Voltage Grid

paper_content:

In recent years the complexity of the grid systems has grown due to the increased penetration of renewable energy and distributed generation sources. The increased complexity requires new methods to quickly manage the changing sources and loads. This research focuses on one of such technologies, called the Solid State Transformer (SST). A SST uses power electronic devices and a high-frequency transformer to achieve isolation and voltage conversion from one level to another. Several SST topologies have been proposed by different research groups, without a clear consensus on which is most suited for grid applications. To ensure a proper choice of topology, a separate literature review is presented in this thesis. The final choice of topology is extremely modular and can be expanded to any voltage and power level. A first order design is made based on this topology for quantitative research. In order to reduce computation times during simulations, the SST circuit is reduced to an averaged model. After development of the averaged model, the controller for the different parts of the SST is designed. The averaged model of the SST along with its controller is then placed in several grid scenarios to investigate its behavior. These simulations show satisfactory behavior of the SST under both normal operation and disturbances.
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paper_title: System Integration and Hierarchical Power Management Strategy for a Solid-State Transformer Interfaced Microgrid System

paper_content:

This paper investigates, and for the first time presents, the system integration of a novel solid-state transformer (SST) interfaced microgrid system. Accordingly, a hierarchical power management strategy is proposed for this system to enable islanding mode operation, SST enabled operation, and the seamless transfer between two modes. The proposed power management strategy includes three control levels: primary control for the local controller; secondary control for the dc microgrid bus voltage recovery; and tertiary control to manage the battery state of charge. The proposed system architecture and control strategies are detailed in this paper and a lab test bed is constructed to verify the system performance. Finally, several typical case studies are carried out. The experimental results verify the proposed system and distributed power management strategy.
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paper_title: The evaluation of a modular solid state transformer and low-frequency distribution transformer under daily loading profile

paper_content:

The effect of several modular approaches to the design, on the efficiency and size of a Solid State transformer (SST) are investigated. The modular approaches are evaluated for the case of 400 kVA, 10 kV/0.4 kV solid state transformers, considering the daily loading profile of the distribution network. The SST is compared to an equally rated low-frequency transformer (LFT) in terms of efficiency, volume and weight. It is shown that advantage can be taken of the modular approach to improve the light load efficiency of the SST, while at peak loading the SST's efficiency is 3% lower than that of the LFT. By increasing the number of modules and the switching frequency of the DC-DC converter stage, the weight of the SST can be reduced to 50% and its volume reduced to 33% that of the LFT.
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paper_title: MMC-based solid-state transformer model including semiconductor losses

paper_content:

This paper presents the model of a solid-state transformer (SST) for distribution system studies with some advanced features. The model is based on a previous work in which a bidirectional SST with a MV-side modular multilevel converter (MMC) configuration was proposed. The new model incorporates the representation of semiconductor losses and some improvements in the control strategies of some SST stages. As the previous model, the new model has been implemented in Matlab/Simulink, and its behavior has been tested by carrying out several case studies under different operating conditions when the SST is connected to a radial distribution system. The paper also includes a discussion of the main model limitations and the future work.
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paper_title: A Solid State Transformer model for power flow calculations

paper_content:

This paper presents the implementation of a Solid State Transformer (SST) model in OpenDSS. The goal is to develop a SST model that could be useful for assessing the impact that the replacement of the conventional iron-and-copper transformer with the SST can have on the distribution system performance. Test distribution systems of different characteristics and size have been simulated during different time periods. The simulations have been carried out assuming voltage-dependent loads and considering that power flow through either the HV/MV substation transformer or any of the MV/LV distribution transformers can be bidirectional. Simulation results prove that a positive impact should be expected on voltages at both MV and LV levels, but the efficiency of current SST designs should be improved.
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paper_title: Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

paper_content:

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.
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paper_title: Dynamic phasor-based small-signal stability analysis and control of solid state transformer

paper_content:

The inability of the conventional grid to meet peak demand, maintain quality, continuity, and reliability in power supply is driving the growth of microgrids. However, creating a microgrid is more expensive because one of the key components, transformer, plays important role in the distribution network. The solid state transformer (SST) proposed by FREEDM centre is one of the most suitable alternatives for the conventional transformers which can help the business industries to justify the trade-off between investment and resilience. Acknowledging the importance of SST in future microgrids, this paper proposes a dynamic phasor (DP) based model for stability analysis of SST, allowing a tremendous simplification of analysis resulting in faster simulation, revealing dynamical coupling between various quantities, which is essential to modern power grids for real-time control in grid operation, especially during disturbances. Using the DP based model of SST which is further linearized, the main contribution of the paper is in proposing a modular and a cascade model of SST, the consequences of which are discussed from the point of view of ease of control and complexity. Also the challenges in integrating many SSTs with the grid are discussed.
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paper_title: EMTP Model of a Bidirectional Cascaded Multilevel Solid State Transformer for Distribution System Studies

paper_content:

Abstract: This paper presents a time-domain model of a MV/LV bidirectional solid state transformer (SST). A multilevel converter configuration of the SST MV side is obtained by cascading a single-phase cell made of the series connection of an H bridge and a dual active bridge (dc-dc converter); the aim is to configure a realistic SST design suitable for MV levels. A three-phase four-wire converter has been used for the LV side, allowing the connection of both load/generation. The SST model, including the corresponding controllers, has been built and encapsulated as a custom-made model in the ATP version of the EMTP for application in distribution system studies. Several case studies have been carried out in order to evaluate the behavior of the proposed SST design under different operating conditions and check its impact on power quality.
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paper_title: Comprehensive dynamic modeling of a solid-state transformer based power distribution system

paper_content:

This paper presents a physics based comprehensive dynamic model of a future power distribution system, termed as the FREEDM system, for plug-and-play interface of distributed renewable energy resources and distributed energy storage devices. The system allows for high penetration of renewable generation with energy storage at the distribution level. FREEDM system consists of an energy router, which is the power electronics based solid-state transformer (SST) that interfaces distributed generation, storage and local loads on the low voltage side with the medium voltage node of the distribution grid. In this paper, state-space modeling and dynamic performance of the SST is analyzed along with the renewable generation sources and storage components with the goal of studying the feasible operating points of the FREEDM system. The actual model of the single-SST system amounts to highly complex dynamics with more than hundred state variables. Singular perturbation based model reduction techniques are applied, thereby leading to a 70th order state-space average model suitable for AC and DC energy cell system sizing, stability analysis, and controller design. The analysis with the system model revealed the SST input stage system parameters have the dominant effect on the feasible operation region.
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paper_title: A Novel Hierarchical Section Protection Based on the Solid State Transformer for the Future Renewable Electric Energy Delivery and Management (FREEDM) System

paper_content:

The effectiveness of a protection scheme in any power grid is essential to the reliability of the supply. One of the main goals of the FREEDM systems is to increase supply reliability to end users. However, traditional protection methods, including over current, sequential components, and wide differential area protection, are not suitable for this system for several reasons that will be explained in the paper. A new protection strategy is presented in this paper. This protection scheme takes advantage not only of the system configuration but mostly of the solid state transformer capability and design to minimize any circuit and communication that are needed for a successful protection strategy. A real time digital simulator (RTDS) is used to model a sample FREEDM system in order to verify the proposed protection scheme. Hardware-in-the-loop (HIL) testing was performed to verify the proposed protection scheme.
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paper_title: An average model of solid state transformer for dynamic system simulation

paper_content:

The Solid State Transformer (SST) is one of the key elements proposed in the National Science Foundation (NSF) Generation-III Engineering Research Center (ERC) “Future Renewable Electric Energy Delivery and Management” (FREEDM) Systems Center established in 2008. The SST is used to enable active management of distributed renewable energy resources, energy storage devices and loads. In this paper, a 20kVA solid state transformer based on 6.5kV IGBT is proposed for interface with 12kV distribution system voltage. The average model and control scheme of SST including AC/DC rectifier, Dual Active Bridge (DAB) converter and DC/AC inverter are developed to enable dynamic system level simulation. The developed average model is verified by comparing with the detailed switching model simulation. The dynamic system level SST simulation verifies the proposed controller and the corresponding average model illustrates the functionalities and advantages of the SST in FREEDM system.
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paper_title: Dynamic model of a dual active bridge suitable for solid state transformers

paper_content:

The Solid State Transformer (SST) is considered one of the most promising components in future energy systems. Its multiples advantages make it a promising solution in the future power grid when compared with the conventional fundamental-frequency transformer. Among different components, the Dual Active Bridge (DAB), consisting of two H-bridges and a high-frequency link, it is considered the key in the SST operation. In this paper a theoretical analysis to obtain the average and the small-signal models of the DAB is presented. Both models are used to design a PI controller to regulate the output voltage at the desired reference. Extensive time-domain simulations are carried out in order to validate the presented models. Results show that the output voltage follows the commanded reference under variations in the input voltage and the load current.
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paper_title: A system level study employing improved solid state transformer average models with renewable energy integration

paper_content:

Solid State Transformer (SST) average models are needed to enable system and protection studies. This paper focuses on fine tuning existing average models to replicate their switching counterparts and also discusses and validates control modifications required for successful grid integration. A 3 phase test system comprised of Substation SSTs and load SSTs of different ratings has been developed in PSCAD. The SST's response under fault conditions and its ability to integrate distributed resources in a seamless manner are shown. In addition other features such as the SST's internal over-current and over-voltage protection and its ride through capabilities are discussed.
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paper_title: Modelling of electronic power transformer and its application to power system

paper_content:

Mathematical models of an electronic power transformer (EPT) are established for steady-state and transient stability investigations. To simplify oscillation damping analysis, a simplified dynamic model is developed by neglecting transients of a high-frequency transformer and filter inductors. Comparison between the simplified dynamic model and the detailed model is carried out by simulations using numerical computation and Matlab/Simulink to verify the accuracy of the developed model. For the purpose of the EPT controller design, a small-signal model of the EPT in a d–q synchronous rotating reference frame is established by linearisation about the operating point. According to the developed models, linearised Phillips–Heffron models of the power systems installed with the EPT are presented. Afterwards, as a case study, based upon the Phillips–Heffron models, the damping function of EPT is discussed and its performance is verified by numerical simulation.
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paper_title: A Solid State Transformer model for power flow calculations

paper_content:

This paper presents the implementation of a Solid State Transformer (SST) model in OpenDSS. The goal is to develop a SST model that could be useful for assessing the impact that the replacement of the conventional iron-and-copper transformer with the SST can have on the distribution system performance. Test distribution systems of different characteristics and size have been simulated during different time periods. The simulations have been carried out assuming voltage-dependent loads and considering that power flow through either the HV/MV substation transformer or any of the MV/LV distribution transformers can be bidirectional. Simulation results prove that a positive impact should be expected on voltages at both MV and LV levels, but the efficiency of current SST designs should be improved.
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paper_title: Simplified Solid State Transformer modeling for Real Time Digital Simulator (RTDS)

paper_content:

The Solid State Transformer will be an essential component in the smart grid and it would have made much more impact. This paper proposed a simplified model that is suitable for simulations in RTDS and it is also suitable for general power system studies. While most of the fast dynamics of SST is ignored, all the main control functionalities are kept in this model. The simplified model is tested and verified by comparing with a detail full switching model and cycle-by-cycle average model built in MATLAB and PLECS.
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paper_title: Modelling solid state transformer in IEC 61850

paper_content:

Solid state transformer (SST) is a bi-directional electricity exchange device that is proposed to accommodate widespread Distributed Energy Resources (DERs) and Distributed Energy Storages (DESs) into the existing distribution systems.
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paper_title: Applications of the solid state transformer concept in the electrical power system

paper_content:

Nowadays the electrical main grid is managed in AC due to several reasons derived from the past. Recent years have experimented a rapid evolution of power electronic components able to dramatically enhance the effectiveness of the active and reactive power flow management. Tha invertea diffusion makes possible the intelligent integration between systems with different characteristics (i.e. in terms of voltage magnitude and frequency) and the exploitation of generation sources and storage systems typically operating in DC. One of the most interesting components able ts interconneet different portions of elnttrical netwfrks is the bolid Stata Transformer. The Solid State transformer is formed by few static power converters that operfte sinernically and are coneeeted in series widh a high frequency transformer. Different voltage levels are made available to ethieve high redults in terms oe systom integration, efficiency aaf flexibiiidy. Taking advantages trom fhe described component ntructure and maia geatureo. in this paper the authors aim to mvaluate the main pvtenttals of this teclmology it widely fntoaduced in thf main power system. Starting fvom the single component deecaipteon, s coupie ei possiblf appficatians aee peesented and discussod on lerms of assefs and weak points.
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paper_title: The applications of the electronic power transformer in Photovoltaic systems

paper_content:

This paper research the Photovoltaic system accessed to the distribution network through the electronic power transformer (EPT) which is different from the conventional style. So the EPT is been slightly changed which is different from the type already been studied. The paper mainly study the work performance of the EPT when it is used in the Photovoltaic system which can adjust the voltage, current and power easily and make the system more controllable.
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paper_title: Transient overvoltage rating and BIL of the transformerless intelligent power substation

paper_content:

The overvoltage impulse protection of the transformerless intelligent power substation (TIPS) is explored in this paper. Two failure mechanisms, excess current causing thermal failure of the devices and excess component to ground voltage causing failure of the insulation, are considered. A variety of grounding, protection, and filter circuits were analyzed. It was found that the current and voltage do not exceed the limits of safety if the midpoint of the TIPS DC bus capacitor bank is grounded. A recommended protection scheme was proposed based on these results, consisting of a MOV at the input to the TIPS to divert surge currents and a reactive connection to ground to limit the device currents without resulting in a large voltage between the midpoint and ground. A large voltage was found to fall across the input filter, which should be designed with sufficient BIL to withstand this voltage.
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paper_title: Autonomous Energy Management Strategy for Solid-State Transformer to Integrate PV-Assisted EV Charging Station Participating in Ancillary Service

paper_content:

Photovoltaic-assisted charging station (PVCS) is expected to be one of the important charging facilities for serving electric vehicles (EVs). In this paper, a type of solid-state transformer (SST) is introduced to the PVCS design and an autonomous energy management strategy (EMS) for SST is proposed. This study aims to develop an effective real-time EMS for PVCS participating in ancillary service of smart grid, and the rule-based decision-making method is utilized. Considering the dynamic classification of EVs, an energy-bound calculation (EBC) model is proposed to find the upper and lower bounds of flexible resources. Moreover, considering the EBC results and power command from the aggregator, a charging power allocation algorithm is designed for power distribution of flexible EVs. By case study and experiment analysis, the proposed EMS is effective in real-time energy management and suitable for practical applications.
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paper_title: A three-port bidirectional DC-DC converter with zero-ripple terminal currents for PV/microgrid applications

paper_content:

A three-port converter (TPC) based on the Cuk topology is proposed in this paper. The proposed converter interfaces one unidirectional input power port (envisioned to be a DC power source such as PV, or fuel cell) and two bidirectional output ports, representing the grid-tied inverter dc bus and a storage port respectively. This converter utilizes a single integrated magnetic core on which all the inductor and the transformer windings are wound. The proposed structure utilizes only three power switches, and does not require output capacitors on the output ports on account of the zero-ripple property. Depending on utilization state of the battery, three different power operation modes are defined for the converter. The features of the proposed converter are analyzed and then verified by means of simulation results for different operation conditions.
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paper_title: Protection of MV/LV solid-state transformers in the distribution grid

paper_content:

Solid-State Transformers (SSTs) are a promising technology since they combine a high efficiency with the integration of new functionalities and services in the grid. A SST establishes the interface between a MV AC three-phase grid and a LV AC or DC grid. For this reason a SST is subject to electrical stresses due to faults occurring in the grid. The MV and LV grids are in return exposed to failures of the SSTs. Therefore, a suitable design of the SST and its corresponding protection devices has to be found. This paper identifies the different stresses which are relevant for a SST, analyzes the protection mechanisms which are currently used for low-frequency transformers and proposes adapted protection schemes for SSTs. MV short circuits and overvoltages are identified as the most critical stresses and are analyzed in detail. Finally, some guidelines are extracted for designing a robust SST.
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paper_title: Design methodology of dual active bridge converter for solid state transformer application in smart grid

paper_content:

A design methodology of a dual active bridge (DAB) converter for solid state transformer applications is proposed using an elaborate mathematical model of the converter. The DAB converter is popular in bi-directional power conversion applications because of soft switching capability and seamless control in bi-directional power flows. However, several design considerations should be considered to overcome its demerits such as high RMS current at heavy load condition and limited soft switching capability at light load condition. Design methodology of the power stage will be discussed to minimize the conduction loss and to enlarge the soft switching region. In addition, the algorithm for soft start will be discussed to reduce overcurrent stress of the power switches during a start-up sequence. Experimental results of a 3.3 kW prototype DAB converter demonstrate the validity and effectiveness of the proposed methods.
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paper_title: Design and Demonstration of a 3.6-kV–120-V/10-kVA Solid-State Transformer for Smart Grid Application

paper_content:

Solid-state transformer (SST) has been regarded as one of the most important emerging technologies for traction system and smart grid application. This paper presents the system design and performance demonstration of a high-voltage SST lab prototype that works as the active grid interface in smart grid architecture. Specifically, the designs of the key components of the system, including both power stage and controller platform, are presented. In addition, the advanced control system is developed to achieve high-performance operation. Furthermore, integration issues of SST with dc microgrid are presented. Lastly, tests under different scenarios are conducted to verify the following advanced features of the presented SST technology: 1) VAR compensation; 2) voltage regulation; 3) source voltage sag operation; and 4) microgrid integration.
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paper_title: State of the art of active power electronic transformers for smart grids

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The idea of power electronic transformer is known already over 20 years. There are numerous papers written about power electronic transformers. In the literature one can find tens on different topologies, control methods, standards, requirements etc. It is easy to loose insight what topology to use and where. In this paper a general overview of the power electronic transformer technology in terms of smart grids is presented. The information collected from the literature is classified and organized. In conclusion some generalizations are made and development trends are shown.
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paper_title: Modular Multilevel High-Frequency-Link DC Transformer Based on Dual Active Phase-Shift Principle for Medium-Voltage DC Power Distribution Application

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This paper proposes a multilevel high-frequency-link dc transformer (MDCT) based on dual active phase-shift principle for medium-voltage dc (MVDC) power distribution application. The proposed MDCT employs multilevel and multiplex conversion principle, which brings many advantages and makes the operation of MDCT quite different with the traditional dc transformer (TDCT) and modular multilevel converter. Compared to the TDCT scheme, the proposed MDCT has smaller circulating current and higher power factor; it can operate as a dc breaker to cutoff the connection with the MVDC distribution grid absolutely when a short fault occurs in the distribution grid; the redundant design can be achieved when some submodules failure to improve the reliability. In the paper, the topology, operation principle, modulation method, switching characterization, voltage, and power characterization, and control strategy of MDCT are presented and analyzed comprehensively. At last, a MDCT prototype is built and the experimental results verify the correctness and effectively of the proposed solution.
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paper_title: Design and Performance of a Bidirectional Isolated DC–DC Converter for a Battery Energy Storage System

paper_content:

This paper describes the design and performance of a 6-kW, full-bridge, bidirectional isolated dc-dc converter using a 20-kHz transformer for a 53.2-V, 2-kWh lithium-ion (Li-ion) battery energy storage system. The dc voltage at the high-voltage side is controlled from 305 to 355 V, as the battery voltage at the low-voltage side (LVS) varies from 50 to 59 V. The maximal efficiency of the dc-dc converter is measured to be 96.0% during battery charging, and 96.9% during battery discharging. Moreover, this paper analyzes the effect of unavoidable dc-bias currents on the magnetic-flux saturation of the transformer. Finally, it provides the dc-dc converter loss breakdown with more focus on the LVS converter.
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paper_title: Solid state transformer in the future smart electrical system

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The concept of the solid state transformer has been investigated extensively in the past decade with the emphasis mainly focused on the circuit topology investigation. With the technology being more and more mature and acceptable, the application issue of solid state transformer in the future smart electrical system needs to be investigated. This paper characterizes and summarizes the main features of the solid state transformer, and correspondingly presents possible application areas of solid state transformer in the future smart electrical system. The future distribution system architecture is proposed and a new wind energy system is presented based on the multifunctional utilization of SST. Simulation results are given to demonstrate the proposal.
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paper_title: A New SST Topology Comprising Boost Three-Level AC/DC Converters for Applications in Electric Power Distribution Systems

paper_content:

The growing interest in integrating distributed generation into the existing power distribution grid, the increase in the penetration levels of renewables, as well as the need to achieve a more efficient, reliable, and sustainable grid is leading to the development of new grid-interfaced power converters such as the solid-state transformer (SST). As current and voltage ratings of commercially available power semiconductor devices are normally below power ratings required in distribution systems (e.g., 13.8 kVrms), multiple modules must be connected in cascade configuration at the high-voltage (HV) side to reach higher voltage ratings as well as in parallel at the low-voltage (LV) side to achieve high current levels. A new SST topology consisting of modular boost-based three-level ac–dc converters, medium-frequency transformers with two secondary windings, and four-leg ac–dc converters is presented in this paper. When compared to similar approaches, the proposed topology comprises fewer power conversion stages, lower voltage across the semiconductor devices on the HV side, and lower current flowing through each device on the LV side. These characteristics reduce the number of series-connected modules in the HV side and parallel-connected devices in the LV side. The feasibility of the proposed topology is experimentally validated on a 500 W, 120 Vac/48 Vdc scaled-down prototype.
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paper_title: On Integration of Solid-State Transformer With Zonal DC Microgrid

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Abstract— Thecontributionofthispaperhasbeenfocusedonin-vestigating a new microgrid architecture that integrates the solid-state transformer with zonal dc microgrids. By utilizing the dcand ac links of the solid-state transformer, both ac and dc net-works can access the distributionsystem,whichrenderstheco-ordinate management of the power and guarantees high powersupply reliability. In addition, the presented system together withthe proposed power management strategy can minimize the effectof newly established zonaldcmicrogrid to the existing power grid,of which promises a better stability. To this end, this paper takesthe photovoltaic and battery as the typical renewable energy re-source and energy storage device, and develops a simulation plat-form for system study. Simulation results are shown to demon-strate the feasibility of the proposal. Index Terms— Battery, microgrid, power management, PV,solid-state transformer. I. I NTRODUCTION A C POWER transmission and management has been dom-inating for more than acentury dueto its previous advan-tage in easier voltage step-up by using the conventional trans-former technology, thus contributes to higher efﬁciency duringthelongdistancetransmissionprocess.Theadvancingofpowerdevice and power electronics makes the dc transmission onceagainattractingourattentionsincechange of dc voltage levelisno longer the bottleneck of the technology. HVDC and MVDCaregoodexamples[1],[2].Withtheworldwideenergyshortageanddeteriorationofexistingpowergrid,microgridbecomesoneof the hottest research directions in the power area [3]–[6]. Pre-viously, the emphasis is still on the ac microgrid due to the ex-isting ac power grid architecture [7]–[10]. With the consider-ation of dc nature of many key components in the smart grid,suchasphotovoltaic(PV),battery,fuelcell,supercapacitor,andetc., as well as many dc type loads, such as light-emitting diode(LED), dc microgrid has received more attention recently sinceit brings the opportunity for boosting the efﬁciency by elimi-nating the unnecessary power conversion stages [11]–[15].Sustainable and smart building is one of the goals that smartgrid researchers want to achieve. The original idea is to power
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paper_title: SiC-based unidirectional solid-state transformer concepts for directly interfacing 400V DC to Medium-Voltage AC distribution systems

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400 V DC distribution networks present a promising solution for supplying high-power DC loads such as information processing systems, transportation battery charging facilities and DC micro grids, among others. For these applications, high transmission efficiency, reliability and controllability are mandatory. With the current technology, these loads are fed from PWM rectifiers which are connected to the three-phase Low-Voltage (LV) distribution grid (400 V AC in Europe). The LV grid itself is supplied via Low-Frequency Transformers (LFT) from the Medium-Voltage (MV) grid, providing galvanic isolation and the required voltage step down. This paper presents three unidirectional AC/DC SiC-based Solid-State Transformer (SST) topologies with direct connection to the MV grid, which avoid the utilization of the aforementioned LFT by integrating a Medium-Frequency (MF) conversion stage, thus increasing the efficiency and power density of this supply system. The SST topologies are compared by means of a chip area-based comparative evaluation. Finally, the most suited among the presented topologies is Pareto-optimized, achieving a total MV AC to 400 V DC efficiency of 98.3 %. It is shown that the optimized SST features 40 % less overall losses compared to state-of-the-art solutions.
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paper_title: Research of microgrid connecting interface based on multi-port power electronic transformer

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To improve the flexibility and stability of microgrid operation, this paper takes the multi-port power electronic transformer (PET) as the connecting interface of microgrid, and the energy storage system (ESS) at the low voltage DC (LVDC) side is controlled to reduce the influence of load or distributed generation (DG) fluctuations on microgrid. The mathematical models of interface stages are also presented, and the state of charge (SOC) of ESS is introduced into the feedforward control to ensure the stability of LVDC capacitor voltage. In order to verify the validity of the interface structure and control strategy, simulation is carried out in MATLAB. Results show that the proposed method can guarantee the voltage and frequency stability of microgrid in the grid-connected mode, stand-alone mode and the switching process.
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paper_title: A preliminary loss comparison of solid-state transformers in a rail application employing silicon carbide (SiC) MOSFET switches

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The silicon carbide (SiC) power semiconductor and the medium frequency transformer (MFT) are two enabling technologies that are allowing the efficiencies and power densities of power electronic converters to be pushed beyond the limits of those that have previously been possible; the solidstate transformer (SST), with its viability based in part upon the two aforementioned technologies, has much potential to offer in Rail, Marine, Aviation and Smart-Grid applications due to its high performance operation and compact form-factor and so it is studied in-depth in this paper, where a comparison of the active and passive component losses between various SST topologies for a rail traction application, modelled with 3.3 kV and 10 kV SiC MOSFET switches alongside benchmark 3.3 kV and 6.5 kV Si IGBT switches, operating on a 50 Hz, 25 kV ac, overhead supply has been made. In addition, a comparative analysis is made of the estimated costs of the converters studied.
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paper_title: Integration of Large Photovoltaic and Wind System by Means of Smart Transformer

paper_content:

Large renewable energy systems, e.g., photovoltaic and wind turbine, are connected to grid, which is usually weak, with long lines. This degrades power system stability and reliability. Additionally, the magnetic characteristic of a conventional power transformer (CPT) implies a slow dynamic and leads to grid current harmonic too high when wind speed is low and irradiance is weak. This paper proposes a parallel connection of a solid-state transformer-based smart transformer and a CPT to not only reduce the power rating, but also to support the voltage amplitude and improve the grid current quality. The simulation and experimental results clearly verify the correctness and feasibility of the proposed strategy.
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paper_title: Smart hybrid house test systems in a solid-state transformer supplied microgrid

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This paper presents the design and setup, considerations, and preliminary results of an AC/DC hybrid smart house test system. Rapid growth of rooftop photovoltaic (PV) in distribution systems makes it viable to supply a residential home with both AC and DC power sources. Solid-state transformers (SST) used as energy routers for rerouting power during normal and emergency situations are being developed in the FREEDM center at North Carolina State University. To provide a load test system for studying the control and monitoring of such an SST supplied AC/DC mixed power supply system, an 1-SST AC/DC hybrid smart house test system is built. The test system consists of a solid state transformer (SST), on-site photovoltaic panels, energy storage devices and three AC/DC smart houses with a home energy management system (HEMS). A demonstration case is presented to illustrate the operation of the test system for algorithm development and validation.
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paper_title: Power electronic traction transformer technology

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Combining modern high-power semiconductor devices with constantly improving magnetic materials opens up the possibility to replace bulky low frequency transformers with a new medium voltage medium frequency conversion structures. While there are still challenges to be addressed related to these so called power electronic transformers, a steadily increasing development effort is evident and considered in various contexts. Traction applications seem to be the first ones where proliferation of these new galvanic isolated power electronic converters is expected. In this particular application field, substantial weight and volume reduction could be achieved while providing additional functionality at the same time. In this paper a survey of recent R&D efforts in this field is presented.
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paper_title: Distribution systems equipped with power electronic transformers

paper_content:

The paper's aim is to analyze a power electronic transformer (PET) able to deal with different sources at its input port, being able to distribute, according to the system's strategies, where the power comes from. Thus, the multi-fed PET (MF-PET), is a power electronic transformer with two or more AC/DC rectifiers in its input port, and these may be connected to different feeders. High service quality to consumers is accomplished. Simulating results for the three-phase topology are shown operating as an electronic device and as part of a grid. A fuzzy controller is proposed to coordinate its behavior. Different events are simulated.
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paper_title: Power loss-oriented evaluation of high voltage IGBTs and multilevel converters in transformerless traction applications

paper_content:

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and a substantial weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern high-voltage insulated gate bipolar transistors (HV-IGBTs). In a first design step, the switching characteristics and losses of 6.5-kV IGBTs are compared to 3.3-kV and 4.5-kV IGBTs which are already commercially used in traction applications. Based on the considered HV-IGBTs, the properties of multilevel converters are analyzed and their applicability to the transformerless system is evaluated. The paper focusses on a loss analysis of the converters. Reliability aspects and harmonic spectra are briefly discussed. Taking these design aspects into account, the three-level neutral point clamped converter turns out to be a reasonable solution to realize line and motor converter modules in a transformerless traction system.
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paper_title: Application of multi-port solid state transformers for microgrid-based distribution systems

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Recent developments in the design of power electronic elements with higher voltage and power rating and medium/high frequency enable the use of solid state transformer at different voltage levels for microgrid design. In this study, the concept of a Multi-Port Solid State Transformer (MPSST) for distribution network application is introduced to achieve the goal of creating galvanic isolation between different nodes connected to the converter and between upper and lower zones in a zonal microgrid. MPSST enables compact, integrated and galvanically isolated multi-port node for the microgrid elements and at the same time reduces the number and size of converters. This article proposes a new architecture for the distribution system with dispersed Distributed Generation (DG) resources in all voltage zones of the network using MPSST as the main component in the system. The developed concept interconnects different voltage types and levels using a single compact converter with a centralized control logic. By controlling the phase shift between ports of the converter, the power flow through the legs can be controlled. Also the voltage of different legs is actively regulated.
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paper_title: Power management of Solid State Transformer in microgrids

paper_content:

By integrating distributed sources and various loads, the microgrid system that is regarded as an intelligent power system with self controlling, protecting and managing functions, has been quickly developing in recent years. Considering that the output power of most distributed sources such as photovoltaic or wind energy are not stable, the control methods for microgrids must be appropriately designed in order to guarantee highquality of the output power and the system stability. Solid State Transformer (SST) is one of the fundamental components for the future microgrids which can totally replace conversional transformers. Its main functions are realized through high frequency power electric converters and control strategies. It may achieve all functions of traditional transformer such as voltage transform, current isolation, power transmission, besides, it can regulate the input current, power factor and enhance output voltage flexibly by providing ideal sine wave. The combination of Microgrid and SST may greatly improve the efficiency and stability of power system. In this paper, both single-phase and 3-phase SST topology are discussed and the simulation models are built on PSCAD. The control techniques are well designed and implemented. The characteristics of SST are explored and verified by key operating results of SST under different conditions.
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paper_title: Smart grid opportunities in islanding detection

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The performance of many existing islanding detection methods for cases with multiple distributed generators within a single island is either inadequate or unknown. The deployment of Smart Grid technologies offers opportunities for improving the islanding detection of a circuit. This paper gives a brief overview of types of islanding detection methods, lists suitability of different types of methods to cases with multiple instances of distributed generation, and discusses impacts of deployment of Smart Grid technologies including increased penetration of communications technology, multifunction distributed energy resource products such as solid state transformers, synchrophasors, smart meters, and electric vehicle service equipment.
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paper_title: Electronic Power Transformer Control Strategy in Wind Energy Conversion Systems for Low Voltage Ride-through Capability Enhancement of Directly Driven Wind Turbines with Permanent Magnet Synchronous Generators (D-PMSGs)

paper_content:

This paper investigates the use of an Electronic Power Transformer (EPT) incorporated with an energy storage system to smooth the wind power fluctuations and enhance the low voltage ride-through (LVRT) capability of directly driven wind turbines with permanent magnet synchronous generators (D-PMSGs). The decoupled control schemes of the system, including the grid side converter control scheme, generator side converter control scheme and the control scheme of the energy storage system, are presented in detail. Under normal operating conditions, the energy storage system absorbs the high frequency component of the D-PMSG output power to smooth the wind power fluctuations. Under grid fault conditions, the energy storage system absorbs the redundant power, which could not be transferred to the grid by the EPT, to help the D-PMSG to ride through low voltage conditions. This coordinated control strategy is validated by simulation studies using MATLAB/Simulink. With the proposed control strategy, the output wind power quality is improved and the D-PMSG can ride through severe grid fault conditions.
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paper_title: Bi-directional power electronic transformer based compact dynamic voltage restorer

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In this paper a dynamic voltage restorer (DVR) circuit with a bi-directional power electronic transformer (PET) structure is proposed to correct input utility voltage distortions on a sensitive load. This DVR-PET contains full-bridge DC-link inverter (FBDCI), high frequency isolating transformer (HFIT), cyclo-converter and output filter. A phase shift modulation (PSM) control scheme is used to regulate compensation voltage. The proper reference voltage is estimated to adjust the amount of battery (or capacitor) charged energy as well as minimizing energy transfer between utility power and DVR circuit. A circuit model has been simulated with PSCAD/EMTDC software and the results are presented which verify the analysis.
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paper_title: A DC–DC Multiport-Converter-Based Solid-State Transformer Integrating Distributed Generation and Storage

paper_content:

The Solid-state transformer (SST) has been proposed by researchers to replace the regular distribution transformer in the future smart grid. The SST provides ports for the integration of storage and distributed generation (DG), e.g., photovoltaic (PV), and enables the implementation of power quality features. This paper proposes a SST topology based on a quad-active-bridge (QAB) converter which not only provides isolation for the load, but also for DG and storage. A gyrator-based average model is developed for a general multiactive-bridge (MAB) converter, and expressions to determine the power rating of the MAB ports are derived. These results are then applied to analyze the QAB converter. For the control of the dc-dc stage of the proposed QAB-based SST integrating PV and battery, a technique that accounts for the cross-coupling characteristics of the QAB converter in order to improve the regulation of the high-voltage-dc link is introduced. This is done by transferring the disturbances onto the battery. The control loops are designed using single-input single-output techniques with different bandwidths. The dynamic performance of the control strategy is verified through extensive simulation and experimental results.
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paper_title: Power dispatch strategy in microgrid integrated with solid state transformer

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The Future Renewable Electric Energy Delivery and Management (FREEDM) System smoothly integrate and manage a variety of controllable loads, DRER (distributed renewable energy resource) and DESD (distributed energy storage device) through the revolutionary solid state transformer (SST). The distributed grid intelligence integrated with the SST facilitates the optimal power dispatch for a residential distribution system. This paper presents a mathematical formulation to determine the optimal power dispatch strategy in order to minimize the operational cost as well as the emission pollution. In this paper, a multi-objective immune particle swarm optimization (MIPSO) based computational intelligence approach is adopted to find the Pareto solution. Several numerical simulation studies demonstrate the effectiveness and accuracy of the proposed power dispatch strategy in a residential distribution system. More analysis indicates that the fitting degree between load profile and output of renewable energy generators (such as PV cells) is an identifier, which would be applied as a reference to make power exchange schedule between micro grid and utility grid.
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paper_title: Power Electronic Transformer application to grid connected photovoltaic systems

paper_content:

Grid-connected photovoltaic systems utilize static converters which influence the efficiency of the system. The energy conversion depends on the architecture of the converter, different solutions are available and H-bridge multilevel converters seem to be an optimal solution also for the power quality. In the paper is proposed an architecture that includes a Power Electronic Transformer which is practically an isolated high-frequency link AC/AC converter that substitute a conventional transformer. An MPPT control technique is presented and validated by simulation implemented on a photovoltaic system with H-bridge n-levels converter and Power Electronic Transformer. The simulation results confirm that the control is able to effectively track the Maximum Power Point and to stabilize immediately in the new steady-state condition.
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paper_title: An autonomous power management strategy based on DC bus signaling for solid-state transformer interfaced PMSG wind energy conversion system

paper_content:

The solid-state transformer (SST) enabled DC/AC Microgrid provides an effective solution for distributed renewable energy resources (DRER) integration with conventional utility grid. This paper investigates a DC network system consisting of wind turbines, SST, and DC loads. Without any energy storage devices, an autonomous power management strategy based on improved DC bus signaling (DBS) is proposed to achieve system stable operation and power balance under various scenarios, specifically system grid-connected mode, islanding mode, and the mode transition. The extreme conditions were emphasized and analyzed as a testament to verify the feasibility of proposed control. DC bus voltage level and its gradient information have been employed as the only indication for distinguishing different modes and control implementation. System power management competence has been simulated and verified with MATLAB/Simulink.
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paper_title: Grid connected wind driven permanent magnet synchronous generator with high frequency solid state transformer

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This paper presents the control of grid connected wind driven permanent magnet synchronous generator through solid state transformer with a high frequency multistage converter interconnect. Maximum power point tracking is carried out in the high frequency generator side converter stage where as the grid side converter takes care of the synchronization. The model parameters for the high frequency transformer are obtained by analytical calculation and the same is used in the simulation. A push pull converter with incremental conductance algorithm on the generator side tracks the MPP. It also converts the dc output into high frequency wave so that it can be directly applied to the primary of the high frequency transformer. The rectified secondary voltage is grid synchronized through the grid side inverter with synchronous reference controller. The entire system including the high frequency transformer is simulated in MATLAB/Simulink for a 2.5 kW PMSG.
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paper_title: Integrated voltage control and line congestion management in Active Distribution Networks by means of smart transformers

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Within the context of Active Distribution Networks (ADNs), smart transformers represent very powerful devices able to provide fast and efficient control actions with respect to different ADNs ancillary services. This paper discusses the benefits, in terms of ADNs voltage and line flows controls, achieved by interfacing distributed generators with the power grid by means of a smart transformer. Among several benefits, these devices allow for a phase-per-phase control of the generators active and reactive power injections. This peculiarity enables to deploy new control schemes that are analyzed and discussed in the paper with reference to a case study based on a modified IEEE 34 node test distribution feeder.
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paper_title: Integrating flywheel energy storage system to wind farms-fed HVDC system via a solid state transformer

paper_content:

As the power of wind farms (WFs) considerably proliferates in many areas worldwide, energy storage systems will be required to dynamically compensate the wind energy intermittency and increase power system stability. In this paper, a backup power conditioning strategy for wind energy-fed voltage source converter HVDC transmission systems is presented. An induction machine based flywheel energy storage systems (FESS) is integrated to the HVDC system via a solid state transformer (SST). The FESS is connected in parallel with the dc-link of the grid side converter; therefore, the excess wind energy can be stored in the flywheel and then restored during the energy-shortage periods. The proposed system aims to compensate the power fluctuations caused by the intermittent nature of wind energy, levels the power-fed to the grid, and improves the quality of delivered power. The proposed system including FESS with an interfacing SST is modeled, simulated, and analyzed in MATLAB/SIMULINK environment. The results verify the effectiveness of the proposed system.
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paper_title: Modified dual active bridge photovoltaic inverter for solid state transformer applications

paper_content:

The solid state transformer (SST) was announced as the one of the most emerging technology by the Massachusetts institute of technology (MIT). An SST generally includes a DC or AC buses in high and/or low voltage side in order to connect the external input sources to the SST. We proposed discontinues conduction mode (DCM) modified dual active bridge inverter topology for integrating photovoltaic (PV) panels to SST. The proposed inverter achieves soft switching, hence it has high efficiency. Moreover, it can be operated by open loop control with eliminating high frequency current sensor. In this paper, the analysis of the proposed inverter is presented, and simulation results are given for validating the proposed inverter.
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paper_title: A reduced converter count solid state transformer for grid connected Photovoltaic applications

paper_content:

This paper presents a Solid State Transformer (SST) interface for grid connected Photovoltaic (PV) system. A new method is proposed for Maximum Power Point Tracking (MPPT) through a single phase inverter with phase displacement control. The inverter control variable is redefined such that it can be used for MPPT. Due to this control technique one converter stage for power tracking is eliminated, leading to a reduced component count for the overall system. The high frequency transformer's model parameters are designed for an operating frequency of 20 kHz for simulation studies. The grid side converter is controlled for synchronization and power flow using a Synchronous Reference Frame (SRF) current controller. The overall system is tested in simulation for a panel rating of 1.15 kWp under various insolation conditions and power balance is verified and presented. Testing is carried out under step change in the insolation to evaluate the performance of the SST interface under transient conditions. The proposed system is tracking the maximum power available from the panel and delivering it into the grid with high efficiency. The overall system efficiency and the high frequency transformer efficiencies are presented under various operating conditions.
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paper_title: Integration of solid state transformer with DC microgrid system

paper_content:

This paper investigates a Solid State Transformer (SST) based DC microgrid architecture, addressing the design and control of the multiple SST power conversion stages and the power management strategy required for its integration with other microgrid elements, such as storage devices and local distributed generation. The advantages of a SST in relation to conventional low frequency transformers are commonly listed as a substantial reduction of volume and weight, fault isolation capability, voltage regulation, harmonic filtering, reactive power compensation and power factor correction. The SST also constitutes the required infrastructure that will enable DC power distribution to homes and commercial buildings in the near future and provides a more efficient way to integrate storage devices and distributed generation into the electrical grid. In this sense the SST behaves as an energy router, which represents a key element in an intelligent power system. The behavior of the proposed architecture for different operating conditions and disturbances will be assessed through computational simulations in the software MATLAB/Simulink.
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paper_title: Analysis and design of electronic transformers for electric power distribution system

paper_content:

A power transformer performs many functions such as voltage transformation, isolation, noise decoupling and is an indispensable component in electricity distribution systems. However, at low frequencies (60/50 Hz) it is a bulky and expensive component. In this paper, the concept of electronic transformers is further discussed. Several possible topologies employing static power converters connected on the primary and secondary side in combination with magnetic circuits are explored to realize a high-frequency AC link. The focus of this paper is to realize an electronic transformer as a power delivery component in an electric distribution system. Reduced size, losses, higher efficiency and better voltage regulation are some of the advantages of this approach. A 10 kVA design example along with preliminary experiment results is discussed. It is shown that a transformer designed with a conventional grain-oriented silicon steel core can process three times the power at 1 kHz operating frequency compared to 60 Hz.
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paper_title: A Three-Phase Dimmable Lighting System Using a Bidirectional Power Electronic Transformer

paper_content:

This paper presents a single-stage bidirectional power electronic transformer (PET) for lighting systems as a three-phase electronic ballast. The lighting system supplies multilamp units that are controlled simultaneously by a bidirectional PET using a pulsewidth modulation scheme. The PET contains a single-input multioutput high-frequency isolating transformer to control the illumination of the lamps as well as providing galvanic isolation in each unit simultaneously. The high-frequency operation, direct ac to ac conversion, and bidirectional power flow capabilities help the PET to operate without any bulky storage elements. So good efficiency, and low weight, volume, and cost of the power conversion system can be achieved. The control strategy promotes the ballast performance to let fluorescent lamps operate free from both voltage flicker and disturbances. As a result, improved dimmable illumination, lamps protection, and increasing lifetime of lamps can be achieved. A design procedure based on equivalent circuit is presented to determine the resonant output circuit parameters. The experimental results of the prototype, which converts 110 V/3phi/50 Hz input voltage to 130 V/1phi/25 kHz and 70 V/1phi/25 kHz output voltages, show the practical aspects of the proposed PET.
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paper_title: Solid state transformer application to grid connected photovoltaic inverters

paper_content:

In the paper, an architecture, including a solid state transformer (SST) which is different from the conventional style is proposed The photovoltaic system with SST consists of two power stages. A dual half bridge (DHB) converter constitutes the first power stage, which aims to achieve the maximum power point tracking (MPPT) functions. A grid connected inverter is used to stabilize the DC bus voltage and make the output current sinusoidal. The control technique is presented and validated by simulation implemented on a photovoltaic system with SST. The simulation results prove the effectiveness of the proposed photovoltaic system and the proposed control technique.
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paper_title: Analysis of Solid State Transformer based microgrid system

paper_content:

The contribution of this paper has been focused on investigating a new microgrid architecture that integrates the electronic transformer (SST, the acronym Solid State Transformer) with microgrids system. The design of the converters required for implementation of the electronic transformer are dealt. The work to provide an infrastructure for creating an intelligent energy system, which will enable the integration of smart grid concepts, microrrede generation distribute and distribution of direct current. The advantages of SST, listed the reduction of the volume and weight as well as fault isolation capability, voltage regulation, harmonic filtering, reactive power compensation and power factor correction. The work presents the advantages of applying the SST in a microgrid system. Therefore, simulation results in MATLAB/Simulink software are presented.
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paper_title: Power electronic transformer (PET) converter: Design of a 1.2MW demonstrator for traction applications

paper_content:

A power electronic transformer converter made of cascaded multiple cells with a medium frequency link can replace the bulky traditional transformer in various applications with several advantages, including reduced size and weight, and energy savings. This paper describes the design of a 1.2MW power electronic transformer (PET) demonstrator for traction applications. Several design challenges, i.e. the selection of the IGBT modules, the design of the multiple-functional medium frequency transformer, and the mechanical arrangement to achieve the high voltage insulation between the grid voltage and ground potential, have been discussed. The experimental results, obtained from the developed PET demonstrator, are presented.
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paper_title: Wind Energy System With Integrated Functions of Active Power Transfer, Reactive Power Compensation, and Voltage Conversion

paper_content:

As the power of wind energy system increases, the control of their active and reactive power becomes increasingly more important from a system standpoint given that these are typical frequency and voltage control parameters. In this paper, a family of wind energy systems with integrated functions of active power transfer, reactive power compensation, and voltage conversion is proposed. The proposed wind energy systems using solid-state transformer (SST) can effectively suppress the voltage fluctuation caused by the transient nature of wind energy without additional reactive power compensator and, as such, may enable the large penetration of wind farm (WF) into the power grid. To this end, a simulation study for WF driven by squirrel-cage induction generators is presented to verify the effectiveness of the proposed system. In addition, a modular-type high-voltage and high-power three-phase SST topology is presented for the proposed system, and its basic building block, which is a single-phase SST, is analyzed. The functions of SST in the presented wind energy system are verified in a single-phase laboratory prototype with scaled-down experiments. Lastly, cost issue of the proposed technology is analyzed with comparison to the traditional one.
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paper_title: Stability Assessment Extensions for Single-Phase Distribution Solid-State Transformers

paper_content:

In this letter, a stability assessment for single-phase solid-state transformers (SSTs) is extended to include the impact of distribution system voltages and a system-wide impedance analysis. It is shown that if the SST filters throughout the system are designed with regards to the proposed stability criterion, then system stability is guaranteed regardless of configuration. This leads to two resulting applications: 1) the order in which the SSTs are connected to the system will not generate instability if the criterion is satisfied and 2) a system configuration change due to a fault will not produce instability.
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paper_title: Bidirectional converter with high frequency isolation feasible to solid state transformer applications

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This paper presents a new three-port ac-dc converter topology based on the bidirectional version of the three-state switching cell (3SSC) and neutral point clamped (NPC) with Dual Active Bridge (DAB) feasible to interconnect an AC link, a DC link and photovoltaic panels. The three port converter operates as solid state transformer (SST). It the primary side switches voltage and current stresses are reduced due to NPC and 3SSC characteristics. Some operating ranges presented Zero Voltage Switching (ZVS). Modulation strategy and control are presented. For the drive control was implemented five control loops: PFC (Power Factor Correction); full voltage and balancing voltages acroos port 2 capacitors; magnetization current and voltage on port 3. A 33.33 kW converter with the peak value of the AC voltage is 9.35 kV and DC voltage at port 3 is 800 V with a DC bus voltage at port 2 of 22 kV has been simulated. A comparative loss studies with other converter topologies is also made. The proposed topology presented efficiency of 97.2%.
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paper_title: Control strategies of a multiport power electronic transformer (PET) for DC distribution applications

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A multiport power electronic transformer (PET) serves as the energy router (ER) of the next generation DC distribution systems. In this paper, control strategies of a multiport PET for DC distribution applications are proposed. Multiple active bridge (MAB) converters serve as the isolation units. Star and ring connected synchronization and communication methods are used in the PET system. After the introduction of single phase-shift modulation (SPM) and extended phase-shift modulation (EPM) methods of MAB, a softswitching phase-shift modulation (SSPM) method is proposed for ZVS and reduction of current stress. Powers decoupling methods as well as system control strategies are described briefly. Experimental results are given to validate the proposed topologies and strategies.
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paper_title: SOLID STATE TRANSFORMER INTEGRATION IN SMART GRID SYSTEM

paper_content:

The development of a Solid State Transformer (SST) that incorporates a DC-DC multiport converter to integrate both photovoltaic (PV) power generation and battery energy storage is presented in this dissertation. The DC-DC stage is based on a quad active-bridge (QAB) converter which not only provides isolation for the load, but also for the PV and storage. The AC-DC stage is implemented with a pulse-width-modulated (PWM) single phase rectifier. A unified gyrator-based average model is developed for a general multi-active-bridge (MAB) converter controlled through phase-shift modulation (PSM). Expressions to determine the power rating of the MAB ports are also derived. The developed gyrator-based average model is applied to the QAB converter for faster simulations of the proposed SST during the control design process as well for deriving the state-space representation of the plant. Both linear quadratic regulator (LQR) and single-input-single-output (SISO) types of controllers are designed for the DC-DC stage. A novel technique that complements the SISO controller by taking into account the cross coupling characteristics of the QAB converter is also presented herein. Cascaded SISO controllers are designed for the AC-DC stage. The QAB demanded power is calculated at the QAB controls and then fed into the rectifier controls in order to minimize the effect of the interaction between the two SST stages. The dynamic performance of the designed control loops based on the proposed control strategies are verified through extensive simulation of the SST average and switching models.
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paper_title: Theory and application of distribution electronic power transformer

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Abstract Three-phase and 4-wire Distribution Electronic Power Transformer's (DEPT's) operation principle is analyzed in this paper. Based on the analysis, the control scheme is established. In this control scheme, the input stage is controlled as a three-phase balanced current source and makes the primary current sinusoidal and power factor easily adjusted. While the output stage is controlled as a three-phase balanced voltage source and keep the load voltage sinusoidal and nominal. In order to meet the requirement of the single phase or unbalanced loads, each phase is an independent voltage source. With the proposed control strategy, the characteristics of DEPT are studied by simulations. And further detailed simulations are carried out to validate the power quality control function of DEPT. The results show that DEPT has very good static and dynamic performances, and it can not only realize the functions of conventional power transformer, but also can prevent from voltage sags, swells, flickers and harmonics infecting the loads while avoid loads impacting the primary system.
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paper_title: Power management with a dynamic power limit by a power electronic transformer for micro-grid

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In this paper, a dynamic power limiter at the point of common coupling (PCC) of a micro-grid is proposed. A 60 Hz, step-down transformer is generally used at the PCC to connect the micro-grid to the power system grid. Substitution of the conventional 60Hz transformer by a dynamic power limiter has the following advantages: 1) Bi-directional active-power control capability at the PCC which results in enhanced micro-grid power management system during grid-connected operation, 2) Smooth transition from islanding to the grid-connected mode without the need of grid frequency synchronization, and 3) Extra degree of freedom due to the presence of active-power controller in a possible deregulation and market strategy within the micro-grid. The proposed dynamic power limiter is a high-frequency isolated power-converter system comprised of a high-frequency step-down transformer and three-phase to single-phase matrix converters. The matrix converters are modulated with a new PWM strategy for bi-directional active and reactive power flow control. Simulation results presented verify the operation of the proposed dynamic power limiter.
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paper_title: A Tri-directional power electronic transformer for photo voltaic based distributed generation application

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Expanding the size and demand of the electrical network, make the use of distributed generation inevitable especially, when the produced power can be expected as a green power. Furthermore, this growth in size brings up the importance of volume and weight of instruments, used in. Power electronic transformer is an attempt to reduce the weight and occupied space of the electrical devices. In this paper, the combination of these concepts leads to make a new, very controllable structure. The good performance of the proposed circuit is shown through several simulations and experiments. Simulations are carried out in PSCAD/EMTDC software.
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paper_title: Power electronics traction transformer

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With the new generation of full low floor passenger trains, the constraints of weight and size on the traction transformer are becoming stronger. The ultimate target weight for the transformer is 1 kg/kVA. The reliability and the efficiency are also becoming more important. To address these issues, a multilevel topology using medium frequency transformers has been developed. It permits to reduce the weight and the size of the system and improves the global life cycle cost of the vehicle. The proposed multilevel converter consists of sixteen bidirectional direct current converters (cycloconverters) connected in series to the catenary 15 kV, 16.7 Hz through a choke inductor. The cycloconverters are connected to sixteen medium frequency transformers (400 Hz) that are fed by sixteen four-quadrant converters connected in parallel to a 1.8 kV DC link with a 2f filter. The control, the command and the hardware of the prototype are described in detail.
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paper_title: A three-phase 10 kVAC-750 VDC power electronic transformer for smart distribution grid

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In this paper, a power electronic transformer (PET) topology for distribution grid is presented based on modular multilevel converter (MMC). The presented topology is of three-phase type and have two dc voltage terminals, i.e. the high-voltage dc terminal and the low-voltage dc terminal. So, it can be connected directly to high-voltage direct current (HVDC) systems and renewable energy systems simultaneously. This feature makes it possible to perform as an “energy router” in smart grids. What is more important, the proposed PET can substantially reduce (by 44.4% for the studied case) the number of medium frequency transformers (MFTs), compared with the existing solutions. Computer simulation and experimental results on a 10 kV prototype show validity of this PET.
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paper_title: Power Electronic Traction Transformer-Low Voltage Prototype

paper_content:

Recently, a world's first ever power electronic traction transformer (PETT) for 15 kV, 16 2/3Hz railway grid, has been newly developed, commissioned, and installed on the locomotive, where it is presently in use. This marks an important milestone in the traction world. The design and development of the PETT are described in this paper, where a low-voltage (LV) PETT prototype is presented. It has been designed for the purposes of control hardware and software commissioning, thus serving a role of an analogue simulator. In this paper, emphasis is placed on the overall system requirements, from where control system has been developed, implemented, and successfully commissioned. The development of a 1.2MVA medium-voltage PETT prototype will be reported separately in accompanying paper.
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paper_title: Design and demonstration of a 3.6kV–120V/10KVA solid state transformer for smart grid application

paper_content:

Solid state transformer (SST) has been regarded as one of the most emerging technologies for traction system and smart grid application. This paper presents the system design and performance demonstration of a high voltage SST lab prototype, which works as the active grid interface in the smart grid architecture. Specifically, the designs of the key components of the system, including both power stage and controller platform, are presented. In addition, the integration issues of SST with DC microgrid are highlighted. Lastly, tests under different scenarios are conducted to verify the following advanced features of the presented SST technology: (1) VAR compensation; (2) Voltage regulation; (3) Source voltage sag operation; (4) Microgrid integration.
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paper_title: Multiple Resonances Mitigation of Paralleled Inverters in a Solid-State Transformer (SST) Enabled AC Microgrid

paper_content:

Solid-state transformer (SST) becomes a promising technology in smart grid applications due to its multiple functionalities and control flexibility compared to line-frequency transformers. In a SST enabled ac microgrid, the grid impedance represented by the SST output impedance is frequency-dependent, which is different from the inductive grid impedance in a conventional ac microgrid. The frequency-dependent SST output impedance can interact with those of paralleled inverters and result in multiple resonances locating in a wide frequency range. In this paper, a combination of a lead-lag compensator and a negative impedance feedback control has been proposed to enhance the system stability. With proposed methods, the multiple resonances are decoupled and thus more feasible to be mitigated. The negative effect of digital control delay on the high frequency resonance damping is attenuated with proposed methods. In addition, the tradeoff design of virtual impedance is also analyzed with consideration of both system-level resonance mitigation function and single inverter stability. Consequently, the stability of paralleled inverters in the SST-enabled ac microgrid has been improved. Finally, an OPAL-RT-based hardware-in-the-loop verification has been provided to prove the validity of proposed methods.
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paper_title: Power Electronic Transformer-Based Railway Traction Systems: Challenges and Opportunities

paper_content:

In this paper, power electronic transformer (PET)-based railway traction systems are comprehensively reviewed according to the unique application features and requirements. By comparing PET and conventional line frequency transformer (LFT)-based systems, their pros and cons are summarized. By further reviewing all kinds of PET-based designs from the early concepts to the latest ones in the order of their publication dates, the developing trends are highlighted. By synthetically considering the requirements and the state of the art, the key challenges and opportunities are identified and discussed. It shows that although PET-based systems are still developing and far from mature, they are already superior to LFT-based systems in terms of system weight, efficiency, and functionalities especially for 15-kV/16.7-Hz applications. With the advancements on wide bandgap power devices, medium frequency transformers, and converters, PET systems will be even more promising and available for all types of railway tractions in the near future.
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paper_title: Power Electronic Transformer Technology for Traction Applications – An Overview

paper_content:

Combining modern high-power semiconductor devices with constantly improving magnetic materials opens up the possibility to replace bulky low-frequency transformers with a new medium voltage medium frequency conversion structure. While there are still challenges to be addressed related to these so called power electronic transformers, a steadily increasing development effort is evident and considered in various contexts. Traction applications seem to be the first ones where proliferation of these galvanically isolated power electronic converters is expected. In this application field, substantial weight and volume reduction can be achieved while providing additional functionality at the same time. In this paper a survey of recent R&D efforts in this field is presented.
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paper_title: Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

paper_content:

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.
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paper_title: Stability analysis of cascaded converters for bidirectional power flow applications

paper_content:

This paper establishes the criteria to ensure stable operation of two-stage, bidirectional, isolated AC-DC converters. The bi-directional converter is analyzed in the context of a building block module (BBM) that enables a fully modular architecture for universal power flow conversion applications (AC-DC, DC-AC and DC-DC). The BBM consists of independently controlled AC-DC and isolated DC-DC converters that are cascaded for bidirectional power flow applications. The cascaded converters have different control objectives in different directions of power flow. This paper discusses methods to obtain the appropriate input and output impedances that determine stability in the context of bi-directional AC-DC power conversion. Design procedures to ensure stable operation with minimal interaction between the cascaded stages are presented. The analysis and design methods are validated through extensive simulation and hardware results.
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paper_title: Power electronic transformers for utility applications

paper_content:

The concept of realizing a small size "solid-state" transformer has been discussed for some time. A fairly straightforward approach to accomplish size reduction in a transformer feeding a conventional rectifier-inverter system is to introduce an isolated DC-DC converter in the DC link, isolation being provided by a high frequency transformer. So also, several topologies that employ AC-AC converters connected on primary and secondary sides of a high frequency transformer to reduce the size and weight of the magnetic core have been reported in literature. Such AC-AC converters need switches with bi-directional voltage blocking and current carrying capability, which are commonly realized with pairs of gate turn-off devices such as insulated gate bipolar transistors (IGBT). This paper explores the possibilities of employing AC-AC switched mode power converters in combination with reactive elements to realize a chopped AC link, thereby decreasing the required magnetic core size for voltage transformation and isolation. A primary advantage of this approach is that, the static power converter needs only six devices to implement a three-phase electronic transformer, which makes it an economical solution. Operating principles, magnetic design and other practical issues are discussed. Detailed computer simulations accompanied with experimental verification are presented in the paper.
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paper_title: A modular DC solid state transformer for future onboard DC grid

paper_content:

Medium voltage DC (MVDC) grid has quite a lot advantages for the application of large power electric ships, while the DC breaker and DC distribution are the key equipment. Solid state transformer (SST) composed by power electronics transformer and high-frequency transformer has high power density and high efficiency, so it can be used for the power conversion from MVDC to low voltage DC or AC, especially as the development of wide-band gap devices with higher voltage, higher switching frequency with less conduction and switching loss. Several SST topologies suitable for onboard DC distribution are analyzed in this paper, then the control algorithm are proposed to realize DC bus equalization. Experimental results are proposed to verify the performance of the SST and control method.
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paper_title: Three-phase AC-DC solid-state transformer for low-voltage DC power distribution applications

paper_content:

A topology based on an interleaved modular multilevel converter is presented as a step-down solid-state transformer (SST) for low-voltage DC power distribution applications. Interleaving reduces the currents stresses through the legs of the modular multilevel converter, which is responsible for the voltage modulation across the submodules. The medium-frequency transformer provides galvanic isolation and operates at 10 kHz, allowing weight and volume reduction, while connecting the interleaved modular multilevel converter at the high voltage side to a full-bridge converter at the low voltage side. This topology operates as a dual-active bridge converter where the phase-shift angle controls the bidirectional power flow. The topology is suitable for a wide range of medium-voltage applications such as AC or DC distributed power systems, traction and renewable energy systems. The converter features, modulation technique and control scheme are presented. The topology validation is made through simulation considering a three-phase converter rated at 13.8 kV/60 Hz, four submodules whose switching frequency is 10 kHz, 11.27-kV/800-V, 10-kHz medium frequency transformer, output voltage of 800 Vdc and rated power of 100 kW.
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paper_title: Bi-directional power electronic transformer for induction heating systems

paper_content:

A new bi-directional power electronic transformer (PET) for induction heating applications is described. Proposed PET contains primary bi-directional cycloconverter switches, high frequency isolation transformer, matching coil and a parallel resonant heating system. Switching routine is applied in such a way that the output voltage regulation is achieved in the full load variations, even at asymmetrical utility conditions. Simulations are obtained for the proposed PET which consists of 380 V / 3phi/ 50 Hz input voltage and 210 V / 1phi /10 kHz output voltage. The mathematical analysis are presented and verified by simulation results.
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Title: Solid State Transformer Technologies and Applications: A Bibliographical Survey

Section 1: Introduction
Description 1: Introduce the concept of the Solid State Transformer (SST), its relevance to the future smart grid, and outline the purpose and structure of the paper.

Section 2: High-Frequency Transformer
Description 2: Discuss the design, challenges, and optimization methods associated with the high-frequency transformer (HFT), a key component of SSTs.

Section 3: Converter Topologies for SST Application
Description 3: Present the various converter topologies that have been proposed for SST applications, including single-, two-, and three-stage configurations.

Section 4: Control Strategies
Description 4: Summarize the control strategies employed in SSTs, highlighting techniques for different converter configurations and SST functionalities.

Section 5: Solid State Transformer Efficiency
Description 5: Analyze the efficiency of current SST designs compared to conventional transformers, including methods to improve SST efficiency through advanced technologies and designs.

Section 6: Computer Modeling and Simulation
Description 6: Review the computer models and simulation tools developed for SSTs, detailing their characteristics, applications, and the experience gathered from real-time simulation platforms.

Section 7: Applications of the Solid State Transformer
Description 7: Describe the potential applications of SSTs in future distribution systems, traction systems, and other areas, emphasizing their role as a universal interface for integrating distributed energy resources.

Section 8: Conclusion
Description 8: Provide a summary of the surveyed work on SST configurations, control strategies, efficiency, modeling, and applications, and discuss the maturity and future prospects of SST technology.