Power Electronic Transformer Research Articles

A Modular Multiport Power Electronic Transformer With Integrated Split Battery Energy Storage for Versatile Ultrafast EV Charging Stations

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

Dynamic model of direct matrix converter and its experimental validation

SummaryPower electronics is a major candidate in the ongoing research of electrical power grids because they can provide better control with more information (voltage ratio, efficiency, power flow direction, etc.) than the all‐passive solutions such as transformers and so forth. Power electronic transformer has the possibility to replace the conventional one by ac/ac matrix converter with high‐frequency compact electronic transformers. In this work, a dynamic model of matrix converter is studied and issues such as efficiency, voltage ratio limitation, and transient behavior are discussed and the equivalent numerical calculations are presented. Moreover, an experimental converter is built to validate the numerical results and also to describe the switching frequency impact on the operation. It is concluded that the control creates voltage ratio saturation and high‐switching frequency results in lower efficiency values. On the other hand, efficiency can be increased by using resonances of the circuit, which can also provide higher operating frequencies. Copyright © 2015 John Wiley & Sons, Ltd.

Extended Unbalanced Operation of Induction Motor Driven System Based on a Single-Phase Electronic Power Transformer

A new control strategy to extend the unbalanced operation of the induction motor driven system based on a single-phase electronic power transformer (EPT) is proposed in this paper. This new motor driven system avoids the traditional input power transformer and only single-phase power network is needed. It has a modular structure and consists of series H-bridges rectifier, output-parallel dual active bridge converters and a three-phase voltage source inverter. In the input stage, a new control strategy is proposed to balance the dc voltages among the series H-bridges and regulate the current. This new control algorithm determines the minimal reactive current to stabilize the converter when the loads are seriously unbalanced. In order to verify the new driven system, a 3 kV/462 V/180 kW EPT driving an induction motor is constructed in MATLAB/SIMULINK. The simulations results illustrate the effectiveness of the proposed unbalanced control algorithm and good dynamic behavior of the integrated control system for this new driven system.

A New Resonant Modular Multilevel Step-Down DC–DC Converter with Inherent-Balancing

Modular multilevel converters (MMCs) have become increasingly interesting in dc-dc applications, as there is a growing demand for dc-dc converters in high-voltage applications. Power electronics transformers can be used for high step-down ratio dc-dc power conversion, with high power rating and efficiency achieved. However, this arrangement requires a large number of high isolation voltage transformers and a complicated balancing control scheme. To provide a simple solution with inherent voltage balancing, this paper presents a new resonant MMC topology for dc-dc conversion. The proposed converter achieves high-voltage step-down ratio depending on the number of submodules. The converter also exhibits simplicity and scalability with no necessary requirement of high-voltage isolation transformers. By using phase-shift control, a much higher converter operating frequency is achieved compared to the switching frequency. Resonant conversion is achieved between the series inductor and submodule capacitors. The operation principle and theoretical analysis are presented in this paper, which have been verified by experimental results based on a bench-scale prototype.

Application of Distribution Power Electronic Transformer for Medium Voltage

In this paper a distribution power electronic transformer (DPET) for feeding critical loads is presented. The PE based transformer is a multi-port converter that can connect to medium voltage levels on the primary side. Bidirectional power flow is provided to the each module. The presented structure consists of three stages: an input stage, an isolation stage, and an output stage. The input current is sinusoidal, and it converts the high AC input voltage to low DC voltages. The isolated DC/DC converters are then connected to the DC links and provide galvanic isolation between the HV and LV sides. Finally, a three-phase inverter generates the AC output with the desired amplitude and frequency. The proposed DPET is extremely modular and can be extended for different voltage and power levels. It performs typical functions and has advantages such as power factor correction, elimination of voltage sag and swell, and reduction of voltage flicker in load side. Also in comparison to conventional transformers, it has lower weight, lower volume and eliminates necessity for toxic dielectric coolants the DPET performance is verified in MATLAB simulation. DOI: http://dx.doi.org/10.11591/ijpeds.v4i4.6395

Damping of interarea oscillations using electronic power transformer

This study utilizes the electronic power transformer (EPT) to enhance the damping of interarea oscillations in a multimachine power system based on active power modulation. The dynamic model of EPT is established, and decoupled vector control schemes of inner current control loops and outer active and reactive power control loops are modeled and designed. A power oscillation damper (POD) applied to the active power control loop of EPT is introduced to improve the stability of the power system. The POD controller parameters are designed based on the frequency response method. A study system based on the classical two-area four-machine system is built for damping controller design and time domain simulation, and the design procedures are presented in detail. The effectiveness of the designed POD controller is verified through eigenvalue analysis and time domain simulations. Copyright © 2014 John Wiley & Sons, Ltd.

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)

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.

A Single-Stage Power Electronic Transformer for a Three-Phase PWM AC/AC Drive With Source-Based Commutation of Leakage Energy and Common-Mode Voltage Suppression

This paper presents a novel topology for the generation of adjustable frequency and magnitude pulsewidth-modulated (PWM) three-phase ac from a balanced three-phase ac source with a high-frequency ac link. The proposed single-stage power electronic transformer (PET) with bidirectional power flow capability may find application in compact isolated PWM ac drives. This topology along with the proposed control has the following advantages: 1) input power factor correction; 2) common-mode voltage suppression at the load end; 3) high-quality output voltage waveform (comparable with conventional space vector PWM); and 4) minimization of output voltage loss, common-mode voltage switching, and distortion of the load current waveform due to leakage inductance commutation. A source-based commutation of currents associated with energy in leakage inductance (termed as leakage energy) has been proposed. This results in soft-switching of the output-side converter and recovery of the leakage energy. The entire topology along with the proposed control scheme has been analyzed. The simulation and experimental results verify the analysis and advantages of the proposed PET.

Power Electronic Traction Transformer: Efficiency Improvements Under Light-Load Conditions

Power electronic transformer (PET), a converter technology that utilizes power semiconductors in combination with medium-frequency transformers, is considered a promising solution for certain applications requiring flexible galvanic isolation. Among are those where space occupied by bulky low-frequency transformers is of concern and/or where advanced power quality control features are needed. In this paper, the PET for a single-phase traction on-board application is discussed with emphasis on the efficiency improvements and reductions of energy consumption during the operation on the vehicle. Several control algorithms devised to improve efficiency under light-load conditions are tested on a low-voltage prototype of the PET, and experimental results are presented demonstrating the effectiveness of the proposed algorithms.

Power Electronic Traction Transformer—Medium Voltage Prototype

Recently, the world's first ever power electronic traction transformer (PETT) for a 15-kV 16.7-Hz railway grid has been newly developed, commissioned, and installed on the test locomotive, where the PETT is presently in use. The design and development of the PETT are described in two parts of this paper. The details related to the 1.2-MVA medium voltage (MV) PETT prototype, including the electrical design of the power circuit, the sizing and characterization of insulated-gate bipolar transistor (IGBT) devices, the design of the medium-frequency transformer, the cooling system design, insulation coordination, and protection, are presented in this part, while the preceding paper deals with the overall system requirements and control system design based on the low-voltage (LV) PETT prototype. The identical control hardware and software from the LV PETT prototype were used for the MV PETT prototype, thus reducing significantly commissioning time. The experimental results obtained during the testing are presented to demonstrate the performance of the developed MV PETT prototype.

Multi-Fed Power Electronic Transformer for Use in Modern Distribution Systems

This paper presents a Multi-Fed Power Electronic Transformer (MF-PET) for use in power distribution electrical systems. The MF-PET is a power electronic transformer with two or more AC/DC rectifiers at its input port, which may be connected to different feeders. Thus, it is possible to share the demand among feeders. Through a proper control, the AC/DC rectifiers may handle active power from the feeders (independently of the load type), may protect the load from sag/swell events, and may transfer power among feeders. This paper presents a three-phase MF-PET model. A small-scale laboratory prototype is shown and its application to distribution electrical system is discussed, exhibiting the MF-PET as a power electronic building block (PEBB) for its integration into electrical grids, enhancing the distribution system flexibility and contributing to the smart grid concept.

A Novel Modulation Method for Modular Multilevel Converters

Modular Multilevel has been widely applied in Flexible HVDC, electric locomotives and PET(Power Electronics Transformers). The modulation method is important for the converter performance. This paper presents an improved modulation method for Modular Multilevel Converter.

Optimal Analysis and Design of Power Electronic Distribution Transformer

This study proposes a new design of Power Electronic distribution Transformer (PET) by using an intelligent program to get an optimum HF operation, which leads to maximum efficiency. This PET has three stages; the input stage consists of three phases, three level PWM rectifier and isolation stage employing three level half bridge convertor connected with high frequency transformer. The output stage has a matrix converter. This new design of PET has the advantage of improving power factor, voltage sags, voltage swell, reactive power compensation, eliminating harmonic and controlling and Protecting its self (PET) with minimum size, compared to the traditional transformer. The software like MATLAB and Artificial Neural Network will be used to analyze and simulate the PET.

Dual active bridge synchronous chopper control strategy in electronic power transformer

The electronic power transformer (EPT) is a novel transmission and transformation device, which consists of a series connection of two voltage-source H-bridge converters and a DC-DC converter with high-frequency isolation transformer. When applied to microgrids, EPT will not only deliver energy to loads from the utility grid but also inject some amount of excess power into the utility grid. Hence, the capability of bidirectional power flow is important for EPT, which depends on the DC-link stage. The traditional synchronous chopper control for a dual active bridge (DAB) converter has some limitations in the application of bidirectional power flow area. This study proposes a novel synchronous chopper control strategy for the DAB converter to implement bi-direction power flow, details the basic principle and steady-state operation and presents the mathematical derivations. A three-phase three-stage circuit configuration of 10 kV/400 V bi-direction EPT system based on the novel control DAB converter is designed, and corresponding control schemes for the system are discussed. The performance of this EPT system is validated by the MATLAB/Simulink-based simulations and the laboratory prototype experiments.

전력전자 변압기로 동작하는 저전압 직류배전 기능을 갖는 Quasi Z-소스 AC-AC 컨버터

This paper proposes a quasi Z-source AC-AC converter with the low DC voltage distribution capability operating as a power electronic transformer. The proposed system has configuration that the input terminals of two quasi Z-source AC-AC converters are connected in parallel, also their output terminal are connected in series. Simple control method of duty ratio was proposed for the in phase buck-boost AC voltage mode and the DC output voltage control. DSP based experiment and PSIM simulation were performed. As a result, the PSIM simulation results were same with the measured results. By controlling the duty ratio under the condition of 100 [Ω] load, quasi Z-source AC-AC converter could buck and boost the AC output voltage in phase with the AC input voltage, and the same time, the constant DC voltage could be output without affecting the AC output characteristics. And, the DC output voltage 48[V] was constantly controlled in dynamic state in case while the load is suddenly changed (50 [Ω]→100 [Ω]). From the above result, we could know that the quasi Z-source AC-AC converter can act as a power electronic transformer with a low DC voltage distribution capability.

Control of Electronic Power Transformer with Star Configuration under Unbalanced Load Conditions

The electronic power transformer has many advantages, such as reactive power compensation, voltage regulation, harmonic suppression, and power factor correction. When a three-phase electronic power transformer with a star configuration is applied in the distribution network, three-phase load unbalance will cause three-phase DC voltage unbalance if the electronic power transformer controller generates improper asymmetric voltage signals, which may trigger over-voltage protection. The effects of three-phase load unbalance to the work condition of the electronic power transformer are analyzed, and a control algorithm is proposed to balance three-phase DC voltages in this article. Meanwhile, input stage control, isolation stage control, output stage control, and individual DC voltage balancing control are presented. Simulation and lab experimental results show that the electronic power transformer still works properly even under serious unbalanced load conditions.

Power Electronic Traction Transformer-Low Voltage Prototype

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.

Development and testing of a bidirectional distribution electronic power transformer model

Transformer size can be significantly reduced by increasing the operating frequency, which may be achieved by means of power electronics converters installed as interfaces to the power frequency systems at both transformer sides. In this work, a model for a bidirectional high-frequency power electronic transformer is presented. Several case studies have been carried out in order to evaluate the behavior of the transformer under different operating conditions and test the impact on the power quality. The results show that the electronic power transformer, also known as solid-state transformer, not only matches the functions of a conventional power transformer, but also provides additional capabilities to mitigate dynamic power quality problems.

A New Power Electronic Transformer Applied to Distribution System

This paper proposes a new power electronic transformer (PET) to be used for distribution system, which is composed of the three-stage power circuits. The input stage uses Modular Multilevel Converter (MMC) to reduce the voltage stress rating of components, meanwhile, the harmonic components in the current will be deduced significantly, to make DC capacitor voltage of MMC rectifier balance, a control method is proposed. The isolation stage use a middle-frequency transformer to realize voltage conversion and isolation of the low-voltage side from the high-voltage side. In the third-stage, a inverter outputs the desired voltage and power to the load. The simulation results are presented to show the validity of the proposed PET system.

Analysis of the Internal Electrical Characteristics of Electronic Power Transformers

The modularized subunit of an electronic power transformer (EPT) is a series connection of two H-bridge voltage-source converters and a DC-DC converter with a high-frequency isolation transformer (HFIT). On the basis of cascading and paralleling the modularized subunits, EPT can be used in high-voltage and large-current applications in the power system. This paper discusses the steady state analysis of the modularized subunit of EPT. Theoretical analysis considers the influences of the two H-bridge voltage-source converters on the two sides of the DC-DC converter. We deduce the formulas of the theoretical calculation on the internal electrical characteristics of EPT (e.g., the voltages of the DC-bus capacitor and the primary side peak current of the HFIT). This paper provides guidance on the design and selection of EPT key elements (e.g., the DC-bus capacitors and HFIT). Experimental results are obtained from a single subunit of a laboratory model rated at 962 V, 15 kVA. All calculations, simulations, and experiments confirm the theoretical analysis of the subunit of EPT.