Shuguang Song

Control and Experiment of an H-Bridge-Based Three-Phase Three-Stage Modular Power Electronic Transformer

Compared with conventional power transformer, the power electronic transformer (PET) or solid-state transformer has many attractive additional features. This paper focuses on an H-bridge-based three-phase three-stage modular PET, which consists of an input stage with series-connected H-bridge converters, an isolation stage with several independent dual-active-bridge converters, and an output stage with parallel-connected H-bridge converters. This PET suffers dc-link capacitor voltage unbalancing issue, and the parallel-connected module current unbalancing sharing issue. In this paper, a system control structure is proposed for the PET to deal with these issues. Different input-stage individual module dc-link voltage balancing control methods are analyzed and compared. It is found that the one implemented by directly trimming module output voltage amplitude is most suitable for PET. Moreover, a downscaled laboratory prototype is designed, built, and tehsted to verify the control strategy.

A modular multilevel converter based Railway Power Conditioner for power balance and harmonic compensation in Scott railway traction system

The modular multilevel converter (MMC) is one of the most attractive converter topologies for high or medium voltage power system. In this paper, a back-to-back modular multilevel converter is proposed for Scott railway traction system. The MMC works as a Railway Power Conditioner (RPC) which realizes active power flow control that balances the Scott Transformers feeder currents, and achieves compensation of load harmonic currents as well. The circuit configuration and corresponding control strategy is discussed. A back-to-back MMC simulation model is built in the PSCAD/EMTDC software environment. The effectiveness of the RPC system is verified by simulation results.

An improved circulating current control startegy for modular multilevel converters through redundant voltage levels

This paper first introduces an existing circulating current controller for modular multilevel converter (MMC) through redundant voltage levels with detailed analysis for the controlled circulating current. To further reduce the peak-to-peak value of the circulating current at switching frequency and decrease the total control error, a fixed-band redundant state transition principle is put forward in this paper, so that the circulating current can be approximately limited within the fixed-band. Compared to traditional control methods, the main advantages of the proposed strategy are unaffected arm voltage reference, simple implementation, reduced switching frequency and very fast dynamic performance. Simulation results verified the effectiveness of the proposed control strategy.

Control Strategy for Arc-Suppression-Coil-Grounded Star-Connected Power Electronic Transformers

The grounding of low-frequency distribution transformers is a critical concern that ensures the proper operation and protection of the distribution system. The power electronic transformer (PET), as a future alternative low-frequency distribution transformer, also needs grounding. This paper studies PET operation with arc-suppression-coil (ASC) grounding in the Y-connected medium-voltage (MV) stage, focusing on the following two aspects. 1) The MV stage current stress brought by the ASC is analyzed, and a negative-sequence current compensation method is proposed to reduce the MV stage current stress. 2) The ASC impedance adjustment range that the MV stage can provide by zero-sequence voltage injection is analyzed, and a virtual impedance method is proposed to achieve this adjustment. The effectiveness of the proposed ASC-PET as well as its control strategy is verified by both simulation and experiment results.

Submodule voltage fluctuation elimination in Modular Multilevel Converter with integrated Super Capacitor Energy Storage System

Modular multilevel converter (MMC) receives wide acceptance in high power, high voltage applications. However, large capacity capacitors are needed in the sub-modules due to low frequency fluctuation power. The voltage fluctuation is even larger especially in the startup of Motor drive or low frequency output application. Besides, the integrated energy storage systems (ESS) in MMC is used for recovering the kinetic energy in motor drive. This paper investigates the feasibility of using the ESS to work as the energy buffer for the low frequency fluctuation energy in the submodule (SM) while achieving the energy storage function at the same time. Simulation results in PSCAD/EMTDC prove the validity of the control strategy.

A modified circulating current suppressing strategy for nearest level control based modular multilevel converter

The second-harmonic circulating current in the phase legs of modular multilevel converter (MMC) leads to additional power losses. Existing circulating current suppressing strategies are realized by injecting the second-harmonic component in arm voltage reference. This paper presents a modified circulating current suppressing strategy including two controllers with a fixed-band submodule (SM) state transition principle. The proposed strategy is different from traditional methods, and especially suitable for MMC with nearest level control (NLC). Operating as hysteresis control, the peak values of circulating current can be approximately limited within the fixed-band. A detailed analysis in this paper for traditional circulating current suppressing controllers (TCCSC) and the proposed controller1 demonstrates that the peak-to-peak (p-p) value of the controlled circulating current for the former is greater than the latter, mainly due to the operation characteristic of MMC in the case of NLC. Moreover, the proposed controllers are simpler to be implemented with very fast dynamic performance. Simulation results proved effectiveness of the proposed control strategy.

Dual active bridge assisted modular multilevel converter allowing low frequency output

The modular multilevel converter(MMC) is one of the most attractive converter topologies for high or medium voltage power system. In this paper, a combination of modular multilevel converter and dual active bridge(DAB) is discussed. The DAB assisted MMC(MMC-DAB) is capable of low frequency output with elimination of capacitor voltage fluctuations in submodules. This paper analyzes the MMC-DAB structure and its control strategy. A MMC-DAB simulation model is built in the PSCAD/EMTDC software environment. The effectiveness of the MMC-DAB topology is verified by simulation results.

Reactive component injection control of the modular multi-output power electronic transformer

Power Electronic Transformer(PET) is a promising component in the future power system. The modular PET is the most attractive due to its high voltage level and flexible function. Unfortunately, when the modular PET is driving multiple loads, there would be overmodulation issue in the AC-DC stage caused by the unbalanced load power. This paper proposed a reactive component injection control strategy to prevent overmodulation for a three-module PET under unbalanced load. Two specific algorithms are proposed. The effectiveness of the proposed control strategy and the two algorithms are verified by simulation results.

Long-distance distribution line compensation using power electronic transformer

Power Electronic Transformer(PET) is a promising component in the future power system. The PET can offer not only the functions of conventional low frequency transformer, but also many additional functions. Among all the additional functions, the reactive power control function is considered one of the most important one. This paper is discussing the application of reactive power control function on the long-distance distribution line compensation. By injecting a proper reactive current, the voltage drop on the long-distance distribution line can be compensated, which minimizes the overall current amplitude. As the current amplitude is minimized, the distribution line power loss and the PET power loss can both be reduced. Simulation results verify the effectiveness of the proposed reactive current injection algorithm.

A single phase power electronic transformer considering harmonic compensation in Scott Traction system

Power electronic transformer(PET) has been the focus of interest in recent years. Compared with conventional low frequency transformers, PETs can offer many additional functions such as phase shifting, frequency shifting and reactive power/harmonic compensation. This paper proposes a single phase PET for the Scott railway power system. The PET achieves active power control which balances the Scotts secondary currents. The PET also achieves reactive/harmonic compensation. The configuration and control strategy of the proposed PET is discussed. Simulation results from PLECS verifies the effectiveness of the proposed PET and its control strategy.