Shaodi Ouyang

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.

Research on Unbalanced-Load Correction Capability of Two Power Electronic Transformer Topologies

Compared with conventional power transformer, power electronic transformer (PET) has many merits and an increasing possibility to replace it in the future. In this paper, the unbalanced-load correction capability of two H-bridge based three-phase three-stage modular PET topologies, the separate-phase-connection (SPC) one and the cross-phase-connection (CPC) one, are analyzed and compared. Both of the SPC and CPC consist of three stages: a modular multilevel ac-dc input stage, an isolation stage with several independent modular dual active bridge (DAB) dc-dc converters, and a dc-ac output stage with single-phase-inverter parallel-connected structure. Based on the control strategy, it is found that the SPC is suitable for dealing with full-range unbalanced load under the condition of increasing the input-stage current stress. For the SPC feeding, one load or three loads with identical power polarity, the current stress shall be theoretically increased up to 115% of the rated one, while the percentage is 153% when the SPC feeds three loads with different power polarity. On the contrary, the CPC is only suitable for dealing with partial unbalanced-load conditions. Extendedly, the third so-called full-range-power auto-balance PET is proposed to deal with unbalanced load, the performance of which is verified by simulation results. A downscaled prototype for SPC and CPC is built and tested. Experimental results verify the unbalanced-load correction performance of SPC and CPC.

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.

One-cycle control of a delta-connected cascade H bridge rectifier

One cycle PFC technique has advantages of AC voltage sensor elimination and simplified current loop, thus it has been utilized on various rectifier topologies. In this paper, a modified one cycle PFC method for the delta-connected cascade H bridge (CHB) rectifier is proposed, which combines the one cycle PFC control, zero sequence current injection control and DC voltage balancing control. With the proposed method, unity power factor, unbalanced phase load compensation and DC voltage balance are achieved without using AC side voltage sensors. The validity of the proposed method is verified by both switching model simulation and experiment results.

Comparison of different IGBT based designs of power electronic transformer

Power electronic transformer (PET) has many merits as compared with conventional power transformer. For high-voltage high-power applications, silicon based IGBT and the cascaded modular structure are still the mainstream power semiconductor device and structure, respectively. In this paper, a basic design process of the power stage of an H-bridge based three-stage PET system is presented. Based on the design process, different PET designs based on different voltage rating IGBTs are done and compared. This is a reference for practical PET system design.

Comparison on unbalanced-load handling capability of two power electronic transformer topologies

Power electronic transformer (PET) has many merits as compared with traditional power transformer. In this paper, the unbalanced-load handling capability of two H-bridge based modular PET topologies, the separate-phase-connection (SPC) one and the cross-phase-connection (CPC) one, are analyzed. The two topologies consist of input stage, isolation stage and output stage. The related operating ranges for the two topologies are obtained if the system is confirmed. It finds that the former can deal with full-range unbalanced load if device current stress of the input-stage is increased to 115% while the latter is suitable for dealing with partial unbalanced-load conditions. From practical point of view, the former is more suitable for practical applications. Simulation results verify the analysis.

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.