The power converters based on a modular structure have gained importance for electric transportation applications, including electric vehicles (EVs), EV charging stations, railway traction, and electric ships. Modular multilevel converters (MMCs) are recognized as one of the promising topologies to improve the energy conversion efficiency and fault-tolerant ability of power conversion systems. This paper aims at a review of latest research contributions regarding the evolution of circuit topologies, operational challenges, control schemes, and fault-tolerant strategies of the MMC. Finally, the current state of the art, opportunities and the future perspective of MMC in electric transportation applications are addressed.

Modular Multilevel Converters for Transportation Electrification: Challenges and Opportunities

In order to eliminate the negative-sequence and harmonic currents in the high-speed train traction systems with three-phase V/V transformers, a compensation strategy based on the railway static power conditioners (RPC) is proposed in this paper. An RPC contains two converters that are connected back to back by sharing the same dc link. In this paper, the structure and principle to compensate negative-sequence currents for the RPC with a three-phase V/V transformer are explained, and a strategy to provide the compensation references for negative-sequence and harmonic currents is proposed. Also, a method to separate active current, reactive current, and harmonic current references from the total negative-sequence and harmonic current references is given. Moreover, a controller is proposed to maintain the dc-link voltage and to compensate the negative-sequence and harmonic currents. Simulation and experimental results are provided to demonstrate that the proposed strategy is very effective.

Railway Static Power Conditioners for High-speed Train Traction Power Supply Systems Using Three-phase V/V Transformers

This paper presents a negative sequence compensation system based on a novel two-phase three-wire converter to eliminate a negative sequence current for the high-speed railway traction power system with a three-phase V/V traction transformer. In this compensation system, the proposed two-phase three-wire converter is fed by two single-phase power sources formed with two step-down transformers connecting to the two feeder sections. The proposed converter contains three switch legs, one of which is connected to the common ground wire of two single-phase voltages. Hence, a switch leg is saved compared with two conventional single-phase converters, and the ratings of the power switches are not increased. In order to enhance the dynamic and steady-state performances of the compensation system, a compound control method composed of hysteresis control and dividing frequency control is presented. Simulation and experimental results are presented to demonstrate that the proposed compensator and its control strategy are very effective.

A Negative Sequence Compensation Method Based on a Two-Phase Three-Wire Converter for a High-Speed Railway Traction Power Supply System

For power quality in V/V traction system of the 350-km/h high-speed railway, a kind of railway static power conditioner (RPC) is discussed, which is used to carry out the comprehensive compensation of negative sequence and harmonic currents in the traction substation. In order to improve the control effect and performance of RPC, a dual-loop control strategy is constructed for RPC. Taking into account the disturbance and variation of electrified railway environment, a recursive proportional-integral control based on fuzzy algorithm is adopted to realize a fast and smooth tracking to the reference current. An energy-balance control is proposed to suppress the fluctuation of dc-link voltage and maintain the stability of RPC, which is an accurate and adaptive feedback control based on corresponding parameters. Finally, the correctness of the analysis proposed in this paper has been confirmed by the simulation and experiment results.

A Dual-Loop Control Strategy of Railway Static Power Regulator Under V/V Electric Traction System

The proliferation of the new high-speed train has resulted in significant distortions in network voltage and current in both traction power supply system (TPSS) and the connected power system. The nonlinear and dynamic natures of the modern trains make the calculation and evaluation of such power quality (PQ) problems quite a difficult work. The typical PQ problems in high-speed railway (HSR) are the unbalance, reactive power, harmonic and harmonic resonance, low-frequency voltage fluctuation, etc. This article presents an overview on these power quality issues based on reported documents and field experiments, and further describes the modelling of the TPSS in China HSR systems, which is used to study the PQ issues. In order to improve the PQ performance, a number of mitigation approaches have been investigated and utilized through railway electrification history. Moreover, with the development of new power-electronics technologies, some novel solutions have been developed and implemented to achieve better PQ performance. Additionally, the article compares and summarizes different PQ improvements with respect to various power quality indices.

https://tandfonline.com/doi/pdf/10.1080/23248378.2016.1169228

Power Quality in High-speed Railway Systems

High-speed AC electric railways, such as Shinkansen, require large single-phase AC electric power. So, if the short circuit capacity of the power system is not large enough, therefore, railway operation causes large voltage unbalance at the three-phase side. In this paper, a railway static power conditioner (RPC), a new system for reducing voltage unbalance and fluctuation, is described. To reduce voltage unbalance and fluctuation, the RPC balances effective power of different phase and compensates reactive power and harmonics. It enables operation of high-speed AC electrified railways even if the short circuit capacity of the power system is rather small.

M. Nishitoba

Papers

Development of railway static power conditioner used at substation for Shinkansen