Harmonic resonances in traction power supply systems (TPSSs) have attracted great attention due to their potentially destructive impact on the safe and stable operation of railways. Based on operation data acquired from Chinese railways, this paper presents a comprehensive review of this issue considering both academic and engineering requirements. Analysing actual incidents, the general patterns and effects of TPSS resonances are derived. The relation between the TPSS and the locomotive is illustrated by circuit models. Relevant methods for modeling locomotives and TPSSs are discussed. Both advanced resonance analysis methods giving general influence factors and a simplified resonance analysis explaining resonance features in a practical manner are discussed. Multiple groundbased and on-board solutions for resonance elimination are presented. At last, pre-identifying resonances in actual systems is investigated for addressing the open topic of resonance prevention.

AC electric locomotives that use a number of interleaved four-quadrant converters (4QCs) generate high frequency switching harmonics which may stimulate certain resonances in traction power supply systems (TPSSs). A windowed selective harmonic elimination pulse-width modulation (SHE-PWM) method is proposed to suppress such resonances. Owing to the windowed design and the precalculated solutions, the proposed method covers the wide potential resonant frequency range and addresses the resonant frequency variation while keeping the low switching frequency of the traction converters. The proposed windowed SHE-PWM is fully tested with a closedloop controller in a simulation model with the TPSS and the ac electric locomotive. Comparative simulation results show that the windowed SHE-PWM is an effective alternative that overcomes the resonance suppression limitations of the conventional phase-shifted PWM (PS-PWM). The performance of proposed windowed SHE-PWM on an experimental equivalent resonant circuit is further evaluated and compared with PS-PWM. Both simulation and experimental results verify the effectiveness and feasibility of the proposed method.

An AC-DC-AC topology based on multi-converter is proposed in this paper. It can match the capacity of the converter for high voltage and large power applications. This structure is not only suitable for solving the power quality problems of electrified railways, but also has its own superiorities. Its architecture and advantages in the traction application are analyzed in depth, and the design calculation of the main elements is presented. The control system of the proposed topology is designed, where dc current feedforward control is adopted to relieve the impact of the load mutation on the system. Its modular structure and simple current sharing control provide convenience for the capacity expansion. The software simulation and experiment based on a 45kVA prototype validate the proposed structure, and they are all compared with the traditional power supply systems. The simulation results indicate that this topology can not only satisfy the voltage and power requirements of traction load and solve the power quality problems, but also have its own superiorities in the application of electrified railways. The experimental results verify the correctness and validity of the scheme. INDEX TERMS Electrified railways, power quality, series-parallel structure, back-to-back converter, current sharing control.

The traction load of a high-speed railway (HSR) may cause significant voltage fluctuations in the traction power supply system and the utility grid. To evaluate power quality, modeling traction load is an urgent need as well as a fundamental work. This paper aims to build a probabilistic model to describe the pattern of traction load changing with time. It is very challenging due to the randomness of train behavior. This paper first investigates the stochastic process of traction load and points out that the size and frequency of load fluctuation, respectively, rely on the slope and length of the rail ramp. Based on this, train power (TP) and headway (TH) are set as two basic components of the probabilistic model, where TP and TH are described by a normal distribution and an exponential distribution, respectively. In addition, a sliding time-window method is developed for generating time-dependent load samples. The proposed method has low complexity, as well as can consider the randomness, giving more realistic simulation results. It can be used to forecast traction load for a planning railway or to establish a measurement-based model for an operating railway. A case study is performed for parameter identification and model verification, and the rapid voltage change (RVC) caused by the HSR loads has been evaluated in different operation scenarios. INDEX TERMS Dynamic headway, high-speed railway (HSR), probabilistic model, rapid voltage change (RVC), traction load.

The harmonic spectra of the AC supply currents of electric multiple units (EMUs) employing three-level neutral-pointclamped converters are discussed in this study. The analytical expressions of high-order harmonics because of high-frequency three-level phase-opposition disposition pulse-width modulation are derived by using double Fourier series and a contour plot in a two-dimensional plane. The carrier phase-shift control to cancel some of the high-order harmonics when two or more converters are interlaced is validated. The impact of converter's DC voltage controller on low-order harmonics is formulated. The harmonic spectra are verified by computer simulations and through actual measured data taken at lower train speed and rated power from an EMU of the CRH380AL and tested on a 9 km long testing railway line in a purpose built rolling stocks plant.