Abstract
Harmonic overvoltage in electric railway traction networks can pose a serious threat to the safe and stable operation of the traction power supply system (TPSS). Existing studies aim at improving the control damping of grid-connected converters, neglecting the impedance frequency characteristics (IFCs) of the actual TPSS. The applicable frequency range of these studies is relatively low, usually no more than half of the switching frequency, and there is a large gap with the actual traction network harmonic overvoltage frequency range of 750 Hz–3750 Hz. In this paper, first, the IFCs of the actual TPSS in the wide frequency range of 150 Hz–5000 Hz are obtained through field tests, and the resonant frequency distribution characteristics of TPSS are analyzed. After that, the aliasing effect of the sampling process and the sideband effect of the modulation process of the digital control of the grid-connected converter are considered. Based on the relative relationships among the inherent resonant frequency of the TPSS, sampling frequency and switching frequency, an impedance matching analysis method is proposed for the wide frequency range of the vehicle–grid coupling system. By this method, the sampling frequency and switching frequency can be decoupled, and the harmonic overvoltage of traction network in the frequency range of two times switching frequency and above can be directly estimated. Finally, the method proposed in this paper is validated by the comparative simulation analysis of seven different cases.
Highlights
Since alternating current-direct current-alternating current (AC-DC-AC) Electric Multiple Units (EMUs) and electric locomotives have been adopted widely in China, the traction power supply system (TPSS) and electric locomotives’ electrical matching instability caused by high-order harmonic overvoltage traction network problems occur from time to time
To address the above research deficiencies, this paper mainly focuses on harmonic overvoltage of the traction network with a wide frequency range
This paper presents the results of the impedance frequency characteristics test and analyzes the inherent resonant frequency distribution characteristics of the traction network
Summary
Since alternating current-direct current-alternating current (AC-DC-AC) Electric Multiple Units (EMUs) and electric locomotives have been adopted widely in China, the traction power supply system (TPSS) and electric locomotives’ electrical matching instability caused by high-order harmonic overvoltage traction network problems occur from time to time. The discrepancy between the simplified circuit model in a laboratory setting and actual system requires more detailed consideration when analyzing real problems with existing methods Researchers reckoned that their proposed method may become invalid if there are two resonances on a TPSS with an approximately symmetrical distribution of Nyquist frequency [8]. The influence of complex factors, such as time delay and LCL filters, is taken into consideration, most of the current research has focused on the relatively low frequency range, that is, below the Nyquist sampling frequency or half of the switching frequency These studies do not apply to real on-board grid-connected converters switching frequency of only a few hundred hertz, while harmonic overvoltage frequency can reach thousands of hertz (several times the switching frequency).
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