Low Frequency Oscillation Traceability and Suppression in Railway Electrification Systems

The low frequency oscillation (LFO) phenomenon has frequently occurred in railway electrification systems (RESs) and deteriorates the safety and reliability of RESs. This article proposes a simplified and analytical impedance model of the electric train and traction network interaction system (hereinafter train-network system) in the synchronous frame (-dq), which reveals that the dc-link voltage proportional-integral controller mainly brings the negative resistor to the d-d channel impedance of the train in the low frequency range. The in-depth stability analysis elaborates that the introduced negative resistor will lead the phase margin of eigenvalues of the impedance ratio matrix to be negative under some conditions and then causes the LFO issue. Moreover, case studies also illustrate that the narrower negative real-part area (NRA) caused by the negative resistor and the higher magnitude of the train impedance in the low frequency range can enhance the stability of the train-network system. Therefore, for suppressing the LFO, a proportion-derivation feedback of the current based on the analytical impedance model is then proposed to decrease the NRA and increase the magnitude of the train impedance at the same time in the low frequency range. Both simulation and experimental results validate the effectiveness and correctness of the theoretical analysis.

I. Introduction

Recent years, the low frequency oscillation (LFO) phenomenon has frequently arose in available RESs around the world. Based on a number of actual tests, the LFO easily happens when multiple trains simultaneously energize in a rail depot [1]. At this time, only four-quadrant converters (4QCs) of the train operate in the low-power mode and the current of each 4QC is only few ampere. With increasing the train quantity in a rail depot, this oscillation phenomenon will arise. The typical voltage and current waveforms of the LFO measured in a traction substation and its harmonic spectra are shown in Fig. 1. It can be seen that waveforms contain massive f₀±f₁ interharmonic components and a low frequency envelope of f₁ composed in oscillatory waveforms. It is the reason why this phenomenon is named as the LFO. The LFO has caused a number of serious consequences, such as the malfunction of the protection system, high-voltage and current that damages the onboard devices, transportation delay, and so on [2].