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.