Impedance Modeling and Mechanism Analysis of Low-Frequency Oscillation in Vehicle-Grid Coupling System Under the Mixed Operation of Locomotives

The mixed operation of locomotives aggravates the interaction between locomotives and the traction network, resulting in the low-frequency oscillation (LFO) for the vehicle-grid coupling system. However, the mechanism of the LFO when different types of locomotives access the traction network needs to be elucidated. In addition, the impact of parameters of locomotives on the system stability is still unclear. To solve the above problems, this paper firstly proposes a circuit model-based conversion method to implement the conversion from multiple-input multiple-output (MIMO) admittance models to single-input single-output (SISO) impedance models of locomotives in the single-phase stationary frame. Secondly, the concepts of dominant negative resistance and dominant phase margin (DPM) are proposed to correctly reveal the mechanism of the LFO and accurately reflect the stability margin of the system, respectively. Then, the impact of parameters on the system stability is quantitatively evaluated for the first time based on the sensitivity index and the variation pattern of DPM. Accordingly, the parameter-tuning criteria are proposed to further improve the system stability and thus increase the number of locomotives that can access the traction network. Finally, the correctness of the theoretical analysis is verified through comprehensive simulation results and hardware-in-the-loop (HIL) experiments.