Harmonic stability analysis of single-phase converters in electric multiple units based on an impedance model with harmonic linearization

Low-frequency oscillation, harmonic oscillation and harmonic instability issues in railway vehicle-grid systems pose challenges to the system stability and safe operation. Previous studies constructed the linear time-invariant impedance model of vehicle-grid system, which ignored the internal frequency coupling and loss of model accuracy. To fill this gap, the impedance modelling method based on the harmonic linearisation method of the vehicle-grid system is proposed. The derived model provides a clear physical insight into the harmonic coupling relations between ac and dc variables and improves the model accuracy. Then, a systematic analysis of harmonic stability in the railway vehicle-grid system is analysed based on the proposed model. The electrical and control parameters on system stability at the frequencies above and below the fundamental frequency are summarised. The mechanisms of these oscillation issues in railway vehicle-grid systems are then discussed. Finally, the model accuracy and stability analysis results are verified by the frequency scan method and hardware-in-the-loop results.