Low frequency oscillation suppression in train-grid system based on co-phase supply unit

Low-frequency oscillations (LFOs) pose a significant threat to the safe and efficient operation of electrified railways. This paper proposes a novel solution for LFO suppression based on a single-phase back-to-back converter namely co-phase supply unit (CSU), which simultaneously achieves continuous power supply and negative sequence compensation for electrified railways. First, a control framework for the CSU is proposed. Based on this, the dq admittance model of the CSU is derived. Then, the specific admittance for each channel is obtainable by model decomposition. With this, the impact of each parameter on the CSU admittance characteristics is clearly expressed, which can guide the parameter adjustments. To further explore the coupling between the train-grid system and the CSU, the dq admittance model is transformed into a single-input single-output system. The stability analysis confirm that the CSU can provide positive conductance in the low frequency range, which eliminates the negative conductance of the train-grid system, thus improving the system stability. Simulation and hardware-in-the-loop experiments both verified the effectiveness of CSU in LFO suppression.