Asymmetric Energy Storage Integrated Modular Multilevel Converter for Unbalance and Power Factor Compensation of Advanced TPSS

Low power factor (PF), high voltage unbalance (VU), and weak regenerative braking energy utilization are the three major challenges of traditional traction power supply system (TPSS) at present. To solve the problems above, an advanced TPSS adopting modular multilevel converter (MMC) based power flow controller (PFC) integrated asymmetric energy storage (AES) is proposed. Among them, the energy storage is only accessed in the traction side of MMC to suppress the phase circulating current, while the battery and supercapacitor are divided into the upper arms and lower arms respectively to reduce the submodule capacitor voltage ripple. Moreover, a hierarchical control strategy is proposed, the first stage of the upper layer establishes an operation optimization model to minimize the electricity cost, for making the energy storage charging and discharging strategy and PFC regulation schedule available. The second stage arranges the power flow distribution of PFC determined by the first stage, which is based on a partial compensation model of power quality, to satisfy the PF and VU thresholds and decrease the MMC capacity. Meanwhile, the reference power flow of PFC drives lower layer controllers, where the passive-based controller is employed to track the MMC port current, and the circulating current suppression controller is designed to decouple the AES power. Finally, the effectiveness of the topology and control strategy is verified by hardware-in-loop experiments. As a result, RMS current can be reduced by 10% and voltage ripple by 30%. According to field-test data, the substation can save 19% in operating electricity costs.