Research on symmetric bipolar MMC-M²Tdc-based flexible railway traction power supply system

The success of the modular multi-level converter (MMC)-based multi-terminal direct current (MTdc) transmission system has been instrumental in advancing the railway traction power supply system (TPSS). Nonetheless, it is beset with issues related to inadequate flexibility and reliability. In this paper, an MMC-based meshed MTdc flexible TPSS (MMC-M ² Tdc FTPSS) with symmetric bipolar network has been introduced, along with the formulation of its control strategies catering to the specific power supply requisites of electric railway applications and operating conditions. To enhance the flexibility and reliability of normal and single-pole operation mode conditions, a master/slave combined voltage droop coordinated control strategy is proposed. Given the uncertain, frequent, and heavy load-varying features of railways, an advanced droop controller, featuring secondary active power-dc voltage compensation techniques, is proposed for the power grid side three-phase MMC to regulate inaccurate current distribution and unwanted voltage deviation during acceleration and deceleration processes. A single-phase ac voltage/ frequency controller is presented for traction drive system side MMCs, which comprises circulating current suppression control and capacitor voltage balance control strategies, ensuring stable and constant voltage, frequency, and phase angle for the electric railway system. Finally, the operational behaviors of the developed symmetric bipolar MMC-M ² Tdc FTPSS during steady-state and post-fault operations are verified using RTDS/RSCAD.