Research on Improved Direct Power Control Method of Urban Rail Flexible Traction Power Supply System

The energy of traditional traction power supply system of urban rail transit adopting diode rectifier can only flow one-way, which causes regenerative braking energy of locomotives inability to be fully utilized. To address the above issues, the fully controlled rectifier is used in the flexible traction power supply system to achieve a bidirectional flow of energy, thus improving the utilization rate of regenerative braking energy. In this work, the control strategy of three-phase diode-neutral-point-clamped (NPC) rectifier is studied, and the structure of the traditional power feedforward decoupling control strategy of the rectifier is derived by combining the mathematical model of the NPC rectifier and the instantaneous power calculation formula. Secondarily, in order to further improve the dynamic performance of the NPC rectifier, an improved direct power control method (IDPC) is proposed, which cancels the calculation of the voltage frequency and phase at the network side by the phase-locked loop(PLL), and the coordinate transformation of the modulation input signal. Aiming at the problem of unbalanced capacitor voltage on the DC side of three-level rectifier, a method of balancing capacitor voltage by controlling the DC component of two capacitor voltages on the DC side is designed. Finally, the traditional power feedforward decoupling control strategy is compared with the IDPC by simulation. The research has shown that the IDPC has the advantage of faster dynamic response speed and the effectiveness of the designed capacitor voltage balancing strategy.