Research on Startup of DC Transformer for Electric Vehicle Cluster Grid-Connection

Abstract

Modular multilevel direct current transformer (MMDCT) provides a feasible solution to realize the efficient and stable integration of electric vehicle charging and discharging cluster into urban DC distribution network. However, there are impulse currents caused by capacitor charging and transformer inrush current in its starting process. Therefore, a fast pre-charging strategy based on compound frequency control was proposed. The independent control of direct current component and intermediate frequency component was used to realize simultaneous charging of the primary and secondary sides. The double closed-loop control was adopted in the primary side, while the variable-step phase shift peak current control was adopted in the secondary side, which solved the contradiction between charging speed and inrush current. Furthermore, a quantitative comparative analysis of various pre-charging strategies was carried out. The effectiveness and feasibility of the proposed pre-charging strategy were verified through simulation. The results show that the magnitude of the inrush current and the charging time are effectively controlled by the proposed method.

Keywords

• electric vehicle
• direct current transformer (dct)
• pre-charging
• compound frequency control