Capacitive Coupling Partial Capacity Medium-Voltage Flexible Power Flow Transfer Device and Control Method
Wang Chengjin1, He Jinwei1, Yang Honglei1, Zhu Shichang1, Li Xiao2
1. School of Electrical and Information Engineering Tianjin University Tianjin 300072 China;
2. School of Electrical and control engineering North University of China Taiyuan 030051 China
Flexible interconnection devices (FIDs) based on power electronic equipment enable power flow transfer between different grid partitions, thereby achieving load balancing and promoting renewable energy integration. Medium-voltage FIDs can generally be classified into full-capacity and partial-capacity types. Full-capacity FIDs are commonly implemented using back-to-back voltage source converters (VSCs), which provide four-quadrant active and reactive power flow regulation with a wide range. However, the reliance on isolation transformers results in disadvantages such as large physical size, high cost, and considerable operational losses.
To overcome the limitations of conventional full-capacity flexible power flow transfer devices, including high cost, significant losses, and dependence on isolation transformers, this paper proposes a partial-capacity power flow transfer device directly coupled with capacitor banks and a three-leg power module. In the proposed topology, the capacitor banks mainly withstand the AC voltage stress, while the parallel port of the three-leg module is connected in series with the capacitors to stabilize the DC voltage of the module. The series port is connected between AC grids to regulate power flow transmission. By dynamically adjusting the number of capacitor banks engaged in operation, the current distribution among parallel branches can be optimized. In addition, active damping control based on the three-leg module is introduced to suppress resonance risks caused by the interaction between the capacitor banks and the inductive line impedance.
Furthermore, to address overvoltage issues that may arise in partial-capacity power flow transfer devices under complex internal and external fault conditions, a fault current limiting circuit composed of a pre-charging diode rectifier bridge paralleled with bidirectional thyristors is proposed. The circuit enables rapid current limiting under scenarios such as grid faults and capacitor short circuits. This paper also investigates the operating characteristics of the proposed device under different grid voltage magnitude and phase differences, as well as variations in line impedance, and performs a comprehensive comparison with conventional solutions. Finally, the effectiveness of the proposed topology and control strategy is verified through RT-LAB hardware-in-the-loop real-time simulations.
Through theoretical analysis and experimental validation, the following conclusions summarized as follows:
(1) The proposed topology features a compact structure, low cost, and a reduced number of switching devices. Through the flexible coordination between the voltage-supporting capacitor banks and the active three-leg circuit, together with the proposed control strategy, the device achieves further improvements in both cost effectiveness and efficiency compared with conventional unified power flow controllers (UPFCs).
(2) The proposed fault current limiting circuit based on a diode pre-charging rectifier bridge enables rapid suppression of fault currents under both internal and external fault conditions. Compared with conventional solutions such as current-limiting reactors, the proposed method provides advantages including higher current limiting capability, lower cost, and smaller equivalent impedance.
(3) By means of the flexible control capability of the three-leg circuit, the proposed strategy not only achieves fast and accurate power flow regulation, but also actively suppresses resonance risks caused by the complex coupling between the voltage-supporting capacitor banks and the inductive line impedance.
王程锦, 何晋伟, 杨洪磊, 朱世昌, 李晓. 电容耦合型部分容量中压柔性潮流转供装置及控制方法[J]. 电工技术学报, 0, (): 368-.
Wang Chengjin, He Jinwei, Yang Honglei, Zhu Shichang, Li Xiao. Capacitive Coupling Partial Capacity Medium-Voltage Flexible Power Flow Transfer Device and Control Method. Transactions of China Electrotechnical Society, 0, (): 368-.
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