Direct Common-Mode Current Closed-Loop Suppression Strategy for Non-Isolated Low-Voltage Flexible Power Flow Controller
Cao Xiaojun1, He Jinwei1, Qiao Xuebo2, Han Junfei3, Wang Chengshan1
1. School of Electrical and Information Engineering Tianjin University Tianjin 300072 China; 2. Electric Power Research Institute of China Southern Power Grid Guangzhou 510663 China; 3. Institute of Science and Technology China Three Gorges Corporation Beijing 100038 China
Abstract：With the continuous improvement of the penetration rate of distributed new energy power generation in distribution grids, there are a series of problems such as disordered distribution of current, reverse heavy overload of lines, and increased voltage fluctuations, which seriously affect the stability and safety of distributed new energy power generation and distribution grids. The flexible power flow controller has the excellent characteristics of two-way controllable energy and continuous current adjustment. In addition, it also has auxiliary functions such as direct access to distributed energy storage and active power quality adjustment. Since the pulse width modulation (PWM) technology used in the converter will cause common-mode voltage (CMV). The CMVwill cause adversely affect to the system innon-isolation transformer. The common-mode current (CMC) will produce in the common-mode loop which make the harmonic distortion rate on the AC side increase and protection act incorrectly, and affect the normal operation of the system. In order to solve this problem, a closed-loop suppression strategy for specified harmonics of CMC based on direct modulation of average common-mode voltage (AVE CMV) was proposed. An accurate common-mode circuit modelof non-isolated low-voltage flexible power flow controller was established to analyze the impedance characteristics and the dominant characteristics of CMC. The mathematical relationship between the AVE CMV and the CMC in a switching cycle was constructed and the correlation between the AVE CMV and the CMC during a switching cycle was revealed. Then, the precise adjustment of AVE CMV was realized through the coordinated distribution of the two-sided converter's zero vector, and then an efficient decoupling method of direct regulation of the AVE CMV and space vector pulse width modulation was proposed, which realized the closed-loop suppression of the specified CMC without affecting the dynamic regulation of the differential-mode current. Finally, the feasibility and effectiveness of this method were verified through experiments. The following conclusions can be draw from the analysis: (1) The connection between the AVE CMV of a switching cycle and the CMC is revealed, and the limitations of the traditional common-mode voltage peak suppression method on the CMC suppression are pointed out. (2) It is clarified that the third and ninth harmonic components of the AVE CMV are the main cause of CMC.ACMC efficient suppression strategy based on closed-loop regulation of the third and ninth CMC is proposed.It is pointed out that the adjustment of the AVE CMV in a switching cycle can be achieved by adjusting the action time of the zero vector, and then the suppression of the overall CMC can be achieved by coordinated control of the action time of the different zero vectors of the two converters. (3) The proposed CMC suppression strategy and the highly decoupled characteristics of differential-mode current and power regulation in the system are analyzed and verified.The transient CMC suppression in the dynamic adjustment process of power and AC and DC voltage is realized.The method proposed can be applied to not only a two-port non-isolated flexible power flow controller, but also a multi-port flexible power flowcontroller through the AVE CMV synergy of the switching cycle between the ports.
曹晓军, 何晋伟, 乔学博, 韩俊飞, 王成山. 非隔离型低压柔性潮流转供装置共模电流直接闭环抑制策略[J]. 电工技术学报, 2025, 40(1): 152-163.
Cao Xiaojun, He Jinwei, Qiao Xuebo, Han Junfei, Wang Chengshan. Direct Common-Mode Current Closed-Loop Suppression Strategy for Non-Isolated Low-Voltage Flexible Power Flow Controller. Transactions of China Electrotechnical Society, 2025, 40(1): 152-163.
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