Abstract：The DC-link capacitor is the core component of the cascaded H-bridge (CHB) static var generator (SVG) system. It withstands complex electrical stress during long-term operation, and an online condition monitoring of its health status is critical to the reliable operation of the system. This paper proposes an online condition monitoring strategy for CHB-SVG capacitors based on module selection. According to the collected control signals, the parameter reflecting the capacitance value is calculated to sort the capacitance value of each module. Then, a special switching mode is used to bypass the module with the smallest capacitance from the system, establishing an independent discharging path. An accurate measurement of the capacitance is achieved through the discharging curve. The proposed method does not rely on extra sensors or affect the normal operation of the system, and no complicated algorithms are required. Firstly, a health indicator of each capacitor is calculated using the existing control signals to select the most fragile module among all CHB modules. Based on the power balancing equation, the health-related indicator K is given, disproportional to the capacitance C. This process can be conducted during normal operation without heavy computational burden. By repeating this process, the degradation trend of each capacitor is monitored in real-time. Once the K is larger than a certain value, the specific module is considered at risk. However, the result can only roughly report potential capacitor failure risk, and the accurate capacitance value cannot be acquired. After locating the risky module, a special switching mode is applied to create an independent internal discharging curve. In this process, the grid-side current is isolated with the capacitor, and an RC discharging path is established. The capacitor voltage is decreasing, which can be measured by the existing voltage sensor. The capacitance value is calculated accurately using the discharging curve. However, extra bypassing and inserting actions are produced, and the power loss during this process is substantial. With the proposed two steps, the risky module is located at the first step, and its capacitance is acquired by the discharging curve at the second step. The overall cost and monitoring accuracy are both guaranteed. The simulation is carried out in Matlab/Simulink, and a three-module CHB prototype is built and operates under SVG mode. The results show that the overall accuracy is above 95%, verifying the effectiveness of the proposed method. The method also works well in similar CHB structures, such as motor drives and PV systems, and is promising to be integrated into existing projects.
王涵宇, 邱荣禄, 吴家伟, 杨柳, 张兴. 基于模块选择的级联H桥型静止无功发生器直流侧电容在线状态监测策略[J]. 电工技术学报, 2025, 40(4): 1169-1179.
Wang Hanyu, Qiu Ronglu, Wu Jiawei, Yang Liu, Zhang Xing. Online Condition Monitoring Strategy of Capacitors in Cascaded-H-Bridge Static Var Generator Based on Module Selection. Transactions of China Electrotechnical Society, 2025, 40(4): 1169-1179.
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