Diagnosis and Ride Through of Single-Switch Open-Circuit Fault in Low-Capacitance STATCOM

This article proposes novel single-switch open-circuit (OC) fault detection and fault-tolerant methods for the recently introduced low-capacitance cascaded H-bridge static synchronous compensator (LC-STATCOM). Considering the superiorities of the LC-STATCOM, it is an appropriate solution for reactive power compensation. In the case of the submodule (SM) failures, however, maintaining the system's safe operation is a distinctive and more challenging subject compared to the conventional STATCOM due to the large twice fundamental frequency oscillations and fast dynamics of the capacitor voltages. By employing an online capacitor voltage waveform estimation and observation strategy, OC failures are quickly determined in less than one fundamental period without additional sensors. The faulty SM is bypassed entirely in the post-fault operation. Afterward, a fault-tolerant method based on the voltage reference modification technique is introduced to ensure continuous operation of the system under the faulty condition. The required converter over-design is minimized by defining an arbitrary clamp level concept rather than the fixed clamp in fault-tolerant methods. However, the voltage reference modification technique leads to unequal active power flow of the converter's phases. This results in capacitor voltages being unbalanced. The proposed method realizes capacitor voltage balance with a zero-sequence voltage injection-based control scheme. Eventually, the validity of the proposed strategy is verified by simulation and experimental test results.