Online Diagnosis and Ride-Through Operation for Cascaded H-Bridge Converter Based STATCOM With a Single Open-Circuit IGBT

To address the most commonly seen single insulated gate bipolar transistor (IGBT) open-circuit fault problems in cascaded H-bridge (CHB) converter based static synchronous compensator, a simple online fault diagnosis method and the corresponding ride-through operation approach are proposed. The method can quickly determine the exact location of the open-circuit IGBT through three steps, via analyzing the relationships between the dc bias in the three-phase output current of CHB converter, the transient dc rail voltage variations with respect to the averaged dc rail voltage, and the response of dc rail voltage and output current when the modulation approach of the faulty power module is adjusted. Afterwards, a fault ride-through approach is adopted to seamlessly transfer the modulation approach of faulty H-bridge module and it behaves as a half-bridge to provide a half of the rated output ac voltage. Meanwhile, the corresponding dc offset voltage from this faulty module is compensated by producing the same dc voltage in the healthy phase legs to avoid the dc bias in the three-phase output current. It has been demonstrated that the proposed fault diagnosis approach and the fault ride-through approach are effective in various adverse situations including grid voltage sudden disturbances and the load power sudden changes. Comprehensive simulated and experimental results have been provided to validate the effectiveness of the proposed approach.