Improved post-fault operation strategy for a cascaded H-bridge based STATCOM

This study presents a multi-fault tolerant strategy for a cascaded H-bridge (CHB) based static synchronous compensator (STATCOM) with star configuration. A new method for post-fault operation of CHB-STATCOM is proposed, where after switch failures, the full-bridge (FB) submodules are used as two different types of half-bridge submodules named as positive half-bridge (P-HB) and negative half-bridge (N-HB). Using a modified level shift pulse-width modulation technique, a combination of FB, P-HB and N-HB submodules is employed in three legs of the converter to generate the maximum attainable ac side voltage in post-fault condition. In order to restore the nominal capacity of CHB-STATCOM, the capacitor voltage of submodules is increased based on the type and number of switch failures. Furthermore, by utilising a voltage balancing strategy, the voltages of submodules' capacitors are kept balanced and regulated at the desired value in post-fault condition. The feasibility and effectiveness of the proposed strategy is validated on a scaled-down 9-level CHB-STATCOM laboratory prototype.