A Coordinated Control Technique for a Faulty Cascaded H-Bridge STATCOM Under Unbalanced Grid Conditions

One attractive feature of grid connected Cascaded H-bridge (CHB) static synchronous compensator (STATCOM) is its ability to run continuously with faulty submodules inside the converter. The CHB STATCOM faces the problem of capacitor voltage balancing when there are faulty submodules inside it. This issue of capacitor voltage balancing becomes more challenging during unbalanced grid conditions. The present article deals with a coordinated control technique which helps the faulty CHB STATCOM (having single or multiple number of faulty submodules) to operate stably with unbalanced grid conditions. The proposed control technique helps the faulty CHB STATCOM to balance the capacitor voltages while injecting both positive as well as negative sequence currents to the unbalanced grid. In addition to the decoupled dq-current control technique in positive and negative sequence domain, a geometrical analysis is implemented in this article. The proposed technique uses a redundant module to regain the pre-fault voltage capability of the converter. This technique helps the healthy operating CHB submodules to operate within linear modulation range thereby operating in presence of severe grid disturbances even with multiple number of faulty submodules. The simulation results and experimental results from a scaled-down prototype are used to successfully verify the proposed concept.