A Novel Open-Circuit Fault Detection and Localization Scheme for Cascaded H-bridge Stage of a Three-Stage Solid State Transformer

Three-stage solid-state-transformer (SST) is a preferred replacement for line-frequency transformer as it provides ancillary services in addition to stepping-up/down of grid voltages. Each active switch in SST is a potential source of failure, and a quick fault detection and isolation help in reducing the down-time. In this paper, a novel fault detection and localization algorithm is proposed to localize open-circuit (OC) faults (be it a single switch fault or multiple switch faults in multiple modules) in the cascaded H-bridge (CHB) stage of SST. The proposed method is designed with one voltage sensor to measure the grid-side voltage of the CHB for fault detection and localization. By comparing the estimated grid-side voltage of the CHB with the measured values, OC fault is detected. Then, the modulation scheme is switched from phase-shifted unipolar SPWM to phase-shifted bipolar SPWM to localize the faults. The difference in the measured grid-side voltage of the CHB before and after each module's state change is calculated and faults are localized. The presented algorithm is substantiated using experimental results from a 1 kVA SST prototype. The proposed algorithm requires only 2n comparisons to localize the faulty modules in a CHB, which completes in less than one line cycle.