A 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 downtime. In this article, a novel fault detection and localization algorithm is proposed to localize the 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 the SST. The proposed method is designed such that only one voltage sensor is needed 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. Once the fault is detected, the modulation scheme is switched from phase-shifted unipolar sinusoidal pulsewidth modulation (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 main advantage of the proposed algorithm is that it requires only 2n comparisons to localize the faulty modules in a CHB with n H-bridges, which takes less than a line cycle to complete. © 1986-2012 IEEE.