Voltage Ripple-Based Fault Diagnosis of dc-link Capacitors and Fault-Tolerant Operation via Power Regulation in Solid-State Transformer

This article proposes voltage ripple-based fault diagnosis of dc-link capacitors and fault-tolerant operation via power regulation in solid-state transformer (SST). An SST configured as an input-series-output-parallel (ISOP) structure generally consists of a cascaded multilevel H-bridge (CHB) converter and multiple isolated DC-DC converters. In a single-phase SST based on ISOP, the dc-link of the CHB exhibits second-order voltage ripples, which are inversely proportional to the dc-link capacitance. As capacitors age, their capacitance decreases, which is referred to as a capacitor fault. When a capacitor fault occurs in the dc-link, the voltage ripple can exceed the rated voltage of the capacitor, necessitating fault diagnosis and tolerant operation. In this article, by tracking the maximum and minimum of the dc-link voltage, the magnitude of the voltage ripple is compared to the value from the sound condition. This method allows for the diagnosis of the capacitor fault without additional circuits. Once the fault is diagnosed, the SST transitions to fault-tolerant operation. In this mode, the power of the faulty module is regulated to be lower than the others by adjusting the modulation of the CHB. As voltage ripples are proportional to their corresponding module powers, the reduction of voltage ripple in the faulty module becomes feasible. The validity of the proposed fault diagnosis and tolerant operation is verified through experimental and simulation results, respectively.