OCNOP Control of Grid-Connected Converters for Single-Phase Open Circuit Fault

An open circuit fault-tolerant operation of a grid-connected inverter requires switching reconfigurations or redundant inverter leg to bypass the faulty device. Such measures led to increased cost and design complexities. In the absence of such reconfiguration mechanisms, a grid-connected inverter with an open circuit fault in one of the inverter legs injects negative sequence current into the grid. The negative sequence current causes oscillation in dc-link voltage and thereby injects lower order harmonic into the grid. The present work proposes a novel control strategy (open circuit nonoscillatory power) to deliver a steady power into a balanced three-phase grid with only two active legs of the grid-connected inverter. The current of these two active phases are controlled to be equal in magnitude and 60° phase apart. This inhibits the flow of negative sequence current and in turn limits the lower order harmonic injection within the permissible limit. A control strategy is also proposed to modify the power references during postfault operation so that the postfault current in two active phases are limited within the converter current rating. The performance of the proposed scheme is compared with the conventional vector control scheme through a simulation study. The simulation results for the proposed control scheme are verified through experimental study in a laboratory prototype.