Short-circuit fault protection strategy of parallel three-phase inverters

In this paper, a five-stage fault protection scheme against the short-circuit fault for parallel high-power three-phase combined inverter to achieve high reliability is proposed. There are two control modes, Voltage Controlled Mode (VCM) and Current Controlled Mode (CCM). VCM operates under normal operation to keep constant output voltage. CCM operates under fault operation to limit the output current, which is combined with hardware current limit circuit and software current limit unit. For parallel inverter system, the output voltage recovers quickly and then switch from CCM to VCM for inverter once the short-circuit fault is recovered. However, A little time difference of switching mode between parallel inverters results in the surge circulating current, which reduces the system reliability. Thus, the pre-synchronized state is added to achieve the current sharing at any time and to achieve simultaneous mode change for each inverter. A prototype with two parallel three-phase 400kVA/50HZ inverters is built to verify the reliability of the proposed method.