A Current Limiting Strategy with Parallel Virtual Impedance for Three-phase Three-leg Inverter under Asymmetric Short Circuit Fault to Improve the Controllable Capability of Fault Currents

Voltage-controlled three-phase three-leg (TPTL) inverters are widely applied in various occasions. The inverters are generally switched to the current-controlled mode to limit the fault currents when short-circuit faults happen. Under symmetrical fault, the inverter can be controlled as a symmetrical, three phase current source. However, the voltage limiting could happen under asymmetrical fault. As a result, the fault phase currents will be distorted and cannot be completely controlled. The switching devices could be damaged while the setting design and reliable action of the protection devices will be also greatly affected. Considering the voltage limiting is mainly caused by large load impedance of healthy phase under asymmetrical fault, the voltage limiting is avoided by reducing the load impedance by means of the virtual impedance in parallel with the output filter capacitor in this paper. As a result, the current limiting loop is not broken and the fault phase currents can be effectively controlled. In addition, the implementation of virtual impedance in control system and the influence of virtual impedance on the stability of system are also analyzed in detailed. The theoretical results are validated by experiments.