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This paper proposes a four-stage fault protection scheme against the short-circuit fault for the high-power three-phase three-wire combined inverter to achieve high reliability. The short-circuit fault on the load side is the focus of this paper, and the short-circuit fault of switching devices is not involved. Based on the synchronous rotating frame, the inverter is controlled as a voltage source in the normal state. When a short-circuit fault (line-to-line fault or balanced three-phase fault) occurs, the hardware-circuit-based hysteresis current control strategy can effectively limit the output currents and protect the switching devices from damage. In the meantime, the software controller detects the fault and switches to the current controlled mode. Under the current controlled state, the inverter behaves as a current source until the short-circuit fault is cleared by the circuit breaker. After clearing the fault, the output voltage recovers quickly from the current controlled state. Therefore, the selective protection is realized and the critical loads can be continuously supplied by the inverter. The operational principle, design consideration, and implementation are discussed in this paper. The simulation and experimental results are provided to verify the validity of theoretical analysis.
Date of Publication: Aug. 2012