Sensitivity enhancement of backup protection for high impedance transformer phase-to-phase faults

During the relay protection setting calculation of high impedance transformer, the sensitivity of the overcurrent protection with composite start-up voltage and impedance protection of high impedance transformer often can not meet requirement. According to an setting calculation example in actual project, this paper analyzes the principle, ratio characteristic curve, calculating method of action current and brake current, action criterion, relay protection setting calculation, the sensitivity when the transformer three sides winding capacity ratio is 100/100/50, the sensitivity when transformer in parallel operation or individual operation, and time delay of the incomplete differential protection. The results show that the high impedance transformer configured with incomplete differential protection can improve the sensitivity effectively and the principle is simple and easy to be applied. This paper also points out the method when the incomplete differential protection used in the direct-ground system to pre