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Voltage sags mitigation with feeder fast-tripping overcurrent protection

Since voltage sags are mainly caused by distribution system faults, it is rational to design the sag mitigation schemes from the point of relay protection performance. This paper evaluates the voltage sag mitigation capability of definite-time overcurrent protection by introducing the protection characteristics into the CBEMA curve and analyzing the essence from the point of energy balance. The result proves the validity of sag mitigation with overcurrent protection. It is also showed that the only effective improvement measure is the available extension of the instantaneous relay protection zone. To achieve it, a non-selective fast-tripping overcurrent scheme is proposed. Cost analysis and case simulations demonstrate that the transient voltage quality can be improved considerably without any cost increases using the novel scheme.