Secured Zone 3 Operation Against Fault Induced Delayed Voltage Recovery Event

Prolonged voltage level reduction caused by the fault induced delayed voltage recovery (FIDVR) event may lead to a decrease in distance relays security. Therefore, some distance relays are prone to mal-operation under severely prolonged depressed-voltage conditions, and their security should be considered in the FIDVR event studies. The present paper investigates the possible operation of distance relay during the FIDVR event and proposes an auxiliary protection algorithm to effectively prevent its mal-operation during short-term dynamic voltage transient. The proposed algorithm estimates the critical time and voltage level at which the impedance trajectory arrives in Zone 3 of the critical relays using the rate of change of the impedance trajectory, relay margin index, and time-variant NERC/WECC severity indices. Then, the critical voltage is compared with the expected voltage level at the estimated critical time calculated by piecewise cubic spline interpolation (PCSI) method and three consecutive voltage phasors to decide about the block of relays which entering the impedance trajectory to their Zone 3 is probable during the FIDVR event. The efficacy of the proposed method is demonstrated by simulating some short-term voltage transient scenarios on the IEEE 39-bus test system.