Emergency Zone 3 Modification as a Local Response-Driven Protection Measure Against System Collapse

This paper proposes a protection scheme that applies two supervisory zones on each distance relay in the transmission system and timers in order to avoid undesirable Zone 3 operations under voltage-stressed system conditions. The speed at which the apparent impedance trajectory crosses the two supervisory zones is measured by the relay in order to determine the rate of change of the apparent impedance. Zone 3 modifications based on the normalized rate of change of the apparent impedance are applied: 1) Zone 3 operating area is reduced by using circle or straight line blinders when the instability is clearly recognized; 2) Zone 3 is blocked when there is no enough time to identify the cause of the apparent impedance transition; and 3) no measure is applied for a fault or a power swing. Simulations performed in a test power system show the effectiveness of this method in mitigating system blackouts due to cascaded outages.