Using Logic Controls in Cascaded Events Prevention for Pilot Distance Relay Zone Three

This paper presents a new method to prevent cascaded events of pilot distance relays caused by zone 3 elements in case of heavy loaded transmission line or during load encroachment by using logic controls. Undesired operation of distance relay may be leaded to activation of another distance relays and cascaded to partial or total blackout in the end. A proposed adaptive distance protective gives a new concept to distinguish between faults and heavy loaded transmission line depending on analyzing system parameters and data transfer between distance devices, and also secures time to make repaired action like load shedding. Logic controls are used to insure the decision to trip or not, through observing the system parameters and compare its values with setting already input to the system. When the distance relay pickup and the logic controls find the system is healthy, that means the resulted pickup came from transmission line overload not from actual fault. Consequently the trip signal is blocked and protects the system from cascaded mal-operation, or send load shedding signal to reduce the current flow through this line.