19-09-2017, 04:37 PM
This is also a special type of overcurrent relay with directional characteristics. This directional overcurrent relay uses the relay drive principle when the fault current flows in the relay in a particular direction. If the power flow is in the opposite direction, the relay will not operate. Normally, the conventional overcurrent relay (without direction) will act for the fault current in any direction. The directional overcurrent relay recognizes the direction in which the fault occurs, in relation to the location of the relay. The principle of directional protection is as follows: Consider an XY feeder, passing through station A. The circuit breaker in feeder AY is provided with a directional relay R, which will trip the CBy circuit breaker, if the fault power flow is in the AY address alone. Therefore, for faults in the AX feeder, the CBy circuit breaker does not trip unnecessarily. However, for faults in the feeder AY, the circuit breaker CBy is triggered, due to the address characteristic of the relays, configured to act in the direction AY. This type of relay is also referred to as the reverse power relay, with respect to the direction of the fault current flow (power). Reverse power flow relays with directional characteristics not only detect the flow of direction, but also measure the magnitude of the power flow. Directional Relay Connections Whenever a near or near failure occurs, the voltage becomes low and the directional relay may not develop enough torque for its operation. In order to obtain sufficient torque on all types of faults, regardless of the locations with respect to the relays, the relay connections must be modified. Each relay is energized by the current of its respective phase and voltage. One of the methods of such connections is connection 30o and another is connection 90o.