The distance relay is suffering from under-reach in its performance. This is because of some transient disturbance, which may happen when the power flows and swings. Besides that the power swings as a consequence of some fault cases. In this paper a new distance relay has been suggested and modeled for detecting stable and unstable power swings.
This relay is known as out of step relay (OOS). The method employed is to measure the rate of change of the positive sequence impedance of the relay which supervise the trip signal that comes out of the distance relay. It allows the distance relay to work, if it detects an unstable power swing or a fault. On the other hand it blocks it if the power swing is stable. The new relay have been modeled and simulated using MATLAB-SIMULINK software programs.
The performance of this relay has been tested on an equivalent two sources system. The results prove that the suggested relay is active and fast. Thus overcoming the limitation of the distance relay in case of under-reach. The results also show that the new relay has the ability in discriminating between the types of faults, that occur during power swings.
Key Word: Power swing, Distance relay , Out of step protection technique.