Lecture 26 :
Power Swing Detection, Blocking and Out-of-Step Relays
Introduction (contd..)
Out-of-Step Tripping Relay (contd..)
As shown in fig 26.4 the basic scheme consists of two modified reactance type units whose characteristics are set parallel to the system impedance characteristic SR with one on each side of line SR.
Consider an unstable swing PQ as shown in fig 26.2. It will cross the first unit at point X when it will pick up, and emerge from relay characteristic at . The two crossing, indicate that swing has crossed the impedance characteristic and hence is a loss of synchronism condition, leading to an out of step trip decision. The scheme will equally well pick up if swing movement was from Q to
. The scheme would also pick up even if the unstable swing is behind the relay.
For example, a swing from to . If the swing is far away from the line characteristic, then currents involved are quite less. It is then associated with very low power reversals which do not characterize unstable swings. To prevent tripping on such condition, the out of step relay is supervised by an overcurrent relay unit. If the current signal is below a preset value, it inhibits the tripping signal of out of step relay.
. The two crossing, indicate that swing has crossed the impedance characteristic and hence is a loss of synchronism condition, leading to an out of step trip decision. The scheme will equally well pick up if swing movement was from Q to
. The scheme would also pick up even if the unstable swing is behind the relay. 
to 
. If the swing is far away from the line characteristic, then currents involved are quite less. It is then associated with very low power reversals which do not characterize unstable swings. To prevent tripping on such condition, the out of step relay is supervised by an overcurrent relay unit. If the current signal is below a preset value, it inhibits the tripping signal of out of step relay. 
