When there is a sudden and large change in the power system i.e., due to removal of a fault or a sudden change in the load, there will be a change in the phase angle between the induced voltage on the line.
Consider a transmission line between two generating stations. The current flowing through the transmission line depends upon the phase difference between the voltage generated at the two ends of the line. The phase difference is equal to the rotor angle.
But due to disturbances in the transmission line, the rotor angles swing and it changes the rotor angle. Thus there will be a change in phase difference, thereby changing the current flowing through the transmission line.
This phenomenon is known as a power surge or power swing. The performance of different relays during power surge is explained as follows. The figure given below will explain the characteristics of some important distance relays and power surge on the R-X diagram.
The impedance seen by the relay during a power swing will not be the true value of impedance, but the power surge seen by the relay appears like a fault changing its distance from the relay location. Hence, the relay will give a false operation. From the figure, it is clear that the greater the area of the relay characteristics on the R-X diagram, the greater will be the period under the influence of power surge.
Mho relay is the least affected as its area on the R-X diagram is small. The impedance relay with directional unit characteristics has more area than the mho relay but is lesser compared to that of the reactance relay. The effect of power surges on impedance relay is greater than that of mho relay and lesser than that of reactance relay.
As the area of reactance relay occupying on the R-X diagram is large, it is largely affected by the power surges compared to other types of distance relays.
Therefore, the influence of power surges on reactance relay is greater than the influence of power surges on impedance relay is greater than the influence of power surges on mho relay. Hence, from the above comparison, we can say that the mho relay is less sensitive to the variations due to power surges and hence more stable.
Therefore, to overcome the malfunctioning of distance relays during a power swing, the angular limits of its pickup characteristics should be reduced so as to enclose only the faulty area. The above conditions can be obtained by using an elliptical relay or by using a rate of rise of the current monitoring relay.