The impedance relay measures the V/I ratio and the relay operates when it falls than a certain prefixed value. The tripping characteristics is simply a line on V/I diagram as stated below:
As, Z= R+jX, and if we draw the characteristics for Z having a fixed value on R & X plane,
we find it as circle with centre at origin as seen on the next figure:
In impedance relay effect of high ac resistance results in under reaching of the relay. A fault in zone 1 with high arc resistance may shift in the next zone increasing the relay operation time and hence the fault clearance time. The effect is sever if the fault occurs in the second half of the zone.
What is Power Swing?
When power system is operating in steady state, all the rotor or machine angles are constant. Whenever there are sudden and large changes of power in the system, the rotor angles undergo oscillations till the system reaches a new stable state. This phenomenon is known as power swing.
During power swing the apparent impedance enters the trip region of the relay slowly, whereas during the fault it enters suddenly The effect of power swing on impedance relays is moderate.
The relay is inherently non-directional. It needs directional element. The relay is suitable for moderate length of lines.
