Distance or impedance relays
Distance relays are those in which the operations are governed by the ratio of applied voltage to current in the protected circuit. It is also called Impedance relay. In this the torque produced by a voltage element opposes the torque produced by a current element. The relay will operate when the ratio V/I is less than a pre-determined value.
Fig.13 illustrates the basic principal of operation of an Impedance relay. The voltage element of the relay is excited through a potential transformer (P.T.) from the line to be protected. The current element of the relay excited from a current transformer (C.T) in series with the line. The portion AB of the line is the protected zone. Under normal condition the impedance of the protected zone is ZL. The relay closes when the impedance of the protected zone falls below the pre-determined value ZL. When a fault occurs at F1 in the protected zone the impedance Z will be less than ZL and hence relay operates. If the fault occurs beyond the protected zone (at F2) the impedance Z will be greater than ZL and the relay does not operate.
There are two types of distance
(i) Definite distance relay, which operates for fault up to pre-determined distance
from the relay.
(ii) Time distance relay in which time operation is proportional to the distance of fault
from the relay.
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Written by arjun on March 16th, 2009 with
1 comment.
Read more articles on classification of relays and Power System and Relays and Switchgear&Protection.
#1. August 2nd, 2011, at 4:36 AM.
content as it is copied from V k Mehta book