![]() ![]() ![]() In our daily life, almost all the items we used, being a computer, television, lift or vehicle we drive consist of some kind of electrical or electronics component inside. Under an internal fault condition, the two currents at both the ends are no longer same, this causes circulating current flow through pilot wires and Under normal working condition, the two currents at both ends are equal and pilot wires do not carry any current, keeping relays inoperative. The pilot wires are used to connect the relays. Pilot wire schemes for feeder protection In differential protection scheme, the current entering at one end of the line and leaving from other end of the line is compared. It is good practice to locate a fault any where among different sections of the feeders and check whether that particular section only is isolated without disrupting the power flow in other sections. ![]() It can be observed that relays placed near the bus connecting the sources, don not have any directional feature, where as the rest of the buses, respond to a direction always away from the source. A similar concept of discrimination is also utilized in the below given ring main feeder and a feeder fed from both the sides. Similarly, the relays placed at the source side do not require any directional element. Application of directional relays to parallel feeders It may be seen from the below given parallel feeders that the relays placed at the load side of both the lines use directional element which respond to a direction away from the bus bars. Different types of feeders employ the over current protection along with the directional relay so that proper discrimination of an internal fault is possible. It is customary to have two elements of over current and one element of earth fault protection system in the most elementary form of protection of three phase feeders. ![]()
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