The probability of faults occurring on the lines is much more due to their greater length and exposure to atmospheric conditions. This has called for many protective schemes which have no application to the comparatively simple cases of alternators and transformers. The requirements of line protection are:

(i) In the event of a short circuit the circuit breaker closest to the fault should open all

other circuit breakers remaining in a closed position

(ii) In case the nearest breaker to the fault fails to open back up protection should be provided by the adjacent circuit breakers.

(iii) The relay operating time should be just as short as possible in order to preserve system stability with out unnecessary tripping of circuits.

Differential method is ideal for protection of lines but it is more expensive to use

as the two ends of the line may be several kilo meters apart and to compare the two currents, a costly pilot wire circuit is required. This expense may be justified by using less costly methods.

The common methods for protection are:

1. Time Graded Over Current Protection

2. Differential Protection

3. Distance Protection

Fig.1 shows the symbols indicating the various types of relays.

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