The protection is organized to provide a ring of security around each and every element of power system. If there is any fault associated within the ring the relays associated with it must trip all the allied circuit breakers so as to remove any faulty element from rest of the power system. This ring of security is called ‘Zone of protection’.
A. Faults that occur within the zone are termed as Internal Faults and that occur outside the zone of protection are termed as external faults .
The farthest point from the relay which is still inside the zone of protection is called reach point and the distance between relay and reach point is called reach.
If the adjacent zones do not overlap there will be some portion of the power system which is left out unprotected.
The relay should allow normal load as well as a certain degree of overload to be supplied. So the pickup value of the relay should be more than maximum load. At the same time, the relay should be sensitive enough to respond to the smallest fault. So, pickup value should be less than the smallest fault current.
Naturally when a fault occurs it is sensed by both the primary and backup protection. As the operating time of primary relay is less than that of the backup relay, it will operate first. There will be an overshoot time .
A Definite time over current relay (DTOC) can be adjusted to trip out at a definite amount of time, after it picks up. It has a time adjustment setting and pickup setting.
The operating principle of IDMT relay is based on the idea that more severe a fault is, the faster it should be cleared to avoid damage to apparatus
A. Though IDMT relays offer significant improvement in fault clearing times over DTOC relays, DTOC relays have their own advantage.
Consider a system where .
Since the fault current as a function of fault location is proportional to , it would remain more or less constant throughout the length of feeder. Therefore, the IDMT relay will not be of much help. So DTOC relays can be used for lines which are short in length.
Directional relays are used mainly in single end fed system of parallel feeders, ring main feeder system etc.
Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.
