Time Current Relay Application:
Time Current Relay Application – Overcurrent and earth fault protective gear can be made discriminative by grading the operating times of successive devices. The pickup currents are adjusted in such a way that the protection nearest the fault operates in a shorter time than the protection in the succeeding section towards the power source.
On feeders each relay backs up the one in the next section further from the power source so that the Time Current Relay Application characteristics of the backup relay should be intermediate between the characteristics of the relays on each side of it. Naturally the scheme is inflexible insofar as system development is concerned, and when ultimate system arrangement is unpredictable its use may ‘prove to be false economy.
Time Graded Protection with Overcurrent Relays:
The time interval necessary between successive relays is governed by the following factors:
- Fault clearance time of circuit breaker.
- Finite contact gap to ensure nonoperation.
- Overtravel of the relays.
- Relay and CT tolerances.
The fault clearance time of modern circuit breakers may be of the order of 0.08 second but it is advisable to allow 0.15 second. The residual contact gap necessary to ensure that the next relay nearer the power source does not operate is represented by a time interval of 0.1 second. The overshoot or overtravel of relays is less than 0.05 second. Tolerances are covered by a further margin of 0.1 second. This gives the total time allowed for the grading of successive relays as 0.4 second.
The time interval for discrimination, with any graded scheme, is usually taken as approximately 0.5 second. Where separate earth-fault and phase fault relays are used they are graded independently.
As overcurrent relays are single input relays substituting the threshold conditions in the general Eq. (4.8) we get