Time grading principles:-
The principle is as shown in figure 1.4. The current is measured at various points along the current path, e.g. at source, intermediate locations, consumer end. The tripping time at these locations are graded in such a way that the circuit breaker nearest the faulty part operates first, giving primary protection. The circuit breaker at the previous station operates only as back-up. The tripping time at station C, B & A are graded such that for a fault beyond C circuit breaker at C operates as a primary protection. Meanwhile, the relays at A and B also may start operating but they are provided with enough time lag, so that the circuit breaker at B operates only if the circuit breaker at C does not.
Fig. 1.4 Back-up Relaying by Time Grading
In this form of protection, the important protective devices (protective transformers, protective systems, relays, circuit breakers, auxiliaries etc.) are duplicated. Both primary and back-up protections are provided at the same station and are arranged to operate at the same speed i e as fast as possible. Such protection is costly and the cost is justified for protection of EHV transmission, bus-bars, large generators, large transformers etc. If the cost of separate circuit breakers is not justified, the same circuit breaker with two independent trip coils can be employed, one for each protection. Sometimes the Main & Back-up protections are based on different principles of operation e g differential & over-current, so that if the main protection fails to sense the fault, the back-up protection does not fail to do so.
The merits of Duplication Back-up principle are:
Fast and almost simultaneous fault clearing, improved stability.
Complete reliability can be assured.
It should be economically justified.
Monitoring: - Monitoring means checking the performance. Monitoring is used as an alternative to duplicate protection. It is a continuous process of monitoring instrument transformers, relays, circuit breaker, trip circuit & other equipment of primary protection. The monitoring devices continuously switch ‘in’ & ‘out’ and determine whether the component is in working order. The monitoring is achieved by means of high frequency signals. It is also used in protection of transmission lines by means of power line carrier telemetering.