Primary Protection

Below is the power system protection scheme which is designed to protect the power system parts and components. As shown in below fig, each line associated with over current relay that protect the lines from faults. So, if a fault happens on any line, it will be cleared by its relay and circuit breaker. This is called primary or main protection and acts as a first line defender. The service record of primary relaying is very high with well over 90% of all operations being correct. But this is not always the case, sometimes faults are not illuminated by the primary or main protection system i.e. circuit breaker and relay system because of trouble within the relays, circuit breakers or wiring system in different conditions. In those conditions, Secondary or backup protection system does the required job.





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Primary protection may fail due to the following reasons

Failure of DC supply to the tripping Circuit

Failure in relay operating current or voltage

Failure in circuit breaker tripping mechanism

Failure of main protective relay operation

Failure in the wiring of relaying system

Failure of CTs or PTs operation

Secondary Protection

Back-up protection is very important for stable and reliable power system

As we know that, it is not possible to design a 100% secure and efficient system because there are possibilities of failure in the connected CTs, PTs, circuit breaker etc. in the system. If it happens, then it will destroy our whole switching system.

If the primary protection operation falls into trouble, then secondary protection disconnects the faulty part from the system. Moreover, when we disconnect primary protection for testing or maintenance purpose, then secondary or back-up protection will act as primary protection. In the above fig, relay “X” (1 Sec time setting) provides backup protection for each of the four connected lines to the main bus.

In addition, a larger part is disconnected then when primary relaying functions correctly. Therefore, greater emphasis should be placed on the better maintenance of primary relaying which is economical.





Types of Secondary or backup protection