The relays are used to detect abnormal conditions like faults in the electrical circuits. If the healthy circuits and equipment are not isolated from the fault, it leads to disconnection of supply to a large portion of the power system. Thus a fault in the power system should be detected in the minimum time. So that the damage caused by it will be minimum and it can be repaired quickly to restore its service.
Hence the protection scheme employed in the power system should be able to clear the fault in the minimum time. There are two types of protection schemes employed in the power system networks, namely,
- Primary or main protection, and
- Back-up protection.
Primary or Main Protection :
The power system is divided into various zones of protection. For each zone, there is a particular protective scheme. If any fault occurs in a protected zone, it is the duty of the primary or main relays to act and isolate the faulty element. Primary protection is one, which immediately senses and responds to the fault. It will take an instantaneous action in order to isolate the faulty part from the healthy part of the power system.
Back-Up Protection :
If due to some reasons, primary protection fails, additional protection is generally provided called backup protection. The backup protection is the substitute for the primary protection and isolates the faulty section from the healthy one. The primary protection may fail due to any of the following reasons,
- The circuit breaker fails to operate
- The tripping mechanism of the circuit breaker fails
- The current or voltage supply to the relay fails
- Failure of CT or PT
- The main protective relay fails.
Back-up relays operate independently of those factors which cause primary relays to fail. It is the one that may sense the fault immediately. But, it should not operate instantaneously. It must operate only when primary protection fails to operate.
In case the main protection is made inoperative for the purpose of maintenance, testing, etc., the back up protection acts as the primary protection. For economic reasons and technical facts, backup protection is usually provided for short circuits only. Different types of backup protection are listed below,
- Remote back-up protection
- Relay back-up protection
- Bus back-up protection
- Centrally coordinated beak-up protection.
Remote Back-up Protection :
In this type of backup protection, the primary and backup protections are located at different stations and are desirably operated independently of the factors causing failure of primary protection. It is most widely used for the protection of transmission lines.
Relay Back-up Protection :
Both primary and backup protections are provided for the same circuit breaker. In case the primary fails, the backup trips the breaker without delay. Separate trip coils are provided for the same circuit breaker. The principles of operation of both primary and backup protection are different. This type of backup is provided where remote backup is not possible.
Bus Back-up Protection :
When a fault occurs on a system and the circuit breaker fails to trip, then the fault is called a bus fault. Such type of backup protection is provided with an appropriate time delay. Bus backup protection is used in case of a bus-bar system with a number of circuit breakers connected to it and all the circuit breakers connected to the bus are opened.
Centrally Coordinated Back-up Protection :
For the systems having central control, such back-up is provided that, main protection is at different stations and backup protection for all stations are at the central control centre. This type of protection is the coordination of protective relaying equipment, high-frequency carrier current equipment and digital computers.
The central control regularly monitors the load flow frequency and if any abnormal condition exists, it readily gets the information and takes the necessary action.
Example :
In the case of alternators, primary protection is provided to the stator, by percentage differential protection or by restricted earth-fault protection, etc. The backup protection is provided by over-current protection and earth-fault protection.
Of course, these backup protection schemes are not for the backup of the primary protection of the stator. Instead, they operate for faults when the corresponding protection system fails to isolate the fault. The figure below shows the operation of primary and backup protection schemes.
In the above figure, the tripping time constants of relays R4, R3, R2, and R1 are graded in such a way that if a fault occurs beyond the D breaker at point F, the relay R4 will operate as primary protection and isolate the faulty section by the operation of the breaker at D.
Now if for some reason the circuit breaker at D fails to operate the faulty section then it would be isolated by the operation of the relay R3 at C, R2 at B and R1 at A. The relays R3, R2, and R1 are graded with enough time lag sequentially. Thus, the operation relays R3, R2, and R1 to isolate the faulty section is due to backup protection.
