The protective relaying scheme is used in a power system network to protect the system. And the relaying scheme is an adequate part of the power system.
The protective relaying equipment provides protection and is classified into two types.
- Primary Protection
- Backup Protection
The primary protection is responsible for protecting all the power system elements. It is used to protect equipment from all types of faults. The primary protection scheme is the first line of defence.
A backup protection scheme comes into the picture when the primary protection system fails to operate.
The primary protection system is also known as the main protection scheme.
When the primary protection system is in maintenance, the backup protection acts as the main protection.
The backup protection system is arranged in such a way that the failure in main protection will not lead to failure in backup protection.
There are the following reasons to fail in primary protection;
- Failure in circuit breaker
- Failure in protective relay
- Loss of supply to the relay
- Failure in tripping circuit
- Failure in DC tripping voltage
- Failure in measuring devices (CT/PT)
The backup protection is located at different stations from the primary protection in most conditions.
For economic conditions, the backup protection is only used for the protection against short circuits and not for any other abnormal conditions.
The primary protection scheme is designed to protect the components of a power system and is the first line of protection.
Each line has a relay that protects the line. If a fault occurs in any line, it will sense by the relay connected in that line. And the relay will send a signal to a circuit breaker and disconnect lines.
The above figure shows the basic connection of the power system network. And this forms the primary or main protection serves as the first line of defence.
The primary protection is highly recommended to operate at first. But due to some abnormal condition, it failed to act. Under this condition, backup protection will come into the picture and disconnect the faulty section.
Backup protection is the second line of defence. If primary protection fails to operate, the backup protection works to disconnect the faulty section.
The backup protection is designed to operate with sufficient time delay, so the primary protection scheme will have enough time to operate.
In the above figure, relay-A is used as a backup relay for all lines. For example, if a line fault is not cleared by its relay and breaker, the relay-A and group of breakers will operate after a substantial time delay.
It will clear the entire group of lines. Hence, it is clear that when the backup protection operates, a more significant portion of the power system will disconnect. So, it is better to operate primary protection in most cases.
Concept for Backup Relaying
Consider a transmission line section as shown in the figure below.
The relays A, B, I, and J are backup relays, and the relays C, D, G, and H are primary relays.
Between stations K to L, relay-E is a primary relay. If the primary relay E fails to trip, backup relays A and B will trip. And it will disconnect line-1 2 and 3.
If primary relay E operates, it will disconnect line-3 only; therefore, it is highly recommended to operate primary protection only.
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