Primary and Backup Protection in Power System

In the previous article on “zones of protection in power system“, we see that in order to ensure the safety of the power system, it is divided into different zones. For each zone, there is an appropriate protective scheme. In this article, we will learn about two types of protection in a power system i.e. primary protection and back up protection. Let us understand both types of protection.

Primary protection

The relays used in primary protection are called primary relays. The primary relays of a zone are responsible for isolating the faulty component in the event of a fault occurring in that zone. The primary relay serves as the first line of defense. If the primary relay fails to operate then back up protection is used to isolate the faulty component from the system.

Back up protection

When the relay used in primary protection fails to operate then back up protection is used to clear the fault. The relays used in back up protection are called back up relays. It is important to note that backup relays are independent of factors that might cause primary relays to fail to operate. In order to give the primary relay sufficient time to operate, a backup relay operates after a time delay. In the event that a back up relay is operated, a large part of the electrical system will be disconnected from the local power source and it cannot be avoided. The back up relay serves as the second line of defense.

Types of back up protection

There are three types of back up protection i.e.

  1. Remote back up
  2. Relay back up
  3. Breaker back up

1. Remote back up

As the name implies, there is a backup relay located at a nearby station. The function of remote back up is to back up the primary protective scheme (equipment such as relays, circuit breaker, CT, VT, bus bar, etc.) in case of a failure of the primary protective scheme. It’s the most cost-effective and easiest type of back up protection. This is a widely used backup protection system for transmission lines.

2. Relay back up

In this scenario, an additional relay is used to provide backup protection in the form of a local backup. When the primary relay fails, the additional relay trips the same circuit breaker, and this operation occurs immediately. It is desirable that additional relays operate according to different principles than the primary protection. The additional relays should be powered by separate current transformers and potential transformers. The relay back up protection is costly. The relay back up protection is recommended in situations where a remote backup is not feasible.

3. Breaker back up

This serves also as a backup on a local level similar to relay back up. Generally, this type of backup is utilized for bus bars that are connected to a number of circuit breakers. In the event that a protective relay operates in response to a fault, but the circuit breaker does not trip, it is treated as a bus bar fault. This is the case when it is necessary to trip all other circuit breakers within the bus bar. If the appropriate circuit breaker fails to trip in the specified time period after its trip coil is energized, the main relay closes the contact of a back-up relay, which trips all other circuit breakers on the bus.

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