Application of Relays – Relay Selection Method:

Application of various relays used in the power system network depends upon various factors like the amount of fault current, protection methods, etc. We know that there are different types of relays available like Over Current relays, Distance relays, and Differential relays, etc. All these relays have their own purpose and their usage is different.

How to choose the relay based on the requirement of the system? is the main criteria for any Electrical Engineer. So we will see the exact application of the relays to know which relay is required to meet our system requirements.

Let’s see the application of various relays:

1. Application of Over current relays:

Over current relays are used for both ground fault protection and phase fault protection. These relays selected based on the amount of fault current.

Where distance protection is costly, over current protection is used. Over-current protection is normally used as back up protection where the primary protection is provided with distance schemes.

Read various schemes of protection using Over current relays here.

Over Current and Earth fault Protection Schemes

2. Application of Distance relays:

Distance relaying should be considered when over current relaying is too slow.

Distance relaying generally used when

1. Where high-speed automatic reclosing is not necessary to maintain stability.
2. Short time delay for fault is permitted.

Unlike over current relays distance relays achieve selectivity on the basis of impedance rather than current.

Distance relay types- Application:

1. For Ground fault relaying:

The ground resistance can be so variable, a ground distance relay must be practically unaffected by large variations in fault resistance.

 Therefore, Reactance relays are generally preferred for ground relaying.

2. For Phase fault relaying:

• Short Length Lines: The Reactance relay is practically unaffected by arc resistance. For Short transmission lines, arc resistance is large compared with Line impedance.

• Longer Length Lines: In longer transmission lines severe synchronizing power surges frequently occurs. Mho relays are used for these lines as they occupy the least space in R-X diagrams compared to other relays.

• Medium Length Lines:  For medium length transmission lines Impedance relays are used. Arcs affect an Impedance relay more than a reactance relay but less than a mho relay.

3. Application of Differential relays:

Large transformers, generators, and motors are provided with Differential protection. Differential relay responds to vector difference between two or more identical electrical quantities, e.g. (I₁-I₂)

Read more about Differential Relay Protection here.

Differential Relay Protection