What is a protection relay?
A relay is an automatic device that detects an abnormal condition of the electrical circuit and closes its contacts. These contacts, in turn, close and complete the circuit breaker trip coil circuit, thereby causing the circuit breaker to trip to disconnect the faulty part of the electrical circuit from the rest of the undamaged circuit.
Now let’s talk about some terms related to protection relay.
Activation signal detection level:
The value of the activation quantity (voltage or current) is on the threshold above which the relay begins to be actuated.
If the value of the activation amount is increased, the electromagnetic effect of the relay coil is increased, and above a certain level of the actuation amount, the relay moving mechanism just begins to move.
The value of the current or voltage below which a relay opens its contacts and returns to its original position.
Relay operating time:
Immediately after exceeding the activate quantity level, the displacement mechanism (e.g. a spinning disc) of the relay begins to move and finally closes the relay contacts at the end of its travel. The time that elapses from the time the actuated quantity exceeds the removal value to the time the relay contacts close.
Relay reset time:
The time that elapses between when the actuation amount becomes less than the preset value and when the relay contacts return to their normal position.
A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The control impedance in the relay is a function of the distance in a distance protection relay. This impedance or the corresponding distance is called the relay reach.
Power system protection relays can be classified into different types of relays.
The types of protection relays are mainly based on their characteristics, logic, actuation parameters and operating mechanism.
Based on the working mechanism, the protection relay can be classified as an electromagnetic relay, solid-state relay and mechanical relay. In reality, a relay is nothing more than a combination of one or more open or closed contacts. All or some specific relay contacts change state when activation parameters are applied to the relay. This means that open contacts become closed and closed contacts become open. In an electromagnetic relay, these closings and openings are made by the electromagnetic action of a solenoid.
In the mechanical relay, the closing and opening of the relay contacts are done by mechanically moving different gear level systems.
In the solid-state relay, it is mainly carried out by semiconductor switches such as the thyristor. In the digital relay, the activated and deactivated state can be called state 1 or 0.
Based on the characteristic, the protection relay can be classified as
Definite time relay
Inverse time relay with defined minimum time (IDMT)
IDMT with inst.
Characteristic in steps.
Voltage limitation on the current relay.
Based on the logic, the protection relay can be classified as follows:
Restricted earth fault.
Reversed power relays.
Loss of excitement.
Negative phase sequence relay, etc.
Based on the activation parameter, the protection relay can be classified as
Power relay etc.
Depending on the application, the protection relay can be classified as follows:
The primary relay or primary protection relay is the first line of protection of the power system, while the backup relay is only used when the main relay does not operate in the event of a failure. Therefore, the backup relay is slower in action than the main relay. Any relay may not be used for one of the following reasons:
The protection relay itself is defective.
The DC voltage supply to the relay is not available.
The tripwire from the relay panel to the circuit breaker is disconnected.
The trip coil in the circuit breaker is disconnected or defective.
Current or voltage signals from CT or PT respectively are not available.
Since the backup relay only operates when the main relay fails, the backup protection relay should not have a point in common with the primary protection relay.
Some examples of mechanical relays are-
OT (travel oil temperature)
WT trip (winding temperature trip)
Bearing temp travel etc.
Water level controls etc.
Pole mismatch relay.
List Different protection relays are used to protect equipment from different power systems