- Inverse Time Relay Definition: An inverse time relay is defined as a relay where the operation time decreases as the actuating quantity increases.
- Operating Time Relationship: The relay’s operating time is inversely proportional to the magnitude of the actuating quantity, meaning higher quantities result in faster relay operation.
- Pick Up Value: The minimum actuating quantity required to initiate the relay’s operation is known as the pick-up value.
- Mechanical Accessories: Inverse time relays use mechanical accessories, such as a permanent magnet in an induction disc relay or an oil dash-pot in a solenoid relay, to achieve inverse time delay.
- Definite Time Lag Relay: This type of relay operates after a set time delay, independent of the magnitude of the actuating quantity, ensuring consistent operation time above the pick-up value.
In this type of relays, the time of operation depends upon the magnitude of actuating quantity. If the magnitude of actuating quantity is very high, the relay operation is very fast. In other words, the relay operating time that is time delay in the relay is inversely proportional to the magnitude of actuating quantity.
The general characteristics of an inverse time relay is shown are in figure below.
Here, in the graph it is clear that, when, actuating quantity is OA, the operating time of the relay is OA’, when actuating quantity is OB, the relay operating time is OB’ and when actuating quantity is OC, the relay operating quantity is OC’.
The graph also shows that if the actuating quantity is less than OA, the relay’s operating time becomes infinite, meaning the relay does not actuate. The minimum value of actuating quantity needed to start the relay is called the pick-up value, denoted as OA.
The graph indicates that as the actuating quantity approaches infinity, the operating time does not reach zero but instead approaches a constant value. This is the minimum time required to operate the relay.
The inverse time relay, where the actuating quantity is current, is known as inverse current relay.
In this type of relay, the inverse time delay is achieved by using specific mechanical accessories.
In an induction disc relay, inverse time delay is achieved by placing a permanent magnet so that when the disc rotates, it cuts the magnet’s flux. This induces current in the disc, slowing its movement. A solenoid relay can be converted to an inverse time relay by using a piston and an oil dash-pot. The piston, attached to the moving iron plunger, is immersed in oil, and when the relay is actuated, the piston moves upward with the plunger.
Viscosity of oil slows the upward movement of plunger. The speed of this upward movement against gravity also depends upon how strongly the solenoid attracts the iron plunger. This attraction force of the solenoid depends upon the magnitude of actuating current. Hence, time of operation of relay is inversely proportional to actuating current.
Definite Time Lag Relay
During relay coordination in electrical power system protection scheme, there is some time intentionally required, to operate some specific relays after some specific time delays. Definite time lag relays are those which operate after a specific time.
The time lag between instant when the actuating current crosses the pickup level and the instant when relay contacts finally closed, is constant. This delay does not depend up on magnitude of actuating quantity. For all actuating quantity, above pick up values, the relay operating time is constant.
