Relay Testing Kit – Theory

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Protective relay is a device which maintains the power system equipments and gives an output under an abnormal condition to ensure protection of the system. The basic objective of the protective relay is to isolate the faulty section in a power system as quickly as possible so that rest of the system functions normally. It is necessary to ascertain security, reliability, quality, sensitivity and speed of relays.

Various tests are recommended in National and International Standards to check the withstand capacity of a relay during fault conditions. During testing, protective relay is made to undergo simulated field conditions to check its performance. The relevant National / International standards recommend many tests to ascertain the relays effectiveness to perform its intended function.

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A Relay receives the input signals from CT’s/ PT’s and its program calculates the fault conditions as per application. It issues a trip command to the Circuit Breaker on sensing a faulty condition. On the basis of application, Relay may be classified into below types:

Step 1:

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Relay Testing Kit injects fault in form of I & V on respective CT & PT points of Relay under test . The Timer of Relay Testing Kit starts by initiating  fault injection.

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Step 2:

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Relay senses the fault & issues the trip command, its trip timing must depend on the time characteristics of relay settings.

Step 3:

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Relay Testing Kit receives the trip command from Relay & Stops the timer.

Step 4:

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The measured tripping characteristics are compared with respect to expected tripping as per settings by Engineer.

The basic connection of protection relay has been shown in the picture.  The secondary of current transformer is connected to the current coil of relay. The secondary of voltage transformer is connected to the voltage coil of the relay. Whenever any fault occurs in the feeder circuit, proportionate secondary current of the CT will flow through the current coil of the relay due to which mmf of that coil increases. This increased mmf is sufficient to mechanically close the normally open contact of the relay. This relay contact actually closes and completes the DC trip coil circuit and hence the trip coil is energized. The mmf of the trip coil initiates the mechanical movement of the tripping mechanism of the circuit breaker and ultimately the circuit breaker is tripped to isolate the fault.

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 Types of Relay as per various requirements:

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• Differential Relay

• Over Current Relay

• Frequency Actuated Relay

• Directional Over Current Relay

• Restricted Earth Fault Relay

• Over Fluxing Relay

• Distance Schemes Relay

• Bus bar Protection Relay

• Reverse Power Relay

• Loss of excitation Relay

• Negative Ph. Sequence Relay

• Energy Meters Relay

• Transducers Relay

• Fault Playback Relay

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