Trip Circuit Supervision

Dependability of protective system in electrical network is the ability to detect abnormal operating conditions and successfully isolate the fault by closing trip circuit through its trip contact. One of the major concern for isolation of fault by tripping of appropriate circuit breaker is the healthiness of whole trip circuit.

To ensure the trip circuit remains intact and monitor the health of trip circuit, trip circuit supervision relays are installed in protection panels. Before understanding the internal circuitry of TCS (Trip Circuit Supervision) relay & how it works, let us see a basic trip circuit consisting of control voltages, trip coil, IED/relay trip contact and circuit breaker auxiliary contacts. 

52a and 52b are Circuit Breaker NO and NC auxiliary contacts respectively. 52a follows the position of circuit breaker whereas 52b is opposite the position of circuit breaker.

Trip Circuit Supervision Relay (Internal Circuit & Working)

Basic internal circuit of a TCS relay consists of three coils may be named as K1, K2, K3 and their NO/NC contacts as shown below:

Coil K3 de-magnetizes if both K1 and K2 coils remain de-magnetized for a certain time interval.

TCS relay connections with trip circuit is also shown in below scheme:

For a healthy trip circuit, K1 or K2 or both the coils remain magnetized and their parallel NO contacts will magnetize K3 coil. As long as K3 remain magnetized, there is no fault alarm on annunciator panel due to open NC contacts of K3.

The two coils K1 and K2 are used to continuously look after the trip circuit for all conditions as listed below:

1. When circuit breaker is ON
2. When circuit breaker is OFF
3. When circuit breaker is tripped and relay trip contact is closed
4. When circuit breaker is ON and relay trip contact is closed (during tripping)

We will now understand the working of TCS relay coils for all above conditions one by one in detail.

    1.     When circuit breaker is ON

In this condition, auxiliary contacts 52a and 52b are closed and open respectively. Terminal A1 of coil K1 is directly connected to positive supply of trip contact at terminal X and negative supply to A2 of coil K1 is passed through trip coil and 52a (terminal Y), both in series as shown in below circuit diagram. Coil K2 remains de-magnetized as there is no negative supply to its A2 at terminal Z.

This condition reflects:

• Positive supply to trip contact is intact
• Trip coil is healthy (unbroken)
• Negative supply to trip coil is intact
• Auxiliary contact 52a is functioning properly

If any of the above fails, K1 will be de-magnetized which will cut supply to K3 and after a certain delay, there will be alarm on annunciator panel i.e., “Trip Circuit Faulty”.

     2.     When circuit breaker is OFF

In this condition, auxiliary contacts 52a and 52b are open and closed respectively. Terminal A1 of coil K1 is directly connected to positive supply of trip contact at terminal X and negative supply to A2 of coil K1 is passed through trip coil and 52b (terminal Z) and coil K2, all in series as shown in below circuit diagram. Both the coils K1 and K2 will remain magnetized during this state.

This condition reflects:

• Positive supply to trip contact is intact
• Trip coil is healthy (unbroken)
• Negative supply to trip coil is intact
• Auxiliary contact 52b is functioning properly

If any of the above fails, K1 and K2 will be de-magnetized.

     3.     When circuit breaker is tripped and trip contact is closed

In this condition, auxiliary contacts 52a and 52b are open and closed respectively. Terminal A1 of coil K1 is directly connected to positive supply of trip contact at terminal X and supply to terminal A2 of coil K1 is also positive, it will not be magnetized. Terminal A1 of coil K2 will have positive potential (terminal Y) due to closed trip contact and negative supply to A2 of coil K2 will be passed through trip coil and 52b (terminal Z), both in series as shown in below circuit diagram. Only coil K2 will remain magnetized during this state.

This condition reflects:

• Positive supply to trip contact is intact
• Trip coil is healthy (unbroken)
• Negative supply to trip coil is intact
• Auxiliary contact 52b is functioning properly
• Circuit breaker is tripped correctly when relay operated

If any of the above fails, K2 will be de-magnetized.

      4.     When circuit breaker is ON and relay trip contact is closed (during tripping)

 In this condition, auxiliary contacts 52a and 52b are closed and open respectively. Terminal A1 of coil K1 is directly connected to positive supply of trip contact at terminal X and supply to terminal A2 of coil K1 is also positive due to closed trip contact, it will not be magnetized. Terminal A1 of coil K2 will have positive potential (terminal Y) due to closed trip contact but there will be no negative potential at A2 of coil connected with terminal Z due to open 52b as shown in below circuit diagram. During this state both K1 and K2 will remain de-magnetized.

If both K1 and K2 are de-magnetized, there will be no supply to K3 but it will be de-magnetized after a time delay. If any of K1 or K2 is magnetized before time delay ends, supply to K3 will be restored and there will be no fault alarm.

This condition monitors the correct and well-timed tripping of circuit breaker, if the circuit breaker takes longer time to trip (more than the time delay of TCS relay) or fails to trip, TCS relay will pass fault alarm to annunciator panel.

For first three conditions, K1 or K2 or both will be in series with trip coil but does not magnetize trip coil, this is because K1 and K2 coils are designed to draw a very low current which is in-sufficient for trip coil to operate.

About the Author

Salman Younus has over 6 years experience in the field of High Voltage Grid Stations. He received his Bachelors degree in Electrical Engineering from NED University in 2011. He started his career with K-Electric in 2013 and is since associated with it. He also holds Masters degree in Power Systems from NED University.