Trip Circuit Supervision

Introduction: Trip circuit supervision relays (TSR) are used for monitoring of healthiness of Circuit Breaker trip circuit. During any fault in Power system, circuit breakers have to interrupt the fault currents. But if trip circuit is not healthy CB will not operate and fault will not be cleared. This will lead to operation of Breaker Failure Protection, causing larger disturbance in Power system and major damage to equipment. Click here for more information on Breaker Failure Protection (BFP).

Post close supervision: When the system is normal working condition the CB will be in closed state. In case any fault is detected by protection relays, it issue tripping command to CB. Trip circuit should be healthy so that CB is tripped without any failure.

Pre close supervision: When the CB is being closed from open state, the voltage will extend to uncharged portion. There may be existing fault in the system being charged which will lead to operation of protection relay. The CB trip circuit should be in healthy state so that any fault is cleared without ant delay.

The typical schematic is shown here

TSR relay is connected in series with trip coil. Trip coil is high energy coil (Low resistance, high current), due to fast operation required for CB. TSR relay is low energy coil (High resistance, low current). 

For example, resistance of CB trip coild is 55 ohm, operating voltage is 110V. Pickup current for CB tripping is 2A. TSR relay has resistance of 1100 ohm. Its pickup current will be low 100mA. All the relays are designed for pickup up at 60-70% os rated voltage / current. Therefore TSR will pickup at ~70mA).

As shown above, during normal condition when CB is close, both coils will be in series. Current through TSR will be 95mA. When tripping command is issued to CB, Post-close TSR will get bypassed. In this case, after CB is opened 52a will open and 52b will close. Now, post-close TSR will get de-energised and pre-close TSR will be energised. 

if any one relay (pre-close / post-close) is energised TSR alarm relay should be in energised condition. If both (pre-close and post-close) relays are de-energised TSR alarm relay gets denergised and alarm is generated to operator. A time delay of ~200ms is given to avoid false alarm during CB close / open operation.