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.

Breaker Failure Protection

Intruduction: Breaker Failure Protection (BFP) is also known as BFR (Breaker Failure Relay) or LBB (Local Breaker Backup) protection.

The idea behind BFP is that in case of any protection operation the Circuit Breaker (CB) should interrupt the fault current within specified time (Normally < 60ms, depending on voltage level). There may be cases where CB is unable to interrupt the current doe to any of the following causes:

1. CB tripping circuit problems
2. CB operating mechanism problems
3. CB interrupter problems

In the first two cases, the interrupter is not at all operated. Therefore there is no risk of prolonged arc inside interrupter, Only risk is continuous feeding of fault current may lead to disturbance in grid operation.

In the third case, due to prolonged arcing inside interrupter, temperature may go high and it may explode due to high pressure buid-up. Normally porcelain enclosures are used for SF6 filled interrupters. Failure may lead to cracking of porceline into pieces, which are dangerous for men working the vicinity of CB. Further SF6 gas will be released to atmosphere, which is not good for environment. In addition, risk of disturbance in grid operation is already there.

Working Principle: The BFP protection is activated when tripping command is issued to CB. If after a pre-set time (typical 200ms), current is still flowing through CB, BFP will operate. Operation of BFP will isolate the concerned CB from all sides by tripping all the CBs connecting to it. For example in the image shown below, following will be the isolation logic:

BFP operated for	CB to be opened
CB-1	CB-2, CB-4, CB-11
CB-2	CB-1, CB-3, CB-11, CB-21
CB-3	CB-2, CB-6, CB-21

There are two methods used for operation of BFP:

1. Current level detection: If current through CT is above set value (typical value 100A) after pre-set time, relay interprets that CB has failed to interrupt the current.
2. CB status from auxiliary contacts: If CB status as per auxiliary contacts (52a) is still closed, relay interprets that CB has failed to open.

First method is more reliable and works for all the three cases mentioned in introduction, therefore used in most of the cases. Only drawback is there must be some significant amount of current flowing for operation of BFP.

Second method does not work for third case (CB interrupter problems). It is used where lower level of current, which is not significant of operation of relay, is also dangerous for health of equipment / person working in the vicinity,

The working principle can be understood from image below: