The trip circuit supervision relay is one of the most important parts of the protection scheme. It is mounted in the Control and Relay panel in a substation. To isolate a fault in the power system, the protective relay senses the fault and gives command to the trip coil of the associated circuit breaker. Now, if the circuit breaker’s trip coil has some internal fault or there is some internal wiring fault in the trip circuit of the circuit breaker then the whole protection idea to isolate the fault via operation of the circuit breaker may fail.
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To prevent this situation and to always supervise the trip circuit of the circuit breaker, a trip circuit supervision relay is used. Unlike other auxiliary relays, the trip circuit supervision relay remains always energized in healthy circuit conditions and only de-energizes when there is a fault in the trip circuit. Unlike other relays, the trip circuit supervision relay is supplied with negative DC and the positive remains conditional as we will see in the later discussions.
CONSTRUCTION:
It has three coils A, B & C along with resistors for each. A & B coils have associated NO contacts which are used for the energization of coil C. The coil C has NO and NC contacts which can be used in the closing circuit of the circuit breaker and for annunciation of the healthy and unhealthy trip circuit. The supervision function is based on the low current 3mA injection principle.
OPERATIONS OF TRIP CIRCUIT SUPERVISION RELAY:
The trip coil of the circuit breaker has auxiliary contacts 52a, and 52b. 52a being NO contact and 52b being NC. The circuit breaker has two operating conditions pre-close and post-close. Preclose is the condition before closing the circuit breaker, which means tripped condition. Post-close is the condition after the closing of the circuit breaker.
PRE CLOSE CONDITION:
In the pre-close stage, the auxiliary contacts of the circuit breaker 52a being NO and 52b being NC conduct the negative DC supplied to the trip coil to coil B of the TSR and coil A. Since coil A and B are in series both get energized and their associated NO contacts close, which energizes coil C of TSR.
As the coil C of TSR gets energized, its NO contact closes and gives an annunciation that the trip circuit is healthy. The NO contact of the C coil is often taken for interlocking in the closing circuit of the CB. It means that if the TSR is unhealthy/de-energized then the C coil’s NO will be non-conducting and hence closing of the CB will not take place.
In pre-close condition both the coil A and B of the TSR get energized as they are in series.
POST CLOSE CONDITION:
In the post-close condition, the auxiliary contacts of CB, 52a the NO contact, will be conductive and 52b the NC contact will be non-conductive. In this condition, the negative DC from the trip coil will reach coil A of the TSR and will get energized. As soon as coil A of TSR energizes, the NO contact of coil A will become conductive and will energize coil C of the TSR. The coil C being energized will give annunciation of a healthy trip circuit via its associated NO contacts.
NB: In post-close condition coil A of TSR and a trip coil of CB is in series but the trip coil will not get energized because of the voltage drop across coil A and its resistors. The trip coil also has resistors and will not get energized until a positive DC pulse is directly fed to the Trip coil via master trip relay 86.
CONCLUSION:
Thus, the trip circuit supervision relay 95 is engaged in continuous supervision of the trip circuit of the circuit breaker. Unhealthy indication of TSR means either:
- Short circuit of the control supply.
- Open circuit of the control supply.
- Loss of supply voltage.
- Failure of the trip coil.
FAQ’s
What is the trip circuit supervision relay?
It is a relay which supervises the healthiness of the trip circuit of a circuit breaker. As the trip circuit of a circuit breaker has to be healthy always to isolate the faults via tripping operation. It is therefore essential to keep a check on the trip circuit.
What is 52a and 52b?
52a and 52b are the NO and NC contacts of the circuit breaker’s auxiliary switch.
What does trip circuit mean?
It is a circuit through which the trip signal from the master trip relay reaches the trip coil of the circuit breaker which initiates the tripping operation of the circuit breaker.
Is 52A normally open?
Yes 52a is normally open contact when the circuit breaker is in open or tripped condition. 52b is normally closed contact.
Is the trip circuit supervision relay always energized?
Yes, it is always energized when the trip circuit is healthy and is de-energized only when there is some fault in the trip circuit.
What is the ANSI code for a TCS relay?
The ANSI code for a TCS relay is 95.
How many trip circuit supervision relay required for a circuit breaker?
For circuit breakers having one mechanism box two trip coils are required and hence two trip circuits and two TCS relay. For circuit breakers with seperate poles and mechanism 6 nos. of trip circuit supervison relays are required. 2nos./pole.
How to know if the Trip circuit is unhealthy?
The coil A and B or coil A of TCS relay will always be energized during preclosed and post closed condition of the breaker which actuates the coil C of the relay and via no contact of the relay which becomes closed gives a healthy indication. When the coil C of the TCS relay gets de-energized, it indicates that the trip circuit is unhealthy. The healthy indication led will be missing in this case.
