Why do we need Trip circuit Supervision relay?
A trip circuit supervision relay as the name indicates , its function is of supervisory nature. A trip circuit supervision relay monitors healthiness of tripping circuit and indicate whether the circuit is healthy or unhealthy so that preventive action can be taken accordingly. All faults are detected by protective relays and signal is sent to circuit breaker to open using tripping circuit and therefore trip circuit should always remain healthy.
In a protection system, the tripping of circuit breaker is very important. If there is some issue in a trip circuit ,a fault would not be disconnected and this fault would have to be cleared by another up-stream protections in the power system and may cause other issues. The supervision relay is used to monitor health of trip coil continuously and it gives an alarm/indication in case there is issue with the coil.
Usually there are two trip coils in a circuit breaker and for each trip coil, a trip circuit supervision relay is provided. Thus for C.Bs with two trip coils two supervision relays 195 and 295 are used.
Trip circuit Supervision relay Drawing
The drawing shown below is of supervision relay circuit of TC (trip coil)-01.
In the drawing explained here , it has been mentioned that +Ve DC supply in the C&R Panel for closing/tripping circuit is taken from the circuit breaker itself and the same has been also shown here. Please observe in the above drawing the contact 3 and 4 of 86 relay is a NO contact, during occurrence of any fault it becomes NC and extends +Ve supply to Trip Coil which trips the circuit breaker.
From ferrule no K607 , Trip circuit Supervision relay receives -Ve supply through the trip coil. If the trip coil is not healthy or there is some issue in wiring between circuit breaker placed in the switchyard and Control & relay panel placed in the control room building, supervision relay will not get -Ve voltage from the coil and thus it will operate and an indication will be displayed on the concerned panel that the trip circuit is not healthy.
The operator in the control room will get this information and then further action can be taken on this. In some panels both healthy and unhealthy indication is provided.It is also to be noted that if supervision relays get tripped ,the breaker cannot be closed from the remote (T&C switch of control panel). In electric drawing of closing circuit , +Ve supply given by moving T&C switch to close position passes through all protective and supervision relays and if any of the relay has operated , the closing coil will not get the positive voltage and thus breaker will not close.
Thus its important that supervision relays needs to be reset or the problem in the tripping circuit needs to be rectified then the closing circuit will normally work.
L227 and L228 are the ferrules being used to indicate healthiness of trip coil-01 and 02 respectively on the window annunciator installed on control panel.
