What is Trip Circuit Supervision (95) protection ? How to implement scheme using Numerical relays ?

In a protection system, the trip circuit of the circuit breaker is very vital as it isolates the faulty section. A failed trip coil or its circuit, during an event of actual fault, where that CB is commanded to clear the fault, will unable to isolate the faulted equipment / section, as results the fault remains in to the system. Hence, fault shall be isolated by the upstream protective relay.

The supervision function is particularly important, when there is only one tripping coil and CB tripping is vital such as Generator circuit breakers or any other important circuit breaker in distribution networks. The 95 is intended for a continuous supervision of circuit breaker trip circuit and to give an annunciation or alarm for he following conditions,

• Short circuit in Control supply (DC)
• Open circuit in Control supply (DC) / Loss of control supply voltage
• Loss of control supply voltage
• Failure of mechanism

For any of the above condition, the circuit breaker could not trip and isolates the faulty part while system has actual fault. To overcome such situation, Trip Circuit Supervision (95) becomes very necessary.

There are many method by which we can implement 95 protection scheme i.e., Post closing monitoring only; Both Pre-closing and Post closing monitoring .

 Typical protection scheme using Electro-mechnical Auxilary relay:

How to implement the TCS using Numerical Relay ?

CB status can be monitor using Binary Input (BI) of relay, since it is connected in series with the CB auxiliary contact and trip coil of CB. BI used to monitor the integrity of the CB trip circuit wiring. As soon as BI gets energized, it confirm the integrity of the auxiliary supply to CB trip coil through associated wiring, the same can be utilized for indicating the of Trip Circuit Healthy (TCH) lamp.

If mapped Binary inputs gets de-energised due to break or short circuit in the trip circuit wiring, the Binary output (BO) shall be configured / programmed to generates Annunciation / Remote alarm, and also used for interlocking in Circuit breaker closing circuit.

Supervision using single Binary Input (BI):

The above scheme is used for supervision of trip coil with CB open or closed condition. However, pre-closing supervision is not detected. Depending on the conditions of the trip contact and the circuit breaker, the binary inputs are energized, or not energized.

During normal operation, the binary input is activated when the trip contact is open and the trip circuit is complete, because the monitoring circuit is closed by either the 52a circuit breaker auxiliary contact (if the circuit breaker is closed) or through the bypass resistor R by the 52b circuit breaker auxiliary contact.

Only when the 86’s (master trip) trip contact is closed, the binary input is deactivated. Hence, pre closing or during master trip relay operated condition, supervision of trip circuit is not supervised.

TCS in Numerical Relay using dual / double Binary Input (BI)

This is little complex scheme, in the above scheme, two BI are used, one BI is connected in parallel to the master trip contact, and second BI is connected in parallel to CB auxiliary contacts i.e., 52a, 52b.

The above scheme is supervised the tripping circuit in both pre closing and post-closing of circuit breaker. Scheme not only detects interruptions in the trip circuit and loss of control voltage, it also supervises the response of the circuit breaker using the position of the circuit breaker auxiliary contacts.

Note : Delay in drop off need to be added for prevent false alarm due to voltage dip caused by faults in other section.

Conclusion:

Hence, The essential requirement of a Trip Circuit Supervision relay is that, to supervise the healthiness of the entire trip circuit comprising DC supply, the tripping cable, CB auxiliary contacts and the CB trip coil, or the tripping relay coil and the DC supply to which it is connected.

If there is any fault in the tripping circuit, both BI shall not gets energized and as result BO gets high and same can utilized for generating the ‘Trip circuit Faulty’ alarm and also prevent closing of the CB.

Reference:

• Relay manual of Alstom make VAX31 relay
• Relay manual of Siemens make 7SJ62 numerical relay
• Relay manual of Schneider make P14x numerical relay