Trip Circuit Supervision In Switchgear
Due to operator's negligence, a catastrophic failure of critical equipment in the installation may occur – even where 'TCH Check Push Button and a TCS Lamp' are present. What is the remedy?
A switchgear is a device or a combination of devices, primarily intended for the purpose of making, carrying and breaking currents in electric circuits, during normal conditions as well as during abnormal situations. Thus, in the event of an abnormality as that of a fault, detection and timely isolation of the faulty portion of the network is the most important function of any switchgear. So, in a switchgear installation, the trip circuit is very crucial.
What is the use, even if an installation has 'world-class' protection relays, auxiliary relays and even 'world-class' switchgear, if the circuit breaker trip coil is faulty and/or the wiring is loose and/or the power supply to the circuit breaker trip circuit is interrupted?
In the event that the trip circuit becomes unhealthy, either the fault remains unisolated, leading to catastrophic damages, or leads to the tripping of an upstream switching device, thereby causing unwanted power outage in a larger portion of the network.
The trip circuit of a switchgear may include the main protection relay, the master trip relay, relay contacts, switchgear auxiliary contacts, fuses, links, wiring, terminal blocks etc. With so many components, the probability of the trip circuit becoming unhealthy is quite high. Thus, it calls for a system to monitor the integrity of the trip circuit. This is known as Trip Circuit Supervision (TCS).
Principle of operation
The trip circuit supervision, in its simplest form, may consist of a 'Trip Circuit Healthy (TCH)' indication lamp, as shown in the figure below. To avoid any unwanted tripping of the breaker, in the event of any short circuit in the lamp, a series resistor (R) is added in the circuit.
This arrangement offers trip circuit supervision only when the breaker is in ON condition.
Trip circuit supervision scheme, when the circuit breaker is closed...
By incorporating a simple modification, the trip circuit supervision facility can be extended to the breaker OFF status also, as shown below:
TCS Scheme, when the circuit breaker is open or closed...
In both the above cases, the trip circuit supervision is continuous. Since this is only a local lamp indication for the trip circuit healthiness, an operator has to continuously be monitoring the glowing of the TCH lamp. This is not practically possible. Also, such a continuous supervision of the trip circuit, with resistors in the circuit, might lead to unnecessary drain on the sub-station battery capacity.
Hence, the circuit can be modified such that the trip circuit healthiness can be checked only 'on demand.' This is made possible by the addition of a 'Trip Circuit Healthy Check (TCH)' push button, with a normally open (N.O.) contact configuration. This is shown below:
Modified circuit with the TCH push button...
But, the problem with the above scheme is that the scheme would give only a local indication. Thus, it is suitable only for manned switchgear installations. Also, the operator has to check the healthiness of the trip circuit, regularly, by pressing the 'TCH Push Button' and ensuring that the TCH lamp glows, while doing so. If there is any dereliction of duty on the part of the operator, then even when the trip circuit is faulty, the alarming situation will not come to the notice of the concerned personnel at all.
I have witnessed a couple of cases, where in such installations (as above) with a 'TCH Check Push Button and a TCS Lamp,' there has been catastrophic failure of critical equipment in the installation, due to operator's negligence. That is, there were failures of trip coils of important breakers in the installation, but this did not come to the notice of any one, as no one had been following the practice to check the healthiness of the trip circuit regularly, by pressing the TCH button.
Trip Circuit Supervision (TCS) relay...
To avoid such an eventuality and also to have a remote alarm for failure of the trip circuit healthiness, a Trip Circuit Supervision Relay can be used.
Refer to the scheme below:
Trip circuit supervision scheme (with remote alarm), when the circuit breaker is open or closed...
In the above arrangement, when the breaker is ON, and if the trip circuit is healthy, the current path will be:
‘+ ve’, Fuse, ‘R’, ‘Coil A of the TCS relay, ‘52a’ contact, Trip Coil (TC), Link & ‘-‘ve.
Coil A of the TCS relay will energise and the normally open (N.O.) contact of this coil, wired in the path of Coil C of the TCS relay, will close. Thus, Coil C of the TCS relay will energise and open its normally closed (N.C.) contact that may be wired to the Trip Circuit Failure Alarm.
And, when the breaker is OFF, and if the trip circuit is healthy, the current path will be:
‘+ ve’, Fuse, ‘R’, ‘Coil A of the TCS relay, ‘R’, Coil B of the TCS relay, Trip Coil (TC), Link & ‘-‘ve.
Now, both Coil A & Coil B of the TCS Relay will energise and the normally open (N.O.) contacts of these two coils wired in the path of Coil C of the TCS relay, will close. Thus, Coil C of the TCS relay will energise and open its normally closed (N.C.) contact that may be wired to the Trip Circuit Failure Alarm.
Thus, when the trip circuit is healthy, no trip circuit failure alarm will be initiated. When the trip circuit is unhealthy, whether the breaker is in ON condition or in OFF condition, an alarm will be initiated.
Again, to avoid any unwanted tripping in the event of an inadvertent short circuit in any of the trip circuit supervision circuit components, resistors are added in series with coils A & B. Generally, these resistors are mounted separately, external to the relay. A very important point to note is that the trip circuit failure alarm supply should be independent of the tripping supply so that the trip circuit failure indication/alarm will be obtained, even in the case of failure of the tripping supply.
Such Trip Circuit Supervision (TCS) relays are intended for a continuous supervision of circuit breaker trip circuit, and to give an alarm for loss of auxiliary supply, faults on the trip-coil or its wires independent of the breaker position, faults on the breaker auxiliary contacts and faults in the supervision relay itself.
Numerical trip circuit supervision relay
The trip circuit supervision relay SPER 1C1 from ABB...
As in the case of any other protection relay and auxiliary relays, technology has evolved – today there are numerical trip circuit supervision relays available in the market place. The numerical TCS relay is designed for the supervision of trip circuits and other important control and monitoring circuits.
Fig. 1: A block diagram of a TCS relay...
Block diagram of the relay is shown in Fig. 1. The supervision function is based on a low-level (~ 3 mA) current injection principle. The injected current is sensed by two opto-couplers (one each wired across relay terminals 13-15 and across 12-11. The supervision function in the two steady states of circuit breaker-trip circuit (pre-close and post-close) can be seen from figures 2 & 3.
Fig. 2: Pre-close supervison...
Fig. 3: Post-close supervison...
The relay monitors the trip coil circuit continuously in breaker ON as well as in OFF conditions – and gives alarm in case of the following faults:
- Open circuit of trip coil or trip coil circuit
- Failure of trip coil supply
- Under-voltage condition of trip coil supply
- Failure of auxiliary supply
- Failure of circuit breaker mechanism to complete a trip operation
And, in most of the modern 'Numerical Protection Relays,' the trip circuit supervision function is in-built. Thus, there is no need for any external Trip Circuit Supervision (TCS )relay, either electro-mechanical or numerical.
It is hoped that the importance and criticality of the trip circuit supervision in electrical switchgear installations is well understood. It is wished that the practice of frequent, continuous, regular monitoring of the trip circuit healthiness is followed in all electrical switchgear installations.
The Author is Manager at Megawin Switchgear Pvt.Ltd.