Pre Closing Trip Supervision | Post Close Trip Supervision:
Pre closing supervision and post-closing supervision circuits are used to sense the trip coil healthiness. Trip coils are nothing but an electromagnetic coil which is used to trip the circuit breaker during the fault condition. So in order to check the condition of the trip circuit is essential in power system protection. Therefor trip circuit supervision relays are implemented to check the trip coil conditions. Also each high voltage circuit breaker consists of two trip coils. So we should implement two number of TCS circuits. However, in this tutorial, we are going to see what is pre close trip circuit supervision and post close trip supervision circuit.
These type of supervision relay circuits are implemented in high voltage circuit breaker, in case medium voltage circuit breaker the TCS circuit is very simple and you can check it here.
Circuit diagram of Pre-close supervision and Post close supervision:
The circuit diagram consists of the following components.
- Two sensing relay coils Ra and Rb and Two NO of each coils
- One TCS coil
- One breaker NO (Normally Open) and One Breaker NC (Normally Closed). These two contacts are used to get the breaker status (either circuit breaker is open or close)
- One TCS NO Which is connected to the annunciator or alarm circuits
- One Breaker Protection Contact
Trip circuit supervision relay working principle:
The circuit consist of two relays Ra and Rb and both are connected as shown in the figure. The Qno and Qnc are the breaker status contact, which means It will be act as NO and NC at breaker is in open condition and The Qno become nc when the breaker is in closed condition.
Trip contact from the protection circuits: It is a NO contact of the protective relays; the relay operates when the fault on the circuit hence NO become NC.
Ra and Rb both relays are connected as shown in the figure. Ra relays connects in series with the Qnc and parallel with the Qno. And the Trip contact NO from protection circuit is connected in the input terminal of Qno. All these arrangements are connected in series with Rb coil.
Pre close supervision:
Pre close supervision means, sensing the trip coil before circuit breaker closed condition, which means checking the healthiness of the Trip coil during circuit breaker open condition. It is called pre close supervision.
Consider Now breaker is in open condition, since the current flow through Rb, Ra and Trip coil.
Therefore, two NO of Ra and Rb will become in NC condition, due to the negative of the relay Ra and Rb will be extended using trip coil. Incase if the trip coil is in open circuit, negative of the relay Ra and Rb will not be extended, since the NO of Ra and NC will be in Normal position, then TCS circuit activates the annunciator.
Note: The resistance of the Ra and Rb coil will be high to reduce the current flow in the circuit. Because flow of high current may lead to operate the trip coil.
Post close supervision:
Post close supervision means sensing the trip coil after closing of the circuit breaker or breaker closed condition. In this, the breaker contact Qno would be NC and Qnc would be NO, Since the Ra relay coil does not carry any current. Therefore, Rb coil takes the responsibility of trip coil supervision.
Now current flows from Rb and TC (Trip Coil).
Therefore, Rb no become NC since the TCS circuit consider as trip coil in healthy condition. In case of, if the TC become open, then the Rb NC become NO, the TCS circuit activates the alarm circuits.
That’s all about Pre-close supervision and Post close supervision circuits.