Control Circuit can be divided into following Circuits:
- DC Supply to the Breaker
- Closing Circuit
- Tripping Circuit
- Indication Circuit
- Spring Charge Motor Control Circuit.
DC Supply to the Breaker:
- There are different MCB’s in DCDB for different Feeders CP/RP. Let say , we want to wire up the Breaker for Feeder 1 . From DCDB MCB 1, Two wires , one for positive (1J1) and one for Negative (1J2) will be brought to the Relay Panel. The prefix 1 in the ferrule stands for Feeder 1.
- In RP, these wires will go to the incoming of Fuse and Link . After Fuse 1J1 Becomes 1K1 and after Link 1J2 becomes 1k2.
- In the Breaker, the Closing Coil and the Trip Coil 1 is given DC Through Source 1 and the Trip Coil 2 is supplied with DC Source 2. Each Tripping coil is supplied with different DC Source for if one one DC fails, other Trip Coil can be operated. However, most Substations, do not have two different DC Sources and so , what we do is, we loop thetwo DC Circuits in the Breaker.
- If there is any Electrical problem in the Breaker, the first hing we, sould check is, wether there DC to the Breaker. First we will check DC on the TB between 1K1 and 1K2. If there is no DC there, we will check DC at the outgoing of the DC MCB. If there is no DC at Outgoing of the DC MCB, we will check if there, is DC at the incoming of the DC MCB.
- If there is no DC at the Outgoing of the DC MCB, but there is DC at the incoming of the DC MCB, it implies that, DC MCB is faulty and needs replacement.
- So, if the Brekaer is not Closing or Tripping elctrically and the TCS is also faulty, we need to check the Braker DC First.
Closing Circuit is used to Close the Breaker Electrically. Some components used in Closing circuit are: Closing Coil, Closing Resistor, Auxillary Contact, Antipumping Relay, TNC Switch, LR Switch, etc…. . Lets see how is the sequence of operation for Closing a Breaker Electrically:
- A continous negative is given to one end of the Closing Coil. To Close the Breaker, we need to extend, the Positive to the other end of the Closing Coil.
- Positive can be extended to the Closing Coil, either Locally or Remotely.
- Closing the Breaker, by extending Positive through TNC Switch in the Breaker is called as Local Closing. In the Breaker, when the LR switch is kept on Local Mode, positive gets extended to the TNC switch. When the TNC Switch is kept on “C”, the Positive is further extended to the Closing Circuit.
- After the Local TNC Switch, there is an NC Contact of the Antipumping Relay and the Coil of the Antipumping Relay is placed in Parallel with the Closing Coil. When a Closing Command is given, the AP Relay gets energised, its contacts open, breaking the Closing Circuit and thus protects the Closing Coil and also prevents the Hunting effect in the CB. For more details on Antipumping Relay in CB Click on link : Antipumping Relay
- After the NC Contact of the AP Relay, we have a Spring Charge Contact, which Closes when the Spring is charged and so the Positive can be extended futher.
- After the Spring Charge Contact, we have NC Auxillary Contact called as 52b, which opens up, once the Breaker is closed, therby breaking the Closing Circuit and protecting the Closing Coil from any continous coming from somewhere to the Closing Coil. Then there is a Closing Resistance , followed by the Closing Coil.
- Now the Breaker can also be closed, through the Control Room using the TNC Siwtch in the Control Panel, which is called as Remote Closing. When we carry out the Breaker Maintaince, we need to ensure that the Breaker cannot be closed remotely. Because, say, if someone is working, on the Breaker, and the Breaker is closed remotely, there can be an accident. So, we need to take an interlock in the Remote Closing Circuit, that the Breaker cannot be closed remotely, when the LR Switch is kept on Local Mode for Breaker Maintaince purpose.
- So the Positive from the Control Panel TNC Switch, first goes to the LR Switch. When the LR Switch is in the Remote Mode, the Positive (Called as Closing Positive K5) is extended to Control Panel/Relay Panel, where there are some more Interlocks, primarily of Master Trip Relay (86) and TCS (195ABC & 295ABC) relay. The Interlock is such that the Breakr cannot be closed, when the Master Trip relay is operated or the Trip Circuit is Faulty.
- If all the above Interlocks in the Remote Closing Circuit are followed, the Closing Positive is extended to the TNC Switch in Relay Panel in the Control Room. Now, when we put this TNC Switch to “C”, the Breaker gets closed, provided, all the interlocks in the Breaker like the Spring Charge Interlock is ok.
Breaker not Closing Locally:
If the Breaker is not Closing we need to take following actions:
- We need to first find out , if its an Electrical Problem or a Mechanical Problem. To find this out, what we do is, we remove the DC supply of the Breaker and try to close the Breaker Mechanically. If the Breaker Closes Mechanically, its an Electrical problem, otherwise its a Mechanical Problem. Lets assume, that the Breaker is closing Mechanically and thus its an Electrical Problem.
- First thing we need to do , is to check , if there is proper DC Supply to the Breaker. So if, we connect, Multimeter between K1 and K2, we should get 220V DC (We have assumed that Breker Control Supply is 220 V DC)
- Now, we are sure that, there is some break in the Closing Circuit. It will be very difficult to find this break physically. But we know one fact that a continous Negative is given to the closing coil. If the Closing Circuit is ok and Spring is Charged, this negative will get extended through the Closing Circuit, till one end of the Local TNC Switch. But if there is a Break, negative wont get extended till one end of the Local TNC Switch. Now, to check the negative, we need either Earth or Positive Voltage.
- The Breaker should be in the OFF Position. We will hold Red probe of the multimeter at K1 (Positive) on the TB and keep it constant there. Now, we will touch the Black Probe of the multimeter to K21 on TB or incoming of the NC Antipumping Contact and check the voltage. If there is no voltage between K1 and K21, there is break somewhere in the circuit.
- Now, keeping Red Probe at K1, we will touch the Black Probe of the Multimeter at the Outgoing of the NC Antipumping Contact (at K15). Lets say there is Voltage between K1 and K15. Now in the previous step we saw that the negative was not getting extended to K21 but the negative is getting extended to K15 and there is only NC Antipumping Contact between K21 and K15. So, it means that NC Antipumping Contact is Faulty and needs replacement.
- Now, say if there is a Voltage between K1 and K21. But still the Breaker is not closing. There can be two problems. Either the TNC Contact is Faulty or the Positive is not getting extended to TNC Switch.
- Thus, for troubleshooting, purpose, the closing circuit can be divided into two circuits viz. tyhe circuit before the TNC Switch where we have to get Positve Voltage and after the TNC Switch, where we have to get Negative.