Power Transmission and Distribution Interview Questions: All practical Questions with Quiz at the end !
Circuit Breaker:
Answer:
K23A & K53A is used for Post Close Supervision for Trip 1 & Trip 2 Circuit correspondingly.
When the Breaker is ON and if the Trip circuit is ok, they should have negative Voltage (coming from the Closing Coil) .
Current Transformer:
Answer:
The Secondary Winding of R, Y and B Phase CT’s are mostly Star connected and then connected to the Relay/Meter due to following reasons:
- CT Secondary Earthing, which is very important for Safety, is possible with Star Connection and not with Delta Connection.
- Metering is easier with Star Connection than Delta connection, as no is required. In Star Connection, Line current is equal to phase current and so the current flowing through the Meter will reflect the Line Current of the System. As the Meter measures exact CT Secondary Current, there is no conversion required. Let say if we use a Delta Connection for CT Secondary, it will reflect the phase current of the system. So we have to calculate the phase current to match the line current. The phase current is equal to root three times of line current. Let’s say if the Line current is 100 A, the Phase Current will be 173 Amps approximately.
- Since all the Protections are directly set with Line current, it is very easy to do the relay Settings for a Star connected CT.
Answer:
While carrying out CT wiring, if the Metering Core gets interchanged with the Protection Core, being a Metering Core, it will get Saturated during Fault and will not provide any protection.
The Main Function of the Metering CT is to carry Full Load Current and so It is designed in such a way so that it gets saturated during faults,
Once it is Saturated, it does not follow CTR i.e. it will not give correct reflection of the Fault current in the secondary side and so the relay will not operate and Fault will not get cleared.
Answer: 20 A.
Since Site CTR (CT Connected Ratio) is 800/1, Load current in secondary will be 40/800= 0.05A.
Now, since the Meter CTR (CTR set in the Meter) is 400/1, 0.05A will be reflected in Meter as 0.05 x 400 = 20 A.
This shows that if Meter CTR is half the Site CTR, it will measure only half the current.
Or we need to apply Multiplying Factor, which is calculate by the formula:
MF= (Site CTR x Site PTR)/ (Meter CTR x Meter PTR) = 2.
where we have assumed that Site PTR is same as Meter PTR.
Distance Protection:
Answer:
Carrier Aided Distance Protection clears Zone 2 Fault instantaneously instead of waiting for 350ms, by using Signaling Channel.
PUR or PUTT scheme of Carrier Aided Protection is used in MSETCL.