PE review Q 48

[yaoyaodes]

Can anyone help me with this question? I am not sure if I understand the tap setting correctly. I am not quite sure what is restraining winding tap setting. I am assuming that because the two currents (4.8A and 8.3A) on the two sides of the relay are not equal, so there is another CT to make the 8.3A equal to 4.8A? if that's the case, can we step up the 4.8A to match 8.3A?

 

[xnuke]

There isn't another CT, only taps to adjust the way the two currents oppose each other in the restraint coils. Think about it this way: the operating coil shouldn't see much current under full load, so the taps selected need to balance the currents through the restraint coils. With one current at 4.8 A and its tap set at 5 A and the other at 8.3 A at full load, the other tap should be set slightly above 8.3 A so the op coil sees no current. Hence, 8.5:5 is the best answer.

Honestly, I hate this question, since:

• it applies to electromechanical relays, which you don't see much of any more, and the ones that are in service already have their taps set
• it uses delta-connected CTs on the wye side of the transformer, which isn't normally done any longer, as with modern digital relays you simply use wye-connected CTs and tell them the transformer connections and they do the math

 

[yaoyaodes]

There isn't another CT, only taps to adjust the way the two currents oppose each other in the restraint coils. Think about it this way: the operating coil shouldn't see much current under full load, so the taps selected need to balance the currents through the restraint coils. With one current at 4.8 A and its tap set at 5 A and the other at 8.3 A at full load, the other tap should be set slightly above 8.3 A so the op coil sees no current. Hence, 8.5:5 is the best answer.

Honestly, I hate this question, since:

• it applies to electromechanical relays, which you don't see much of any more, and the ones that are in service already have their taps set
• it uses delta-connected CTs on the wye side of the transformer, which isn't normally done any longer, as with modern digital relays you simply use wye-connected CTs and tell them the transformer connections and they do the math

Thank you so much for explaining. That helps a lot.