NCEES Power #140 Practice Exam

[jnspark]

Can anyone provide an explanation for this problem?  I presume it's something to do with the ratio, but what is Vng?  Is this simply asking for what voltage (line to neutral) is necessary, in KV, on the primary to trip?

Gotta say, stuff like this is not making me look forward to Friday.

 

[Drewism]

The relay setpoint is 5V as given in the problem. The relay is tied to a transformer. The transformer turns ratio is N1/N2 or V1/V2. So, the turns ratio is 120.

The secondary voltage would be 5V whereas the primary voltage would be 5V*120 = 600V. Therefore the contacts in the relay will change state when 600V is applied on the high side. 600V = 0.6kV.

The fact that i's a WYE connected capacitor bank or the fact that it's neutral to ground seems irrelevant.

 

[jnspark]

So not a relay problem so much as a transformer problem?

I guess what got me was the Vng part.  Thanks.

 

[BirdGrave]

Yea, whenever you see the vocabulary of "Vxy" it means the voltage from point x to point y.  You can literally trace the points from N to G along the line and see that is just the voltage on the other side of the transformer.  

Hence N1/N2 = V1/V1 = 120/1 = VNG/5