capacitor bank arrangement

[144x]

Hi,imagine a double star ungrounded medium voltage capacitor bank connected together by unbalanced CT in their neutral.the star arrangements are not of equal size.one having 2 *180 KVAR and the other 3*180 KVAR units at each phase.while the contractor could use 150 KVAr unit capacitors to make it balanced he refuses to do so and insists on his design.Is there any hidden problem in this design?
thanx for your consideration

 

[elektroeng]

Hi,

The purpose of the CT in the starpoint is to detect malfunctioning capacitors due to an unbalance current flowing in the starpoint. Installing an unbalance from the beginning makes this detection method useless.

Either take out the CT in the starpoint or ship out your contractor and find one that does what you ask him to.

Good luck !!

 

[supermacc]

144x: Are you referring to a grounded single star bank or an ungrounded double star (wye) bank. If it is a single bank, then I agree with elektroeng but in case of a double bank the CT may actually be able to detect unbalance. However, there may be other unbalance protection relay installed that will not work. Please see

http://pm.geindustrial.com/faq/documents/general/shunt.pdf

for a good tutorial.

Regardless of single or double, I agree with elektroeng that the design appears to be a bad idea, this is not the way things are usually done. Why is your contractor insisting on this design? I am aware of one case where the customer wanted more capacitance in one phase due to that the voltage was lower compared to the two other phases. We refused to design such a bank and recommended the customer to correct the underlying cause for the unbalance. In the end, we found out that one of the CCVTs was faulty so there was never any voltage balance.

 

[supermacc]

EDIT: I now see that the OP mentions that it is indeed a double ungrounded wye bank.

 

[waross]

The contractor is probably saving money. I imagine that 5 x 180 KVAR banks are cheaper than 6 x 150 KVAR banks. There is possibly an additional saving in labour. If the same hardware is used to mount both banks, you may have room to add another 180 KVAR bank if needed in the future.
Under normal conditions including some line to line voltage imbalance there should be no difference between the voltages at the star points of the capacitor banks and no current. If any capacitor in any bank fails, the resulting imbalance will cause a current to flow between the capacitor banks. This current may be used to alarm a faulty capacitor.
respectfully