Big change in current draw on either side of a capacitor bank

[MikeinOK]

I am in the business of selling high voltage test equipment and am trying to help out a customer. The customer is a small municipal utility. They have two places where there are capacitor banks installed on a 4160v feeder. In both places they are getting widely varying readings on either side of the capacitor bank. I went out and verified this with two different meters.

Here are the readings from one feeder.

Phase A Upstream 42.2A +.04PF Downstream 5.2A +.71PF
Phase B Upstream 41.9A +.35PF Downstream 15.4A +.85PF
Phase C Upstream 39.7A -.37PF Downstream 15.9A +.85PF

Any clues what would cause this? Any idea how to confirm it and fix it?

Thanks in advance for anyone who can help.

 

[collies99]

Are these Wye connected or delta ? I am assuming wye connected because the cause below would work for a delta bank.

Is there a detailed 3-line cap bank configuration diagram you can post ?

The two banks are of different sizes judging by the phase currents.

Upstream phase C may have a cap or two with a blown fuse, thus drawing less current. Same reasoning for downstream phase A but in a big way, perhaps 2 of 3 groups per phase of the cap bank are not energized.

I am guessing that individual caps are not tested, just current load readings taken.

 

[collies99]

Make that "the cause would NOT work for a delta bank"

 

[waross]

Turn the capacitors off. They are grossly oversized. You have a fairly good power factor on the downstream side. The addition of the caps is taking a normal power factor and pushing it way into the leading quadrant. The increase in current upstream is the capacitor reactive current drawn by the caps. With a properly sized capacitor bank, the upstream current will be less and the power factor will be improved instead of being pushed to very low values.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Is there a problem or is the customers power system just unequally loaded.

 

[waross]

Phase A Upstream 42.2A +.04PF Downstream 5.2A +.71PF

I think that the OP's issue is the difference in incoming and outgoing current in each phase, not the unbalance between phases. The power factor going from 71% to 4% and the current increasing by a factor of 8 is an indication of over-correction.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pwrtran]

Information is not clear -
What is the bank size?
What do you mean upstream and downsteam?
Does the downstream mean the load side of the tapping connection point of the cap bank?
Or, it means the second cap bank at downstream of the first bank?

 

[waross]

I'm guessing about 300 KVAR or a little less. Possibly 275 KVAR or 250 KVAR if the voltage is a little low.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeinOK]

Sorry, I should have been clearer. By upstream and downstream I meant the supply side and load side of the capacitor. As far as the size of the cap bank, I don't know. This is a very small municipal utility that doesn't employ an engineer. The longest time employee has been there 25 years and he said that the cap banks were already there when he started. It is in an area which has a lot of oil and gas drilling and production so approximately 10 - 20% of the total load in the town is inductive motors on drilling rigs, pumps, compressors etc. so the power factor fluctuates all over the place.
The general consensus seems to be that the capacitors are oversized and that makes sense with the available data. I am going back to see them late this week or early next week and I will see if they can take the cap bank out of service and read the current again.

 

[MikeinOK]

Oh and thank you very much everyone for your input. I'll come back and let you know what I find out.