Interaction of power transformers and capacitors

[Planguy]

Is there an inherent problem associated with having a bank of power factor correction capacitors on an otherwise unloaded power transformer? The particulars are:

12/16/20MVA, 69,000 volt delta - 6,640/11,500 volt wye transformer with resistance grounded neutral (11 ohms)
5,400 kVAR capacitor bank, ungrounded wye located in the substation.
No other load on the transformer.

 

[musicguy1800]

Why is the transformer necessary? Was the utility unable to find a cap bank that could operate at the nominal 69 kV voltage?

There are inherently large transients associated with cap banks, especially during switching. An EMTP simulation should be done to determine the severity of the transients in your situation, and therefore the likelihood of damage done to the transformer in question.

As a sidenote, the control cables and other cables in the vicinity of the cap bank should be shielded, with the shield grounded every ten feet or so to break possible induced current loops and ensure proper operation of the PT's and other equipment associated with your cap bank.

 

[busbar]

One rule of thumb for AC shunt capacitors is the their resonant harmonic order is equal to the square root of ratio of short-circuit MVA to capacitor MVAR.

For instance, with an 8%Z transfomer, resonant harmonic order figures about 5.3. Higher transformer impedance will move the resonant order undesirably closer to 1.

 

[Planguy]

Thanks for your comments. This would be a very temporary situation only for testing the new capbank installation. The load usually on this transformer has been transferred off so that it will not be lost if something causes the relaying to trip the bus. I was concerned that this might somehow be a ferroresonance problem.