Blown Fuse on Capacitor Bank and Unbalanced Voltages 1

[djs6588]

I recently asked some questions about capacitors in general and got some answers to really help clear things up but I have another question. I was told by a linecrew putting up a new transformer that the secondary seemed high, was between 126 to 127V. It is normally a blown fuse or stuck switch on a capcitor bank so I went out to check a 3 phase 600 kvar cap bank nearby and two of the oil switches are in and one has the handle hanging down. I take the bank out of service, recheck the voltage and it is down around 122 volts. I was just curious what causes this large unbalance if one of the phases is opened and the other two phases are online. Normally this bank would give a 2 maybe 3 volt rise but since one of the oil swithces was stuck open the rise was higher. If anyone doesn't feel like answering and just has some reading material I have no problem trying to read through and figure it out on my own but it seems the only thing I find when searching is how to protect cap banks or how they should be wired, not what happens when one fails. I appreciate any help.

 

[jghrist]

Are the capacitors connected grounded wye, ungrounded wye, or delta?

 

[PHovnanian]

What is upstream of this bank? Voltage regulators, single phase or three phase? What sort of regulator(s)? How far away?

 

[djs6588]

It is a grounded wye bank and I forgot to mention this is the first step of a 3 step bank all 600kvar grounded wye. The step with the blown fuse was the first step and im not sure why but a lot of our cap banks with several steps have the second step then first then third. So upstream there is a cap bank about 200 ft and downstream there is a cap bank about 400 ft. There are no voltage regulators on this feeder and this is fairly close to end of the feeder.

 

[PHovnanian]

Its possible that the kVAR imbalance on the circuit could be interacting with the line's zero sequence reactance to shift the neutral point. This would cause a voltage rise on one or two phases and a drop on the others.

The other possible cause would be a delta tertiary in the substation transformer circulating the kVAR imbalance and causing a rise on all phases.

How (on which phases) was the voltage rise measured?