Capacitor Tripping Problem

[robb1]

Hi all we have a 110kV star star Capacitor bank that is tripping on "out of balance" (There is a current transformer in the neutral) we have tested the capcitance of each leg and balnced the capacitance as best we can. Using an AVO LCR Meter (this method has worked for us in the past) The bank still trips when it is put back into service

Just a thought will another Bank in parallel (selected to the same bus) have any effect on this?? as the bank trips as soon as it is put in

 

[Skogsgurra]

robb1,

I do not think that I would use an AVO LCR meter to measure the capacitance of a 110 kV capacitor bank. The reason is that parasitic effects like test lead and contact resistance introduces errors that are hard to estimate and this would more often than not lead to false conclusions regarding actual capacitance values.

If a "real" capacitance tester is not available, you can use a transformer and a voltmeter plus an accurate clamp ammeter to check the value of each capacitor. Use at least 100 V at50 or 60 Hz (depending or where in the world you are) and be sure to disconnect any discharge resistors or reactors before taking the readings.

Check that V/A = 1/wC (w = 2*PI*f) and - if needed - disconnect/change capacitors that give wrong readings.

One caveat: Make sure that your mains does not contain too much harmonics. They will influence the measurements. But you can still use the method for comparative purposes if the THD is constant during the measurement.

And of course: make sure that the capacitors are fully discharged before starting anything!

 

[robb1]

Thank you for your quick reply we will try this
when you say a "Real" capacitance meter what would you class as this.. is there any other problems that could cuase this apart from the obvious out of balance as we have taken six sets of measurements and are pretty certain it is balanced

Thank you

 

[digitrex]

Please forgive my stupid questions. I am not familiar with this "110kV star star capacitor bank". I thought there is only one star-point.

1. You have one current transformer at the capacitor neutral point, then it should be located at the neutral grounding cable. It seems to me that the current transformer will pick up earth fault currents in the 110kV grid.

2. Is the capacitor's neutral point connected to neutral points of other equipment (eg transformers, capacitors etc) and connected multiple grounding pits?

 

[rbulsara]

Are you saying you have two capacitor banks in parallel and the second one trips when its put on line (while other is already engergized)??

 

[robb1]

rbulsara
yes this is true c11 (out of balance bank) can be in service and when c12 (the parallel bank ) is switched in c11 trips

but with c11 in service and then c12 switched in they both stay in service

we currently have a monitoring device hooked up to the secondary to monitor voltage and current

any ideas would be greatly recieved

 

[Skogsgurra]

robb1,

OK. You get the same readings with the AVO. And the readings agree with the capacitor ratings? Then your measurements seem to be OK.

Is your current transformer connected to measure current between Y-point of bank 1 and bank 2? (I assume it is).

Re-reading your original question, I find that you are pointing to the problem in the latter part of the question: "...will another Bank in parallel (selected to the same bus) have any effect on this?? "

Yes, it surely will. When you connect the first capacitor, the impedance of the bus keeps capacitor charging currents low. But, when the second bank is connected, the impedance will be extremely low (impedance of the two capacitors) and the corresponding current will be high. This will trip modern breakers with electronic/digital tripping devices.

The standard remedy is to incorporate a set of series reactors (usually air cored) to limit the switching currents. Such reactors are usually needed to avoid problems with the breakers as well. Without them, you will have premature aging and also a risk that the breakers weld together as a result of the high and short (a few tens of microseconds) current that results from the low impedance in the circuit.

So, if you do not have a clear indication that the unbalance relay trips, you may have a problem with low impedance when the second bank is switched in. Especially if your breakers have modern (fast) tripping devices and if your capacitor banks are without reactors.

I am very interested in some feed back on this. We have had the same problem with Siemens and also SACE breakers. Installing reactors cured the problem.

 

[rbulsara]

robb1:

skogsgurra addressed what I was about to say regarding higher inrush caused by parallel switching of caps and adding reactor should help.

Only puzzling part is, this would happen in both directions, meaning the second one coming on line trips. Is your C12 bank is much larger than C11 (or vice versa?)

You said there is CT in the neutral, it it connected to any relay that trips the breaker?

One disclaimer: I am no expert in systems of 110kV or larger..but this issue is true for any voltage systems.

 

[rbulsara]

I think your isse is may be more related to sensitive setting of the trip...either add some time delay or higher setting.

May be both reactor and adjsutment of trip setting shold resolve the isse..