Substation Capacitor Bank Problem

[Brad B]

Hello everyone,

I'm no engineer, just a guy that works in the electrical industry. Today I was doing some substation switching on a 138kv shunt capacitor bank, which consists of 11 capacitors connected in parallel forming a group, then four of those groups connected in series per phase, making 44 total caps per phase. The three phases are then wye connected.

We were called out to repair said capacitor bank due to an imbalance lockout. Generally when this happens we find a single fuse has blown, we replace the capacitor and the fuse and away we go. Today we found that an entire group of 11 capacitors on one phase had blown their fuses, but the other groups on that phase were normal. We tested each capacitor, and they all seemed "normal", as in not shorted and capacitance matching nameplate values.

We refused all 11 and reenergized the capacitor bank and, yep, all 11 fuses blew. So, me being as green as I am, am not sure what would cause a single group to blow while leaving the rest of the phase untouched.

Anyone with any experience care to offer any insight? Anything would be appreciated.

Thanks very much.

 

[n.karthick]

Even though I am not sure, it could be due to resonance between your capacitor bank and a nonlinear load connected to the same point. Since nonlinear load draw harmonic currents, resonance may happen at higher frequencies resulting in huge current and fuses may blow up.

 

[famousken]

Remove the capacitors, ship them to me.

 

[jmaniglia]

Please don't take anything I say as an insult, I'm not sure of what you have tried to do to fix the situation.

Possible problems (that I can see):
- If the capacitors are electrolytic and polar, than the fuses could be installed to prevent the capacitors from being damaged when seeing a negative, or backwards, voltage across the terminals.

- The fuses may be in backwards, causing them to see a reverse, or negative, voltage.

- There may be a damaged connection where that set of eleven parallel capacitors is connecting in series with the other parallel capacitors which can cause, at such high voltages, an arc to form across a gap. Arcs approach infinite amounts of current, theoretically, and can easily cause a back-EMF (another form of reverse voltage) that can easily blow your fuses.

- Another thing that can cause an arc is just the wiring being in close proximity to something else conductive (metal casing, chair, really any form of metal).

Other than that, I wouldn't be sure without knowing more about the circuit.

On a side note, can you post the product # and manufacturer of those fuses?? I would really like to use them in a project I'm working on currently...