High voltage capacitor banks.

[kda3310]

I work at a sawmill plant. We have capacitor banks on the high-voltage side of our transformer 34.5 KV. We are not qualified to work on the high-voltage side so we contracted it out to have them replaced. The old capacitors swelled up and blew out. Within 3 days of installing the new capacitors they blew up with a spectacular light show. I do not know of any anomalies that could be causing this. Does anyone have any idea where I should even start to look?

 

[mayanees]

Harmonics and resonance

Harmonics and resonance

You likely have a situation where the load side of the transformer is now seeing Variable Frequency Drive Loads where it once saw motor loads. That capacitance is likely there for Power factor correction and for standard motors you can get away with about 25% of the size of the load for an across-the-line started motor. 100 hP motor, 25 kVAR capacitor.
But now the harmonic distortion associated with VFD loads finds a resonant point with the capacitor and faults at that point.
The first thing to do is find out why the caps are there by investigating the power factor without them. Determine the facility's billing structure relative to power factor and plan accordingly. It may be prudent to install a tuner capacitor that gives the necessary power factor correction without the occurrence of resonance.
I'd suggest contracting with an engineering firm to do a Harmonics analysis and make recommendation for power factor correction, if correction is warranted based on the billing tariff.

 

[Besoeker]

You likely have a situation where the load side of the transformer is now seeing Variable Frequency Drive Loads where it once saw motor loads. That capacitance is likely there for Power factor correction and for standard motors you can get away with about 25% of the size of the load for an across-the-line started motor. 100 hP motor, 25 kVAR capacitor.
But now the harmonic distortion associated with VFD loads finds a resonant point with the capacitor and faults at that point.
The first thing to do is find out why the caps are there by investigating the power factor without them. Determine the facility's billing structure relative to power factor and plan accordingly. It may be prudent to install a tuner capacitor that gives the necessary power factor correction without the occurrence of resonance.
I'd suggest contracting with an engineering firm to do a Harmonics analysis and make recommendation for power factor correction, if correction is warranted based on the billing tariff.

I agree with that. A couple of thoughts to add.
We used a Dranetz harmonic analyzer when we had demonstrate compliance with a particular standard.
If it determined that PFC is required, de-tuning reactors in series with the capacitors. We tuned ours to 177Hz on a 50Hz supply. Others have used 225Hz. Too close to 5th Harmonic for my liking.

 

[kda3310]

Thank you. That makes sense. We have been adding VFD drives over the years. I was in the mind set to just repair what was there.

 

[wbdvt]

Thank you. That makes sense. We have been adding VFD drives over the years. I was in the mind set to just repair what was there.

You may find with all the VFDs installed, you may not need PF correction or it may only be needed locally at a bus or motor.