Earlier in the year I posted the following issue=>
About once a month a QA200 fuse clears on the utility riser at our plant. There has been no indication as to why it blows. We have a 2000 KVA 12470 -> 480volt transformer. On the load (480v) side beyond the main CB there is a fixed bank of 400 KVar caps installed. We have had no indication of faults in the load side, and in fact, as soon as the fuse is replaced we can start up and run everything as normal. The Utility is at a loss to explain the problem. My question is, "can the fixed caps be creating problems on the line side of the transformer causing the fuse to heat?"
See the thread http://www.control.com/thread/1026230518#1026237537

The utility’s answer was to replace the transformer and they did so in February.
Since the transformer was replaced we have not had the outage issues that we had in the past. However, two banks of capacitors on the load side of the transformer have ruptured and blown all three phase fuses protecting the caps. The banks are 100 kVar each and the ruptured caps comprise ½ of the total fixed bank. The capacitors did not rupture at the same time, in fact the last bank to rupture appears to have happened within the last week. What might cause the caps to rupture and blow the fuses?

 

The issue with blowing capacitor fuses is they simply are drawing too much current. The amount current to a capacitor is proportional to frequency. At 60 Hz the capacitor will draw current based on its capacitive reactance, at the 3rd harmonic it will draw current based on its capacitive reactance at three times the frequency and so on.

The most likely answer to your question is harmonics.

You did not state if your power transformer is a Delta or Wye but with these problems it is most likely a Wye and you have a lot of nonlinear loads. Many times the harmonics issues must be resolved to prevent or fix problems like this.

I hope this helps

 

Get a power quality analyzer or a scope with a wideband current probe and you should be able to figure this out. Not that many possibilities. Either you are breaking over the caps with voltage transients, then damaging things or the caps are carrying excessive current. Since the sizing calcs are done at line frequency and aren't rocket science, I'd suspect that you have a harmonic problem and/or possibly a resonance problem. The brute force diagnosis is to measure the cap current and if it is higher than it should be per formula, find out why. Since the capacitive reactance varies inversely with frequency the higher harmonics can cause very high currents and heating in the ESR.

Regards
cww

 

Responding to Kevin’s 28-Jul (00:04) query… some additional food for thought that may help:

A) Usual Suspects.
o What is age of capacitor bank?
o Are capacitors oil-or Askarel-filled?
o Are all bleed-resistors intact.

B) Installation Information.
o Indoor or outdoor?
o Bank connection, wye or delta?
o If wye, is neutral connection grounded? Solidly? Or via a resistor?
o Is bank equipped with its own CB?
o What is size, type, and length of feeder from CB to bank location? In open race-way? Conduit? Direct-burial cable?

C) Operational Information.
o Nameplate data. Especially voltage!
o Is capacitor switched? Randomly? Periodically? Manually? Automatically? How often?
o Do failures occur randomly? Or are they coincident with a unique process requirement, e.g., large motor starting.
o Do fuses “blow” without cap failure?
o How does present failure rate compare failure-rate prior to xfmr change-out?
o Did problem appear soon after xfmr change-out?
o Does problem appear to coincide with bad weather?”

D) Measurements.
o Phase-to-phase voltages; phase-to-neutral voltages; bank phase currents.
o Plant and bank (if metered) energy usage (kWh) and demand (kW).
o Has oil (or Askarel) ever been tested?
o Does utility contract include a pf penalty?

Regards, Phil Corso ([email protected])

 

Check harmonics (if you recently installed Variable Frequency Drives, Soft Starters). remember that the power consumed by the capacitors is rleated to the square of the harmonic number (N) because the Nth harmonic generates N-time more current than the fundamental, and the power in the caps is related to I square times R (their Effective Series Resistance = ESR). For example, only 1% amplitude of the 40th harmonic generates 16 times the power of the 100% amplitude fundamental (40 x 40 x0.01).

Meir