Damage in MT capacitors

[koruma]

Six months ago we installed in our factory some power factor correction capacitors for 2.3 KV motors and they have been operating properly since then, but, recently we installed one capacitor of 160 KVAR, 2.3 KV for power factor correction of a 625 HP, 2.3 KV, 890 rpm motor, at first power up of this motor, there was a big undervoltage condition, and the fuses corresponding to phases A and B blew out, simultaneously, the fuses of another motor connected at that time also blew out, this motor was connected in parallel with another power factor correction capacitor of 160 KVAR, 2.3 KV and the motor is a 450 HP, 1789 rpm, 2.3 KV synchronous motor, this motor and its capacitor have been operating already 6 months.
Both capacitors are shorted internally between phases A and B and their tanks are expanded.
Can anyone tell me what could have happened?

 

[electricuwe]

Judging from the rpm ratings stated in your post I assume that all the motors involved are asynchronus motors (=induction motors) an not synchronus motors as mentioned for the second motor.

The problem you describe may be related to harmonics:
A system consisting of transformers, capacitors, cables and busbars has several different resonant frequencies in the range of 5 to 50 times line frequency. Adding the last unit might have shifted one of the frequencies to an integer multiple of the line frequency and harmonics in the line voltage might have initiated large resonat currents in the capacitors.

As a very rough rule of the thumb it is not recommended to use local capacitors for power factor correction if there is more then 20% of converter load (or other harmonic generating equipment) on a system.

Centralised power factor correction equipment consisting of capacitors in conjunction with series chokes tuned slighly below the 5th harmonic avoids such problems.

 

[dpc]

I agree with electricuwe that this sounds like some type of harmonic resonance condition.

Adjustable frequency drives on the bus can cause this as well.

What was your basis for sizing these capacitors? If the caps are too large, the motor may be over-excited at low load conditions.

Are the cap banks separately fused or otherwise protected?

dpc

 

[electricpete]

Doesn't quite make sense. It is reasonable to presume that the initiating even was failure of the new capacitor for whatever reason (was the cap tested beforehand?). I would also suspect that this initiating event somehow created a transient which applied high voltage to the other capacitor... either due to the initial short, the fuse blowing, or the subsequent single-phase operation of that motor (?). Those aren't very precise explanations, but I have seen some unusual things after a fuse blows.

What is the grounding scheme?

 

[koruma]

You are right, electricuwe, they are asynchronous motors.
How big the harmonic problem in the MV side can be, since all the electronic converters are powered downstream by 460v?
The capacitors are not individually fused, they are conected in parallel with the motor.
dpc- can you explain to me how can the motor be overexcited at low load?- I have heard that this problem arise only when the motor is disconnected from the source.
The capacitors are new and tested at factory.
Thank you very much for your enlightening answers!

 

[dpc]

Regarding over-correction of pf, it is true that the main danger occurs when a running motor is disconnected from the line and left connected to the caps. However, an overcorrected motor at no-load may have excessive voltage applied to it. This will increase the current drawn by the capacitors and their kVAR output as well. It is a good rule-of-thumb to limit correction to about 0.97 at no-load.

Capacitors should have individual overcurrent protection on each phase - (this is required by Code in the U.S.)

Your LV drives could have a considerable impact on the medium-voltage system, despite the buffering impedance of the transformers. It depends on the drive size and type.

I would still suspect some type of resonance condition created by the additional capacitance, resulting in excessive currents and subsequent failure of the cap banks, due to the lack of adequate overcurrent protection.

You may need to do a harmonic analysis of the system. Have you ever measured the harmonic distortion on the MV system?

dpc

 

[busbar]

{Dumb question--what does MT stand for?} As far as resonance, there’s that old trick to figuring potential problems; i.e.,

h = (MVAsc/MVAR)^0.5

Where the short circuit MVA for the point of capacitor connection is known, and MVAR is the capacitor rating. (h is the harmonic order.) If it works out to 49, chances are there’s not a problem, but if it’s 5.1, then more study is warranted.

MV capacitors are often rated and configured in ungrounded wye. If there are multiple cans per phase, then a fuse operation will cause overvoltage in the remaining cans. ANSI/IEEE 37.99 discusses this.

 

[electricuwe]

Even if VSDs are only connected to the 460 V system, the harmonics generated by them can excite severe oszillations on the 2,3 kV system if a resonance condition exists.

 

[electricpete]

Lots of great ideas so far.

Another thing to consider - switching capacitors causes oscillatory voltage transients.