Supply resonance theory.

[Marke]

I am working with a customer who has experienced premature PFC capacitor failure in a pumping installation.

My customer has a number of large pumps operating off dedicated transformers and an 11KV feeder.

The supply authority are insisting on power factor correction, and the motors are controlled by soft starters.
Each motor has the correct amount of static power factor correction that is controlled by the soft starter via a capacitor rated contactor that is switched in once the motor is up to full speed.

The capacitors are not detuned and are only connected when the motors are operating at close to full load.
The capacitors lasted around three months. The capacitor cans bulged at the ends and the capacitors failed.

One theory is, that the capacitors, motors and supply are forming a high Q harmonic resonant circuit which is causing high currents to flow through the capacitors and this is causing the failure.

The other theory is that during the irrigation season, the harmonic voltage on the supply is seriously elevated and this is causing elevated harmonic currents to flow and overloading the capacitors.

The capacitor failure was during the peak of the irrigation season when I would expect a significant harmonic current load in the region due to VFD driven pumps. In another region of a similar nature, I have measured THDv of 12% - 16%.
Tests in the Off season, yield harmonic voltages on this network of 3.5% +/-. At this time, there were no VFD controlled pumps running.

While I have certainly seen many examples of harmonic supply resonance and also non harmonic supply resonance on lightly loaded supplies with long 11KV and 22KV lines supplying the transformer, my experience is that when the transformer loading and motor loading is significant, the Q is so low that there is no significant resonance present.

Has anyone experienced premature capacitor failure on a loaded circuit that can definitely be shown to be due to harmonic resonance?
At light loads this can definitely happen due to higher Q factors.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[Skogsgurra]

Hello Mark

Are the soft-starters thyristor or IGBT types? Do they have a by-pass contactor that pulls in when start is over?

If there is no by-pass, I don't think that resonance is the problem. And especially not if the SS are IGBT types. No - that couldn't be. I guess you would have a problem as soon as you switch the PFC in then. So, thyristors and no by-pass? That could be bad enough because few SS have zero delay and thus cause steps in the order of 10% of peak voltage. Those steps are steep, one or two microseconds, and can easily heat the capacitors. Seen that on smaller systems like snow cannons.

Otherwise, unexpected resonances do exist. I think that you have my report from NamWater a few years ago where we tested a system for resonances (and found a quite unexpected one) using a stereo amplifier, a sweep generator and a recorder. I think that same technique should be applicable here. But you can't have the line live while doing the test

If there is no by-pass, I would bank on a thyristor SS killing the capacitors.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Marke]

Hi Gunnar

Soft starters are SCR based, reverse parallel SCRs phase controlled during start and bypassed once up to speed, connecting the motors directly across the line.
Only when the motors are drawing a loaded current, are the capacitors connected across the line. There is no sign of any disturbance at the point where the capacitors are corrected which suggests to me that there is not a major resonance problem.

We monitored for a week (up to around 10 KHz) and found no issues during that time.
The capacitors all failed within three months during the time where there are many VFDs operating on the same network. I do not have measurments of the THDv during this period, but suspect it may be high.

The big question really is, could there be a resonant circuit formed by the loaded motors, the supply and the PF capacitors that would cause the capacitors to fail?
I would have expected that if this resonance was serious to cause the capacitors to fail, other failures would be expected but there were none.

Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[itsmoked]

If the capacitors are bulging that would indicate to me they're heating up. Can it be, not resonance, but just too much ripple? Possibly from the line harmonics?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marke]

Hi Keith

That is a real possibility, but at present, the THDv is OK.

Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[Skogsgurra]

Mark

Not sure if 10 kHz shows all what is going on. With one or two us risetime, one would traditionally look for components up to a few hundred kHz. Then, standard power Rogowski (like LEM and Fluke) don't work. You need PEM that easily covers MHz, but also tend to ring heavily in certain cases.

I would use a voltage divider and measure differentially against ground with a recorder or scope with at least 1 MHz BW. That is very low for a scope, but sometimes a challenge for a recorder. I would also use an HP filter to cut out out everything below a few hundred Hz. That will increase resolution in the frequency band you are interested in.

Why? Do you ask. Why do that if there are by-pass contactors on the soft-starters?

It depends. If there are several drives on each transformer and those motors start and stop randomly and the starting time is long - there will be influence from the starting motor with its rather high current with very sharp notches from thyristor (OK, SCR) switching. That can heat the capacitors and make them a little fatter than expected. And, as you know, obesity shortens life...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

The capacitors are not detuned and are only connected when the motors are operating at close to full load.

You told us about the startup sequence (caps are not switched in until soft starter is bypassed...very good).

I wanted to clarify doublecheck the shutdown sequence: are the caps disconnected from the motor at the same time the power is removed?


=====================================
(2B)+(2B)' ?

 

[Marke]

Hi Pete

The caps are disconnected first, and then the motor is stopped, either by soft stop (ramping down the voltage) or fast stop (disconnecting the supply). There is no time that the capacitors are connected without a loaded motor at full speed, drawing a high current.

The capacitors are controlled by a separate contactor with built in charging resistors.

The theory is that while the capacitors are connected, and the motors are running under high load, the capacitors are resonating with the motor and supply reactance and generating enough voltage to damage the capacitors.
The interesting thing is that there are no other voltage related failures on site which I expect to see when there is a supply resonance issue.

Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[waross]

Hi Mark. Have you considered what the effect of the high frequency harmonics have on the already loaded capacitors?
I am under the impression that third harmonic voltages will cause three time the current for a given voltage level, five times for fifth harmonics.
I am not on site and I am unsure of the actual effect. However if this was my issue to solve, I would investigate this possibility before rejecting it as a possible cause.
When capacitors are involved, I understand that over voltages cause over current, and over frequencies also cause over current.
My gut feel is that any heating caused by higher frequencies will be in addition to the heating caused by the 60 Hz(or 50 Hz) fundamental supply.
The harmonics may not have to be at a particularly high voltage to add heat to the capacitor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marke]

Hi Bill

Thank you for your comments.
Yes, that is one of the two scenarios.
At present, it is the OFF season for irrigation and the background voltage distortion coming in on the 11KV is in the order of 1.5% which would not cause a problem, but I suspect that during the ON season when all the irrigation pumps are running on VFDs, the voltage distortion on the feeder will be well in excess of 5% and would cause problems.

The supply authority claim maximum voltage distortion from their network is 3% and are looking for a resonance problem. Hence my question.
There is a pile of blown capacitors and they want them all replaced so that they can do some tests. Customer wants some surity that they will not fail again in three months before he agrees to replace them.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[Skogsgurra]

You didn't read my post?
Or did you not understand?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Marke]

Hi Gunnar

Yes certainly did read and understand your post.

At the moment, I am unable to restore all the capacitors and retest, but when we do, I will certainly be following the Guru's recommendations.
The start time is actually 2 seconds and the manufacturer of the capacitors has stated that the short term overload due to the harmonics during start will not be an issue. The THDv during start does reach around 10% when a motor is started. If this was motor number 2, then the capacitors on motor 1, already running, will see a two second burst of 5th and 7th harmonic current. The total number of starts is around four per day. If the capacitors on motor 1 failed and not motor 2, then that would tend to validate the damage caused by the start harmonics from starting 2, but the motor 2 capacitors failed as well.

The voltage waveform is normal, no sign of resonance that I have seen so far.
The suggestion is that the resonant frequency of the capacitors, motors and supply is around the 7th harmonic and that the Q is high.
If this was the case, I would expect to see ringing voltages triggered by the closing of the capacitor contactor. I have not managed to find this, but will look again when we can conduct some more tests on site. (Three hours drive away)
At present, there are no capacitors fitted and there is a reluctance on behalf of the end user to purchase another set until he is confident that they will not fail again.

In another region, about 100KM away from this site, we were experiencing problems during summer because the THDv on the 22KV network was commonly between 12% and 16%. Harmonic mitigation on each VFD on that network has cured the problem. High THDv causing the capacitors to fail is a very real possibility in this installation if the THDv is high during the irrigation season, but we have no fact and data to work with at present and will have to wait till the height of the irrigation season before we can make measurements of the background voltage distortion.

I know that high THDv will cause the capacitor failure. I have seen this many times in many installations. The power utility claim this is not the cause, rather it is the harmonic resonance of the capacitors with the motor and network and that it will be around the seventh harmonic.
I have certainly experienced supply resonance issues with power factor correction capacitors, but only when the supply is unloaded and always with automatic bank correction units. When a capacitor bank closes, an unloaded supply can seriously ring. I have measured in excess of 2000 volts on a 400 volt supply when capacitors are switched in, but I have never experienced capacitor failure on loaded networks when the capacitors are switched in after the motor reaches full speed and a significant load, except where there is a significant THDv on the network.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[Skogsgurra]

A simulation (

[code][img]https://res.cloudinary.com/engineering-com/image/upload/v1504692034/tips/Mark_capacitors_LT_Spice_1_ofkurt.png[/img]

[/code]the exact data are not available) shows what I mean. During start of a motor, there will be short pulses injected to the line. These short pulses/ringing will then affect all capacitors that are connected to that transformer. It is a real possibility. And I have had that problem in light fixtures with PF capacitors. Dimmers in another part of the building (for heating, that was) killed the capacitors.

Shown in the diagram is also the 10 kHz LP filter and the voltage you will see with a 10 kHz recorder.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Guilhermebr]

Normally capacitor for this voltage tolerate high current and voltage harmonic. I had in the past problems with energization capacitor before discharge voltage, some capacitor failed. do you have some protection for re-energization witout no voltage in capacitor?

GS