Use of PFC with Non Linear Loads (VSD) 1

[JimDaly]

A Client has had one of their Power Factor Correction Units 'burn out'. They now plan to replace it with added protection - smoke and arc detection as well as earth leakage and circuit breakers instead of fuses. They have a high non linear load, predominantly VSD's.

I noticed that with the PFC now disabled the pf was 0.97 even though there is a fair amount of rotating plant - I assume due to the proliferation of VSD's and electronic lighting ballasts. I am now wondering what is the right advice to give them. Do you replace the PFC with the same kVAr as before - or fit VSD chokes/filtering - or invest the money in active filtering.

I'd really value some advice on the subject.

Best
Jim

 

[ScottyUK]

I think the logical step is to quantify what harmonics are present in both your load currents and the supply voltage, and then make an informed recommendation. Hire a Dranetz or Voltech analyser, and gather some data. If you're not confident in evaluating the data from the instrument then there are companies who specialise in that kind of consulting work.

 

[jraef]

You typically install PFC caps to correct a poor power factor to 0.95 or better. VFDs, by virtue of having capacitors on the DC bus, essentially correct the displacement power factor to 0.97 or better by themselves. So if the PF is at 0.97 now without the PFC cap system, I'd say it was unnecessary. There is also a danger of causing resonance with the caps that are inside of the VFDs to be concerned about and with little else to go on, I'd venture a guess that this may have contributed to the demise of your PFC system to begin with. So replacing it without first doing a study (as Gunnar suggested) that not only justifies the need for further correction but also looks at the effects of having a PFC system in place where the majority of loads are non-linear, might be throwing good money after bad.

"Will work for (the memory of) salami"

 

[Skogsgurra]

If Life was that easy! Here, I get credit for things I didn't. How different from the more normal situations - where I get blamed for things I didn't!



Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

That said, I think taht I should add some useful insight to my unnecessary and useless comment. Like this:

The rather "good" PF of a VFD does not tell the whole truth. The 0.97 is the PF for the fundamental, so the "complete" PF, including distortion PF and displacement PF can be a lot worse.

Like transformers, I think that UPS-es should be derated according to the once so touted (by Fluke) k factor that seems to be more or less forgotten nowadays.

Or, better, look at the true RMS value of the current and make sure that the UPS can handle it. I once had a case where that well-known Three-letter Company starting with the first letter of the alphabet nearly sued a supplier because his UPS'es couldn't supply the load in an application in hot South East Asia. They were not, according to the Three-letter Company, built for the temperature, which had been carefully specified together with the voltage and current needed.

It turned out that the load was mostly computers with very high current distortion, which hadn't been mentioned at all by the purchaser. He had ordered according to kW/V current rating. New and bigger UPS'es were delivered and the supplier was well compensated. No hard feelings.

Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

OK, I didn't read all the text. PF correction - are they capacitors or some Active device? In both cases, the actual current needs to be assesed. And it would be very good to put 3 or 4% percent line reactors Before the VFDs. It will save VFDs and PFC.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

I have been mistaken for worse people.

 

[jraef]

D'oh!

You foreigners all sound the same to me I guess...

Sorry Scotty.

"Will work for (the memory of) salami"

 

[Skogsgurra]

Hehe.. (Foreign sound of laughter)

Gunnar Englund

http://www.gke.org

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

 

[jraef]

I guess it sounded like something you would say Gunnar...

Maybe we are all blending into an amorphous mass of collective EE knowledge now!

Resistance is futile...

"Will work for (the memory of) salami"

 

[LiteYear]

Just to tidy up some misconceptions:

VFDs, by virtue of having capacitors on the DC bus, essentially correct the displacement power factor to 0.97 or better by themselves.

Not at all. The DC bus capacitors are DC, that is, they are fed by a rectifier, so do nothing for displacement power factor. Note that they may seem to have a positive effect, but only by virtue of isolating the influence of their own inductive motor load from the supply, not by interacting with any other loads on the system.

So if the PF is at 0.97 now without the PFC cap system, I'd say it was unnecessary.

Agreed, but as has been suggested, it would be worthwhile double checking that's the case. You have two axes of PF to be concerned with: displacement and distortion. They have both different causes and different solutions. Be sure you're aware of both.

There is also a danger of causing resonance with the caps that are inside of the VFDs to be concerned about

Again, not so. Not being AC connected, the VFD caps cannot contribute to resonance. Effectively they can only sink current from the supply, not source it, hence no resonance.

To the OP, PFC burn out is common with high distortion PF. Often sites will confuse their distortion PF for displacement PF, install a bunch of PFC and get nothing but smoke. It's likely that your displacement PF is 0.97 and therefore PFC is unnecessary. It's possible that your distortion PF is low and therefore you'd be well served by line reactors or multi-pulse drives or similar.

Remember:
[ul]
[li]displacement PF: caused by inductive loads, mitigated by PFC et al.[/li]
[li]distortion PF: caused by non-linear loads, mitigated by line reactors et al.[/li]
[/ul]

There was a recent webinar by Mirius International that did a very good job of explaining this phenomena. Despite Mirius' obvious self interest, given that they sell a sophisticated line reactor called the Lineator, their webinar was a commendable presentation of facts.

http://www.mirusinternational.com/index.html

 

[ozmosis]

Gunnar
Not all types of VFDs out there will be too happy with 3% or 4% line reactors. Those with the 'slim DC capacitance' on the DC link will potentially introduce bigger problems by adding an inductor. The > IZ, the > the problem, usually.

@JimDaly: better understanding the network configuration first and start with what you know and have, and be clear about what it is you want to achieve. Then you can take corrective action to solve whatever it is you see as the problem.
As a general rule, PFC with little or no inductance,are not too happy on a load that is non-linear.

 

[Skogsgurra]

OK, Patrick, but a reactor would still reduce current distortion, wouldn't it?

What VFDs can't tolerate a 3% reactor? Those with AFE?

Gunnar Englund

http://www.gke.org

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

 

[JimDaly]

I was very pleased and surprised by the many comments and advice, many thanks.
I do have a Power Analyser and have collected 24 hours of data and will send to anyone interested. Below is a brief summary of PF with no correction and VTHD which according to EN 50160 is within limits.

From the comments I understand the difference between PF displacement due to reactive and Non Linear Loads, one being compensated by PFC and the other by active or passive filters. I am uncertain of the science behind selecting the right solution. I would appreciate being pointed in the right direction Best Jim

POWER FACTOR, PF
A B C D TOTAL
Min 0.911 0.878 0.833 -0.201 0.876 on 2014/07/02 02:00:00
Max 0.994 0.994 0.992 0.315 0.993 on 2014/07/01 14:10:00
Median 0.978 0.970 0.968 0.202 0.972
Average 0.977 0.969 0.967 0.251 0.971

PHASE A HARMONIC TIMEPLOT

Harmonics
All shown figures are 95% values
Limit(% of Un) A B C Status
THD <8.00% 2.73% 2.60% 2.35% PASSED
H02 <2.00% 0.06% 0.08% 0.07% PASSED
H03 <5.00% 0.42% 0.56% 0.57% PASSED
H04 <1.00% 0.05% 0.08% 0.08% PASSED
H05 <6.00% 2.00% 1.98% 1.85% PASSED
H06 <0.50% 0.04% 0.04% 0.03% PASSED
H07 <5.00% 1.47% 1.46% 1.21% PASSED
H08 <0.50% 0.03% 0.05% 0.04% PASSED
H09 <1.50% 0.71% 0.60% 0.52% PASSED
H10 <0.50% 0.02% 0.03% 0.03% PASSED
H11 <3.50% 0.83% 0.69% 0.60% PASSED
H12 <0.50% 0.02% 0.03% 0.03% PASSED
H13 <3.00% 0.47% 0.38% 0.46% PASSED
H14 <0.50% 0.01% 0.02% 0.02% PASSED
H15 <0.50% 0.46% 0.31% 0.28% PASSED
H16 <0.50% 0.01% 0.02% 0.02% PASSED
H17 <2.00% 0.42% 0.42% 0.38% PASSED
H18 <0.50% 0.01% 0.02% 0.02% PASSED
H19 <1.50% 0.40% 0.38% 0.37% PASSED
H20 <0.50% 0.01% 0.02% 0.02% PASSED
H21 <0.50% 0.28% 0.29% 0.18% PASSED
H22 <0.50% 0.01% 0.02% 0.02% PASSED
H23 <1.50% 0.18% 0.22% 0.17% PASSED
H24 <0.50% 0.01% 0.02% 0.02% PASSED
H25 <1.50% 0.26% 0.24% 0.22% PASSED

 

[Skogsgurra]

I see absolutely no problem with these data.
You should be OK without any further mitigation.
There must be some other problem than harmonics.
Or are there occasions where harmonics could be a lot higher? Occasions that were not covered by your measurements?

Gunnar Englund

http://www.gke.org

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

 

[ozmosis]

I agree, I see no major issue with the data presented. If the PFC are out of circuit, and you are seeing a PF on average of .97, that seems pretty good to me.
THD seems ok if this represents a 24hr average.


Gunnar: some drives out there use the plastic film caps on the DC link. Overall, they have lower THiD (30%-35% THiD)than traditional pwm drives (45%-120% THiD), but they 'push' distortion levels higher up the spectrum that is not typically measured. Adding impedance to these types of drives or operating these drives on a high impedance supply, can create problems with resonance in high orders. If you read the small print in some drive docs, caution needs to be taken on supplies >1%iz, and take the comments of "you do not need a line reactor..." to mean "do not fit a line reactor..." However, getting slightly off topic now from OP.

 

[Skogsgurra]

Very interesting!

It takes someone "from the inside" to know such things. I shall study that. Any references?

Gunnar Englund

http://www.gke.org

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

 

[mikekilroy]

LiteYear (Computer)
Just to tidy up some misconceptions:

VFDs, by virtue of having capacitors on the DC bus, essentially correct the displacement power factor to 0.97 or better by themselves.

Not at all. The DC bus capacitors are DC, that is, they are fed by a rectifier, so do nothing for displacement power factor. Note that they may seem to have a positive effect, but only by virtue of isolating the influence of their own inductive motor load from the supply, not by interacting with any other loads on the system.

Actually, yes at all.

As stated, that is a perfectly true statement, The caps make sure the dc bus stays near the peak of the AC rms input voltage, so the ONLY time the rectifiers CAN conduct (sink as you say) ,is in phase with the voltage. Hence displacement PF >.97

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

JimDaly, is this a case of PFC installation to correct for all those running motors - BEFORE the VSDs were later installed? Then someone just forgot to REMOVE the PFC stuff since it was no longer needed? Might be a good explanation to the customer why they should just toss 'em.

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

Skogsgurra
OK, Patrick, but a reactor would still reduce current distortion, wouldn't it?

What VFDs can't tolerate a 3% reactor? Those with AFE?

Siemens Servo and Spindle drives with AFE. Very specific requirement for <3% input impedance. If used with higher they generate a LOT more line noise and distortion; once saw clocks run backwards when they are turned on. Their AFE actively shorts their own series inductor at 2khz rate to use the voltage spike when released to regulate the Dc bus among other things....

http://www.KilroyWasHere<dot>com