Use of PFC with Non Linear Loads (VSD)
Use of PFC with Non Linear Loads (VSD)
(OP)
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
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





RE: Use of PFC with Non Linear Loads (VSD)
RE: Use of PFC with Non Linear Loads (VSD)
"Will work for (the memory of) salami"
RE: Use of PFC with Non Linear Loads (VSD)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
RE: Use of PFC with Non Linear Loads (VSD)
You foreigners all sound the same to me I guess...
Sorry Scotty.
"Will work for (the memory of) salami"
RE: Use of PFC with Non Linear Loads (VSD)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
Maybe we are all blending into an amorphous mass of collective EE knowledge now!
Resistance is futile...
"Will work for (the memory of) salami"
RE: Use of PFC with Non Linear Loads (VSD)
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.
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.
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:
- displacement PF: caused by inductive loads, mitigated by PFC et al.
- distortion PF: caused by non-linear loads, mitigated by line reactors et al.
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
RE: Use of PFC with Non Linear Loads (VSD)
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.
RE: Use of PFC with Non Linear Loads (VSD)
What VFDs can't tolerate a 3% reactor? Those with AFE?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
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
RE: Use of PFC with Non Linear Loads (VSD)
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
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.
RE: Use of PFC with Non Linear Loads (VSD)
It takes someone "from the inside" to know such things. I shall study that. Any references?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
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
www.KilroyWasHere<dot>com
RE: Use of PFC with Non Linear Loads (VSD)
www.KilroyWasHere<dot>com
RE: Use of PFC with Non Linear Loads (VSD)
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....
www.KilroyWasHere<dot>com
RE: Use of PFC with Non Linear Loads (VSD)
I found out that little tidbit just before I left the Big S fold a few years ago. They have been pushing that technology here with a vengeance lately, but not telling customers of this little issue (because there is nobody left on staff there that would understand it...). It reared its ugly head just last week when a local user tested them out on their backup generator (with something like 25%z). It was apparently a spectacular event, albeit a bit too early for the 4th of July.
"Will work for (the memory of) salami"
RE: Use of PFC with Non Linear Loads (VSD)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Use of PFC with Non Linear Loads (VSD)
You need to know what problem you're trying to address before you can come up with a recommended solution.
RE: Use of PFC with Non Linear Loads (VSD)
Ah yes, I think I see what you're saying. The draw of the VSD itself has high displacement PF. No argument there. I took objection to the phrase that they "*correct* the displacement power factor". As my note about seeming to have a positive effect states, they do indeed improve the displacement PF compared to a DOL motor. But they do nothing to "correct" displacement power factor due to other loads. The insinuation I read was that being capacitive, they correct inductive loads elsewhere. As I think you'll agree, that's not true.