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

 

[jraef]

... and take the comments of "you do not need a line reactor..." to mean "do not fit a line reactor..."

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....

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"

 

[Skogsgurra]

Love that! "It was apparently a spectacular event, albeit a bit too early for the 4th of July"

Gunnar Englund

http://www.gke.org

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

 

[LionelHutz]

JimDaly - What was the client trying to fix by installing the PFCC bank?

You need to know what problem you're trying to address before you can come up with a recommended solution.

 

[LiteYear]

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

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.