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Discussion on distortion PF correction.
5

Discussion on distortion PF correction.

Discussion on distortion PF correction.

(OP)
If one has a couple of thousand horsepower of motors all on VFDs and they need to "correct the power factor" is a truckload of capacitors the correct solution for the wad of distortion PF or is there something better? Yes, yes, besides replacing all the VFDs with ones having active PF correcting front ends.

Seems to me, fighting distortion with caps would be pretty far from an efficient solution.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Discussion on distortion PF correction.

Hi Keith, maybe, maybe not...

At the macro / power system operation level, I'm aware of at least one case where, at an asynchronous inter-power-grid tie facility, "filter capacitors" [ which I understand to be shunt capacitors with band-pass wave traps in series with them ] are provided and switched in and out of service in concert with changes to the requested power transfer amount. As I understand it, the wave distortion created by the firing of the various devices within the AC-DC-AC conversion equipment [the information provided at my level doesn't spell out what these are] changes according to the power transfer level prevailing at any given instant. These are addressed by switching said filter capacitors in and out of service to mitigate the values of specific harmonics as required to normally limit the THD on our side of the interconnection to < 1.5%, and in the event of the unavailability of some filter capacitors to a maximum of 2.5% THD.

So in at least one instance the distortion power factor [DPF?] is in fact corrected not by changing all of the devices causing the distortion but by injecting counter-harmonics by separate means.

Hope this helps, although it's not part of my skill set to say whether such an approach can be scaled down to the application you're citing...

Carl

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Discussion on distortion PF correction.

If the caps are part of a de-tuned cap bank then no they are useless at removing the harmonics, which means they are also useless at correcting the poor power factor caused by the harmonics.

If the caps are part of a trap filter then yes they could be used to filter the harmonics, which then corrects the poor power factor caused by the harmonics. The trap filter can be a decent solution. We used one on a DC drive that was part of a motor load test setup. They just require some decent engineering to get right as opposed to something like an active harmonic filter that could just be connected and turned on.

RE: Discussion on distortion PF correction.

Keith,
Are you actually measuring that the distortion PF is worse than .95? That would be unusual if so. Before spending a lot of money chasing a perception, I would recommend measuring it. Many end users are unaware that modern VSI Drives don’t run at a lagging displacement PF and only amount to less than 5% distortion PF. Harmonics is another issue of course and often what you do to meet IEE-519 will also help with that distortion PF.

Along the lines of what LionelHutz said, I’ve had really good success the past few years with an Active Harmonic Filter unit made by TCI (Trans Coil Inc). Their latest version of firmware not only filters the 5th, 7th and 11th harmonics, it also can correct power factor up to .98. Im my cases we’ve put them on MCC lineups where we were mainly correcting the harmonics from the VFDs and lighting, but there were also a lot of other small induction motors running at low PF connected to it. The AHF was fixing both issues.
https://transcoil.com/wp-content/uploads/2017/12/H...

I’ve had another company, Metsa, claim they can offer that as well, I’ve just not used them yet.
http://www.mesta.com/Mesta%20DPM%20Active%20Harmon...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Discussion on distortion PF correction.

(OP)
It was a question asked of me that I'd never thought about before.

Thanks CR. Rubber actually meeting the road.

LHutz, Interesting solution. Might work with a large application too.

Jeff, Thanks for the links.
The question to me was; "We have to do PF correction".
"OK"
"And, we have a bazillion VFD controlled motors."
"Whoa"

My thinking is that there shouldn't be much displacement PF but a sh*t-ton of distortion PF in that case.

THanks all, this was what I was looking for. Great info!

Keith Cress
kcress - http://www.flaminsystems.com

RE: Discussion on distortion PF correction.

My pleasure.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Discussion on distortion PF correction.

Usually in my experience if they have a "commandment" like "... have to do PF correction.", that comes from one of two places:
1) The utility bill is showing a penalty them for having poor PF.
2) Some managerial type with zero knowledge of the subject reads some click-bait type article that convinces them that poor PF is the plague of the industrial world and EVERYONE should worry about it.

As far as I can tell, PG&E stopped penalizing customers for poor PF in Northern CA decades ago. A friend of mine used to be the Sprague (now Eaton) PFC Capacitor rep for Northern CA, he gave up the line because he couln't find enough people interested in spending money to solve a problem that nobody cared about. The purported reason PG&E stopped worrying about it was that the power grid in NorCal was built at a time when we actually had industry here, then industry left, and they get reimbursed for residential PF through higher rates anyway. Then what came next was that PG&E basically became a distributor so they had to give up their power generation business (other than the Diablo Canyon nuke plant) and instead takes a service fee for the infrastructure from everyone. So they no longer have a reason to worry about equipment utilization and they just gave up bothering about PF. I think it's more likely that they found this situation to be a way to make it APPEAR that they gave up the penalties, but are managing to get the $$ in those infrastructure service fees. Bottom line though, they are not penalizing people for PF.

So it's MORE likely that someone got a wild hair you-know-where and PERCEIVES that they "have to do PF correction" based on some erroneous info or someone with an agenda to push has their ear.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Discussion on distortion PF correction.

Your thinking is exactly right. A 6-pulse drive without any reactor or choke could have a 97% displacement power factor but a 70% power factor.

Adding a 5% line reactor is generally enough to get the power factor above 90%. You shouldn't add a capacitor trap filter or an active filter without first adding 5% line reactors since the reactors significantly reduce the size of the filter that is required. This means at the end of the day, you generally don't need any kind of capacitor or active filter to fix the power factor of 6-pulse drives.

RE: Discussion on distortion PF correction.

Hello Keith

Traditionally, we have always described power factor as the cosine of the angle between the voltage and current in a network.
Provided that the current and voltage are both sinusoidal, then this is still correct.
On our electrical networks today, we have an increasingly significant harmonic current due to the high level of electronic devices connected to the supply.

In some regions, this high level of harmonic current has proven to be a major problem, resulting in severe regulations being put in place to limit the harmonic currents and more particularly the resultant harmonic voltages.
In other regions, there has not yet been any significant move to penalize harmonics as they have not yet been seen to present major identified problems.

Harmonic currents can significantly increase the transmission and distribution losses in the network in the same way that reactive currents can, but unlike reactive currents, harmonic currents in a finite network impedance, can cause harmonic voltages that can interfere and damage other connected equipment.

In the region where I am based, we have had major harmonic problems with MV harmonic voltages as high as 16%. This has resulted in major damage to connected equipment. - failed induction motor rotors due to excessive slip losses, capacitor failures, transformer and switchgear failures, and electronic equipment operation and reliability compromised.
There has been a very strict regime put in place here to limit harmonic currents and the problems are now reducing.

In some regions, there has been no recognician of harmonic problems and so there are no enforced regulations and/or penalties.
If we look at electronic inverters with no reactors in the rectifier circuit, they can exhibit current distortions of 85% to 115%. The addition of reactors, either AC line reactors or DC Bus chokes can reduce the full load current distortion down to 35% - 40%.

By definition, Power factor is the ration between KW and KVA. A high current distortion results in a low distortion power factor.
VFDs with not reactors can exhibit distortion power factors below 0.7 and VFDs with reactors typically exhibit a distortion power factor of around 0.91

If you are in a region where there are no enforced regulations covering harmonics, and the power factor metering is based on displacement power factor, then there is no advantage in distortion power factor correction unless you are experiencing problems.
Many modern installations are using modern smart metering and if this is set to measure True Power Factor rather than displacement power factor, you can be heavily penalized for a bad power factor even though your displacement power factor is close to unity.

True Power Factor correction is best achieved by the use of active filters as these can correct both distortion currents and reactive currents.
Capacitors, detuned or not detuned, should not be used on circuits with significant harmonic currents present, or with active filters or active front end drives present. They can amplify frequencies other than line frequency due to circuit resonances.

If your client is penalised for poor True Power Factor, then you must apply active or passive filters, capacitors will not help much and could cause resonance problems.

Mark Empson
Advanced Motor Control Ltd

RE: Discussion on distortion PF correction.

No first hand experience with this technology for PF correction, but this company and the owner is out of Pittsboro, North Carolina, USA just down to road from me. Looks interesting.

https://3dfs.com/innovation

It uses high speed DSP technology to analyze distortion in the power waveform and applies stored energy to clean up the waveform with using a power silicon technology developed in Germany according to the last time I talked to the company owner. This was a few years ago and he was rather closed mouthed about it due to patent pending status.

I know from the conversation its not IGBT technology due IBGTs having turn on/turn off speed limitations.


RE: Discussion on distortion PF correction.

Active filters have been around quite a while, I worked on one at ERA Technology in the mid-1990's, and Merlin-Gerin had commercial products on the market a year or two later. The power circuitry itself is little different from the output stage of a voltage source VSD, but the switching strategy is radically different. Not sure why they would need fancy switches, we were running IGBT-based systems handling a few hundred kVA back then and switches are a lot more capable now.

RE: Discussion on distortion PF correction.

(OP)
Greetings Marke, hope all's well with you in the sorta-down-under.

Thanks for filling in some of the poorly illuminated corners on this subject with some real numbers.

Thanks for the link Dan

Keith Cress
kcress - http://www.flaminsystems.com

RE: Discussion on distortion PF correction.

Was the closed mouthed actually because he was trying to trademark the term "software defined electricity"? I find it hard to believe IGBT technology isn't fast enough for active filter switching on a 60Hz network. I'd also like to know what's is being used that is faster.

RE: Discussion on distortion PF correction.

(OP)

Quote:

I'd also like to know what's is being used that is faster.

Certainly these days that would be silicon carbide semiconductors; faster-hotter-HV.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Discussion on distortion PF correction.

That 3DFS thing is basically an inverter based power conditioner with PFC control, Sola was selling those back in the 80s. If you read between the technobabble in the patent application, you can see that it's nothing all that different from a standard double conversion UPS, without the UPS battery pack portion, WHICH, by the way, they sell separately.



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Discussion on distortion PF correction.

Quote:

Certainly these days that would be silicon carbide semiconductors; faster-hotter-HV.

Not likely when discussing this "a few year ago".

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