VFD's and power quality references

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steve66

steve66

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I'm looking for a good reference covering how VFD's and non-linear loads (UPS's) affect harmonics, power factor, and power quality.

Free, instant links are preferred (like online links & PDF's).

A couple specific questions I'm hoping to answer:

1. I know VFD's and UPS's cause harmonics, but do they affect power factor? If a motor with a low PF is supplied through a VFD, will I have harmonics and a low PF?

2. What about a large chiller (maybe 500 HP) that was provided with a factory installed VFD. Would you expect it to generate a lot of harmonics, and cause a low PF? Or would you expect it to have some filtering on the input?

3. Similar question for a large UPS. Would you expect it to generate ugly harmonics, or would the manufacturer usually provide some filtering?

Thanks in advance:
Steve
 
I can speask to the UPS part of the question. The answer is that it depends.

Newer IGBT rectifier front ends will result in lower harmonics (without an input filter) and higher power factor than older units.

for example Mitsubishi 9900B (Toshiba G9000) has

reflected input current total harmonic distortion (THD):
1. 3% typical at 100% load.
2. 5% typical at 50% load.

Power factor:
1. 0.99 typical at 100% load.
2. 0.99 typical at 50% load.

SCR front end types need input filters and can do pretty close on the THD as the Mitsubishi unit but not as low. Their input power factor with the harmonic filter engaged (above 30% load) is approximetly 90-95% depending on load.
 
steve66 said:
I'm looking for a good reference covering how VFD's and non-linear loads (UPS's) affect harmonics, power factor, and power quality.

Free, instant links are preferred (like online links & PDF's).

A couple specific questions I'm hoping to answer:

1. I know VFD's and UPS's cause harmonics, but do they affect power factor? If a motor with a low PF is supplied through a VFD, will I have harmonics and a low PF?

2. What about a large chiller (maybe 500 HP) that was provided with a factory installed VFD. Would you expect it to generate a lot of harmonics, and cause a low PF? Or would you expect it to have some filtering on the input?

3. Similar question for a large UPS. Would you expect it to generate ugly harmonics, or would the manufacturer usually provide some filtering?

Thanks in advance:
Steve
Click to expand...

There are a lot of variables......
But a couple of points.
Variable frequency inverters and UPSs draw non-linear current from the supply. That, in turn, causes supply voltage distortion. How much depends on the VSD or UPS rating and the supply characteristics. A small VFD and a very stiff supply will produce hardly any voltage distortion.

As a very rough guide the harmonic current drawn is the reciprocal of the harmonic number. So for example, a 100A fundamental would have about 20A of fifth harmonic.

Here are the numbers I use for a six-pulse system:

FF5 0.19
FF7 0.13
FF 11 0.08
FF 13 0.065
FF17 0.05
FF19 0.045


Power factor is kW/total kVA
That total is rather important.
PF comes in two flavours.
Displacement and distortion.
For VFDs the displacement is close to unity. Not so for distortion. That's typically 0.95 or worse and can't be corrected using traditional PFC measures.
 
Last edited:
Thanks for the input.

I dug up the cut sheets for the UPS and the large chiller, and also some cut sheets for prospective VFD's. They all listed .95 or better PF for the input, and they all mentioned something about having low harmonics, which pretty much matches what Ron said.
 
There are two types of Power Factor; Displacement Power factor and Distortion Power factor. Displacement Power Factor, which is what your utility measures, is a function of an inductive load, like an induction motor. So from the VFD to the motor, the motor displacement power factor is what it is based on the loading of the motor. But once you have a VFD, there is no direct relation to the motor and incoming line other than as the source of energy. The VFD converts the AC line to DC, then recreates a new AC source to the load. So the displacement PF of the load becomes only remotely related to the line. From a displacement PF standpoint, the VFD is NOT an induction machine, so the PF is not substantially changed, hence the VFD specs that say it is .95PF.

The non-linear load of the VFD, because of HOW it draws current from the line, is however going to create more DISTORTION power factor. Distortion power factor is not as readily seen by PF meters used by utilities (yet) because it takes place at frequencies that are multiples of the fundamental (60Hz) and the meters work off of the fundamental. So its contribution to lowering your overall PF is more than outweighed by the benefits it provides in correcting the much lower motor PF.

As to harmonics, the Rule of Thumb I give people that relates to Besoeker's comment regarding a small drive on a stiff system is this:

If your aggregate Non-Linear load* is greater than 30% of your TOTAL load (linear + non-linear), then you should consider doing a harmonic analysis to determine the best mitigation strategy. If it's below 30%, you are not likely to suffer any serious negative consequences of the harmonics, at least not worthy of the mitigation costs.

* Non-linear loads are of course VFDs, but also ANY form of Switched-Mode Power Supply (SMPS), which is now, basically everything; electronic lighting ballasts, PC power supplies, UPS systems, newer DC drives, servo amplifiers, etc. etc. etc.
 
steve66 said:
Thanks for the input.

I dug up the cut sheets for the UPS and the large chiller, and also some cut sheets for prospective VFD's. They all listed .95 or better PF for the input, and they all mentioned something about having low harmonics, which pretty much matches what Ron said.
Click to expand...
Two points.
Is the power factor total power factor or just displacement power factor.
And does the claimed low harmonics refer to voltage or current?

Read the detail - if it's given.
 
Sorry for double posting, but I had some thoughts in regards to your questions:

1. AC drives should improve overall power factor. There is a sidebar at the end of the ABB white paper that demonstrates this qualitatively.

Related article: http://www04.abb.com/GLOBAL/seitp/s...r+Factor+Improved+Via+AC+Drives+-+Release.pdf

2. I would expect mitigation installed on a drive of that size. In my industry (water-wastewater) we typically specify 6-pulse drives for 50HP-200 HP and 18-pulse for anything higher. Check if the drive in the chiller is of the 18-pulse variety. These types of drives drastically mitigate the up to the nth-order harmonics, where n is the # of pulses. They also come in 6-pulse. The damaging harmonics are 3rd and 5th. Maybe if you read the first 3 links about harmonic mitigation techniques, it will help you understand.

3. I don't have experience with large UPS systems. Personally, I would expect a critical (e.g. large) UPS to have harmonic mitigation integral to the unit. If that isn't the cast, at least the manufacturer should support you in dealing with harmonics their equipment is (known to) creating.
 
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