VFD Low Load Power Factor
VFD Low Load Power Factor
(OP)
Can anyone point me to a reference that can help me estimate power factor of a VFD running a fan load but only slightly loaded? The red book says VFDs have poor power factors at low loads, but that's all the farther they take it.





RE: VFD Low Load Power Factor
thread237-202478: How is the generator sizing with driving VFD
RE: VFD Low Load Power Factor
The VFD manufacturer, a very large German comapny, advertises, through the technical people that I have access to, and through their manuals, that their new hyfalutin rectifier presents whatever power factor I want it to present to the line using a vector control algorithm. They also say that they keep distortion low. I'd be interested in any unbiased opinions on that one.
Ideally, I wish there was a curve that showed inverter PF over the range of its loading, like some motor manufacturers give you. That way I can generate a realistic worst case for apparent power requirement if I chose to go with slightly older but proven technology (the VFD from the thread), which I'd like to do just for reliability and simplicity reasons. I cannot use the fully loaded blower as the worst case, because at this point in our envisioned startup procedure the blower will not be fully loaded and we're miserly with the power.
The generator winding's ability to withstand the current harmonics is a completely different problem than the one I'm trying to figure out at this point - though it looms in my future too. If I can get them to form wind it then it should be ok.
Regards
RE: VFD Low Load Power Factor
RE: VFD Low Load Power Factor
Unfortunately, I suspect I am another employee of the "very large German company", except if the VFD you are referring to is the one I'm thinking of, I might be able to help clear up a couple of things, having helped to design it..
It's not an Active Front End drive in the classic sense, as the input bridge does not use PWM. However, it's true that the harmonics are less of a problem than with conventional drives, especially at the lower harmonics.
Cheers,
Mort
RE: VFD Low Load Power Factor
RE: VFD Low Load Power Factor
Calculate the real power required. That will have a direct correlation to the amount of steam energy required. You should be concerned with the efficiency of the VFDs and motors, not the PF.
Once you have calculated the amount of steam required to supply the reactive power or VARs of a system, you often never do it again.
You may need a tiny percentage more steam to supply the losses occasioned by the reactive power.
respectfully
RE: VFD Low Load Power Factor
RE: VFD Low Load Power Factor
The powers are pretty small - the generator will be 250 kVA but we don't expect a 250 kW load - more like 180 kW at full load.
My question was originally about the power factor of the VFDs, but I think I was asking the wrong question. My question should have been "what is the relationship between mechanical power I need to supply on the input shaft of a generator and known reactive power demand for the loads." Those last couple posts answered this question quite well: very little mechanical power. Surely I need to arrange the electrical circuit for the total apparent power, but the steam power I need to produce will be much closer to the sum of real electrical power delivered (including, of course, losses).
On the point that the VArs get supplied from the excitation system - does this include if the generator is permanent magnet synchronous?
Thank you all very much for your insight.
By the way - no offense to the German company or it's people. Actually I enjoy their control system products very much and meant no ill will toward you guys. It's just that during the sales cycle it's sometimes hard to get all the info you need. Further, during a sales cycle you rarely get to talk to the designers.
RE: VFD Low Load Power Factor
RE: VFD Low Load Power Factor
The issue is as previously described, that the harmonic currents can be significant. This can result in distortion power factors less than 0.7
Drives with no DC bus reactor or AC line reactors have very high peak currents at the input of the drive. These peak currents are very narrow and occur on the crest of the voltage waveform.
The addition of AC line reactors and/or DC Bus reactors will widen the current pulses to a maximum of 120 degrees per half cycle, depending on the value of reactance added.
Active front end technology can reduce the harmonics down to a very low level.
The issues with the harmonic currents are:
Increased losses in the supply (important for smaller generators where the increase in I2R losses can be significant) and interference with AVR operation.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: VFD Low Load Power Factor
There will probably be a load rate limiter which will define a maximum rate at which you can increase the load on the turbine. If you try to increase the load beyond this limit the turbine will normally throttle back and slow down if it is island mode. Whether this action would actually shed any load with a rectifier or AFE drive is highly questionable but it might well cause either a turbine trip or the drive to reject the supply if it has sufficient intelligence to monitor it.
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If we learn from our mistakes I'm getting a great education!
RE: VFD Low Load Power Factor
RE: VFD Low Load Power Factor
An option which gives better motor starting and much less probability of voltage collapse is the Permanent Magnet Generator option. This is a small permanent magnet alternaor fitted to the shaft on the back of the main alternator and exciter. It produces the power for the main field. The output of the PMG is fed to the AVR where it is rectified and controlled.
In both cases the small amount of energy required is supplied by the prime mover.
respectfully
RE: VFD Low Load Power Factor
what are your thoughts on using the AFE as a power factor correction capacitor for lagging induction motor loads connected across-the-line to the same bus as the AFE, parallel to it?
what I would want to do is modify the AFE's power factor during runtime into a leading one to counter the motor's as the motor starts, analogous to switching on a capacitor.
I'm thinking I would figure out what the reactive current in the parallel motor is, then add this to the requirement for the AFE.
i would do this instead of a capacitor because i have a footprint size issue
rock
RE: VFD Low Load Power Factor
If this is an good idea depends on the alternatives you have:
If your supply voltage waveform has low harmonics and you can connect power factor correction capacitors without concern, this will be the more effective option. The power loss of a power factor correction capacitor is much lower than the additional losses you have to expect in the AFE.
If your supply volate is heavily distorted and you would need power factro correction capacitors with choke or harmoncic traps (with the inductances causeing a lot of losses), the use of the available AFE kVA might be an intresting option.