Setup: generator supplying balanced single phase loads. Loads are non-linear, generating substantial amount of harmonics currents, distorting voltage to the degree. Total power factor is about 0.7, when displacement power factor is close to 1. Q: Would application of the capacitors lower the current supplied by the generator? Art.
No. Capacitors change the displacement power factor. If it is 1.0 now, it will only get worse (more current) if you add capacitors. You either need filters or to seriously oversize the generator.
Charles Perry P.E.
Note that application of standard harmonic filters (series-tuned circuit with reactor in series with a capacitor bank) will still send the displacement power factor into the lead. This can overexcite the generator, leading to saturation or tripping on overvoltage. See
"Charles Perry" wrote in news:buq4sg$kvtaa$ snipped-for-privacy@ID-103962.news.uni-berlin.de:
Coming in from the cold. What are the harmonic levels for 3,5,7,9,11? Amps and volts please.
Have you tested the grounding of the generator? If the generator is code grounded,
Well, traditional pf correction using capacitors won't help here. It would only shift the displacement into the 'leading' quadrant. But if the 'capacitors' are part of a harmonic filter installed between the loads and generator, then yes *that* would help.
If the three-phase currents are balanced, check the neutral current. Unlike linear loads that would have nearly zero neutral current, non-linear loads can actually have more current in the neutral than any one of the lines.
daestrom
Thanks you all for the input. My understanding was that it is not possible to correct the power with the conventional PF correction equipment factor when it is due to harmonics , but just one of the books I read sparked a bit of doubt. I wanted to get some confirmation from you guys that I was corrrect in my understanding. The actual values are different than originaly posted, displacement PF is not 1, but closer to 0.7. Any way, if someone is interested I posted the "Export.zip" file on my web side
Just curious:
What would happen if you used a parallel tuned circuit in parallel with the load (or three if you are using thee phases.)? It should not have any effect on the PF at the fundamental frequency.
I don't have a dog in the fight, I'm just curious.
If you have a three phase system, consider installing (or replacing an existing) transformer with one having delta winding (primary or tertiary), designed to bypass load harmonic currents.
P.S. Your .zip file comes back as a 'Page not found' error.
Thank you Paul for pointing the "not found" problem, its fixed now. As to replacing the transformer, this is not an option since the load is supplied by the generator directly. Art.
I'm not sure... But isn't there a way to connect a wye-delta transformer onto a line to act as a harmonic 'trap' of some sort? It doesn't have to supply the load, just have the primary connected on the line and the secondary unloaded. Of course, the size has to be based on the amount of harmonics you're trying to 'trap'. But if connected on the line at the same point as the load, it would prevent the harmonic currents from being felt by the generator. But maybe this is only for even harmonics, I can't remember.
Unfortunately, the ninth edition of "Standard Handbook for Electrical Engineers" has a printer's error and thirty some pages were omitted. Including the section on harmonic transformer connections :-( If I remember, I'll look it up at my copy at work (newer edition).
daestrom
Yes, this can be done. The Y (connected to the feeder phases and neutral) will provide a path for harmonic as well as zero sequence currents to bypass the generator windings. Some harmonic and unbalance current will still flow through the generator, the amounts depending on the impedances of the Y-delta transformer and the generator. Introducing a higher zero sequence impedance into the generator circuit (the ultimate high zero sequence impedance being an open neutral) will cause more of these currents to flow through the transformer.
I looked it up in a newer edition at work today. A wye-delta with the wye connected across the line and the delta unloaded. The 'delta connected secondary provides a low impedance path to third harmonics'. Not sure which harmonics are generated from non-linear loads, probably a series of them. So this would 'trap' some, but not all.
daestrom
I have updated the zip file on my web site, I included wave forms, harmonic analysis, currents and voltages (kind of fun stuff). If any one wants to comment, you are very welcome to do so, I will be grateful for all hints.
Art.
"> > > > > posted, displacement PF is not 1, but closer to 0.7. Any way, if
Looking at the voltage vs current waveform, current is lagging voltage indicating mostly inductive load. Adding a capacitor bank in parallel with the inductive load should bring your displacement power factor to near unity. There are several capacitor bank makers (AEROVOX is one of them) who should be able to help you.
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If you are adding caps, I would highly recommend a 'detuned' capacitor bank. It will help you avoid parallel resonances (caps in 'parallel' with the source) and the possibility of serious harmonic problems. Also the inductors in a detuned cap bank do a good job of protecting the capacitors against voltage transients. There are many considerations when dealing with harmonics and power factor, particularly when the system may be supplied by a generator.
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