Power factor correction and harmonics mitigation

[arisakri]

Hello to everyone!

I am little bit confused with power factor correction and harmonics mitigation and I wanted to talk to you. I know that VFDs produce harmonics and i can not install capacitor banks, right? let's assume that i have 5 motors controlled by vfds and they are fed from the same panel. Using an power analyzer i measure on the panel cosφ 0.85 and THDi=12%. How can i correct the power factor to 0.96 and THDi=8% without using active filters?

And another concern for harmonics mitigation. I read that is suggested to install a DC bus choke or line reactor. But a DC bus choke should be installed inside VFD ,between the input diodes of the rectifier and the DC bus link. So on existing installations i can not use a DC bus choke to reduce THDi but only replace the VFD with a new one with DC bus choke right?

 

[waross]

Power Factor is the ratio of real power to apparent power.
There are two reasons that the apparent power appears higher than the real power.
The first reason and the historical reason is that reactive loads cause a phase shift between the current waveform and the voltage waveform.
This is called displacement power factor and in most cases is caused by inductive loads.
The remedy for this is capacitor banks.
As the use of non-linear loads has grown over the years we have become aware of a second reason that the apparent power is greater than the real power.
Non-linear loads tend to create harmonics and the higher frequencies of the harmonics tend to have more losses and issues than the base frequency.
This is called distortion power factor.
The remedies for distortion power factor are filters and special transformer arrangements.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Also as a general rule, utilities only require the displacement power factor to be corrected to .95. Most of the time their instrumentation cannot accurately read distortion power factor, so then they will separately require that you meet IEEE 519 guidelines for harmonic mitigation, which will then indirectly take care of the distortion power factor anyway. My suspicion is that you have an analyzer here that is "too accurate" in that it is determining the combined PF; both displacement and distortion. I have yet to see a modern VFD that shows worse than a .95 displacement power factor. Now, your 12% THDi is high, which is what I would focus on correcting, as Bill said, with a filter of one sort or another or by a phase shifting transformer into a 12 or 18 pulse drive rectifier or by changing the VFD to a Low Harmonic version.

As to your question on the DC bus chokes, most good quality VFDs will have DC bus chokes, unless they are one of the several lines designed in Japan (or their offspring) where capacitors are cheap. But even then, the better quality brands and models without chokes usually provide provisions (terminals) for connecting them in the field after the fact. If you have a VFD with neither, you likely purchased it solely on price.


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

 

[mparenteau]

As I am reading this...the OP's post is talking about current distortion. 12% is not high IMHO. Unless you are seeing systemic electrical problems, I wouldn't believe mitigation is needed. If this is supposed to be voltage distortion, then yes...there is a problem.

Mike

 

[arisakri]

Thank you all for your answer. It wasn't a realistic problem. I used it just to help us to discuss. But still i don't understand if capacitor banks can used with VFDs.

Furthermore could you suggest me a guide for measuring powe quality on the field, like measurement duration, limitations etc. Something like

https://www.google.com/url?sa=t&sou...Vaw16alSP_1ZSr0aD-qEAzME-&cshid=1589213537434

 

[SolarPrestige]

If you want to install PF correction in front of the supplies to VFDs, then you either need harmonic chokes with the capacitors to stop them absorbing VFD harmonics, or line chokes on the VFD inputs to stop them producing harmonics to the line side. The capacitors should only be absorbing your mains frequency and not much of the higher harmonics (1/wC). They will draw high currents at higher frequencies, if allowed to, and they will get very hot and have a short life.
Remember fires are an issue with PF correction due to items getting hot. Always due to poor design or poor understanding. I have investigated one automatic PF correction system after a fire, where the fuse holders have caught fire (bad design/poor understanding).

In a similar vein, we used to put harmonic chokes on PF capacitors to reduce their absorption of power company ripple frequencies. Old technology now, as ripple frequencies are no longer used here, but I do remember this usage.

Always consider whether the capacitors are better installed at the motor terminals. But not the ones fed by VFDs, the ones that are DOL or similar.

 

[LionelHutz]

Capacitors are often used on the line side of a VFD or rectifier type load as part of a harmonic trap filter.

Installing them as simple 60Hz power factor correction will be a failure.

 

[SolarPrestige]

Lionel, when caps are installed as harmonic trap, they are acting as filters and accept harmonics by design. When caps are installed to correct PF elsewhere in the system, and there are VFDs on the same bus, they must be installed with anti-harmonic chokes or detuning reactors that create a detuned capacitor bank. Any non-detuned capacitors are to be removed, not allowed. The rejection starts below the lowest harmonic expected, accepting only the fundamental frequency of the supply. Different applications, different protections.

 

[LionelHutz]

SolarPrestige - The original post was about installing capacitors to correct the distortion power factor of VFD's. The hypothetical question posed was 5 VFD's on a panel with no other loads mentioned. That was the question I was addressing. My statement stands with regards to that question.

 

[SolarPrestige]

OK, There were two question in the original post. Specifically I was answering the one about PF correction and more specifically OP's 12 May follow on question : "But still i don't understand if capacitor banks can used with VFDs." (Sorry I don't know how to do quotes, I will look later to see how that is done.)

I take capacitor banks to mean PF correction.

 

[waross]

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Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I assumed that the question can capacitors be used with VFDs = can capacitors be used to correct the power factor of VFDs.

There are a number of passive filters available that will correct the harmonics of a VFD to below 8% THDi without resorting to an active filter.

 

[SolarPrestige]

Thanks waross, I will use that next time.

I think enough said now, except be careful with harmonics and capacitors. Good debate.

arisakri: if you want good general guides on power quality, then any of the reputable general electrical equipment manufacturers will have guides. I usually go to ABB, Schneider, Siemens, Eaton, Rockwell and so on. There's also the education providers that often have online stuff available. If you go to Megger or other test equipment manufacturers, you will get a guide that works with what they sell, which can be OK, but may be a bit biased.