PFC for linear power supplies, hybrids and test equipment

[Someone]

1. Bring the phase angle of the current waveform in sync with the voltage waveform (whatever that voltage waveform may look like!).

Isn't that what I just said?

THD and PF are two different things, thats why when you test a PFC converter they spec PF and THD. You can have good PF and poor THD and poor PF and good THD. A Pure sine wave has no harmonics but if its a reactive load then the sine current waveform is either lagging or leading the voltage sine wave. That means you have zero THD (pure sine wave) but poor PF (V & I out of phase). Thats what I recall without looking through books so I could be wrong.

I think there are special PF requirments for lighting, at least in the EU. This makes sense, as lighting is one of the largest loads on the grid, what with mass conversion to LED lighting, poor PF would negate about 40% of the benifit of LED lighting as far as the utility is concerned anyway.

When you may want PFC is if you want to maximise the number of loads off a circuit branch. A typical residential branch is good for 12 to 18A or so rms so the better the PF the more loads you can run off one branch. Other then that you derive reallly no benifit, the utility does. I guess you could be considered enviromentally conscious if that matters to you.

Not completely, Power Factor is a combination of many things and you geometrically sum the contributions of harmonics to the traditional lead-lag component. You cannot have a poor THD (harmonic content) and a good Power Factor, although many PFC systems rely on calculating the harmonics of the long term average rather of individual cycles on the assumption there will be many of them in operation cancelling each other out.

Here there is no real difference between a SMPS (without extra PFC) and a linear stabilized supply as the source is the rectifier and not the regulation.

This is the crux of it, its how the DC is first created that is causing power quality issues. The intermediate DC voltage could then be used for any arbitrary purpose. Although highly pulsed loads could have problems with active PFC as typical loads rely on the averaging above to achieve their ideal figures.

[Kleinstein]

To get a good power factor the waveform of the current has to be the same as the waveform of the voltage. So if the voltage is in crazy shape, the best power factor is having that same crazy waveform as well. However normally the waveform in the grid is still reasonably close to a sine, maybe sometimes going towards a rectangular shape.

The PF combines the effects of harmonics and phase shift in one number.

[b_force]

Not completely, Power Factor is a combination of many things and you geometrically sum the contributions of harmonics to the traditional lead-lag component. You cannot have a poor THD (harmonic content) and a good Power Factor, although many PFC systems rely on calculating the harmonics of the long term average rather of individual cycles on the assumption there will be many of them in operation cancelling each other out.

I could swear I seen some pretty funky VI  waveforms that still had good PF, are you sure?

edit
nevermind I found this

http://engineering.electrical-equipment.org/power-quality/4-8-electrical-equipment-harmonics-power-factor.html

Makes sense.

That blog makes it very understandable.
It's only not completely clear to me what the difference is between PF and DPF?
(doesn't say anywhere)

Another thing I don't completely get, is why a small but higher current spike is worse.
At the end we are talking about energy (or kWh).
The total energy of a small spike can be a whole lot less than a low, but longer perfect sine wave shape.

[SeanB]

PFC for anything with a SMPS inside is more to reduce harmonics in the neutral in the 3 phase distribution system. As the neutral typically is a thinner cable ( as it only has to handle the difference currents in the system, so can be smaller than the phase conductors in a supply) the extra harmonics tend to add up and overload it. Thus the need to reduce harmonics of the mains in a SMPS.

With inductive loads like transformers and motors you can add either a simple capacitive correction , or for larger transformers with a diode rectifier you add a series LC circuit tuned to 3 times mains frequency, to reduce those harmonics. Alternatively you use a ferroresonant transformer, which provides enough extra leakage inductance to isolate the power factor.

The power companies ( and large industrial users) have to add power factor correction to the supply side so this is not reflected down the power transmission lines, so as to reduce RF emission from the lines. Thus they also have power factor compensation capacitor banks, harmonic trap filters and synchronous capacitors in the power yards so as to keep the power factor over 0.9 lagging with only low levels of harmonics on the incoming feeds. This is a non billable power consumption, so they ideally want to keep this loss as low as possible.

[oldway]

A very good explanation:

https://us.tdk-lambda.com/ftp/other/pfc_switchmode_powersupplies.pdf