Power Factor requirement by utilities, how do inverters maintain power factor?

[unseengundam]

Looking through Utility document I found almost all of them in Texas require a power factor of 0.95 or better. In fact one city has .97 power factor requirement: www.weatherfordtx.gov/index.aspx?NID=1004

However, most of information I have found online doesn't give how Grid-Tie Inverter can property handle power factor. Seems like they only generate real power not reactive power, is this correct?

How can I get grid-tie inverters to keep a high power factor?

If I don't keep the right power factor, at best I will get hefty surcharges and at worst they will disconnect me from the grid!

[unseengundam]

Also, note the Solar Edge 3 Phase inverter's spec I wanted to get has no mention of power factor at all:

http://www.solaredge.com/files/pdfs/products/inverters/se-three-phase-us-inverter-datasheet.pdf

[SunEagle]

Looking through Utility document I found almost all of them in Texas require a power factor of 0.95 or better. In fact one city has .97 power factor requirement: www.weatherfordtx.gov/index.aspx?NID=1004

However, most of information I have found online doesn't give how Grid-Tie Inverter can property handle power factor. Seems like they only generate real power not reactive power, is this correct?

How can I get grid-tie inverters to keep a high power factor?

If I don't keep the right power factor, at best I will get hefty surcharges and at worst they will disconnect me from the grid!

While I am not an expert on this subject I would say that unless your inverter is grossly over-sized it should be generating at a power factor close to 1.

Also according to that "weatherfordtx" document that 97% PF requirements is for co-generation > 20kw.

While I also could not find any PF rating for the Solar Edge equipment I did see the rating for a SMA 20kw & 24kw 3 phase inverter with a PF = 1 at rated load.

If you are looking at Solar Edge equipment maybe contacting their factory will get you a PF rating.

[Sunking]

Power Factor is not determined by the generation source. PF is a mathematical relationship between resistance and reactance of the load which has nothing to do with a Inverter or Generator. If what you say were true, you could not connect anything up to the grid except resistors.

[SunEagle]

Power Factor is not determined by the generation source. PF is a mathematical relationship between resistance and reactance of the load which has nothing to do with a Inverter or Generator. If what you say were true, you could not connect anything up to the grid except resistors.

While I will agree with you I was surprised to find PF ratings on a lot of the single phase and 3 phase inverters. While just about all were around 99% - 100% there was some mention of them having a PF = 80% depending on the loading of the inverter.

My guess is that some of these inverters, if too big, can generate losses resulting in a poor PF similar to an induction motor being over sized and creating poor PF.

[Sunking]

Sun Eagle when you see a Genny or Inverter with a rating like 10 Kva @ 80% PF is telling you worse case the genny can supply. It means it can deliver up to 10 Kw at PF = 1 down to 8 Kw @ PF = .8. It cannot produce 7 Kw @ PF = .7, or 5 Kw @ PF = .5.

Your transformerless GTI cannot generate VARS, only pure power. It does not need to as the Grid is a very stiff source and can easily absorb a punny GTI. Stop and think about it. Your POCO genny is producing some 900 Mw, and a GTI 10 Kw is not even a drop in the ocean and has no effect on the utility whatsoever. Does not save the utility one drop of fuel or change PF.

[SunEagle]

Sun Eagle when you see a Genny or Inverter with a rating like 10 Kva @ 80% PF is telling you worse case the genny can supply. It means it can deliver up to 10 Kw at PF = 1 down to 8 Kw @ PF = .8. It cannot produce 7 Kw @ PF = .7, or 5 Kw @ PF = .5.

Your transformerless GTI cannot generate VARS, only pure power. It does not need to as the Grid is a very stiff source and can easily absorb a punny GTI. Stop and think about it. Your POCO genny is producing some 900 Mw, and a GTI 10 Kw is not even a drop in the ocean and has no effect on the utility whatsoever. Does not save the utility one drop of fuel or change PF.

That is what I thought also concerning GTI equipment. PF for a solar pv generation should never even come into the equation.

What was strange was that document the OP posted that goes over the requirements that must be met for any type of co-generator. The >97% PF stimpulation was for a co-gen producing > 20kw. So that POCO had some concern that a co-generator might install equipment that would produce a low PF which would affect the POCO's transmission power. That is something pretty common with most POCO's.

I worked with a POCO in New Mexico that had added PF correction caps to their 12.5kv lines feeding a single customer's 9 transformers. They did this because their Transmission dept did not like the low PF coming from the plant. Since the plant had a very small financial penalty in their contract for low PF they did not do anything to mitigate the problem. Of course the PF caps that were installed ended up generating some pretty bad harmonics into the plant causing equipment failures. My job was to install LC filters in the plant to not only mitigate harmonic distortion but also improve the PF to > 98%.

[jflorey2]

However, most of information I have found online doesn't give how Grid-Tie Inverter can property handle power factor. Seems like they only generate real power not reactive power, is this correct?

They can generate both, but in practice they only generate real power.

A grid tie inverter synthesizes an AC output waveform from a DC input. Thus they can make the waveform look like whatever they want to. If they force more current to the grid earlier in the voltage cycle, they appear as a "leading" source, and will thus can cancel out an inductive load somewhere else on the grid. However, most inverters try to match the voltage waveform exactly, resulting in a close to unity power factor.

Some larger inverters have the ability to artificially create a reactance shift to cancel another poor power factor load elsewhere on the grid. This makes economic sense for large installations because utilities are often willing to pay for power factor correction. I have not heard of any small inverters that do this though.

(General note - people refer to two kinds of power factor, linear and non-linear. Linear refers to phase shifts between voltage and current caused by capacitance or inductance on the grid; nonlinear refers to current waveform distortion caused by things like phase-control lighting or large rectifiers converting AC to DC.)

How can I get grid-tie inverters to keep a high power factor?

In general there's nothing you can do (or need to do) on the inverter side; modern grid tie inverters are already generating high power factor power in order to meet utility requirements. However there is one case where your power factor will go down after installing a GT inverter, and that's when you have a large reactive load somewhere else in your home (huge compressor that runs all the time, something like that.) In that case, the GT inverter will provide unity-power-factor power (i.e. all real power) and cancel out some of the real power demand of the compressor - but it will not cancel the reactive power demanded by the compressor. Thus from the utility's point of view, even though your load goes down, the power factor gets worse because the percentage of reactive power being delivered goes up.

If I don't keep the right power factor, at best I will get hefty surcharges and at worst they will disconnect me from the grid!

If your other loads (like compressors or other large motors) are already causing you headaches, it would be worth it to talk to the utility about adjusting your power factor (by adding a capacitor bank or something similar) to ameliorate the problem. Equal capacitive and inductive loadings will cancel. Ideally they go on the switched side of the power so that the capacitance is present only when the motor is running. Inverter motor drives will also solve the problem; modern inverter motor drives include power factor correction.

But if you don't already have problems, then adding a GT inverter in general won't cause you new power factor problems. And residential loads rarely have issues with power factor.

[sensij]

SolarEdge inverters have extensive documentation of power factor control. They can actually receive an external signal (from the poco) and adjust power factor as needed. In the default mode of operation, at startup and shutdown the power factor is much less than 1, although it pegs out at 1 once a couple hundred watts are being generated ( 3kw inverter).

[DanS26]

SolarEdge inverters have extensive documentation of power factor control. They can actually receive an external signal (from the poco) and adjust power factor as needed. In the default mode of operation, at startup and shutdown the power factor is much less than 1, although it pegs out at 1 once a couple hundred watts are being generated ( 3kw inverter).

I see the same performance with my two Fronius 7.5 kW string inverters. Monitored with a TED system....startup and shutdown low PF but as soon as normal radiance is achieved then they will peg at 100% PF. Now a passing cloud will cause a 98 to 99% PF but I think that is due to the measurement delay in the TED system and not a fluctuation at the inverter.

I monitor my 1980's SubZero refrigerator and the PF on that dedicated line is always ~60% PF......yeah, I know, time to scrap that device but it just keeps chugging along.

[Sunking]

I monitor my 1980's SubZero refrigerator and the PF on that dedicated line is always ~60% PF......yeah, I know, time to scrap that device but it just keeps chugging along.

Your Iverter cannot fix that. No Inverter can correct the Utility PF. It is like you trying to raise the ocean level by peeing in it. A .01 Mva generator cannot change the PF of a 1000 Mva generator. The only thing a GT Inverter can do is make your load look like a 1 PF to the POCO which only helps them, and does nothing for you.

[jflorey2]

I monitor my 1980's SubZero refrigerator and the PF on that dedicated line is always ~60% PF......yeah, I know, time to scrap that device but it just keeps chugging along.

If that bugs you, power factor correction capacitors are a cheaper/easier solution than compensating with an inverter. But as others have pointed out, it won't bother your utility much.

[inetdog]

They did this because their Transmission dept did not like the low PF coming from the plant. Since the plant had a very small financial penalty in their contract for low PF they did not do anything to mitigate the problem. Of course the PF caps that were installed ended up generating some pretty bad harmonics into the plant causing equipment failures. My job was to install LC filters in the plant to not only mitigate harmonic distortion but also improve the PF to > 98%.

This is a misinterpretation of what was going on.
The problem would not be that the source had a low power factor but rather that the source had little capability to supply a low PV load.
You were adding PFC to adjust your load to a high power factor so that the source could supply it properly.

You will often see large generators rated to supply, for example, 10kW at a PF of .8. What that means is that the prime mover of the generator can only actually supply 8kW continuously but that it has the additional current capability to supply a low power factor load (10kW) which requires more current than a resistive load of the same true power.

Some large scale GTIs for commercial use do have the ability, triggered by specific control inputs, to supply reactive power to help the utility deal with low power factor loads on their network. This is an advantage for the utility and saves them money they would otherwise have to shell out for switched PFC banks.

But residential scale GTIs do not have the control input or that capability and only, by design, deliver current which is synchronized to the POCO voltage waveform.
When you have local reactive loads, the entire reactive power comes from the POCO source, not the GTI.
That can make the load seen by POCO appear to have a lower power factor than it actually does since the GTI will supply part of the real power and leave the POCO to supply all of the reactive power. In theory the local load plus that kind of GTI in combination could appear to have a zero power factor and only consume reactive power from POCO.

OK, there is one way in which a power generation system can be considered (stretching it a bit) to have a low PF and that is when the voltage waveform that they deliver is not sinusoidal. The same distortion PF (not correctable by phase shifting) that is seen in the current of some non-linear loads could be found in the voltage output of a generator. A Modified Square Wave inverter would be an extreme example of that.
A generator which is supplying non linear loads from other customers could well be delivering to you a voltage waveform which already has a high harmonic content. PF correction capacitors do not have any ability to mitigate the incoming voltage harmonics! They can just make the problem worse, leading to your need to install LC filters, which can correct for voltage waveform distortion.

Also, except when they are being switched in and out, PF caps should not generate any harmonics. When connected to a load which is not linear (bad distortion PF) as well as having a low displacement PF (cos theta), PF caps could increase the harmonic current for a given harmonic voltage from the source. Or when connected to a source with high harmonic content they could generate high harmonic currents which would make the voltage waveform even worse!

[Sunking]

If that bugs you, power factor correction capacitors are a cheaper/easier solution than compensating with an inverter.

Yep and that scam is alive and well. I can sell you a PFC capacitor bank. Three parties will be Tickled Pink.

• 1st Party is myself, I made a nice fat profit.
• 2nd Party is the POCO, they will loose a little less money because you invested your money, and they did not have to use their money. Companies really like it when you give them money and expect nothing in return.
• 3rd Party is you because with the PFC Capacitor Bank I included a Pink Flamingo Feather to tickle yourself pink with.

[DanS26]

If that bugs you, power factor correction capacitors are a cheaper/easier solution than compensating with an inverter. But as others have pointed out, it won't bother your utility much.

It doesn't bug me, since as a residential customer I only pay for "real" power but when it is running off the inverter I have to produce that "real" power. I was just making the point, maybe off topic a bit, that an older 1980's motor/compressor is very inefficient. I'm sure the motor tech has improved somewhat from 35 years ago.

In the summer months it takes about 250 kWh per month to run, somewhat less in the cooler months where it ONLY uses ~160 kWh per month. I keep it running since it would cost me ~$10k to replace.