Power Factor effects of an inverter on the Grid

Status
Not open for further replies.
Grouch

Grouch

Senior Member
Location
New York, NY
I found this article on how the power factor of an inverter can affect the power factor of the grid. It's from 2015, so I'm assuming this is mostly accurate to this day?


If you go to page 3 it has an explanation (and nice pictures) on how a unity power factor inverter only delivers real power (watts) to a building. That decreases the real watts that the utility delivers; however, the reactive power, Vars, is still being delivered at the same amount to the building. Since the real power being delivered by the utility has decreased, the total apparent power has also decreased. and when you divide the watts by the apparent power, the power factor in the example on page 3 went down to 0.77.

My questions are:
1. For small residential projects... are most inverters only available at unity power factor?
2. Do larger projects, such as commercial, offer inverters where you can modify the inverter power factor, so the inverter can deliver both real and reactive power? That way it won't affect the utility power factor as much.
3. If you click on this inverter by Solaredge, it shows a power factor range between 0.85 and 1 for both models. Does that imply that the power factor on these inverters is manually adjustable?

Thanks again.
 
5 years ago almost every residential inverter always output at unity. However, larger central inverters (like, 50kW plus probably) were already getting features to adjust power factor, which helped with interconnection issues. These days just about all newly manufactured inverters for the US can adjust power factor under certain conditions because it's required in California which is the largest market. The features (along with others) are part of UL1741SA, an addition to the original UL1741 covering inverters and other solar equipment. An inverter with these features can be called a 'Grid Support Inverter' as opposed to a mere 'utility interactive' inverter. The features typically have to be enabled by the installer.

I have to say that it's probably really spotty how much residential installers are actually aware of and implement the features (see ggunn's incorrect answer to question 1). And I have no idea how often they actually kick in during typical operation.
 
jaggedben said:
5 years ago almost every residential inverter always output at unity. However, larger central inverters (like, 50kW plus probably) were already getting features to adjust power factor, which helped with interconnection issues. These days just about all newly manufactured inverters for the US can adjust power factor under certain conditions because it's required in California which is the largest market. The features (along with others) are part of UL1741SA, an addition to the original UL1741 covering inverters and other solar equipment. An inverter with these features can be called a 'Grid Support Inverter' as opposed to a mere 'utility interactive' inverter. The features typically have to be enabled by the installer.

I have to say that it's probably really spotty how much residential installers are actually aware of and implement the features (see ggunn's incorrect answer to question 1). And I have no idea how often they actually kick in during typical operation.
Click to expand...
I stand corrected, but as ben says, if it's there it's rarely used.
 
jaggedben said:
For Solaredge you could start with this application note.

You could probably also google 'Solaredge Grid Support' or 'Solaredge Rule 21'. (The latter applies to California but might be instructive in general.)

This isn't the simplest stuff.
Click to expand...
Great, I can try to look thru it. If I can't find anything, i can also call Solaredge tomorrow and see what they say about the power factor they show on their cut sheet. Thanks.
 
My understanding is Solaredge power factor can be set at a value in that range, or it can change in response to commands, or it can respond to voltage and maybe frequency conditions. Not sure if it can do more than one of those at the same time.
 
Most inverters operate at unity power factor but are rich in harmonics. Displacement power factor doesn't fix that.
 
jaggedben said:
5 years ago almost every residential inverter always output at unity. However, larger central inverters (like, 50kW plus probably) were already getting features to adjust power factor, which helped with interconnection issues. These days just about all newly manufactured inverters for the US can adjust power factor under certain conditions because it's required in California which is the largest market. The features (along with others) are part of UL1741SA, an addition to the original UL1741 covering inverters and other solar equipment. An inverter with these features can be called a 'Grid Support Inverter' as opposed to a mere 'utility interactive' inverter. The features typically have to be enabled by the installer.

I have to say that it's probably really spotty how much residential installers are actually aware of and implement the features (see ggunn's incorrect answer to question 1). And I have no idea how often they actually kick in during typical operation.
Click to expand...
A couple of things...

See the composite data sheet for SMA residential inverters:

All of the inverters show AC nominal output and maximum apparent power as the same value in W and VA; does that not imply a power factor of 1.0? There also isn't the mention of adjustability of PF that I have seen on commercial inverter data sheets. That's what I based my answer on; if PF adjustability is in there somewhere that I haven't seen or understood, then of course I stand corrected.

Also, there is a plot of output power/rated power vs efficiency; it's pretty flat down to 0.1 (10%); underloading them isn't a problem over most of the available range.
 
ggunn said:
All of the inverters show AC nominal output and maximum apparent power as the same value in W and VA; does that not imply a power factor of 1.0?
Click to expand...
Not necessarily; it just means that if the inverter supports non-unity power factor, its real maximum power output will decrease with decreasing power factor, as the VA limit will control. I.e. the limit is a maximum current, be that in phase with voltage or slightly out of phase with voltage.

However, the spec sheet you referenced does also say "Power Factor 1" so it sounds like those inverters don't support altering power factor.

Cheers, Wayne
 
Grouch1980 said:
That's my question as well actually.
Click to expand...
It means that if you do set it up with non-unity power factor to support a reactive load, then you reduce the maximum amount of real power you can produce. You can correct for power factor by taking advantage of the power factor adjustment range, but there is a drawback to doing so. In other words, you have a maximum KVA and Amp rating available to you, and you can either use it entirely for real power, or partially for both real power and reactive power. By default, it has a power factor of unity or close to it, when at full power, unless you program it otherwise. There are harmonics involved and a distortion power factor as Post #8 mentioned, but it usually is limited to 3% or less by the design of the filtering.

Some inverters have headroom on their KVA rating so that the unit can run at the full intended KW power rating, and still support a reactive load. The utilities benefit from inverters that correct for reactive power, so it is an advantage for the grid as a whole to have inverters that can do this. The big question is, how are they going to account for this when defining your system size, and will the application reviewer understand the difference between kW and KVA? You may need to submit a letter from the manufacturer clarifying that the kW rating will not be exceeded, if you depend on the sum of only the KW ratings to define the system size.

Note that when the DC section states "Maximum DC Power at cos ϕ=1", the power factor is what cos ϕ refers to. It is unrelated to angle of incidence, as I first thought when I saw that on the datasheet. It is the maximum usable DC power at unity power factor, and any power greater than that would be curtailed, and remain in the modules in the form of thermal energy.
 
Last edited:
Carultch said:
It means that if you do set it up with non-unity power factor to support a reactive load, then you reduce the maximum amount of real power you can produce. You can correct for power factor by taking advantage of the power factor adjustment range, but there is a drawback to doing so. In other words, you have a maximum KVA and Amp rating available to you, and you can either use it entirely for real power, or partially for both real power and reactive power. By default, it has a power factor of unity or close to it, when at full power, unless you program it otherwise. There are harmonics involved and a distortion power factor as Post #8 mentioned, but it usually is limited to 3% or less by the design of the filtering.
Click to expand...
Ah. So these SMA inverters can be programmed to decrease their power factors from unity to say, .85? It doesn't look like the spec sheet lists the available range. is that what the solaredge spec sheet was implying (from my post #1)? it showed a PF range from .85 to 1... meaning that's the available range?

Back to the SMA inverters... so you decrease the KW rating from its listed max rating... now does the max KVA rating go down as well, since you pulled down the KW rating, or can you also adjust the KVars rating, and therefore adjust the KVA output to whatever you want (without exceeding the KVA max rating)?

Carultch said:
Some inverters have headroom on their KVA rating so that the unit can run at the full intended KW power rating, and still support a reactive load. The utilities benefit from inverters that correct for reactive power, so it is an advantage for the grid as a whole to have inverters that can do this. The big question is, how are they going to account for this when defining your system size, and will the application reviewer understand the difference between kW and KVA? You may need to submit a letter from the manufacturer clarifying that the kW rating will not be exceeded, if you depend on the sum of only the KW ratings to define the system size.
Click to expand...
When you say headroom, do you mean the inverter cutsheet would show a max KW rating of 5, for example, and show a max KVA rating of 7 KVA? Just using random numbers.
 
Grouch1980 said:
Ah. So these SMA inverters can be programmed to decrease their power factors from unity to say, .85? It doesn't look like the spec sheet lists the available range. is that what the solaredge spec sheet was implying (from my post #1)? it showed a PF range from .85 to 1... meaning that's the available range?
Click to expand...
Yes. This is the available range of power factor you can program, or dynamically adjust through a controller.

It typically can be adjusted in either direction, so you can either adjust it to support an inductive load like motors, or a capacitative load like the power supplies of most electronics. Inductive loads cause current to lag behind voltage, and capacitative loads do the opposite. Inductive loads used to be the only kind of loads that did this, and power factor problems used to always be lagging, but with modern loads, power factor problems can happen in both directions.

Some datasheets may incorrectly assign a negative sign to indicate the direction, but the truth is that both kinds of power factor should be positive with the word of either "leading" or "lagging" to follow it. Displacement power factor PF = cos(phi), where phi is the phase angle between voltage and current, and there is symmetry to this function about phi=0. You would have to have phi exceed 90 degrees for power factor to really be negative. The phi can be positive or negative, but cosine of phi is always positive unless real power is flowing in the opposite direction than we assigned as positive. Phi being negative would mean the inverter is a power sink instead of a power source.

Grouch1980 said:
Back to the SMA inverters... so you decrease the KW rating from its listed max rating... now does the max KVA rating go down as well, since you pulled down the KW rating, or can you also adjust the KVars rating, and therefore adjust the KVA output to whatever you want (without exceeding the KVA max rating)?
Click to expand...
The max KVA rating would remain fixed, unless you also program this to be reduced.

The equations involved are P^2 + Q^2 = S^2, and pf = P/S. P is real power with units of kW, Q is reactive power with units of kVAR, and S is called apparent power with units of kVA. Power factor (pf) is unitless. These are the traditional symbols used for these quantities, based on P's alphabet neighbors. R is reserved for resistance, so it was skipped.

You can see that increasing Q, requires decreasing P. There is a right triangle that relates all of these terms called the power triangle.

If you are curious about the theory about this from first principles, I have further information in post 24 in the following link:

Power Triangle Challenge

Another approach. Just a brief section of it. Motor pf (pu) 0,646 0,699 0,744 0,781 0,810 0,834 0,852 0,865 0,875 Motor kW (kW) 1371 1487 1610 1737 1871 2011 2157 2310 2469 Motor kVAr (kVAr) 1620 1521 1445 1390 1353 1333 1328 1339 1366 PFC (kVAr) 1400 1400...
forums.mikeholt.com

Grouch1980 said:
When you say headroom, do you mean the inverter cutsheet would show a max KW rating of 5, for example, and show a max KVA rating of 7 KVA? Just using random numbers.
Click to expand...
Yes indeed. With this example of numbers, 71.4% would be the minimum limit of adjustable power factor, that would not require sacrificing your 5kW maximum rating. That's not a realistic example that I've seen, but it could exist in theory. The most common amount of headroom on the kVA rating that I've seen, is where the kVA rating is about 1.1 times the kW rating.

Given a kW rating of 5 kW and 5.5 kVA as an example consistent with the 1.1 factor, this would mean that 90.9% is the minimum power factor that you could program without sacrificing the 5kW rating. Programming an 0.8 power factor, would limit your real power to 4.4 kW.
 
Last edited:
Since the SMA spec sheet says the inverters are UL1741SA compliant, that means they must support the volt-var feature that "provides dynamic reactive power (VAr) output (absorption or injection) through responses to voltage measurements." (Google it.) So the spec sheet is definitely not telling the full story when it simply says the power factor is 1. Maybe that is just the default. Seems like they need to update their spec sheets with at least a footnote. Whether the SMA single phase line has other power factor settings a la Solaredge is something I can't speak to.
 
Carultch said:
If you are curious about the theory about this from first principles, I have further information in post 24 in the following link:

Power Triangle Challenge

Another approach. Just a brief section of it. Motor pf (pu) 0,646 0,699 0,744 0,781 0,810 0,834 0,852 0,865 0,875 Motor kW (kW) 1371 1487 1610 1737 1871 2011 2157 2310 2469 Motor kVAr (kVAr) 1620 1521 1445 1390 1353 1333 1328 1339 1366 PFC (kVAr) 1400 1400...
forums.mikeholt.com
Click to expand...
Thanks for the link... I'll have to sit down later tonight or tomorrow to follow the math all the way thru.

On small residential projects, I'm assuming it's not typical / common to add a note onto the drawings to have the power factor of the inverter adjusted from its default value of 1... anything lower than unity PF, now the owner won't be generating the full amount of real power that his system could be capable of to offset the utility. At the expense of the utility's PF dropping. Would this be correct? PF correction would make more sense on larger commercial / industrial projects.
 
Grouch1980 said:
Thanks for the link... I'll have to sit down later tonight or tomorrow to follow the math all the way thru.

On small residential projects, I'm assuming it's not typical / common to add a note onto the drawings to have the power factor of the inverter adjusted from its default value of 1... anything lower than unity PF, now the owner won't be generating the full amount of real power that his system could be capable of to offset the utility. At the expense of the utility's PF dropping. Would this be correct? PF correction would make more sense on larger commercial / industrial projects.
Click to expand...
If the utility is going to ask about power factor they will do it in the course of processing your interconnection application. On larger projects you want that application approved in principle before you build the project. On typical resi size projects I've never been asked about it other than with regard to the UL1741SA 'smart inverter' requirements discussed above. (And notwithstanding that the utility demands that the inverters have that capability, I've never seen them follow up to ensure that it is enabled!)

The buidling department won't care whatsoever.
 
The variable PF in modern grid-tied inverters is mainly there to comply with CA Rule 21 requirement for Volt-VAR control of reactive power to support grid voltage stability. So, it's generally not possible to manually set a specific PF.
 
Status
Not open for further replies.
Top