Negative power factor and PV systems

Status
Not open for further replies.

tallgirl

Senior Member
Am I correct in assuming that the POCO would prefer a PV system produce a more-negative (closer to -100%) power factor than just a small (further from -100%, but still negative) power factor?

There's a lot of stink about PV and power factor and I'm trying to understand what the heck the utilities want. Normally "smaller" is bad, but is it smaller in absolute value, or just plain smaller?
 
Am I correct in assuming that the POCO would prefer a PV system produce a more-negative (closer to -100%) power factor than just a small (further from -100%, but still negative) power factor?

There's a lot of stink about PV and power factor and I'm trying to understand what the heck the utilities want. Normally "smaller" is bad, but is it smaller in absolute value, or just plain smaller?
This would be a good question for our solar installation class....but I am not really sure what you are asking.

A poor power factor is a result of a reactive load. I am having difficulty trying to understand how a grid tied PV inverter can produce power with a poor power factor.
 

tallgirl

Senior Member
This would be a good question for our solar installation class....but I am not really sure what you are asking.

A poor power factor is a result of a reactive load. I am having difficulty trying to understand how a grid tied PV inverter can produce power with a poor power factor.
It doesn't.

The inverter produces power with a unity power factor, but the loads attached downstream from the service panel still require reactive power. However, since the system is producing power at a credit, the utility is providing reactive power and not receiving revenue -- which they hate doing.

So ... from a POCO perspective, do they want the power at the service panel to have a power factor of -1, or what? I'm assuming they want -1 since that means they aren't providing any reactive power at all to the loads downstream from the service panel.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
I am going to go out on a limb and say you have lost me on this one. "Power Factor" is defined as the cosine of the angle between voltage and current. That angle can range from minus 90 degrees (for a purely capacitive circuit) to zero (for a purely resistive circuit) to plus 90 degrees (for a purely inductive circuit). The cosine function is positive throughout that range. I infer that PF cannot be negative.

But I will admit that I know nothing about PV systems. Can you explain what you mean by a PF of -1?
 

tallgirl

Senior Member
Charlie,

That all depends on the direction of current flow. What happens when the power is turned around and goes the other way? The voltage is still whichever sign it was, but the direction of the current is opposite what it would be for a load.

For what it's worth, products such as the Shark 100 (ElectroIndustries/Gauge Tech) or the WattNode (Continental Control Systems) will report negative power factor, and the Shark reports phase angles from -180 to +180 degrees. Between -90 and -180 degrees, and +90 and +180 degrees the cosine is negative, and that's where the backwards current flow is happening.

And I think I've answered my own question, so if someone can confirm that higher absolute value of power factor, regardless of the direction the power is flowing, is better from the POCO perspective, that would be great.
 
It doesn't.

The inverter produces power with a unity power factor, but the loads attached downstream from the service panel still require reactive power. However, since the system is producing power at a credit, the utility is providing reactive power and not receiving revenue -- which they hate doing.

So ... from a POCO perspective, do they want the power at the service panel to have a power factor of -1, or what? I'm assuming they want -1 since that means they aren't providing any reactive power at all to the loads downstream from the service panel.
You have me lost, too. Power factor is expressed as a number between 0 and 1, with 0 being all reactive and 1 being all resistive. I don't see how a power factor of -1 can exist.

"However, since the system is producing power at a credit, the utility is providing reactive power and not receiving revenue -- which they hate doing."

What does a PV system have to do with the VARs that the load is requiring? If the utility is producing VARs it will do so with or without a PV system.

If you are suggesting that the Pocos require a reactive load to be corrected before connecting a grid tied inverter that is new to me. If that indeed is so, I would like to be able to bring some nomenclature to class for discussion.

I did find this in my text:

"Power Factor"

Interconnection standards require interactive inverters to maintain an output power factor between 0.95 leading and 0.95 lagging. Most interactive inverters produce AC output with a power factor of 1 under all conditions. Output power factor for stand-alone inverters is a function of the load, which may or may not operate with a power factor of 1."

As I read it, the requirement put forth by the poco is how the inverters are designed, not how they are installed or how reactive the load is. You may want to get with the field engineer for the poco and ask if this is what he or she is talking about.

Also, when we design grid tied systems, we have to design them so they will not produce power in excess of what is used. Only pocos can sell electric energy. Customers can not as they are not under the oversight of the public service commission.
 

mivey

Senior Member
The closer to unity the happier the POCO, for the most part. Most are happy with 90-95% or better and would rather see reactive than capacitive.
 

mivey

Senior Member
Marky,

With reduced real units on the bill (what the POCO uses) there are reduced revenues to cover the var costs they still incurr.
 
Marky,

With reduced real units on the bill (what the POCO uses) there are reduced revenues to cover the var costs they still incurr.
But would that not happen if any other means to reduce revenues were used, such as more efficient devices, better insulation, etc?

The meter doesn't (and can't) run backwards all the time. The net sum will still be positive revenue for the pocos, just a reduced positive revenue.

But back to Julie's original question, "There's a lot of stink about PV and power factor and I'm trying to understand what the heck the utilities want." I am curious as to what she is asking and why. I have not heard of any such stink. I also would like to know what the utilities want, especially if it is something we are not being taught in class.

If the utilities she speaks of require a pf of -1, I would like to first see it in writing and then see it in practice. REMF from an auxiliary power source is not reactance. At least that is what I was taught. Since it is not reactance, it cannot be a part of the power factor ratio.
 

mivey

Senior Member
But would that not happen if any other means to reduce revenues were used, such as more efficient devices, better insulation, etc?
Better insulation means a motor runs less. When the motor runs less it means less kWh and kvarh.

Better efficiency might mean we can use a smaller motor. A smaller motor means smaller kW and smaller kvar.

The meter doesn't (and can't) run backwards all the time. The net sum will still be positive revenue for the pocos, just a reduced positive revenue.
The problem is, part of the load is powered by the PV array and part is powered by the POCO. You have reduced the kWh the POCO must deliver but not the kvarh that is normally associated with those kWh. The POCO uses the kWh and kW billing units with an assumed or average power factor to determine what to charge to cover the kW, kWh, kvar, and kvarh costs.

Since the real component has been reduced without an accompanying reduction in the reactive component, it is the same as if they were serving a load with a horrible power factor. They can compensate for this with an excess reactive load penalty if they measure the reactive load.

But back to Julie's original question, "There's a lot of stink about PV and power factor and I'm trying to understand what the heck the utilities want." I am curious as to what she is asking and why. I have not heard of any such stink. I also would like to know what the utilities want, especially if it is something we are not being taught in class.
They want the customer to manage their power factor. For a simple example, imagine a 1,000 kW customer with ten 100 kW motors with a 90% pf. These each would have a var load of 48.43 kvar. So we have 1000 kW and 484.32 kvar for a 1111.11 kVA total load at a 90% pf. The utility would base their bill on 1000 kW and include enough in the rate to cover the cost of var loss and the 1111.11 kVA of equipment.

If we were more efficient in our process, we could use one less motor. We would have 900 kW and 435.89 kvar for a 1000 kVA total load at a 90% pf. The utility would base their bill on 900 kW and include enough to cover the cost of var loss and the 1000 kVA of equipment.

But suppose instead of being more efficient we added a PV array to crank out 100 kW. We would have 900 kW and 484.32 kvar for a 1022.04 kVA total load at a 88.1% pf. The utility would base their bill on 900 kW. They would have var loss associated with 484.32 kvar and the equipment would have to be sized to handle 1022.04 kVA of load but they would only collect enough revenue to cover var losses associated with 435.89 kvar and equipment sized for 1000 kVA of load.

It is not just a PV thing. It is the same for any generator behind the POCO meter. The generator needs to pick up enough of the var load to keep the metered power factor within acceptable limits or the POCO will either lose money or have to charge an excess var load penalty.
If the utilities she speaks of require a pf of -1, I would like to first see it in writing and then see it in practice.
Forget all that negative stuff as it is just confusing the issue.
REMF from an auxiliary power source is not reactance. At least that is what I was taught. Since it is not reactance, it cannot be a part of the power factor ratio.
But it does because the load has a kVA need made up of kW and kvar. If the aux source supplies the kW the utility is left to supply the kvar. The utility could be in the kvar business, and are to some degree, but the retail rates are not really designed for kvar only sales. The var load is normally just a parasitic part of the main business. The utility tweaks the rates to account for this parasitic component but the main business is to sell energy, not supply vars.
 
Last edited:

mivey

Senior Member
That all depends on the direction of current flow. What happens when the power is turned around and goes the other way? The voltage is still whichever sign it was, but the direction of the current is opposite what it would be for a load.
An easy way to say it is that when the voltage and current have the same sign the load is absorbing energy but when they have opposite signs the load is releasing energy.
 

robbietan

Senior Member
Location
Antipolo City
Utility Side

Utility Side

Being from a distribution utility, it can be safely said that utilities approve of customers that take care of their pf - the closer to unity, the better - the more discounts to the customer.

that being said, I do understand that PV arrays are something like generators - that they can generate VARs. now this gets tricky, residential customers are billed only in Watt-Hrs and generally have low p.f. if ever a residential unit produces more VARs that it uses, the amount may be so small the utility may look the other way. but it will certainly not compensate for the VArs produced.

if the amount of VARs (and Watts) produced is not-so-small, an agreement with the utility is needed with regards to connection requirements, billings, etc. in short, a buyer-seller agreement
 
An easy way to say it is that when the voltage and current have the same sign the load is absorbing energy but when they have opposite signs the load is releasing energy.
?????

Voltage and current signs?

If you mean polarity, only voltage has polarity. Current doesn't so you lost me.....

Also, a load always uses up energy, if it were supplying energy it would not be a load, it would be a source.
 
If the aux source supplies the kW the utility is left to supply the kvar.
Huh??

How can the aux source ONLY supply kWh and not kVArs? If the load is reactive, kVArs will be supplied by the aux source along with watts just like the primary source supplies them.

I don't see how the pF can be changed by the type of machine than supplies it's energy.

I think I know what you are thinking is happening, I just don't agree with it, which means one of us doesn't understand it.

This will be a good topic for solar class. I will be printing this discussion out and giving a copy to my instructor. Nowhere in our text does it tell us that part of a site survey is power factor correction.

Solar panels are rated in watts and not VA because they are DC. That I understand. Inverters are rated in watts, not VA (as far as I have seen). I think that this is misleading. If inverters can only supply watts, how do they run inductive motors and fluorescent lights in a stand alone configuration?
 
Last edited:
Since the real component has been reduced without an accompanying reduction in the reactive component, it is the same as if they were serving a load with a horrible power factor.
Exactly how (functionally, mechanically, physically) does a PV system reduce the real component without reducing the reactive component? That is what I just can't feature. If the PV system stops the meter on a load with a pf of 75 percent, the PV system is supplying all the power, not just real power. In order for the meter to stop and have the poco keep supplying the reactive component the PV system would have to transform the load into a purely reactive one on the line side of the meter. First, there is no such thing as a purely reactive load (it's only theoretically possible) and second there is nothing in the PV system that can do it.
 
Last edited:

tallgirl

Senior Member
?????

Voltage and current signs?

If you mean polarity, only voltage has polarity. Current doesn't so you lost me.....

Also, a load always uses up energy, if it were supplying energy it would not be a load, it would be a source.
Current has a direction -- from higher to lower voltage. Just because the voltage rises and falls (we're talkin' AC here ...) doesn't mean the current always goes in the same direction.

Right now I'm sending 868 watts back to the main service panel from the inverters (where it is promptly consumed by the A/C compressor ...) When current comes FROM that panel it has a "positive" sign, when it goes TO that panel it has a "negative" sign. Just like with DC power ...

(And yes, AC current has a sign -- don't let your Fluke be your teacher.)
 

tallgirl

Senior Member
Exactly how (functionally, mechanically, physically) does a PV system reduce the real component without reducing the reactive component? That is what I just can't feature. If the PV system stops the meter on a load with a pf of 75 percent, the PV system is supplying all the power, not just real power. In order for the meter to stop and have the poco keep supplying the reactive component the PV system would have to transform the load into a purely reactive one on the line side of the meter. First, there is no such thing as a purely reactive load (it's only theoretically possible) and second there is nothing in the PV system that can do it.
PV inverters make power where the current and voltage are in phase. The load consumes power with some angle between the current and voltage. Whatever is different between the PV inverter and the load -- the PoCo has to supply it. The inverter doesn't do anything to the load, the load is still the same. But it is consuming amps when the PV inverter isn't providing enough of them.

I hate to ask this question because it's akin to asking "do you know how electricity works", but do you understand that there are two different components to AC power -- voltage and current?

For a non-reactive ("linear") load, current is a function of voltage only. If it's a 120 watt non-reactive load, on a 120 volt AC system, it draws 1 amp at 120 volts. 1 Amp x 120 volts = 120 volt-amps, and conveniently 120 watts. When the voltage is 60 volts, it draws 0.5 amps (I = E / R, R is constant). For a reactive load it may draw 0.5 amps at 120 volts and 1 amp at 60 volts when the voltage is either rising or falling (depending on whether it is inductive or capacitive reactance).

The situation with PV inverters is that they produce power so that the current and the voltage rise and fall together. There is no difference. In the above example, the difference between the 1 amp at 120 volts being produced, and the 0.5 amps at 120 volts being consumed (and so on for the 60 volt case), is the responsibility of the PoCo.
 
Status
Not open for further replies.
Top