Negative power factor and PV systems

[SG-1]

Picture worth a 1000 words ?

Picture worth a 1000 words ?



To make the negative 1 Power Factor a little clearer look at the attached picture.

When 1000 Watts is flowing in from POCO at unity PF the Watt Meter reads 1000 watts & the PF meter points straight up 12 0'clock. If 1000 VARs are flowing reactive the PF meter points to the 9 o'clock position "0" and the watt meter reads zero also.

When 1000 watts of power flows back toward POCO the the PF meter points to the 6 o'clock position which is sometimes refered to as negative unity or a negative 1 PF. The wattmeter would now read 1000 watts again.

If you only apply voltage to the meter it spins round & round & round like a synchro-scope.

 

[mivey]

Mivey,

Thanks for the replies, since there are so many with so much information I think it would be best to bring this info to my instructor's attention. If you are correct, this should be taught to students studying PV installation.

If? :grin:
Seriously, I am a utility engineer and am well versed on power systems, costs, and rate design.

You mentioned that you thought the author may have purposely left that info out. I don't think so, especially with so much info that is nearly superfluous in nature being discussed.

I would be surprised that an author with enough knowledge to write a textbook would not be aware of the issue. It would not be the first surprise I have ever had though.

I am also going to strongly suggest that a test be run. Of course, I will post the results.

The test will show either you have an inverter capable of supplying the vars or not. The rest is just math and if the test shows anything different then you need to check the conditions of the test.

I think it would be an interesting way to clarify the issue for the class. Let us know how it goes.

One issue I would like detail on, though. I asked to be shown where a customer has been penalized for poor power factor as a result of the effects of a PV system. I still want to know if it has ever really happened. I know that if I were an industrial customer and spent big bucks on a PV system only to be penalized for it, I would be PO'd. Big time.

It happens although the penalty is relatively small with most rates (I'm thinking of a case where the penalty amounted to about 1% of the bill and was about 1/10th of the cost that the PV system added to the monthly costs). If your POCO has low rates, the PV system may actually be costing you money: a lot more than the pf penalty.

A rough estimate for the cost of solar is probably 30 cents/kWh. With tax incentives you might get this subsidized down to around 10 cents/kWh. Do the calcs yourself and don't always believe what the PV salesman tells you. These numbers will vary greatly from state to state and utility to utility.

For residential, try this calculator:
http://sharpusa.cleanpowerestimator.com/default.aspx

This is not a minor issue and if it is occurring in the wild, the PV students need to know about it.

Overall, it probably is a minor issue other than the case where the POCO is going to require that you fix the pf yourself rather than charge you a small fee. Fixing it yourself will eat into the PV economics, which is an issue if your return is marginal.

 

[mivey]

I'm pretty sure that absolute values are involved in the power factor formula. I don't have time to look it up right now but I'm sure a web search would show the relevant result.

The S = P + jQ formula is calculated by S=VI* where the "*" means the complex conjugate

S = VI* = |V||I|e^(jθ) = P + jQ
P = {Re}VI*
Q = {Im}VI*

 

[tallgirl]

Not only am I curious, I may be able to do some lab measurements.

I do "lab tests" all day long. One of my pieces of "lab test" gear is a Shark 100 from ElectroIndustries/GaugeTech. The other a WattNode from Continental Control Systems. Feel free to Google either.

This ain't theory for me.

 

[tallgirl]

Yes. The inverter will shut down.

The traditional grid-tied inverter will not run without the grid. An inverter that can run in island mode can supply both the kWs and vars.

... and it's for this reason that I have batteries. We had a power outage yesterday morning around 7:30 and if it weren't for my inverters tattling on the PoCo, I never would have known it.

What is it with people who pay all that money for solar and don't install a battery backed system? Do they love power outages so much that they want more of them?!?

 

[mivey]

... and it's for this reason that I have batteries. We had a power outage yesterday morning around 7:30 and if it weren't for my inverters tattling on the PoCo, I never would have known it.

What is it with people who pay all that money for solar and don't install a battery backed system? Do they love power outages so much that they want more of them?!?

The cost is out of range for many apps. It could be cheaper in some cases to put a smaller UPS on critical equipment. For many a backup diesel or NG generator makes more sense than a huge battery bank.

 

[iwire]

What is it with people who pay all that money for solar and don't install a battery backed system?

Are you serious?

If you want back up spend the money on a generator but IMO a battery stack with a grid tied inverter is just a money pit for a residence.



(Guess I should have read Mivey's response before I posted)

 

[K8MHZ]

If?
Seriously, I am a utility engineer and am well versed on power systems, costs, and rate design.

Yes, if.

Consider my position. I have two engineers telling me two different stories. You have proven yourself (to me, anyway) to be very knowledgeable and down to earth. My instructor is being paid to teach the subject and I just shelled out 600 bucks for the class.

I have decades of technical experience. Here is a list

1976 - Optical technician for Aden Safety Labs
1977 - Quality control technician for Brenneman Inc.
1979 - Research technician for Howmet jet turbine casting facility
1985 - Automotive, heavy equipment and marine electrical repair technician (and owner of the business)
1990 - Automotive repair tech for Chrysler corporation
1994 - Maintenance technician for Port City Die Cast
1996 - Entered NJATC apprenticeship. Grad #1 in class of 40. Ran conductor testing crew for the build of the Zeeland gas turbine facility (Zeeland, Mi). Tested every conductor and sub unit in the last two turbine units, including the iso- phase buss.
2001 - Elec. tech for G3 Electric. Traveled around MI installing transfer switches at DMUX sites owned by Verizon and still do troubleshooting for G3 (Verizon folded and stopped the DMUX expansion)

Now, looking at that list, do you think that this is the first time I have had conflicting stories from engineers? When I was at Howmet, we had a saying, "Engineers design all the wonderful things in life and technicians can make them work anyway."

At first, I didn't doubt my instructor. Now I am very doubtful and am leaning toward the opposition.

I do want to thank you for spending so much time on this for me. When the dust settles, I am going to ask my instructor if some of the money I gave the college could be passed on to the people that are actually teaching me the subject I paid for.

And yes, I would actually do something like that.

:roll:

 

[K8MHZ]

I do "lab tests" all day long. One of my pieces of "lab test" gear is a Shark 100 from ElectroIndustries/GaugeTech. The other a WattNode from Continental Control Systems. Feel free to Google either.

This ain't theory for me.

As much as I love theory, the convincer is the real world.

 

[K8MHZ]

The cost is out of range for many apps. It could be cheaper in some cases to put a smaller UPS on critical equipment. For many a backup diesel or NG generator makes more sense than a huge battery bank.

If you are clever, you can get residential sized UPS systems very cheaply.

I have a 600, a 400 and a 300 watt UPS. I also have four 100 Ah SLA batteries.

The UPS's were purchased for about 5 dollars each. Two I got at a yard sale and the other was a rebated deal. I removed the internal batteries and connected the 100 Ah batteries to them.

The 100 Ah SLAs were pulls from Verizon sites. I got them for free with the promise that when I was done with them I would recycle them properly. Actually, I was given 8 and sold four.

 

[ohmhead]

Recharge system

Recharge system

Well if someone could figure out how to use that part of the AC cycle to recharge the batts or regenerate energy by using a smaller separate inverter to back feed into the system using the inductive effect by the inductive dump of energy back into the system for just that purpose . Na iam just thinking thats not right !


PF is controlled by the load online .

A PV system is ok if its design to not give more than needed mostly less .

The effects of inverter switching battery charging what our we saving not Harmonics .

Solar alone no battery system maybe better sun goes down PF goes up .

I see more negative than positive to become positive .

Sorry no formulas iam just a simple electrician with no engineering background but i well enough said .

 

[iwire]

If you are clever, you can get residential sized UPS systems very cheaply.

I have a 600, a 400 and a 300 watt UPS. I also have four 100 Ah SLA batteries.

The UPS's were purchased for about 5 dollars each. Two I got at a yard sale and the other was a rebated deal. I removed the internal batteries and connected the 100 Ah batteries to them.

The 100 Ah SLAs were pulls from Verizon sites. I got them for free with the promise that when I was done with them I would recycle them properly. Actually, I was given 8 and sold four.

So basically you are saying if you can get batteries for free the ROI is high. :grin:

 

[K8MHZ]

Mivey

I checked out the online calculator you posted.

The cost per DC kW was confusing. The lowest it would go was $4,000 per kW and up to $12,000.

I haven't got to pricing inverters and installation, but our cells are VERY nicely built and cost about 400 bucks for each 220 watt panel. That would be $1818 per kW just for the cells. I really cant see another $2200 per kW for the rest of the system.

Torrenson Marina in Muskegon, MI installed an array a short time ago. They were told to expect a 10 year pay off. So far, the output has been better than expected and they are looking at an 8 year pay off.

 

[K8MHZ]

So basically you are saying if you can get batteries for free the ROI is high. :grin:

The rate is even higher when you add in the 35 bucks each I got for the four I sold plus the 27 bucks each I will get for the scrap value. (They weigh 102 lbs. each). The only cost incurred was about 20 bucks for gas to drive to Hudsonville and get them.

ROI for back up units is really hard to calculate as most of the variables are based purely upon speculation.

Now, in our lab we are increasing the ROI by using everything we can out of my scrap pile and left over equipment stash to install the four systems. Also, since the 20 of us each paid for the class, we now are looking at a negative installation cost. If the SLA's I have weren't so bloody heavy, they would get one of those for free as well. (I am only using one or two at at time).

We are also saving a few bucks on permits and inspection fees, since we have opted to forgo them altogether.

 

[skeshesh]

The S = P + jQ formula is calculated by S=VI* where the "*" means the complex conjugate

S = VI* = |V||I|e^(jθ) = P + jQ
P = {Re}VI*
Q = {Im}VI*

It's been a few years but I still remember the basics of my power system courses fairly well. What I was talking about is that the power factor is calculated as an absolute value between 0 & 1 as far as I remember. The value can be lagging or leading based on the characteristics of the load impedance, but I don't recall at anytime seeing a negative pf.

 

[kingpb]

The ROI for an inverter system without batteries is longer than the equipment will even last, assuming proper maintenance was done.

The only way these project can get off the ground is because of subsidies. California has figured out how much subsides they need to provide just to make the project halfway feasible from an economic standpoint.

I'm convinced the only people really having them installed are either very naive about the payback, or have so much money they don't really care, it's more of a bragging right. Add batteries and it's even worse.

As far as batteries, for residential, very dangerous because of the high current discharge that would occur upon short circuit.

 

[mivey]

Consider my position. I have two engineers telling me two different stories...
At first, I didn't doubt my instructor. Now I am very doubtful and am leaning toward the opposition.

Don't just take my word for it. Listen to both sides and figure it out. That is where you will get the most benefit. It really is just math.

The point about the PV inverter producing current in phase with the grid voltage can be verified at any number of sites.

I do want to thank you for spending so much time on this for me.

Glad to. I enjoyed it.

When the dust settles, I am going to ask my instructor if some of the money I gave the college could be passed on to the people that are actually teaching me the subject I paid for.

And yes, I would actually do something like that.

After your final grade I hope. :grin:

 

[mivey]

Mivey

I checked out the online calculator you posted.

The cost per DC kW was confusing. The lowest it would go was $4,000 per kW and up to $12,000.

I haven't got to pricing inverters and installation, but our cells are VERY nicely built and cost about 400 bucks for each 220 watt panel. That would be $1818 per kW just for the cells. I really cant see another $2200 per kW for the rest of the system.

Torrenson Marina in Muskegon, MI installed an array a short time ago. They were told to expect a 10 year pay off. So far, the output has been better than expected and they are looking at an 8 year pay off.

I can't vouch for the calculator inputs as it was just one I found on a search but the formulas seemed fair enough for a rough calc. Not sure why they did not have a maintenance factor or degradation factor. I'm sure there are better calculators out there.

The grants I have seen for some helped but the payback was way over 20 years.

If you are getting a nice subsidy to help with the fixed cost and some great rate subsidies then it may very well work out. I know it can be worth it in areas of the country with high rates.

 

[mivey]

It's been a few years but I still remember the basics of my power system courses fairly well. What I was talking about is that the power factor is calculated as an absolute value between 0 & 1 as far as I remember. The value can be lagging or leading based on the characteristics of the load impedance, but I don't recall at anytime seeing a negative pf.

The negative is just being used as an indicator of whether power is being absorbed or delivered, not really a negative power factor (there is no such thing).

The ROI for an inverter system without batteries is longer than the equipment will even last, assuming proper maintenance was done.

The only way these project can get off the ground is because of subsidies. California has figured out how much subsides they need to provide just to make the project halfway feasible from an economic standpoint.

I'm convinced the only people really having them installed are either very naive about the payback, or have so much money they don't really care, it's more of a bragging right. Add batteries and it's even worse.

As far as batteries, for residential, very dangerous because of the high current discharge that would occur upon short circuit.

Your points are consistent with my understanding as well.

 

[jaggedben]

The ROI for an inverter system without batteries is longer than the equipment will even last, assuming proper maintenance was done.

That is a sweeping statement. Others in this thread have pointed out, correctly, that ROI varies by state and utility (and I would add rate schedule, and local weather). In some areas 'grid parity' is close or has even arrived, without subsidies.

We also don't know how long today's panels will really last. Cost estimates are typically based on 25 years but only time will tell how much mileage you can really get out of the current technology.

I'm convinced the only people really having them installed are either very naive about the payback, or have so much money they don't really care, it's more of a bragging right.

Environmental motivations account for a lot, especially considering that for most people the overall cost difference is not severe even if the payback period is long.

I haven't got to pricing inverters and installation, but our cells are VERY nicely built and cost about 400 bucks for each 220 watt panel. That would be $1818 per kW just for the cells. I really cant see another $2200 per kW for the rest of the system.

That seems like a wholesale price or even a cost-of-production price for panels. Average retail prices are going to be around 50% greater. Inverters are going to be from $1/watt (small) to $.25/ watt (large). Basically a retail price for panels and inverters for a residential system is minimum $3/watt. So, with BOS, $4/watt is really only a little bit above minimum for a very savvy DIYer who makes no mistakes.

Now if you are contracting with a solar installer, they have to cover labor, vehicles, tools, warehouse, insurance, management, marketing, profit... you get the idea.

...

Sorry if I'm encouraging this thread to go off topic. I've been lurking, trying to understand the power-factor discussion. About the only thing I can add is that if small inverters have any var controls, they don't appear in any manuals I've looked at. I somewhat doubt that micro-inverter var controls can be changed anywhere except at the factory, if at all, (although I do know that Enphase inverters have other unadvertised capabilities).