AC causing PV inverter to cycle into wait mode

[old dirtbeard]

Hi,

My 6 kW grid-tie photovoltaic system is going into wait mode when my five ton AC cycles on. I have 200 amp service to the house, but these summer days the line voltage will drop from 240 to 225. The inverter is an SMA 6000U with a minimum grid tie operating voltage of 213 VAC. I think that when the AC cycles on, it either momentarily depresses line voltage below 213, or it just causes the line voltage to change too quickly for the inverter to maintain the sync (either one of these conditions will cause the inverter to "detach from the grid" and go into a wait mode monitoring the state of the grid).

Questions:

- Maybe the line drop from the pole is undersized? (does not seem likely given that Edison must have connected it to the 200 amp panel).

- How likely would the gauge of the line drop be the cause of this problem?

- Does anybody know what gauge a 75 foot 200 amp line drop ought to be?

- Has anybody had success in getting their utility to replace the line drop with a larger gauge drop?

- Is there anyway to add something to the AC circuit so that it has a "softer start" so it does not depress the line voltage so much?

This is a real pain as the AC cycles about every 20 minutes and causes the array to go off-line for five or ten minutes each time. I am losing a great deal of electrical production and I can't believe it is good for the inverter, either.

thanks,

doug

[Uncle Monster]

Your 5 ton AC unit will be fed by a 50 to 60 amp 240 vac circuit. You must understand that the LRA (locked rotor amps) for such a unit can be between 120 and 150 amps and the RLA (run load amps) will be between 17 and 25 amps. Your 6 kw power system only puts out around 25 amps continuous current. I don't what the surge rating for your inverter is but I can guess that it is not enough to start a 5 ton AC unit thus it will drop out and wait for the load to stabilize before kicking back in.

[8~{} Uncle Monster

[Telstra]

Can you give the MAKE and MODEL NO of the AC Unit

[old dirtbeard]

Hi,

Yes, thank you, it is a Lennox HS26-060-2P.

The way this grid-tie system works is that during the daytime when the array is producing more elecricity than the house is consuming, it sends the "excess" current back to the grid and spins the meter backwards. During the day if we need more elecricity than we can generate, it draws the shortfall from the grid. At night the meter spins forward just like there was no PV system.During the day, though, the inverter needs to "lock on" to the fequency and voltage of the grid to invert the 380 VDC to 60 cycle, 240 VAC. The inverter can tolerate a grid voltage range of 213 - 262 VAC and a frequency range of only about 59-61 hz.

The panels do not need to fully drive the AC, but when the AC cycles on, it either is dropping the line voltage below 213 VAC or is dropping the voltage so quickly that the inverter loses it's sync on the line. I am thinking that the line drop from the pole cannot supply the start-up volt-amps required by the AC condenser.

Thanks for any help you can provide.

doug

[valvejob]

You have grossly undersized your power unit.

You need to at least double the size or get you some window units and turn one on at a time.

Who did the horrible design of your system?

[old dirtbeard]

Sorry I was not clearer -- please see the response to Telstra -- the PV system is a grid-tie, it does not have to power the entire house. We generally run a surplus (I have a negative balance of $180 with Edison), but we will draw whatever additional current we need from the grid. When the AC is on, the PV array and the grid are powering the house.

The utility meter runs backwards during the daytime (except when the AC is on) and runs forward at night when we are not generating. The idea is overall to zero-out so you do not have an electrical bill.

doug

[Telstra]

As can be seen from the Service manual 1ph LRA is 107A and RLA 20A 3ph LRA 88A RLA 12.5A. and unfortunately Modifying the refrigeration circuit to provide unloading wil not significantly reduce those figures. In addition the motor starting can only be XL across the line.There are various approaches to this problem ( Resistance start ) ( Auto transformer ) ( VSD ) but the manufacturer Lennox should be consulted before these are applied.

[old dirtbeard]

Telstra,

Thank you -- it is single phase and your information is very helpful in determining the gauge required for the drop loop from the pole.

The AC cycling on does cause the lights to dim when it starts up, and at 107 amp start-up and whatever the furnace blower takes, I can see why I am having issues.

Thanks again, I appreciate you taking the time to research this for me.

best,

doug

[Bob F]

Have you talked with the manufacturer of the inverter?

Bob

[old dirtbeard]

Hi Bob,

Great idea, but I have not done so yet.

I wanted to make certain that my grid feed infrastrucure was up to par before I started down other channels. If Edison will not upgrade the line drop, I will contact SMA to see if we could adjust the lower VAC threshold for the inverter.

There is a very clear voltage drop at the inverter when the AC cycles on. As Telstra described, the AC unit draws 107 amps on start-up, and I believe that I just do not have "thick enough copper" between the panel and he pole for the surge that the AC places on the line drop.

It was OK before I installed the PV system, but now I have a more "discriminating consumer" on this side of the line drop. :)

thank you,

doug

>

[valvejob]

Fantastic System!

Keep us informed when the solution is found.

[Bill]

One possible solution (if no other fixes) might be to get a 10,000 BTU window A/C or two, then run these along with your central A/C fan on all the time.

This would keep your central A/C compressor from cycling on as often.

Window A/C units are quite inexpensive these days and very energy efficient.

Also I suppose you could place these units in the rooms you use the most during the day and/or one in your bedroom. Then turn the temp up on your central air, possibly turn central air totally off at night.

[Bill]

You may have just said a magic word. If the lights dim, and it's more than just instantaneous, you might want to check with your HVAC guy to see whether the unit - the AC that is - could benefit from a "start assist". If there currently isn't one on there, or it's just a PTC, or it's bad, it could be taking alot longer to start the compressor than it needs to - long enough that your inverter control sees it as a UT side voltage sag.

[unclemon]

I read somewhere about a power company that didn't like the idea of the meter running backwards. The management considered it buying back power at the retail rate. A ruling by the courts compelled the power company to buy back power so the utility installed two meters, one for power going in and one for power coming back. The customers were credited for power coming back at a lower rate.

[8~{} Uncle Monster

[old dirtbeard]

In Southern California (I think it is Federal now), the utilities are required to have meters that turn both ways. SoCal Edison has a graduated billing structure where the first 10kWh/day (baseline) are billed at $.07, the next at $.10, and so on until you get 200% over baseline and the rate is over 40 cents per kWh -- it is a killer if you have a pool, central AC, koi pond (pumps run 24 hours a day), etc.

Here anyway, they have to buy it back at the same rate they sell it, so if you produce baseline amounts, they buy it back at $.07, if you produce more, they have to buy it back at the higher rates.

Bottom line, though, is that my billing cycle is 12 months -- if after 12 months I have used more electricity than I produced, I have to pay them. If after 12 months I have produced more than I used, they zero out the account, say thank you (maybe) and we start all over again. They will not actually pay you after 12 months for any surplus. I don't think it is right, but that is where it stands today.

Because we were running 200% over baseline on a significant portion of our bill, it made the ROI on the PV system about seven years assuming no rate increases. The panels are warranted for 25 years, so it is a pretty attractive proposition if you are on a graduated billing structure and you are a heavy user.

More than anything, though, I installed the system because I felt bad about the amount of energy my house was consuming. I installed compact fluorescent bulbs, timers, motion activated outdoor lights, etc., but with the pool and the ponds and the AC, no matter what I did, I was going to consume 800 kWh to 1.5 mWh per month ($200 - $400 a month).

Now I am a net producer of electricity -- I am actually supplying my neighbors with electricity. The nice thing about PV here in SoCal is that AC is a huge consumer of electricity, and that consumption occurs at peak solar hours, so the PV systems are helping to limit the peak consumption spikes when the utilities need it most (ahem, that is if your AC does not cause your PV system to go off-line for 15 minutes out of the hour).

Also if you are a tool head, it is kind of cool to watch the meters, and monitor what you did each day as far as production (before I go to work in the morning I check the inverter and utility meter, and then when I come home I do it again to see how much electricity we surplused for the day.

Cool bright days produce the most power -- the panels lose efficiency at higher temperatures. On my 6kW system, we produce just a little less than one million watt hours per month

[old dirtbeard]

Thank you. Maybe I can address this by addressing the AC unit. I will call the AC contractor who installed it and see if he will come out and assess the situation. Do you have any ballpark ideas what a start assist should cost to install if there is not one? Or what it should cost to replace one if it is bad? This is the first house I have had with central AC and I know very little about it. I do appreciate your help and advice.

It is 100 degrees today and the inverter is going into wait mode every time the AC kicks in. The default timeout for the inverter is 5 minutes and since it happens every 20 minutes, so I am losing 25% of my generation during peak hours (when I need it most).

thanks again,

doug

[John Grabowski]

It is possible that the power company transformer size is marginal compared to the load of the neighborhood. You might try giving them a call and mentioning the voltage drop that you are getting. Also, power companies are known to cut back voltage during peak demands such as the summer. That may be a contributing factor. I would also contact the manufacturer of the inverter to see if the minimum voltage setting can be changed.

[unclemon]

When I worked out in The Marshall Islands 20 years ago, one of the computer rooms for a deep space radar had motor/generator sets equipped with large flywheels as a sort of UPS for the electronic equipment. I wonder if a setup like that could work as a surge buffer for starting an AC unit. Could be an interesting project.

[8~{} Uncle Monster

[unclemon]

I want my own "Mr. Fusion" reactor.

[8~{} Uncle Monster

[old dirtbeard]

Thank you John,

All great suggestions, thanks again. Edison changed out the transformer about a year ago when we lost one leg of the 240 -- I would have hoped that they put in one with "growth capacity," but who knows? I'll check on this one.

We definitely are suffering from low voltage (it dropped from 268 in the a.m. to 208 midday yesterday, so we are wattage challenged. Someone sent me the tech support number for SMA to check if I can alter the minimum voltage (I am told that it is a regulated issue and they probably will not be able to alter it.

Thanks again, John, I appreciate you feedback.

best,

doug