Hi, can I just tap my PV system to an Autamatic Transfer Switch I use for my Generator? Since inverter will not work without the utility power then the PV system is automatically isolated during power outage. My PV System does not have battery backup. Thanks
PV Array (microinverter) with Generator
- Thread starter Cudal81
- Start date
- Status
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Unless the PV system and generator are specifically designed to work together, you cannot use them both at the same time.
Common PV systems require connection to the grid in order to absorb excess power production. A generator is likely not capable of handling this.
So if you have a common grid tie PV system with the generator transfer switch 'downstream' of the PV system (meaning that the PV system gets disconnected when the grid is disconnected) then your system will work fine, but the PV system will not supplement the generator power.
If you have a common grid tie PV system 'downstream' of the transfer switch, and try to use the PV system and the generator at the same time, then if you are lucky things will simply shut down. If you are not lucky then you will let the magic smoke out of the generator or inverters.
-Jon
Common PV systems require connection to the grid in order to absorb excess power production. A generator is likely not capable of handling this.
So if you have a common grid tie PV system with the generator transfer switch 'downstream' of the PV system (meaning that the PV system gets disconnected when the grid is disconnected) then your system will work fine, but the PV system will not supplement the generator power.
If you have a common grid tie PV system 'downstream' of the transfer switch, and try to use the PV system and the generator at the same time, then if you are lucky things will simply shut down. If you are not lucky then you will let the magic smoke out of the generator or inverters.
-Jon
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
If you mean tapping the utility side feeder to the ATS, yes you can. But your downstream conductors and ATS should be rated for the combined current of the utility and PV output.
Carultch
Senior Member
- Location
- Massachusetts
I disagree with this one. If the load side circuit is ultimately protected against the current adding up as you described, it should only need to be sized per the maximum of the two sources. Not the sum total of both sources.
Here's a sketch of an example:
Worst case scenario, the loads draw 200A, regardless of what source feeds them. Any greater than that, and the main breaker of the main panel would trip. Any breaker in this diagram, could alternatively be a set of fuses, and the same reasoning will apply.
At points A and B, the current Ia and Ib will be no greater than 200A, otherwise, the main panel's breaker would trip.
At points C and D, the current Ic and Id will be no greater than 200A, otherwise the service disconnect will trip
At point E, the feeder tap of interconnection, the PV OCPD limits current Ie to 80A. This really corresponds to the inverter limiting it at 64A or less, but that's a detail for another time.
If Ie = 80A and Ia = 200A, then Ic = 120A. The PV subtracts current from the main supply, so that only the remaining current comes from the grid.
If Ie = 80A, and Ia = 0A, then Ic would equal -80A, indicating that we are exporting 80A to the utility.
If Ie = 80A, and Ic = 200A, you could not have this happen without a fault or an additional load.
In order for loads in the downstream panel to draw 280A, you'd need this to be an MLO panel without a 200A OCPD between it and the point of interconnection. Then in that case, you'd need to upgrade the main panel amps to meet or exceed 280A (which would mean a 400A busbar). It is statistically unlikely that this would happen (given how conservative load calculations can be), but still a possibility that requires protecting against. Alternatively: you'd need to connect on the supply side of the service disconnect, so nothing downstream of the point of interconnection draws more than 200A.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I agree that the downstream 200A breaker satisfies article 705's rules on taps, although it is a bit vauge. Hopefully the OPs AHJ doesn't get too persnickety about the placement of the breaker.
Thank you Jon. so we'll just need to tap at the line side tap of the ATS.
Thanks this helps a lot
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Please note winnie's response. You will NOT be able to use any PV power when the generator is supplying your loads. The hookup will look like Carultch's diagram.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I assumed that is what Cudal81 meant by tapping the PV at the line side tap of the ATS. The PV is connected on the utility side of the ATS, the generator is connected on the alternate input to the ATS and the loads are on the output.
Also I want to emphasize the possibility of the PV system and generator being designed to work together. This is something that people are asking for (even though it likely doesn't make economic sense) and something that some PV manufacturers are beginning to implement. There is no physics reason why a PV system and generator cannot work together; it is simply the case that most available hardware won't work together.
-Jon
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Tesla PowerWall 2's would have to sell for about $100 per in order for the numbers to work out. Folks may have other reasons to pull the trigger on such a solution; every situation is different.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
AFAIK a Tesla Powerwall will not function with a generator. No matter how much it costs.
Jon
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Caribconsult has almost this arrangement. He charges his PowerWalls with either the PV array or from the utility. I can't see why the generator couldn't sub for the utility, but I could be wrong.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
The difference is that you can backfeed a utility.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
The difference being, he doesn't. The PV array and inverter are not grid-tied.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
If your PV array and control system is designed so as to _never_ backfeed the 'utility', then I believe that using a generator is likely to work.
My understanding of the problem with combining a common PV array and a common residential generator is that there is no control to prevent trying to backfeed the generator. If PV production exceeds consumption, then the PV system will attempt to export power, and the generator is not capable of absorbing such power. A PV system that never causes a backfeed should work with a common generator.
-Jon
Carultch
Senior Member
- Location
- Massachusetts
It's not just backfeed that is the issue. A zero export control system for preventing backfeed is OK for a utility, where policy doesn't allow you to backfeed. Presuming the utility accepts your method of export power limitation, which you'd have to confirm.
By contrast, a generator really needs a simultaneously operating PV system to be controlled for "minimum import" when in generator mode, rather than just zero export. A generator has a certain range relative to its nominal power, where it can safely/efficiently operate. If the generator runs with too little load, it can mean long term damage to the generator. In diesel generators, the issue is known as wet stacking. You don't want a PV system to exacerbate this issue. When load on the generator gets close to its limit, you want the control system to reduce PV power to stay in the power range where the generator is meant to operate.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
That's only because he disconnects from it. When he reconnects his solar inverters will backfeed the utility, especially when his battery is full. it would be the same with a generator. I don't think his M250s could have been configured otherwise. Coincidentally Enphase is releasing firmware updates on Monday that might be changing that, for compatability with their batteries and generator support, although I don't know the details of their implementation. Point is, not all solar inverters can be configured to avoid backfeed as Carultch describes.
caribconsult
Senior Member
- Location
- Añasco, Puerto Rico
- Occupation
- Retired computer consultant
That's a good question: if we can charge the battery from the grid, why not from the generator? The only answer I ever got on this topic was that the sine wave output from the gen is not 'clean enough' or some paltry explanation like that. Today's gens put out a pretty clean low-noise sin wave that we've never had any problems with. I'm not sure what the inverter gens put out but it has to be something clean enough to run a computer. But so as to not violate our guarantee, if the battery is low and there's not much sun left then we close the grid breaker, feed some grid into the gateway, charge the battery to perhaps 85%, which is more than enough to last through the night, we turn the breaker off and go to bed hoping the following day there might be sun. This has happened a lot recently, because we've had lots of cloudy and rainy weather. A normal solar harvest for a day of good clear day of sunshine might produce 15+Kwh to the battery and the house....I've seen 17 on occasion, and average of almost 14/day, and leave a solid safety margin for over night, but some cloudy days we might get only around 11Kwh , and that' s cutting it close. We might need a drop of grid on those days. It's work out well, we never let the battery discharge to the shutdown point, and we're living off the sun I'd say 99.95% of the time. Our grid bill averages $13/mon compared to about $100 pre-Tesla, and a big chunk of that charges is just for having a connection, whether you use it or not. They have meters here they read from the central office and the billing is right.
Plus being almost full time disconnected from the grid, we are isolated from the vicissitudes of that grid system, which is full of surges, brown-outs, black-outs, voltage fluctuations, induced lightning surges, spikes of unknown origin. We used to have a pile of household electric and electronic things fried from the grid. Phone chargers that exploded from a wall socket! All of that is gone by not being grid tied. In essence, we use the grid to backup the solar, not vice-versa.
caribconsult
Senior Member
- Location
- Añasco, Puerto Rico
- Occupation
- Retired computer consultant
Do think it's safe to relay on the Enphase Enlighten website to tell you when you need a software update? The numbers there vary quite a bit from the GUI readout, but the Enlighten has it's advantages in being able to inspect each panel and microinverter remotely
caribconsult
Senior Member
- Location
- Añasco, Puerto Rico
- Occupation
- Retired computer consultant
That's because we don't leave the grid breaker on for extended periods of time...it's used as needed, and we never let if reach the point where the system would have surplus power to sell back. You can't do that through the same meter you get your supply from, just by jamming it back into the circuit from your controller...it won't generally pass through the meter, and in some cases like ours, it's a digital meter they read from the office and this reverse push generates an error message on the meter. In order to sell back, you have to have a sell-back system with the local grid, usually involving a separate meter and lines for this reverse flow, or perhaps it can pick it up the reverse flow from the grid connection and sell that back at some predetermined (but usually a pittance) rate through the 2nd meter. It costs extra for this gear and it's more to go wrong and for the tiny amount you earn from sellback, it just isns't worth the grief you have with the local utility, PREPA.
- Status