Using two solar inverters on a grid tie setup

[Dforster67]

I currently have an Outback Skybox with around 5kw of panels. It is working great, but it is about at its limit as far as PV input.

I am thinking of installing a second inverter out in my shop with another 3 or 4kw of panels that I picked up for a good deal. Are there problems with this arrangement? I wondered about the safety settings on the Skybox that detect a grid outage and immediately stop any grid interaction. During an outage my 2nd inverter would continue supplying power and the skybox might not know the grid was down and would continue to try to backfeed.

Any ideas are apprecaited.

 

[MisterSandals]

During an outage my 2nd inverter would continue supplying power and the skybox might not know the grid was down and would continue to try to backfeed.

When the grid goes down, both would (better!) stop back feeding and neither would know the other existed.

 

[BentleyJ]

If you install a second grid-tie inverter than it has to be UL1741 compliant which means if the grid goes down then your second system stops producing power also. The second option is to install the shop system as a stand-alone system with batteries or a hybrid inverter with internal ATS. Third option is something like the new Enphase IQ8 inverters and Smart Switch. If the grid goes down the switch disconnects from the grid and the inverters keep producing. They work with or without a battery. Of course, without a battery the output would be very limited on cloudy or rainy days.

 

[Dforster67]

If you install a second grid-tie inverter than it has to be UL1741 compliant which means if the grid goes down then your second system stops producing power also. The second option is to install the shop system as a stand-alone system with batteries or a hybrid inverter with internal ATS. Third option is something like the new Enphase IQ8 inverters and Smart Switch. If the grid goes down the switch disconnects from the grid and the inverters keep producing. They work with or without a battery. Of course, without a battery the output would be very limited on cloudy or rainy days.

Ok. That's what I was thinking. I just want to be sure I wasn't creating some sort of artificial "grid" by having two grid tie inverters. After reading some replies and thinking more about it, I think I understand how it would work. In my setup, the 2nd inverter will be downstream of the Skybox and the skybox will shut off its grid connection during an outage and switch to solar/battery only. So the skybox has severed the grid connection and the 2nd inverter isn't tied into the grid directly anyway so it won't matter if it stays on or not. And even if the 2nd inverter were tied into the main panel it would detect the grid being down so would shut off as well.

I will have batteries on the 2nd system if I can. And I do currently have batteries on the skybox for backup only. Eventually I hope to get off grid completely but need more batteries for that to happen.

 

[BentleyJ]

Having the second system downstream of the Skybox would satisfy the anti-islanding requirement. HOWEVER, keep in mind if the systems are connected and the Skybox is providing an AC power source to keep the 2nd system active, there may be a conflict. A grid-tie inverter is designed to produce as much AC as it can based on MPPT solar panel output. If your consumption in the shop from the second inverter is less than what it wants to push out, the excess will get forced back through the Skybox. THIS IS NO BUENO.
My suggestion would be that the shop system be designed around an MPP Solar Off-Grid type of hybrid inverter (or similar brand) This way it could be connected to the "house" system in case it ever needed utility power to charge the batteries on a cloudy day but would have no capability of supplying power back to the grid or Skybox.

 

[Ampster]

@Dforster67
There is so much activity on this forum I did not see your post until now. Glad you got some other opinions. There is a wealth of information here about some of those less expensive hybrid inverters.
I think the point @BentleyJ is making is that if they are not both UL1741SA compliant they may not interact well. The beauty of UL1741SA is that when the grid is down and the Skybox is providing the grid forming AC, then backfeeding the Skybox through the subpanel should work because they would be AC coupled. If your production exceeds your house loads and battery charging loads the Skybox would increase the frequency of its AC which would cause the other inverter to modulate its production to a level consistent with the loads. When the grid is up the grid would be backfed and the grid presents an infinite load so there would not be an issue.

 

[BentleyJ]

@Dforster67
There is so much activity on this forum I did not see your post until now. Glad you got some other opinions. There is a wealth of information here about some of those less expensive hybrid inverters.
I think the point @BentleyJ is making is that if they are not both UL1741SA compliant they may not interact well. The beauty of UL1741SA is that when the grid is down and the Skybox is providing the grid forming AC, then backfeeding the Skybox through the subpanel should work because they would be AC coupled. If your production exceeds your house loads and battery charging loads the Skybox would increase the frequency of its AC which would cause the other inverter to modulate its production to a level consistent with the loads. When the grid is up the grid would be backfed and the grid presents an infinite load so there would not be an issue.

You are absolutely correct, thanks for filling in that blank. I didn't get into the details of AC coupling because he mentioned wanting to go Off-Grid eventually anyway. Also, I wasn't sure about the Skybox being a frequency shifting Inverter. Last time I looked into Outback, the Radian series used "load contol" instead of dithering. In fact, the reason I chose a Schneider Conext XW+ was specifically because it does have the frequency shift capabiltiy. In reality it works and will shut down the Enphase microinverters and the Fronius IG plus at 60.5Hz but tends to cut back in too soon then the batteries reach full charge in a couple of minutes and it frequency shifts again and so on. In the few cases when we had extended power outages during the day, I ended up just shutting off the grid-tie systems for a couple of hours to discharge the batteries.

 

[Ampster]

Last time I looked into Outback, the Radian series used "load contol" instead of dithering.

That was one of the reasons I sold my Radian in 2017. Since then I think they have updated the firmware in the Radian to do frequency shift and be UL1741 compliant.

 

[Dforster67]

That was one of the reasons I sold my Radian in 2017. Since then I think they have updated the firmware in the Radian to do frequency shift and be UL1741 compliant.

Thanks both of you! So if my 2nd inverter is UL1741 compliant then it will not be an issue with backfeeding too much energy to the skybox as BentlyJ mentioned in his previous post? My shop loads will not be heavy or consistent most of the time. Although I am toying with the idea of heating water for my radiant floor system with excess after batteries are charged.

 

[BentleyJ]

So if my 2nd inverter is UL1741 compliant then it will not be an issue with backfeeding too much energy to the skybox as BentlyJ mentioned in his previous post?

Yes, that is correct. ANY UL1741 compliant inverter system must stop exporting if the line frequency reaches 60.5Hz or above. So that will prevent dangerous back-feeding from your secondary system into your Skybox. However, it may also mean that your secondary system just shuts down completely for a period of time. What if your second system was also a Skybox? Would it shut down at 60.5Hz or simply disconnect via the internal ATS but still continue to power your shop? Maybe Ampster can help here.
You probably need to sketch your proposed system on paper and choose the preferred hardware then run through a few scenarios to verify the user settings per the user manual will give you the desired outcome for each condition. Grid is normal. Grid goes down short term, Grid goes down long term. What happens day vs. night vs. partial sun.

 

[Ampster]

So if my 2nd inverter is UL1741 compliant then it will not be an issue with backfeeding too much energy to the skybox as BentlyJ mentioned in his previous post?

The way I understand it is the latest spec is UL1741SA and that iteration includes modulation in response to frequency changes by the AC forming inverter. The Skybox is 1741SA compliant so if your other hybrid is UL1741SA compliant it should be able to modulate power. Modulation is preferable to on/off shut down. Backfeeding per se is not the issue since AC coupling is backfeeding. Controlling that backfeed is the important issue when the grid is down. This is important since the grid acts as a buffer when loads decrease and when there is no grid the batteries have to be that buffer.

 

[BentleyJ]

OK, that makes sense. The Schneider Conext XW+ does actually have that functionality, frequency ramps up at 0.1Hz per step, but our other 2 grid-tie invertes are too old. They just stay on at 100% until the limit is reached then shut down completely. Thanks

 

[BentleyJ]

I've only skimmed over the major points of the new SA (Rule 21) requirements. Where I get confused is with regard to the "Ride-Thru" provisions that are supposed to use some kind of as yet incompletely defined algorithm to help support the grid in a brown out rather than just abruptly shuting down leading to even more fluctuation. Ultimately, they seem to want full 2 way communication forming a sort of micro-grid configuration but on a macro level. Kind of sounds like an oxymoron to me.

 

[Ampster]

Where I get confused is with regard to the "Ride-Thru" provisions that are supposed to use some kind of as yet incompletely defined algorithm to help support the grid in a brown out rather than just abruptly shuting down leading to even more fluctuation.

Yeah it does not make sense that there could be a brownout when a lot of solar was already supporting the grid. I understand why California adopted Rule 21 because of the high percentage of solar and if it all went offline at the same time it would be a big jolt to the grid. Kind of like the Superbowl phenomenen of all the toilets in the country flushing at the same time. That turned out to not be an issue. Bureaucrats have to worry about stuff like that. Years ago I was at a conference where the head of the CPUC at the time warned that a solar eclipse could jolt the California grid. Some one in the audience reminded him that the shadows of eclipses move across the state and the entire state will not see the same darkness at the same moment. LOL