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Gridtied AC coupled batt inverter/charger, and contingency operations

offgrider

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lets say things really go downhill, power is out for an extended period of time. is it possible to put any of the GT ac-coupled batt inverter/chargers into some kind if "offgrid" mode that will just signal the microinverters to just stay on. of course you'd disco the house main grid connection in this mode.. for just in case purposes, if someone needed that capability, which item would be best suited?
 
A few have that feature, an AC outlet that can be enabled when grid is down. SMA Sunny Island (newer models) has "Secure Power".
Some hybrid inverters are batteries optional.

People always ask this question, and UL1741 inverters are designed to not put out power unless they see correct voltage and frequency applied.
People have connected them to a battery inverter and fooled them into running. But what happens when PV output exceeds house loads? Maybe it works so long as battery inverter plus PV inverter both deliver power to support loads.

You are best off assembling a system designed to support your critical or important loads. That could mean reallocating some PV panels to a different charge controller or inverter for a small battery system. As I mentioned in another thread, if your inverters support frequency-watts, there is a very nice battery inverter setup you can implement, which will automatically manage everything.
 
yes thanks very much. I believe I am getting very close to the full picture now, where excepting the POCO meter, which connects to the mains panel, for a microinverter based system, EVERYTHING connects to the ac-coupled batt inverter/charger:
  • mains panel
  • critical loads panel
  • PV 240VAC input from my microinverters
  • battery
  • generator
this is a fun project to go along with the house build I have in progress, so i can just get it all right and optimized the firs time. its a sizeable microinverter based array, nearly 40 panels, simply too big for me as a first run to do a 45degree roof, so its getting contracted out. But I do plan on coming in afterwards myself with the batt inverter/charger system. for starting off though, I'll have the PV contractor just wire the critical loads panel directly to the mains panel, and also wire the microinverters to either panel. then when im ready to AC-couple, nearly everything else needed is done.

i dont like single points of failure though (i love the microinverter model) so Ill prob get a few ac/couple batt intverter/charger systems. I believe I can just wire them both to the same critical loads panel, as well as both to the same batt bank? and then just split the PV panels between the 2?
 
With some brands of battery inverter, you can connect multiple in parallel for more watts (and more pass-thru current from grid), and if 120V some can stack for 120/240V.
You could also have multiple systems that don't talk to each other.

For this to work, you should make sure your PV inverter (microinverter if you get that) is designed to play nice in a battery system.
The early model Enphase were not. The later model Enphase are, and Enphase has a battery system to work with them.
Depending on how many watts, even high-voltage string inverter you might have more than one, which provides redundancy.

I suggest designing the system with both PV and battery upfront, decide what equipment you want that works together. Easiest if they are all one brand, and that may work best. You could mix brands, but then you're more on your own.

I suggest having the critical loads panel with some sort of transfer switch so it can go to the inverter system, or just wire straight to main panel if inverters fail. Rather than a separate transfer switch, I have a main breaker and a backfed breaker with sheetmetal interlock between them. Typically this is intended as a generator input. Mine are square-D, but many brands have them.
I also have a similar interlocked breaker on the main panel (feeds my garage). Normally it is fed from grid, but I can manually switch to feed grid from inverter as well (because inverter is fed from the same panel, I manually turn that beaker off so inverter doesn't feed inverter.)

Sounds like you want backup when grid is down.
A different feature is shifting time when you draw from grid or feed grid. There are systems for that as well. Some overlap in functionality, but each is optimized for its main purpose.

If you want backup to run anything with a motor (well pump, A/C) it needs to deliver enough surge current, about 5x nameplate rating.
 
Enphase's Ensemble gives you a grid-tied microinverter system that works with AC batteries (either their own Encharge or Tesla). That's how I'm setup, you can read about it here. The IQ8 microinverters aren't out yet, but supposedly can do that without batteries.

The downside at the moment is you get locked into a vendor. AC batteries are a simple enough concept, but no one has figured out to make a DIY solar compatible version as the system needs to accommodate surges due to appliances (e.g., air conditioning) shutting off by a combination of throttling panel output and "absorption". There's a thread on that here.


...critical loads panel...

With that much solar, why have critical circuits at all?

I'm only just over 7 kW and mine is setup as shown to the right, no critical circuits.

Currently I manually switch things off at the breaker panel if I don't want power
there. Eventually I plan to use a "smart" system to turn on/off things depending
on if excess solar is available (e.g., turn on the hot water tank or eCar if the
system would otherwise throttle back the solar panels).
1614168524132-png.38387
 
yes, def want power when the grid is down, just to critical loads, which in my case will be kitchen, master bed/bath, and living room.
ill likely be going with IQ7A microinverters since my currently-panels' STC just match its 350va ability. I guess my 2 main choices for island control are the Sunny Island or Outback Radian?

thanks all i think the plan here is really starting to come together
 
If you use Enphase microinverters consider using their battery inverter.


GXMnow uses Enphase with Schneider, and has had issues with them disconnecting and remaining off line. Various software updates and workaround. Maybe that is due to his trying to shift when his production/consumption is seen by the grid.

Tesla PowerWall of course is an AC battery. I'm not clear on what if any additional hardware is needed for backup operation (a disconnect switch from the grid is obviously required somewhere.)

SMA Sunny Boy Storage is similarly an AC battery, using 3rd party batteries with compatible BMS. It requires "Automatic Backup System" device (transfer switch and 120/240V transformer) to provide backup. Rated 6kW with anemic 7kW surge so wouldn't start larger than 1400W motor.


Unless your battery is huge or you are OK with everything shutting off at night, consider distinguishing "critical" loads from "important" loads. Small battery can keep internet and an LED lamp going, vs. what it takes to run a single heating appliance in the kitchen. A relay to disconnect "important" loads and a system able to control that based on battery SoC would be good.

I still recommend high-voltage string inverters. They are competitive in price, efficiency, reliability. (Although RSD cuts deeply into price competitiveness.) PV isn't your only power source; you have the grid, so only inverter failure during a grid outage impacts you. You can have two inverters, redundancy without as fine granularity as microinverters.
 
I just really like the ability of panel-level monitoring that the microinverters give. but 2 different MPPT inputs on 2 different systems for a total of 4 MPPT strings would be acceptable. But I just dont want either SMA or Enphases "complete solution" I dont want to be tied to a vendor like that for the complete solution. even the tesla powerwall is too complete for my taste - if im going microinverters Id just rather have a different vendor do the island control, and even that as split up as much as possible (ac inverter, batt controller/bms, batteries). make sense?
 
Solar Edge has optimizer per panel, which provides panel level monitoring.
With their optimizers, panels of multiple orientations are all connected in series. Up to 24 panels in a single string (the optimizers limit max voltage seen by string inverter.)


They also have a battery backup system. I don't know how their scheme works.


A number of brands should work with each other, using frequency/watts for battery inverter to communicate with PV inverter.
I've heard some brands tried for some time before getting it working well (somebody said that about Skybox, I think.)

SMA has been doing this for decades, so using all their products should work well (within their compatible list.)
That isn't a closed system and you can use other brand inverters that respond to frequency. I just don't see a benefit to deliberately mixing if you could start with all one brand.
At least for a while I saw SMA advertising their battery inverter works with any grid-tie inverter, but I don't think it works nicely with a basic UL1741 because battery has to cycle as PV inverter kicks on and off. What works cleanly is a PV inverter that reduces watts as frequency increases.
 
I've heard some brands tried for some time before getting it working well (somebody said that about Skybox, I think.)
The Skybox AC coupling was implemented in firmware after it was released. I purchased my Skybox knowing Outbacks reputation and it has worked seamlessly since they implemented AC coupling. Some other brands may still be trying but often the issue is with the older GT inverters. My Skybix works well with my IQ7 micros.
 
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So Skybox + IQ7 would meet OP's goals well?

How about shifting when power is drawn from/delivered to grid, does it perform that function?
 
..........
How about shifting when power is drawn from/delivered to grid, does it perform that function?
Several modes do that. Backup with Export and Self Consumption primarily. When you say shifting, what specifically are you referring to? Load shifting when on grid or managing load and AC coupled GT inverters when grid is down. As far as the scenerio outlined in original post it easily handles that without an issue. The only solution to multole points of failure is Enphase Ensemble but not sure about DIY or pruce.
 
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I mean charging batteries from PV during times with utility rates are low (perhaps avoiding charging from grid at night, waiting for PV to come on-line in the morning),
followed by supplying loads from batteries during times when utility rates are high. Optionally backfeeding grid from batteries during those hours.

Eventually all customers in some markets will be placed on time-of-use rates. For now, regular customers pay same rate regardless of time of day, but pay more per kWh if above some allocation. PV customers are put on a schedule where cost per kWh is higher during certain hours (3x the price 4:00 PM to 9:00 PM in my case.)

Using Skybox to charge from PV during morning when rates are low, and to generate power from battery late afternoon when rates are high, would be worth $0.30/kWh in my case. That could backfeed the grid, earning $0.45/kWh credit during early evening hours which would provide 3x as much power at $0.15/kWh in the morning.

Second desirable feature is zero export, store surplus power from PV so no power is consumed from or delivered to grid, and other times supply as much as possible/needed from battery to avoid drawing from grid. This could let someone avoid grid-tie fees and maximize PV usage where grid-tie isn't allowed.

And then, signals to control loads based on available power is useful for both those grid-tied cases and for off-grid.
 
I mean charging batteries from PV during times with utility rates are low (perhaps avoiding charging from grid at night, waiting for PV to come on-line in the morning),
followed by supplying loads from batteries during times when utility rates are high. Optionally backfeeding grid from batteries during those hours
All of the above except optional backfeeding grid from batteries. As far as I know backfeeding grid from is not something I know how to do but it is capable as far as meeting Sunspec requirements. Selling excess solar to grid is native in two modes I mentioned.
I think selling battery energy to the grid is more of a utility restriction issue than a capability issue.
Export is configurable from zero to 5kW.
I do not know of a hybrid inverter that can control other loads. I am reading about some breaker panels that can do that but I am not sure what signalling protocal they require.
 
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I tried to run my GT system off my portable generator just to see if it would work. They never came online. Portable generators probably have pretty dirty power and the inverters didn't like that. That is my guess.

If you do decide to try something like this, be very careful. I am not sure what would happen if the solar output exceeded the system load. I would hope that the frequency would increase and trip the solar, but the generator may not be very happy and do undefined random bad things.
 
Fortunately most grid tie inverters can't be fooled that easily. You are correct bad things could have happened.
 
I mean charging batteries from PV during times with utility rates are low (perhaps avoiding charging from grid at night, waiting for PV to come on-line in the morning),
followed by supplying loads from batteries during times when utility rates are high. Optionally backfeeding grid from batteries during those hours.

Eventually all customers in some markets will be placed on time-of-use rates. For now, regular customers pay same rate regardless of time of day, but pay more per kWh if above some allocation. PV customers are put on a schedule where cost per kWh is higher during certain hours (3x the price 4:00 PM to 9:00 PM in my case.)

Using Skybox to charge from PV during morning when rates are low, and to generate power from battery late afternoon when rates are high, would be worth $0.30/kWh in my case. That could backfeed the grid, earning $0.45/kWh credit during early evening hours which would provide 3x as much power at $0.15/kWh in the morning.

Second desirable feature is zero export, store surplus power from PV so no power is consumed from or delivered to grid, and other times supply as much as possible/needed from battery to avoid drawing from grid. This could let someone avoid grid-tie fees and maximize PV usage where grid-tie isn't allowed.

And then, signals to control loads based on available power is useful for both those grid-tied cases and for off-grid.
What I’m looking for is a net-zero smart battery charger - anyone know if such a thing exists?

What I want it to do is monitor net generation from a Microinverter-based PV array net of self-consumption and use that potential export to instead power a battery charger (so export remains at zero).

It would need to generate the grid signal to the Microinverter and have throttling capability to reduce Microinverter array output as the battery approaches full charge.

It would need to be a generic battery charger like MPPT SCCs with a clamp-sensor input like GTILs to monitor export to the grid and to support the generic AC throttling standards to throttle-back microinverters and shut them down if needed (or they are older-generation).
 
How close to true zero export do you need to be? There are several ways this can be done. What is more important... Having no export at all, or being more efficient and pushing some power out once in a while?

The easiest way to be true zero export is to go with a DC coupled battery charging setup. It is less efficient as it will just throttle back the charging to hold the batteries full during the day, and you run on the batteries at night. If the batteries run low, you can buy grid power to keep you going.

If you are going to do it with Microinverters, Enphase has a setup where the Envoy measures the grid current and commands the microinverters to throttle down to limit any export. But then your battery charger needs to adjust the charge current to maximize charging to use the "extra" power to keep the microinverters from throttling down. There is no add on system that I know of that has the control you are asking for.

My Schneider XW-Pro has a lot of capability, but when it is tied onto the grid, some of it is not able to function. You can't frequency shift the grid. So while I am on grid, any extra production from my solar array is still being exported. I have to manually set how much charge current I want to to use. But if it goes off grid, it becomes a whole different animal. It will let the microinverters power all the loads, and any extra power will be used to charge the batteries. If the extra power exceeds the maximum charge current, or if the batteries become fully charged, then it will start to frequency shift to tell the microinverters to throttle back. If I had enough solar and battery to completely run my home, then it would work perfectly like this. But only about half of my home is on the inverter output. The rest is still back in the main panel. So I run the system on grid. This was the extra power does flow back to my main panel and helps power the rest of my home, and does export some when I am not using all of it. And I end up buying power for about 5 - 10 hours overnight once the batteries are back down to 40% where they started the day before. While on grid, I still need to manually trigger it to start the charge cycle each morning, but I am working on a simple fix for that. When off grid, this is not a problem, as it just charges from any extra power from the microinverters, but while on grid, it will never run the battery below the "Recharge volts" setting. If you have DC coupled charge controllers, that also "fixes" the issue. This is one case where I think the Skybox actually works without external intervention. I am still trying to convince Schneider that this function is needed and could help them sell more units. But they just don't seem to care.

I have not done a test since the software update, but Enphase has assured me that it should fix the issue I had with the microinverters going offline when I had a power failure. Basically, when the power failed, the Schneider XW-Pro did it's job. It disconnected from the grid, and started producing the local micro grid. But during the brief switching time, the microinverters saw it as a grid frequency error. They disconnect and should monitor the grid and if the grid is good for 5 minutes they should reconnect. But there was a logic error that was preventing them from connecting again. I hope to try a power fail test soon, but I want to shut down my PC's and such first, just in case it fails.

Since the system does work so well while off grid, I have looked into having a controller take the system off grid for part of the day. If the power fail disconnect issue is fixed and does not cause any problems, this may be the easiest fix. I can just have a simple time disconnect and reconnect the grid input to get the best of both worlds. This will likely cause a bit deeper cycling of the batteries, but that is adjustable to where I could make it use less while on grid to balance it out.

It really comes down to your power usage, you rules for power export, system cost, and solar production capability. For the last 10 days, I am very close to net zero. I have been buying some cheap overnight power, but then exporting some cheap power, and a few KWHs of peak rate power as well. Half way through a month billing cycle, SCE current owes me $3.00 as I have exported a little more than I bought. My peak export power did hit 2,000 watts when my battery topped up and the sun was still shining strong. I have delayed my charging start time a bit to reduce that. My peak power usage rate hit a maximum of 1,200 watts from the grid. 2 PC's on, watching Dish Network TV, and the furnace was running. Might have put something in the microwave as well. And of course, this was at night, no sun, and after the battery inverter had shut down as we were past the end of the peak rate time, and the grid power is cheaper.

I have done the math several times, and I do want to keep time shifting power, but let's be totally honest here. Even with a fairly big difference in the rates, from $0.22 overnight to 0.44 peak rate, I can only "save" about $2.00 per day without my air conditioner running and not exporting a lot from battery during peak rate, which is not allowed in many areas. I am dialing in mine to export less than 500 watts from battery back to grid. Without the battery, I would be buying over 1,100 watts ( for 5 hours = >5.5 KWHs) from the grid during that time. And my consumption can go over 4,000 watts , when the A/C compressor is running. Once the weather starts getting hot again, I am going to try and pump up my battery power so that most of the air conditioning energy is coming from solar and battery stored solar power. If I had the A/C drawing it's power from my backup loads panel, the XW-Pro could self compensate and just pull the extra power from the battery, but I am keeping that big load back in the main panel. The compressor only draws 14 amps while running, but the starting surge can hit 105 amps. That is a bit much to pull through the XW inverter. So as far as the XW is concerned, it needs to export towards the grid to "help" power it. One feature I could add is using a "WattNode" box connected into the XW-Pro and the Gateway. It can self adjust the grid sell power to keep the grid current close to zero. This will only work during the time it is exporting to the grid anyways, and will not make any adjustments while charging.
 
How close to true zero export do you need to be? There are several ways this can be done.

Appreciate the extensive response.

I have a NEM agreement for 3kW peak and there are those who say it is ‘better to ask forgiveness than to ask permission’ but my current priority to to be certain I stay within my 3kW peak (meaning no export beyond what my existing grid-tied array is producing).

The other atypical constraint of my system is that I do not want to require my main panel or create a critical-loads panel.

I have 2 SUN 1kW zero-export inverters which will convert 24V battery power to up to 1kW per leg to offset self-consumption (monitored with clamp sensors).

So all I’m looking to do is get my new partially-shaded PV array to charge my 24V LiFePO4 as effectively as possible.

DC-coupled is obvious and easy but not well-suited to partial shading. Microinverters would be better for getting more output from a partially-shaded array, but need to either be throttled-back of have a sure on-premises destiny for the power produced (which is where my idea of a zero-export AC battery-charger to consume all Microinverter-generated power beyond self-vonsumption

The easiest way to be true zero export is to go with a DC coupled battery charging setup. It is less efficient as it will just throttle back the charging to hold the batteries full during the day, and you run on the batteries at night. If the batteries run low, you can buy grid power to keep you going.
Yes, that’s my current plan. And as long as the battery is oversized versus the daily generation and fully-drained from self-consumption overnight, it is more efficient than any other alternative I have found (into the battery, the losses add up when converting that DC battery energy to AC for consumption).

The biggest loss of efficiency I’m seeing with DC-coupled is how poor it will be in handling shade. MPPT-per-panel is much less practical with DC strings versus microinverters...
If you are going to do it with Microinverters, Enphase has a setup where the Envoy measures the grid current and commands the microinverters to throttle down to limit any export. But then your battery charger needs to adjust the charge current to maximize charging to use the "extra" power to keep the microinverters from throttling down. There is no add on system that I know of that has the control you are asking for.

That’s what I was afraid of. But if you agree it is a good idea, I think it will only be a matter of time.

The GTIL inverters I’m using didn’t exist a few years ago. They have completely changed the game as far as installing zero-export solar. They have a clamp sensor to tell them how much power is being consumed on their specific leg and then use that to limit the amount of energy they produce, feeding it into the same leg (literally through an outlet).

You can supply your self-consumption with literally no rewiring (and if their is a failure in the zero-export system, the grid and all rewiring remains unchanged and unaffected. Truly a game-changer as far as the cost and complexity of adding some solar production to offset a portion of self-consumption.

So on the AC-world, where conversion to AC happens in the array, what you need is the GTIL-equivalent for consumption rather than generation. Monitor generation and anytime it goes positive, increase battery charging current to offset export and bring back to zero. It would also need the ability to throttle-back microinverters once the battery is full, which can be a brute-force switch on the grid signal for once-per-day shutdown (rarely needed as long as the battery is oversized).

If this idea is as good as I think it is (which is why I value your opinion), I think it will only be a matter of time before one of the Chinese companies puts something like this on the market.

My Schneider XW-Pro has a lot of capability, but when it is tied onto the grid, some of it is not able to function. You can't frequency shift the grid. So while I am on grid, any extra production from my solar array is still being exported. I have to manually set how much charge current I want to to use. But if it goes off grid, it becomes a whole different animal. It will let the microinverters power all the loads, and any extra power will be used to charge the batteries. If the extra power exceeds the maximum charge current, or if the batteries become fully charged, then it will start to frequency shift to tell the microinverters to throttle back. If I had enough solar and battery to completely run my home, then it would work perfectly like this. But only about half of my home is on the inverter output. The rest is still back in the main panel. So I run the system on grid. This was the extra power does flow back to my main panel and helps power the rest of my home, and does export some when I am not using all of it. And I end up buying power for about 5 - 10 hours overnight once the batteries are back down to 40% where they started the day before. While on grid, I still need to manually trigger it to start the charge cycle each morning, but I am working on a simple fix for that. When off grid, this is not a problem, as it just charges from any extra power from the microinverters, but while on grid, it will never run the battery below the "Recharge volts" setting. If you have DC coupled charge controllers, that also "fixes" the issue. This is one case where I think the Skybox actually works without external intervention. I am still trying to convince Schneider that this function is needed and could help them sell more units. But they just don't seem to care.

I have not done a test since the software update, but Enphase has assured me that it should fix the issue I had with the microinverters going offline when I had a power failure. Basically, when the power failed, the Schneider XW-Pro did it's job. It disconnected from the grid, and started producing the local micro grid. But during the brief switching time, the microinverters saw it as a grid frequency error. They disconnect and should monitor the grid and if the grid is good for 5 minutes they should reconnect. But there was a logic error that was preventing them from connecting again. I hope to try a power fail test soon, but I want to shut down my PC's and such first, just in case it fails.

Since the system does work so well while off grid, I have looked into having a controller take the system off grid for part of the day. If the power fail disconnect issue is fixed and does not cause any problems, this may be the easiest fix. I can just have a simple time disconnect and reconnect the grid input to get the best of both worlds. This will likely cause a bit deeper cycling of the batteries, but that is adjustable to where I could make it use less while on grid to balance it out.

It really comes down to your power usage, you rules for power export, system cost, and solar production capability. For the last 10 days, I am very close to net zero. I have been buying some cheap overnight power, but then exporting some cheap power, and a few KWHs of peak rate power as well. Half way through a month billing cycle, SCE current owes me $3.00 as I have exported a little more than I bought. My peak export power did hit 2,000 watts when my battery topped up and the sun was still shining strong. I have delayed my charging start time a bit to reduce that. My peak power usage rate hit a maximum of 1,200 watts from the grid. 2 PC's on, watching Dish Network TV, and the furnace was running. Might have put something in the microwave as well. And of course, this was at night, no sun, and after the battery inverter had shut down as we were past the end of the peak rate time, and the grid power is cheaper.

The Conext solution is beautiful, as is Solark and even Magnum PAE, but they all require getting wired into the critical path, which means more $$$ and an additional potential point of failure.

I was headed down that path before I realized it made little sense for my use-case.

I’ve already got a 4kW grid-tied system under NEM1 that covers my annual consumption (at least before they start reducing credit by shifting peak TOU hours this summer).

2-3 years from now, we’ll have an EV, so by then, I want this battery+ GTIL system powered by a new array to offset a major portion of self-consumption (and 100% of peak TOU consumption) which will leave plenty of generation credit from the existing grid-tied array to power an EV.

My fall-back for dealing with shading issues is just to add more panels on another part of the roof, so that kind of sets my budget for how much I’m willing to spend to get back a portion of the ~40% of potential generation I lost to shading.

But it is annoying to me that there is not yet a solution to allow AC-coupled battery charging designed for zero-export (full consumption of excess generation). If there were, I’d be using microinverters in the new array rather than looking at using more MPPTs and possibly Tigo optimizers...

Oh well, I suppose I’ve got a couple years to see how things evolve...
 
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