AC coupling off grid

I wanted to check in to hear about DIY installed and working systems with off grid micro inverters.

Particularly, what exactly is involved in getting the communication protocols of voltage regulation working.

For example, Midnite charge controller, Enphase micros, fortress battery. What inverter will bring all that together and support the micro inverters?

Any thoughts appreciated.

Curious as to why you’d want to use both a charge controller and AC couple with micro inverters? Especially if your off grid, at that point to makes the most sense to just use a DC coupled system.

AC coupling doesn’t really have a protocol per se… but rather your inverter charger will use frequency shifting to tell the micro inverters to throttle their output or shut down entirely.

Sorry. Should have said charger/inverter. But also grid forming.

My goal is to get from DC to AC as quickly as possible (microinverters). And have traditional AC systems throughout. The charger/inverter/battery bank can be located in the basement. This will allow me to minimize DC runs, arc fault, fire hazard.

I can’t seem to pin down the equipment and settings to do this.

AC coupling is not completely straight forward and communications between equipment offered by different manufactures is virtually nonexistent because there really are no "Industry Standards" in use at the moment. Bluetooth, WiFi, Ethernet, RS485, CAN bus, Xanbus, Power Line Modulation are all in play.
Based on your description, I would take a look at Magnum Energy. Their Grid-Tie microinverters communicate with their grid forming Battery/Inverter [MicroGT500 microinverter and MS-PAE inverter/charger] I have not personally tested or seen any reviews of these products, I am only aware of them due to articles I've read online.

Not really sure your fears about DC are warranted. DC coupled systems are perfectly safe.

The added costs, complexity, and loss in efficiency with AC coupling only make sense if you already have a grid tied installation.

So basically you are looking for a UL listed Inverter capable of doing AC Coupling. Having built in charge controller not necessary.

1. The XW Pro 6848 does AC Coupling, is split phase 240/120 in a single unit and would be a good choice.
2. SMA Sunny Islands can do AC Coupling, but is 120v per inverter, so 2 inverters needed to do 220v. That's double the cost.
3. I think Sol-Ark inverters can AC Couple, but they are about twice as much and come with charge controllers built in, though you don't have to use them.

I'm sure there's some others, but all 3 of those are solid choices for AC Coupling.

That’s helpful, thanks.

In terms of actually designing a system, I can get the IQ8s, panels that are suitable to the IQ8s, the SMA Sunny Island (only need 120), and the Fortress battery. And a 120v Fortress Power approved LFP charger.

If I have components sitting in my living room, hook up the panels, micros, inverter, and battery, I have a feeling that I’m going to be out of luck without first figuring out how to make them play well together. I haven’t found that playbook yet.

Regarding the safety of DC, I’m thinking of putting this on the rooftop and the inverter/charger/battery are going to be in the basement, three stories down. I’d like to isolate the DC components as I am able. AC safety systems are comfortable and reliable. I’ll have to go with a power shack if I can’t use the big, flat, 30x30 roof. Would be an issue for security in my area.

Bluedog225 said:

That’s helpful, thanks.

In terms of actually designing a system, I can get the IQ8s, panels that are suitable to the IQ8s, the SMA Sunny Island (only need 120), and the Fortress battery. And a 120v Fortress Power approved LFP charger.

If I have components sitting in my living room, hook up the panels, micros, inverter, and battery, I have a feeling that I’m going to be out of luck without first figuring out how to make them play well together. I haven’t found that playbook yet.

Regarding the safety of DC, I’m thinking of putting this on the rooftop and the inverter/charger/battery are going to be in the basement, three stories down. I’d like to isolate the DC components as I am able. AC safety systems are comfortable and reliable. I’ll have to go with a power shack if I can’t use the big, flat, 30x30 roof. Would be an issue for security in my area.

Click to expand...

People run DC hundreds of feet from their arrays to their equipment. 3 stories isn't that big of a deal. Fuses, breakers, and properly sized wires, just like with AC, are all you need to make this a safe system. Your current with a 250ish vdc string is going to be about the same as with your 240vac micros.

And as far as finding a playbook, well that's going to be hard with AC coupling (unless you go Victron & Fronius) and especially if you mix and match manufactures.

Really the only unique integration consideration in this design is the AC Coupling control between the Hybrid battery inverter and the micro inverters.

Under normal grid up conditions the inverter will just pass the extra power from the micro inverters to the grid "selling with net metering". This is assuming you can/will get Net Metering from your utility company and have a inspected/approved interconnect with them. Otherwise this doesn't apply.

During an outage though with grid down or if you don't have Net Metering, the inverter will push the extra energy into the batteries. This is where things get interesting. The batteries will eventually get full and then the frequency curtailment feature kicks in to try to control output from the micro inverters. This is the integration part that may or may not work that smoothly, depending on how the micro inverters get along with the hybrid inverter. I only have experience with the XW Pro doing this and it ramps frequency SLOWLY from 60hz (100% output) to 62hz (0%) output. My SMA Sunny Boys are setup with freq/watt gradient and do as commanded. It works pretty well together, but not perfect.

A few things to consider when picking equipment and designing an AC Coupled system:

1. Not all hybrid inverters can handle AC Coupling. Usually high end Low Frequency inverters are the only one's capable of this.
2. The hybrid battery inverter has to be big enough to handle the FULL output of the grid tie inverters.
3. The battery has to be big enough to handle the full current that the grid tie inverters could put out until full. For instance if you maxed a XW Pro out with 6,800 watts of grid tie solar hooked to it, your battery needs to be able to handle 140 amps of charging.
4. The micro-inverters need to support Freq/watt curtailment. This is a requirement included in California Rule 21, so if they are compliant with that standard, they have that feature.
5. Its best to have your hybrid inverter set at a lower charge voltage than normal. This gives the frequency curtailment time to ramp up without overcharging "high voltage" happening on the battery bank. So your battery's will probably be only charged to 90% max at any given time.

AC Coupling is a somewhat more advanced configuration, due to the integration with a Hybrid inverters. While there is some efficiency gains, there are also some drawbacks with less exact charging control to the batteries.

Me personally, if your not grid interactive and don't have Net Metering, I would not AC Couple. I would DC Couple by hooking the panels in series (up to 11 in some cases) and run a single 10 gauge PV cable to a high voltage solar charge controller. If you have more than 11 panels, run (2) pairs of 10 gauge PV cable and make 2 arrays hooked to 2 charge controllers. Lots of all in one inverters options or charge controller inverter options in this configuration.

Several points to think about.

Power has to go somewhere. The power from GT micro-inverters has to either go to house loads or into battery when operating without grid to push it to.

Freq control shutdown or reduction control on GT inverters can take up to a few seconds to happen.

LF hybrid inverters are better able to handle any back surge.

Worse case is a lot of PV GT production being consumed by house loads then suddenly a heavy house load is shut off leaving inverter to handle a lot of back surge overproduction power.

I would not use AC coupling on most HF inverters. SolArc runs GT inverter input through generator connect relay so it can pull the plug on GT inverters if it loses control on excess power generation.

Most all HF inverters have to switch DC to HV DC boost converter mode to switch direction of power flow which takes some time. This, and poor surge capability, can make them vulnerable when using AC coupling with them.

Battery size must be appropriate to take back surge power until hybrid inverter gets GT inverters to reduce their output power.

Most common mistake is thinking you don't need much battery capacity because you have so much PV panel power to supply your needs.

On DC coupling,

DC coupling for daily grid pushing through hybrid inverter puts a lot of 120 Hz ripple current on batteries. The batteries act as a large filter capacitor between DC charge controller and inverter. If you don't have large enough battery capacity it can be quickly damaged by this high ripple current. For 48v inverter, roughly 100 AH battery capacity per kW of DC PV power is minimum battery sizing to prevent excessive battery stress.

Thanks for all the detailed responses. I realize I’m out of the mainstream here.

My goal is to avoid the risk of a long, unregulated, DC wire run through the structure and the associated fire risk. I’m willing to pay extra to mitigate this risk.

I need some sort of rapid shutdown device on the panels to accomplish this. Micros seem 100 percent reliable. No chance of DC arc anywhere downstream. My DC risk is isolated at the panel, and isolated at the inverter charger in the basement. All the rest is plain vanilla AC with the associated safety devices and track record.

Micro inverters seemed to fit this requirement. What are my other options? Are optimizers or some type of panel level rapid shut down device less complicated? And more reliable that an IQ8?

My other option is to run the DC lines from the roof to the basement in metal conduit to avoid damage from animals (rats/squirrels) or weather exposure. I’m not sure what happens with a prolonged DC arc to that conduit. I‘d rather minimize the DC arc issue in my design.

I‘m open to other solutions. The is a lot of appeal to a roof mounted array in my situation. But I’d like it to be as safe as a traditional utility feed. Otherwise, I’ve got to build another structure, run lines underground, etc. with a fair amount of extra expense and risk of theft.

Thanks all!

With your concerns of PV Arc Faults, you should read up on NEC 2017 and Arc Fault detection certified equipment that meets UL 1699B.
Most Solar manufactures have equipment that addresses Arc Faults available.

Of course this is not needed with the Micro Inverter route. I have NO experience with Microinverters, so can't offer you any advise on them. It just makes me cringe when I hear the words roof mount solar panels with micro inverter. Make sure you think about how you will replace one or more of those when they fail. They WILL fail eventually....

Thanks. It’s a big, flat, southwest facing 30x30 standing seam metal roof on a 1:12 pitch with a hatch. No access issues at all. It begs for solar….

If you follow the NEC rules, you need to run metal conduit for all the DC wires exclusively (can't mix AC and DC) inside the house to the inverter. In addition you need to have a rapid shut (RSD for short) down device on each panel, such as a Tigo TS4. You also need a DC disconnect close to an accessible location within your structure. The rapid shutdown devices will basically disconnect the panels if there is a short or if there isn't a 'keep alive' signal coming from the rapid shut down 'brain' which can be connected to a emergency switch, the inverter signal, etc. Other than that the DC wires should be relatively safe inside the metal if there is a short, the RSD will break the signal, and the inverter should have fault protection as well and you can even add fuses or breakers as well in line if you need to (aren't required unless paralleling more than 2 strings). You will probably get better efficiency with higher voltage and DC for charging batteries with DC strings. I had a choice to use micro inverters, which are excellent and have a lot of advantages over DC strings, but opted for DC strings going into a Sol-Ark 12K. It also supports AC coupling to microinverters (or grid tied 1741SA type inverters) as well if needed.

Thanks for this detail Keith. That’s along the lines of what I was thinking. Good to have the specifics.

My global impression is that either way (DC as you describe or micro inverters) is more effort and expense.

Micros intuitively strike me as more elegant and fundamentally safer. I can’t back that up. Just an intuition that they would fail safe and the whole of the wiring installation would be nice, tame, familiar, AC.

Bluedog225 said:

Thanks for this detail Keith. That’s along the lines of what I was thinking. Good to have the specifics.

My global impression is that either way (DC as you describe or micro inverters) is more effort and expense.

Micros intuitively strike me as more elegant and fundamentally safer. I can’t back that up. Just an intuition that they would fail safe and the whole of the wiring installation would be nice, tame, familiar, AC.

Click to expand...

I think you are somewhat correct about microinverters being marginally safer in that with DC strings there will most likely be higher voltages and DC in general tends to arc more powerfully if for whatever reason the insulation is breached between DC and ground or the opposite polarity conductor and there is an arc, it might be harder to extinguish with a breaker or the RSD system. However, the DC system will most likely have lower amps. For your roof, I'm not sure how many panels or strings you'd have, but remember with the microinverters you could have lots of AC amps (40A-60A or more), whereas with DC your max amperage would probably be around 10A. The microinverters have built-in RSD and wifi monitoring with a unified system whereas with DC you probably will have multiple monitoring, for example, both Tigo's and Sol-Ark's which have different parameters to view, Tigo's being a panel level info, and Sol-Arks being MPPT, Battery and Home AC info. I think the only advantage might be efficiency of the DC-DC when charging batteries. With microinverters charging batteries, you would be going from DC-AC-DC with a bit of loss for each conversion. Overall though I think you are right, microinverters, with all things being considered, are more elegant.

ChrisFullPower said:

People run DC hundreds of feet from their arrays to their equipment. 3 stories isn't that big of a deal. Fuses, breakers, and properly sized wires, just like with AC, are all you need to make this a safe system. Your current with a 250ish vdc string is going to be about the same as with your 240vac micros.

And as far as finding a playbook, well that's going to be hard with AC coupling (unless you go Victron & Fronius) and especially if you mix and match manufactures.

Click to expand...

But to get permitted in the future, or get grid hookup, would require rapid shutdown, dc arc fault and more headaches. Not to mention one cloud and the entire string of series panels go down, and a single point of failure in your string, vs a single micro inverter failure.

Bluedog225 said:

That’s helpful, thanks.

In terms of actually designing a system, I can get the IQ8s, panels that are suitable to the IQ8s, the SMA Sunny Island (only need 120), and the Fortress battery. And a 120v Fortress Power approved LFP charger.

If I have components sitting in my living room, hook up the panels, micros, inverter, and battery, I have a feeling that I’m going to be out of luck without first figuring out how to make them play well together. I haven’t found that playbook yet.

Regarding the safety of DC, I’m thinking of putting this on the rooftop and the inverter/charger/battery are going to be in the basement, three stories down. I’d like to isolate the DC components as I am able. AC safety systems are comfortable and reliable. I’ll have to go with a power shack if I can’t use the big, flat, 30x30 roof. Would be an issue for security in my area.

Click to expand...

With a single sunny island you will need a transformer to work with most (240v) micro inverters.
Ideally get one with a split phase secondary built in (low freq).

ChrisFullPower said:

AC coupling doesn’t really have a protocol per se… but rather your inverter charger will use frequency shifting to tell the micro inverters to throttle their output or shut down entirely.

Click to expand...

"Generally, inverters that comply with IEEE 1547 and UL 1741 will be acceptable for interconnection."

This pdf is from 2019, but it's a good start.

How do delete this post goshhhdarrrn