Here is a real kicker. If you had a GridTie inverter connected to the AC Out leg of a Battery Based inverter and it was producing power, but the batteries are charged. It should pass thru one side and out the other. NOW, what happens when you have 2 or 3 charge controllers and more panels connected thru the standard DC-Coupling method, and maybe some SWWP Air403 windmills. Then WHAT would happen when the grid tie inverter is producing and the Battery Based inverter is Selling power to the Utility at the same time? Can the Battery Based inverter pass the current of the Grid Tie inverter through it to the grid while still selling power to the utility? This is a theoretical situation since we are dealing with an "Undocumented Feature".

iboc,
i'm having trouble visualizing what you are saying so could you word it more precisely using the pdf as the start example and add to it?

Perhaps, I could draw up an autocad model, I cant do it right now though. I think that what I mentioned may be possible. In this scenario I described I think the only dangerous situation would be if the grid went out, then there would be no place for the excess current to go and the batteries would overcharge unless a method of disconnecting or disabling the gridtie inverter in the event of max bettery charge reached was designed. I figure thats what the communication cable between the GT and the XW are for...

By the way, I got our new power meter to read a bit of a credit this past weekend by switching the house to solar (the old system) for the day and letting most of the power from the Skystream feed back in to the grid. The windmill tower and nacell were dropped 6 feet durring installation accidentally by the contractor. We just had to make sure it wasn't broken.

Ok, I went to the Xantrex forums and began asking about AC-Coupling there. The first response wasn't promising, but them one of the moderators answered my question and said YES, Actually Xantrex Inverters can do this. This is what Eyedid had to say.

"Yes, you can connect the GT inverters to the output of the XW system in an AC-coupled configuration.

The XW will provide the “grid referenceâ€￾ for the grid-tie units to continue supplying power.
When grid is present, the soalr inverters will just sell back across the terminals of the XW.

Because the XW does not regulate charging when power is flowing from its AC output to the battery, a shutdown line is run from an interconnect board on the GT invetrers(RJ11 adapter, plus an optocoupler) to the XW inverter's auxiliary output, configured to trigger on high battery voltage condition.

The RJ11 on the GT inverters is originally used for 3 phase systems. 3 inverters are linked together through RJ11. When one unit disconnects (or 1 phase is lost) the rest of the units shutdown as well. We use this same principle to shutdown the GT inverters when a high battery voltage is triggered through the XW auxilairy line.

Are you planning to connect any AC source on either or both of the XW AC inputs (Grid, or generator)?"

For the full thread look here.---> http://www.xantrex.com/forum/forum_post ... D=597&PN=1

Well, Since Xantrex has publicy let the cat out of the bag ...

www.solar-guppy.com/downloads/AC%20Coup ... Draft4.doc

Reading the doc, it looks like when the GT output (+3KW) exceeds the XW charging capacity of ____ , magic smoke gets released from somewhere.

And I had stocked up on 500W, 400V DC-DC 28V converters to charge a makeshift battery bank.

Well I guess its good that I only have 3KW worth of positive ground Sunpower Panels. - And Solar Guppy, thanks for th info. Very enlightening.

I don't know about the XW or other inverters specifically, but assuming you have AC input and and inverter that automatically transfers the load when AC input fails (and restores the AC in to AC out via transfer switch) and turns off when the AC input is back on again--I see no reason that you could not push through grid tied inverter power back to the AC input (utility line).

There could be an issue with limitations of the AC battery charging circuit. For example, many inverter chargers allow you to set a maximum current through the inverter... Say 10 amps. If you have zero load on the inverter, then 10 amps (120 vac) is available to charge the batteries. If your inverter draws 5 amps, then only 5 amps would be available to charge the batteries (etc.).

What happens when your Grid Tied Inverters starts pumping power back through the line? It may look like a "load" (assuming the inverter/charger does not detect direction of current flow) and start to limit the amount of power available to charge the batteries.

This is probably not the worst thing in the world--at worst, your battery charging may be reduced during 9am to 3pm--and shift your maximum charging to off-peak times when the grid is up. In my case, that is actually a good thing as I have Time of Use metering and this would shift my optional power usage to off peak times...

In terms of safety, using the GT inverter shutdown to control battery charging voltage is OK--but I would probably recommend that a second dump controller be installed on the batteries too to prevent overcharging in the event the primary battery monitor or GT shutdown circuit failed or the control cable were cut (you might want to hook up an alarm (light/bell) on the Dump Controller to indicate a primary circuit failure).

Basically, 99% of the time, a Grid Tied / Off Grid Inverter/Charger setup like this is going to trickle charge (and equalize) based off of the function of the internal battery charger in the off-grid inverter (or separate AC battery charger if you go this route).

For the 1% period of time when you are off-grid (utility lines failed), then you will be using the charge through the inverter back-feed process. Having a second dump controller might be a bit of overkill--you could probably achieve similar results by just using a over voltage alarm and manual kill the GT inverter if you are home most of the time. Would depend on size of solar array and battery bank (might just be an unexpected battery equalization function).

Where I see more work needing to be done is with generator backup... You could not want to feed much (or any) power back to the generator and unless is it an Inverter/Generator, the AC power would probably not be stable enough to allow the Grid Tie inverter to function (frequency out of range failure?). Hence, my earlier suggestion of using the generator to directly recharge the batteries and forget the GT/Inverter-charger/Generator interaction.

I still wonder about NRTL/UL/Building code violations since none of this has probably been tested/approved for this type of usage. You want to make sure that everything (fuses, equipment rating, wire runs, batteries, etc.) are all properly designed and interconnected. Don't want somebody killed or loss of insurance over this "off-spec." usage of equipment.

-Bill

Thanks for the pointers, But as I have discovered. Codes and inspections dont seem to be much concern in this area, and the insurance company knows what we are doing, they gave us the loan! We even asked the town and utility co. and they told us we dont need any inspections or permits. So I will do my best to be sure that the system is installed in a safe manner. Looks like Xantrex is going to be my choice for inverters and hardware after all though, which Im happy about. And the GT inverter costs close to the same as the 3 MX60s or 3 XW controllers I would have needed anyway if I went battery connected. I figure the overal costs of the system should be the same. I will just need to keep an eye on it. And we wont be using a generator. The whole Idea is to have adequate backup for at least 1 day before batteries need recharging, and to do it all RE.

Just an Update to this thread, the system is now online and has been generating power for the past week. We get between 15 and 21 kwh per sunny day, but its been cold so that may change. Thanks for all the help guys!