AC Coupling Question

[jmktexas]

Just wondering if this will work. My older gvfx 3648 works fine but is really underutilized now that the radian is up and going. I'm wondering if I can run both inverters to my critical load panel without either inverter having problems? Then I could add more loads to panels and use output of gvfx better. Right now I'm selling most of the gvfx production. Presently I'm using 2 separate critical load boxes, one for radian and one for gvfx. Grid zero on radian has worked really well for me so far but I'm guessing I would have to give up my grid zero function for this to work. I'm not sure if I could stay grid tied and sell from gvfx either.
Ideally I'd like to put central air on critical load panel but it would take everything radian could do by itself if any other loads were being used, especially at start up. I know I can use a "soft start" capacitor to help with that but I'd still be pushing envelope with just radian production so I was wondering about coupling gvfx output to load center? All wiring diagrams I've seen so far show a GTI without battery backup as coupling device. I have attached a very basic wiring diagram to show what I'm thinking about.

[jmktexas]

Well I thought I attached pdf. Guess not. Try again.

AC Coupling.pdf

(14.3 KiB) Downloaded 35 times

[Mike Curran]

Outback inverters have an AC-in (from grid) and an AC-out (to loads) connection. Only the AC-in connection synchronizes itself - the AC-out doesn't know what it's connecting to. So in your diagram, the GVFX connection to your protected loads panel would have to be the AC-in (grid) connection to insure it is in synch with your Radian output (during grid outages) or with grid when it's up. Basically, you would always be "selling" to your protected loads, with no loads on the GVFX output (unless you wanted to add another protected, isolated load panel). And if your existing protected loads aren't using it, it would sell to grid.

This would be a pretty unconventional setup and I doubt OB would give its blessing. Problems might arise during grid outages when excess production from your GVFX has nowhere to go - you risk overcharging your lithium battery - the Radian may try to dither its output frequency to get your GVFX to disconnect, but the GVFX may not respond to that.

Maybe someone else has a better idea, but long story short, I would not advise it.

[jmktexas]

Outback shows diagram for an inverter tied to a radian but it isn’t grid tied or have battery backup. That’s why I was wondering about using the gvfx with grid tie. Could be I’m not understanding everything I’m looking at but the only difference I see is being grid tied and battery backup. Don’t see how that makes a difference to the radian but I’m certainly no expert. Just asking for input from forum. I absolutely won’t try this without OB advice/blessing.

https://www.outbackpower.com/downloads/ ... radian.pdf

[Mike Curran]

It could work, it's just very unusual for the GVFX to be set up that way, with no loads on its AC output. And it does have frequency limits on accepting AC at its grid input, where you'll be connecting it to your Radian output through the load panel. So it just might disconnect if the Radian sees its lithium battery getting overcharged and raises its AC output frequency.

I mentioned OB but I personally wouldn't worry too much about their opinion unless they can point out a fatal flaw. If you try it let us know how it turns out. Good luck!

[jmktexas]

Okay. I’m pretty confused between info I have read and info outback is giving me. Everything I’ve read implies ac coupling a gvfx to radian should work and act as a parallel connection. I.e. 3.6kw+7.2kw=10.8kw. Outback’s application note shows this also. I’ve talked to techs twice at Outback and both of them say I can’t do this. Best explanation I have gotten is it will be a “glitchy” setup. But neither could give me a good reason why or what that means. Would it be a settings nightmare or would it damage something or both? I absolutely will not ac couple setup without their guidance but they couldn’t give me an example of what would work either. So I’m guessing ac coupling is off the table for now.

[Mike Curran]

Too bad. I would've liked to see how it worked. Not sure I would try it myself, though, unless my GVFX were expendable.

[jmktexas]

What’s puzzling to me is even my 8 year old gvfx has settings on its mate for ac coupling. I just hate to keep calling them and become “that guy”. I have figured out the gvfx can’t be connected to grid, I think it syncs up with radian output and if it loses the radian then it shuts off because gvfx thinks radian is its grid. Not really sure. Application note has a wiring diagram on it showing what I was hoping to do but techs keep telling me it won’t work.
I’ll probably stare at it for a while longer then try asking the question a different way. I’ll post if I ever get bold enough to try.

[Mike Curran]

even my 8 year old gvfx has settings on its mate for ac coupling

I don't think those settings apply in your desired use case, they're for other grid-tied inverters to AC-couple to the GVFX AC output.

I think it syncs up with radian output and if it loses the radian then it shuts off because gvfx thinks radian is its grid.

That's absolutely correct.

[jmktexas]

Yeah, maybe the terminology is what’s getting me in trouble. According to wiring diagram, gvfx output does tie into radian output. They are both ac output so I’m not always sure which one they’re talking about. I haven’t looked at the ac coupling settings on radian yet. I need to update firmware before I go there. Haven’t done that yet because I loathe fixing stuff that’s working. But I will when I’m confident it will work.

[Mike Curran]

I took a screenshot from the app note https://outbackpower.com/downloads/docu ... radian.pdf and marked where your GVFX would be.

Note, the connection to the Radian output is from the GVFX grid input, NOT its output. Only the GVFX grid input will sync with the Radian's AC output. The GVFX AC output will not. If you connect the GVFX AC output to the Radian's AC output you WILL destroy one or both of them.

Understand that the original, unmarked diagram is shown for a strictly grid-tied, batteryless inverter ("GDI", or grid dependent inverter) which has only one AC connection. Your GVFX has two and if you're going to do this, it's VERY important that you make the right connection: GVFX grid input (syncs to grid, or in your case, to Radian) to Radian AC output.

Edit: Also understand, when you AC couple in this fashion, all the PV power going into your GVFX that's not being used to charge its battery (it does have its own battery, right?) and/or support your loads [edit: or sell to grid] will flow uncontrolled into your Radian and through to your lithium batteries. The Radian AC-coupling settings must work to force disconnection from the GVFX when this unthrottled power exceeds your lithium batteries' limits, else your lithium batteries will be destroyed. Not to scare you or anything but this is how it works.

[jmktexas]

Ah-h-h. I'm a little slower than average. I will need to draw pictures in my mind on this. I'm betting you're right. What has thrown me was in the paragraph just above this drawing.

"By connecting (coupling) the output of the GDI to the output of a storage based inverter (SBI),

I took that to mean gvfx output to radian output. I know normally that would blow a power module on the radian but I figured the ac coupling settings would prevent that somehow. You know after I get this all straight in my head I might just bite the bullet and stack another radian. Be a lot more expensive but I know that will work.

Thanks for all your explanations. I admit what you're telling me is way different from what I was understanding the notes. Let me think on it.

[raysun]

Mike has drawn the correct picture, both conceptually and schematically.
Your notion about stacking a 2nd Radian is a viable approach to getting additional available power on line. If an 8kW increase isn't needed, the 4kW GS4048a will stack with the existing 8kW unit. In any event, both need the same firmware revision.

[jmktexas]

Yeah, after listening to Mike, I think I’m going to blow off the whole ac couple idea. Just too much risk for the limited reward. Saying that if one looks at Outback’s wiring diagram, gdi output is definitely wired into radian output. It doesn’t say gdi ac input anywhere that I’ve seen. You just gotta know that stuff intuitively or through bad experiences.
Mike probably saved me a lot of money. I was absolutely positive I had it right, but I’ve blown up a power module before doing what I thought I was absolutely right.
So now I ask a lot of dumb questions before I start.

I’m sure I’ll have more questions when I start adding another radian.

[Mike Curran]

In the app note and in general, typical "GDI"s do not have both an input and an output. Just a single grid tie through the loads panel. Your GVFX obviously has both. Easy to understand the confusion.