Will OutBack GVFX3648 trick an SMA grid tie inverter?

[Murby]

I'm investigating which off grid inverter to purchase with the intention of going full 240 split phase with at least 7kw of power.

I've learned that the Sunny Island can "trick" the Sunny Boy SB 6.0 Grid tie inverter into producing power in the event of a grid failure. I've also learned that the Conext and Outback Radian inverters can do the same thing.

Does anyone have any experience with doing this? Can a GVFX3648 do it too?


Thanks,

[jflorey2]

I'm investigating which off grid inverter to purchase with the intention of going full 240 split phase with at least 7kw of power. I've learned that the Sunny Island can "trick" the Sunny Boy SB 6.0 Grid tie inverter into producing power in the event of a grid failure. I've also learned that the Conext and Outback Radian inverters can do the same thing. Does anyone have any experience with doing this? Can a GVFX3648 do it too?

Well, the very first problem there is that the GVFX generates 120 volts. You need 240 volts for an SMA inverter. Which means either a very large transformer or stacked inverters.

Problem #2 is that you have to have an inverter that is at least as large as the system you want to accept power from. Two stacked inverters MIGHT work (7200 watts) but it would be close. A more conservative system design would use four. And at that point you are better off with a Radian or two anyway.

Problem #3 is you are going to need a huge battery bank. Using a max charge rate of C/8 means you will need a 56kwhr battery bank, or 1200 amp-hours of lead acid batteries. That's around $9000.

Problem #4 is that the Sunny Island uses a frequency control scheme to reduce generation as the battery nears full; the Outback products won't do that. You can still do it (there's an application note from Outback as to how to do that) but in general it works like this:

-System comes up, generates 240 volts
-SMA inverter comes on line. GVFX rectifies 240VAC to 48VDC.
-Batteries charge at 7kW until they hit constant voltage (which is NOT full charge)
-ROCB or relay trips and the solar inverter goes off-line
-System runs on batteries for some time until battery charge drops low enough to accept full rate again

Needless to say, keeping your batteries at 60% charge (or whatever point you switch to absorb gives you) will not be good for them, so you'll either be running a generator a lot to keep them fully charged (even though it's sunny out) or you will be replacing batteries a lot.

[Murby]

Well, the very first problem there is that the GVFX generates 120 volts. You need 240 volts for an SMA inverter. Which means either a very large transformer or stacked inverters.

Yes.. I'm considering stacked inverters...

Problem #2 is that you have to have an inverter that is at least as large as the system you want to accept power from. Two stacked inverters MIGHT work (7200 watts) but it would be close. A more conservative system design would use four. And at that point you are better off with a Radian or two anyway.

My solar system is 7.56 KW on the DC side but only 6.0 KW on the AC side. I just realized why the inverter has to be as large.. its because the SMA is an all or nothing output scheme absent any frequency controlling that a Sunny Island has right? I didn't think of that.. thanks,

Problem #3 is you are going to need a huge battery bank. Using a max charge rate of C/8 means you will need a 56kwhr battery bank, or 1200 amp-hours of lead acid batteries. That's around $9000.

Problem #4 is that the Sunny Island uses a frequency control scheme to reduce generation as the battery nears full; the Outback products won't do that. You can still do it (there's an application note from Outback as to how to do that) but in general it works like this:

-System comes up, generates 240 volts
-SMA inverter comes on line. GVFX rectifies 240VAC to 48VDC.
-Batteries charge at 7kW until they hit constant voltage (which is NOT full charge)
-ROCB or relay trips and the solar inverter goes off-line
-System runs on batteries for some time until battery charge drops low enough to accept full rate again

Needless to say, keeping your batteries at 60% charge (or whatever point you switch to absorb gives you) will not be good for them, so you'll either be running a generator a lot to keep them fully charged (even though it's sunny out) or you will be replacing batteries a lot.

So there's really no (practical) way to do this without a standard run of the mill DC Charge controller like a Flexmax 80 right?

Looks like the Radian and the Flexmax is the only practical and reliable way to go yes?

[jflorey2]

So there's really no (practical) way to do this without a standard run of the mill DC Charge controller like a Flexmax 80 right?
Looks like the Radian and the Flexmax is the only practical and reliable way to go yes?

Maybe. The problem you may run into there - if you want to do an 'easy' conversion - is that the Flexmax 80 is only good to 145 volts, and most SMA grid tie systems are designed to operate at 400-500 volts. You'll have to rewire your array at least, and you may end up with some panels that are unusable depending on your panel topology.

The way around that is to go with a 600V charge controller - Schneider and Morningstar have them - but they are pricey.

Outback is soon coming out with their Skybox, which is an integrated grid tie inverter (600V strings) and battery system. Might be worth looking into. It's intended as a replacement for grid tie inverters.

[inetdog]

Even if the input voltage ranges are compatible, you generally will not be able to connect two MPPT devices (such as a GTI and a CC) to the same PV array output. The MPPT algorithms will fight each other trying to steal the largest share of the available array power.

[ButchDeal]

Even if the input voltage ranges are compatible, you generally will not be able to connect two MPPT devices (such as a GTI and a CC) to the same PV array output. The MPPT algorithms will fight each other trying to steal the largest share of the available array power.

There would be no need for the old grid tie inverter. It could be sold to help pay for the charge controllers.

[jflorey2]

Even if the input voltage ranges are compatible, you generally will not be able to connect two MPPT devices (such as a GTI and a CC) to the same PV array output. The MPPT algorithms will fight each other trying to steal the largest share of the available array power.

If they are both on - agreed. However:

1) If you only enable the CC during a blackout, there won't be a fight
2) The Morningstar 600V CC comes with a high voltage DC switch for exactly that reason

As another note, the grounding scheme on some CC's vs some transformerless grid tie converters is not compatible. They rely on the whole array floating. (Which is another argument for the Morningstar.)

[SWFLA]

I see this subject pop up somewhat frequently here.

1 - it seems as though if you want to AC couple batteries you need to do it right the first time. It also appears as SMA and Schneider are the only two that have a solid systems put together to do it, at the moment. Frequency curtailment to control charging seems iffy to me. Too many bells and whistles that need to be spot-on. Possible battery Bank sizing issues. And big $$$

2 - why in the world does anybody want to go from DC to AC to DC and back to AC again? Equipment is getting pretty efficient these days but it's flushing good watts right down the toilet, when you need them most because the grid is down. I understand that one already has the grid tie inverter but that still doesn't sell me on AC coupling.

The string/grid-tie method is the way to go for a stationary set up (IMHO). Not going to get into shading here but with 2 or 3 mppt inputs and some whiz-bangs I'm sure that can be dealt with. No problems with that here. If I decided that I had to have the luxury of battery backup I would go this route.

Upgrade the DC disconnect to a transfer switch. Grid-tie/off/charge controller.
Install a
Conext MPPT 80 600
Conext SW 120/240V (one or two) and 48v worth of batteries.
The inverter(s) feed the main panel through the mechanically interlocked back-feed where the generator would normally be wired. I do not believe in critical load sub panel's. Having 240 split phase is a must. I'll be damned if every light bulb and receptacle doesn't work in my house,some pumps, and a 1 ton ducted mini-split for the master bedroom 900w max. Not to mention the wiring headache of said sub panel. Nothing is going to be automated but we're talking about an extended power outage anyway. I don't see any cheaper or simpler solid way to do it. Besides having a nice heavy transformer the SW series inverters have some nice features. Generator load shaving is one that sticks out for me. All you need is a 3k or 5k generator and you have the surge capacity to start a pool/well pump or a/c unit, with in reason of course.

Just my thoughts
( insert your favorite charge controller / inverter)

[SunEagle]

I see this subject pop up somewhat frequently here.

1 - it seems as though if you want to AC couple batteries you need to do it right the first time. It also appears as SMA and Schneider are the only two that have a solid systems put together to do it, at the moment. Frequency curtailment to control charging seems iffy to me. Too many bells and whistles that need to be spot-on. Possible battery Bank sizing issues. And big $$$

2 - why in the world does anybody want to go from DC to AC to DC and back to AC again? Equipment is getting pretty efficient these days but it's flushing good watts right down the toilet, when you need them most because the grid is down. I understand that one already has the grid tie inverter but that still doesn't sell me on AC coupling.

The string/grid-tie method is the way to go for a stationary set up (IMHO). Not going to get into shading here but with 2 or 3 mppt inputs and some whiz-bangs I'm sure that can be dealt with. No problems with that here. If I decided that I had to have the luxury of battery backup I would go this route.

Upgrade the DC disconnect to a transfer switch. Grid-tie/off/charge controller.
Install a
Conext MPPT 80 600
Conext SW 120/240V (one or two) and 48v worth of batteries.
The inverter(s) feed the main panel through the mechanically interlocked back-feed where the generator would normally be wired. I do not believe in critical load sub panel's. Having 240 split phase is a must. I'll be damned if every light bulb and receptacle doesn't work in my house,some pumps, and a 1 ton ducted mini-split for the master bedroom 900w max. Not to mention the wiring headache of said sub panel. Nothing is going to be automated but we're talking about an extended power outage anyway. I don't see any cheaper or simpler solid way to do it. Besides having a nice heavy transformer the SW series inverters have some nice features. Generator load shaving is one that sticks out for me. All you need is a 3k or 5k generator and you have the surge capacity to start a pool/well pump or a/c unit, with in reason of course.

Just my thoughts
( insert your favorite charge controller / inverter)

One component that could also be needed (depending on the inverter type) would be some type of auto transformer that helps keep the voltage between the pv system and grid matched. It doesn't do everything but can help eliminate one of the variables for a safe grid tie system with batteries.

[Murby]

Hey guys.. I want to clear something up here because I think there might be some confusion...

I DO NOT want to change my current SMA grid tie setup... What I want to do is to have the equipment necessary to BE ABLE TO DO IT if the grid ever goes down for an extended period of time and the normal resources are not available.

Currently, when we lose power here, and we do lose power frequently for three to five days at a time, I just plug in my trusty Coleman 5kw generator that I converted to run on natural gas.. it powers the entire home like nothing was ever wrong... and the natural gas allows it to run flawlessly even in -10

[Murby]

So now I think I'm going with the Radian 8048.. only thing left to decide is if I should use a MorningStar charge controller or go with the OutBack Flexmax...

[ButchDeal]

So now I think I'm going with the Radian 8048.. only thing left to decide is if I should use a MorningStar charge controller or go with the OutBack Flexmax...

There is no need for the flexmax. You will be AC coupled with your SMA

though if you got two flexmax charge controllers you could dump the SMA, The outback will grid tie and feed extra power into the grid just like the SMA does with the batteries full most if the time.

[jflorey2]

Hey guys.. I want to clear something up here because I think there might be some confusion...

I DO NOT want to change my current SMA grid tie setup... What I want to do is to have the equipment necessary to BE ABLE TO DO IT if the grid ever goes down for an extended period of time and the normal resources are not available. Currently, when we lose power here, and we do lose power frequently for three to five days at a time, I just plug in my trusty Coleman 5kw generator that I converted to run on natural gas.. it powers the entire home like nothing was ever wrong... and the natural gas allows it to run flawlessly even in -10 . . .

So now I think I'm going with the Radian 8048.. only thing left to decide is if I should use a MorningStar charge controller or go with the OutBack Flexmax...

If you are going to do this you have to use a switch that breaks both connections to the array and makes them to the new load. Both the SMA and the Flexmax want the panels to float. One version of the Morningstar 600V charge controller does this, so that's a good way to get it. The other way is a high voltage DPDT switch; these can be hard to find.

Will your array work with the 145V limit of the Flexmax?

[SWFLA]

Jflorey2 -The other way is a high voltage DPDT switch; these can be hard to find.

600v is not high voltage. How many would you like?

[Mike90250]

Can the candidate inverters sink the Grid Detection Impedance Test Pulse ? That's another test the UL spec calls out, in addition to Fq, & voltage