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Old 03-01-14, 07:33 PM   #1
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Default Using a solar system as a generator (to be code compliant)

I have an Outback GVFX inverter which is a Grid Interactive Unit, It has a bi-directional (AC In circuit), which allows the user to sell PV output to the grid. If the grid goes down it will drop the connection to the grid and begin to power a separate AC out circuit that will power user determined critical loads. I would like this system to be code compliant.

I understand that the GVFX inverter has a built in transfer switch to enable it to switch from it's by-directional (AC in) circuit to (AC out) in the event of a power outage. Rather than use a sub panel for critical 120v loads, I would like to use house's service panel ONLY, for connecting to the inverter's AC hot out circuit.

I plan on using a generator interlock mechanism that is available for the new breaker box I am having installed to replace the existing house box which makes it impossible for the main to remain switched on in order for the house panel to be back fed by the other breaker which is interlocked to the main.

The interlock kit has a 50 amp 240 volt breaker for back-feeding. I would like to substitute a 30 amp 120 volt breaker in it's place, but if need be I also have an Outback PSX 240 auto transformer I could use to bump up the GVFX's output to 240 volts so as not to be guilty of altering the interlock kit's original design, which may cause it not to be code compliant. This would also eliminate the need of keeping all of the 120 volt circuits on one leg of the breaker box seeing that it would power both legs.

Of course I know that I would need to switch off all of the 240 volt breakers before operating the interlock function, and manage my 120 volt loads to stay within the inverter's 30 amp capability.

All of the power equipment is located in a shed and the shed will have it's own 240 volt service from the house breaker box. I understand that if the interlock breakers are operated so that the main is off the inverter will not know when the power comes back on.

This whole set-up basically allows using the batteries and inverter as a generator. From looking at the various interlock kits I don't see why it would not be a legal setup. The only difference from a generator is the GVFX's bi-directional line (AC In) which will lose its connection when the interlock breakers are switched to generator positions.

Anyone versed in Solar grid tied / battery backup systems that can comment?



Last edited by verdigo; 03-01-14 at 08:23 PM..
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Old 03-02-14, 07:31 AM   #2
Steve Hull
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A great project!

I think you are going to need a dump load system. If house demand is less than PV production AND if batteries are fully charged, then power must be dissipated.

Not hard to do (a big resistance coil), but I believe this may need to be incorporated in order to be code compliant.

I know of people that just use a 240 V heating element from a stove.

consulting on geothermal heating/cooling & rational energy use since 1990
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Old 03-02-14, 10:00 AM   #3
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Actually when the grid is present the inverter acts as a grid tied unit. If production was such with the grid down I could probably rig a relay to turn on the hot water heater.

The inverter's output is only around 3000 watts and no more than 30 amps though.

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Old 03-02-14, 11:04 AM   #4
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Dennis, can you draw a simple three line sketch of what you are trying to incorporate into the panel in the shed, and also how that shed panel will connect to the house panel?

Sometimes it is easier to find the flaw in what might make a system non-compliant when it is drawn. I think I understand what you are trying to do, but it sounds like it is going to be a challenge.

As I understand it, you have a house where you have a service panel you are replacing with a new one that offers a supplemental generator lockout option. However, your PV system and grid-tie inverter components live in a remote shed. How many feeder wires do you have buried between the shed and the house? Do you have any sub-panels between your meter box and your house service panel?

I ask these questions because from what I understand of what you described, it sounds as if your GVFX needs to sense the grid is alive before it gets into your house panel where you will have the generator lockout. Although I don't consider myself a NEC compliance expert, or grid-tie inverter expert, I'm willing to look at what you propose and see if I can understand the logic behind how it would function and where it might get problematic.

I'll have to see if I can find some other documentation on the GVFX line from Outback. The document I found last night during a cursory look was geared toward programming the stacking abilities of GVFX inverters and wasn't very helpful in wiring diagrams.
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Old 04-25-14, 02:06 PM   #5
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Seems like a lot of work when you already have an automatic device for doing this in the GVFX. Feed a 120V service back to the house from the shed, install a smaller panel next to the main panel and move the 120V loads you want to use during an outage to it.
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Old 04-27-14, 09:07 PM   #6
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The Outback folks can tell you exactly how the GVFX's work. They also have a forum.

I have a pair of VFX3524's. When the batteries are full, the charge controllers turn the solar panels off or at least reduce the charge to float the batteries. If you are using wind, you do have to divert power elsewhere and use it or else Murphy will be your best friend.

You might check the Outback PSX 240 auto transformer. I'm thinking it will make 20 amps unless you cool it. Could be wrong about that.

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