[RE-wrenches] Back feeding battery inverter with grid-tie inverter

[Dana Brandt]

I agree that whether you go with AC or DC coupling you'll want to run the DC
from the array to the battery inverter location to take advantage of the
higher voltage. If you go with AC and the Xantrex XW you'll want to talk
with them directly about backfeeding the inverter from the load side. I
don't know about their current equipment, but in the past they said not to
backfeed them. However, it worked just fine given the H-bridge inverter
design. I wouldn't go with a design that works but is not recommended by the
manufacturer for a client, though. I'd get Xantrex to ok it first.

Do you already have the XW? If not, another option for AC coupling would be
to talk with Outback. They've done a few of these systems and can help you
put it together.

Dana

-- 
Dana Brandt
Ecotech Energy Systems, LLC
www.ecotechenergy.com
dana at ecotechenergy.com
360.510.0433

On Sun, Jan 18, 2009 at 9:31 PM, Jeff Yago <jryago at netscape.com> wrote:

> I know this subject has come up before, but there have been several
> different suggested ways to handle the problem and I was hoping for easiest.
>  We install lots of both on grid and off grid systems.  However, we have a
> client that will require a ground mounted array much futher away from the
> inverter and batteries in the garage than we normally would want to do since
> the voltage drop for this long distance would be excessive at low voltage
> even with large cables.  We have decided to locate a weather-proof sunnyboy
> grid-tie inverter at the array, and supply 120/240 volts to the critical
> loads sub-panel which is supplied by a Xanrex XW inverter and battery bank
> in the garage. I realize there are several design issues here when the grid
> is down,  since we want the grid-tie inverter to remain on but be blocked
> from the grid by the Xantrex inverter.
>
> The question is, what is the best way to keep a load on the sunnyboy since
> it will not be grid connected during an outage.  Also, is there a best way
> to feed the output from the Sunnyboy to the Xantrex that does not just power
> the sub-panel loads during an outage, but also will re-charge the battery
> bank through the Xanrex?
>
> If you back-feed the Xanrex from the sub-panel it is normally supplying
> with output from the Sunnyboy, will the Xanrex see this input and think the
> grid is up and be able to re-charge the battery bank?
>
> Will we need any separate transfer switches or power relays?
>
> Thanks,
>
> Jeff Yago
>
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