Hi Dave,

Depending on the house loads, you might very well do without an additional (very
expensive) auto transfer switch. The inverters, of course, already have one.
Just leave the Sunny Islands connected 24/7.  The transfer from grid to battery
is then milliseconds and flawless. The 56 amps continuous, and deep SI surge
capacity will most likely, comfortably, handle the whole shebang. I've got a
single SI5048 doing this now through a transformer. Works great.

Put a manual transfer switch at the barn in the event there's ever a problem
with the SI's. But, Iota makes a 100 Amp auto transfer switch, if you decide
you've got to have one. It is attractively priced compared to anything else I
could find. Beware, it may not be UL listed. I've got one, haven't used it yet.
Let me know if you're interested :-)

During a recent severe weather event, our customer's utility was down for 5
days. Battery lasted 24 hours before shutting down the system to self-protect,
as designed and programmed. We had to round up a generator in a hurry to "backup
the backup". A previously installed a manual transfer switch, with the exterior
generator inlet, made that relatively painless, but it was something we hoped
would never be needed. As you know, you also need SMA communication boards and
the SMA programming cable to get the Sunny Boy to work with the SI's when the
grid is down.

Regards,
Dick Ratico
Solarwind Electric


I

--- You wrote:
We have a new (for us) application for grid connected net-metered PV with
battery back-up as follows. The twist is that the entire load center in the
house will be the "protected loads panel". 

-          5kW PV ground mount located 250 feet East of their barn.

-          Will utilize AC Coupled SB5000US inverter to (2)Sunny Island
5048's for 120/240VAC back-up capability with 24.5kWh back-up battery bank.
All inverters/batteries to be located in barn.

-          Barn has 200A service from grid (PV system AC point of connection
will be there), house is located 175 feet West of barn and has a 100A load
center (sub panel from barn).

-          Intention is to back up house as the protected loads panel fed by
the Sunny Islands in case of grid failure. 

-          The Sunny Islands will be able to pass through up to 56 Amps at
240VAC. 

-          I want to supply the 100A house load center with up to 100A
during normal operation with grid available. I am proposing to use an
automatic contactor/transfer switch to feed the 100A house load center.
Normally Closed contacts from grid power, N.O. contacts fed by Sunny Island.
Therefore normal operation will be for grid to be connected directly to
house 100A load center and under grid failure the transfer switch will
connect the Sunny Islands to power the 100A house load center. Under grid
failure condition we will have an LED signal light come on to warn the
homeowners that they are on battery power.

Questions for wrenches: 

1)      Is that whole thing a viable idea?

2)      100 Amp automatic transfer switch recommendations?

3)      LED signal light recommendations?  Or is there another method to
warn homeowner of grid down status?

 

Thanks,

 

Dave

 

David Palumbo, NABCEP Certified PV Installer
--- end of quote ---

