Hi Wrenchies,
Yes and if you use a GENTRAN or equivalent circuit-by-circuit transfer
switch add-on for the output from the offgrid inverter to your AC panel
- then there is no need for a separate dedicated AC loads panel and
rewire like in Larry's original drawing. I've used this trick for years
to avoid a dedicated subpanel for the output of an off-grid inverter.
Combined with the DC transfer switch idea that high voltage MPPTs
enable, (we've been waiting for this) you have a really simple retrofit
for an on-grid system.
And now with Magnum and others offering low-power 120/240 split phase
outputs in one inverter - the grid-tie hybrid retrofit market is getting
more attractive all the time. DC Transfer Switch to MPPT Charger to
120/240 split phase inverter/battery to GENTRAN Switch. Getting cheaper
all the time.
And hopefully one day not too far off SMA will bring their "Backup Set
S" they show in their European catalog to the States- this is basically
a 2.2 KW version of the Sunny Island - 2 of them would achieve 120/240
stacked and be a nice cheaper SB enabler as well.
Also your standard 100 AMP 4 pole DPDT transfer switch may not be rated
for 600 VDC explicitly but think of it - the old knife style units
"throw" is like 12" - no way that won't work with 600 VDC.
Hope that helps!,
Jeff
Larry Crutcher, Starlight Solar Power Systems wrote:
Right.... a major rewire and many components...and sacrifices. The Outback is
limited to 30 amps AC and cost much more than you would need to spend.
With my idea you just add a transfer switch....Done. When the grid is down, PV
feeds HV CC. Then you can use ANY size inverter, with or without a transfer
switch. No rewiring the grid inverter. No 240 volt transformers or dual
inverters. No relays, switches or diversion controllers. Batteries are always
properly maintained. No phantom loads. Very simple.
Thanks for your input. Now if I can just find that darn 600 volt transfer
switch.
Larry Crutcher
Starlight Solar Power Systems
On Nov 9, 2011, at 1:55 PM, Drake wrote:
Any amount of power will AC couple.
If you use an off grid inverter, such as an Outback, to feed the protected load
panel, the inverter should be fed by grid AC power to supply loads, when the
grid is available. Just feed your grid tie inverter into the protected loads
panel. You will need a relay to shut off power from the grid tie inverter
when the batteries are full. This can be triggered by the fan relay in the
Outback.
The on grid inverter will push power into the grid through the battery
inverter's internal transfer switch during normal operation and AC couple when
the power is out. If your grid tie inverter is 240 volts, you will need to use
a transformer or two inverters.
The phantom load of the inverter can be eliminated by using a transfer switch
to send PV power straight to the grid, during normal grid availability, keeping
the inverter turned off. AC coupling seems much easier than using a charge
controller, but there is no conditioning of the batteries. The charge is just
on and off at the voltage set point. For a back up system, that is not much of
a problem, as the battery inverter can maintain the batteries when power is
available.
At 02:04 PM 11/9/2011, you wrote:
Nice Find, Drake! Thanks.
What is the minimum system for an AC coupled inverter?
Again, the idea is for low cost battery backup of an existing grid tie. The AC
coupled schematics I have looked at require a major rewire and many components.
I would like to hear if someone has a simple add on solution compared to just
switching the array to the high voltage CC.
Larry Crutcher
Starlight Solar Power Systems
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