[RE-wrenches] High Voltage MPPT units; Adding battery backup

[Jeff Clearwater]

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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