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

Thu Nov 10 10:49:21 PST 2011

Hi Drake,

A GENTRAN or equivalent is standard equipment for adding a backup 
generator to a house or small business as an alternative to one main 
manual or automatic transfer switch.   It's simply a box with 8-12 
individual transfer switch breakers some 120 and a few 240 - mostly 15 
and 20 amp.  They sell plug-in types and hard wire types where the 
output of a small gas generator is either plugged into the face of the 
unit or hardwired into the buss.

Then there is a bundle of wires that run from the GENTRAN to the house 
breaker panel.  For each house circuit you want to backup you simply 
interupt the load line coming out of that main panel circuit breaker 
with the 2 lines from the individual transfer switches in the GENTRAN.  
So on the GENTRAN you switch each circuit between "GEN" or "Utility".  
Then you simply use the output of your new off-grid solar inverter to 
feed the GEN side and during a power outage you can not only switch your 
house over to your solar inverter with no dedicated critical loads 
subpanel but you can control how much load you want to put on your 
backup solar.

I've used 2 GENTRANS to cover all the circuits in a house - so IMHO it's 
better than a dedicated subpanel or an all-house automatic transfer 
switch in that you can pick and choose how much load you want to expose 
your inverter and batteries too.  The only disadvantage is they only 
usually come in 15 and 20 amp circuits and usually offer only one or 2 
240 Volt circuits.  If you want to empower a 30 or 50 Amp 240 circuit - 
there are other companies that sell GENTRAN type switches with 
customizable breaker options but they are much more expensive.

Do watch your neutral and grounding circuits though.  Since the neutral 
is bonded at the Main panel you want to be consistent with the "grounded 
conductor grounded at one and only one point rule" when the GENTRAN is 
installed and switched - a point many electricians miss when they 
install backup generators.

Hope that helps.

Jeff Clearwater
Village Power Design
NABCEP^(TM) Certified PV Installer
C-46 CA Solar Contractor

Drake wrote:
> The GENTRAN sounds very interesting.  The Internet has some for sale, 
> but so far no detail.   Will the 12 circuit panel shed one circuit at 
> a time?   In a nutshell, how does this work?
>
>
> Thanks,
>
> Drake
>
>
>
> At 02:32 AM 11/10/2011, you wrote:
>> 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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>
> Drake Chamberlin
> ATHENS ELECTRIC LLC
> OH License 44810
> CO license 3773
> NABCEP Certified PV
>
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