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

Wed Nov 9 13:55:08 PST 2011

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
>
>
>On Nov 9, 2011, at 6:14 AM, Drake wrote:
>
>>
>>I vote for the AC coupled system.  But if you 
>>need to make a 600 VDC transfer switch, I think 
>>you could likely make one from Tyco relays and 
>>voltage controlled relays.  http://relays.te.com/datasheets/ev200.pdf
>>
>>At 03:16 PM 11/7/2011, you wrote:
>>>Ray,
>>>Yes, the idea was for a low cost way to add 
>>>battery backup to existing high voltage PV 
>>>grid tie systems, not new construction.
>>>
>>>The second inverter does not need to be able 
>>>to sell since that is handled by the grid tie 
>>>inverter. It simple passes grid power to loads 
>>>and maintains the battery. That means any 
>>>off-grid inverter/charger or even low cost 
>>>separate inverters and chargers may be used, a 
>>>very economical solution. However, the idea 
>>>may be of no value because it appears no one makes a transfer switch.
>>>
>>>Larry Crutcher
>>>Starlight Solar Power Systems
>>>
>>>
>>>
>>>
>>>On Nov 7, 2011, at 11:32 AM, Ray Walters wrote:
>>>
>>>>Just curious, what's the advantage of 
>>>>switching at the array vs. just using a Sunny 
>>>>Island or Outback system? Seems this would 
>>>>cost more, and have less efficiency, since 
>>>>you would still have the battery float 
>>>>losses, backup inverter idle losses, in 
>>>>addition to the 2nd grid tie inverter losses. 
>>>>Are there site specific conditions driving this that I'm not understanding?
>>>>I've done quite a few GT systems w/ battery 
>>>>backup, so I'm always up for new ideas though.
>>>>
>>>>Ray
>>>>
>>>>
>>>>On 11/7/2011 12:15 PM, Larry Crutcher, 
>>>>Starlight Solar Power Systems wrote:
>>>>>Hi Glen,
>>>>>That model is a disconnect switch. What the 
>>>>>system needs is a 4 pole transfer switch, 
>>>>>automatic or manual, to connect the PV source to the charge controller.
>>>>>
>>>>>Hello Gary Willit,
>>>>>Yes, you've grasped my concept precisely and 
>>>>>I agree, getting a high voltage DC transfer 
>>>>>switch may nix this design. Sounds like a product for Midnite Solar to build.
>>>>>
>>>>>
>>>>>
>>>>>Larry Crutcher
>>>>>Starlight Solar Power Systems
>>>>>
>>>>>
>>>>>On Nov 6, 2011, at 1:42 PM, Glenn Burt wrote:
>>>>>
>>>>>>Siemens makes a line that are UL listed.
>>>>>>Their VBII series like HNF361PV – 30A
>>>>>>
>>>>>>-Glenn
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>>Drake Chamberlin
>>ATHENS ELECTRIC LLC
>>OH License 44810
>>CO license 3773
>>NABCEP Certified PV
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Drake Chamberlin
ATHENS ELECTRIC LLC
OH License 44810
CO license 3773
NABCEP Certified PV
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