[RE-wrenches] AC Coupled Re-visited

[Nick Soleil]

Aloha Solar Bozos:
    I thought that this was answered pretty  well in previous posts (for six years now,) but I figure that I should offer my experience here, and can possibly help provide some clarification.  I believe that I was the first person to install an AC coupled system.  In 2003, I had a project that had a Sunny Boy SWR-2500U that was 250' from a main meter, and the customer wanted to add battery back-up.  At an SEI training, at SMA headquarters in Grass Valley, Christopher Freitas and I spoke about the possibility of AC coupling on the Outbacks AC output.  A couple of weeks later, I put together a design, and sold an AC coupled system.
    Before beginning, I called Greg Thomas at Outback, and I mentioned my plans.  I asked what would happen when the inverter was in 'Inverting' mode?  He went deep into thought, as we discussed what we would do.  Greg said, "What would I do if I was the inverter?  Let's see, as the Sunnyboy backfeeds the inverter's AC output, the voltage would rise I up.  What would I do with that voltage?"  Greg said that the inverter was not prepared for this, and would do whatever it could to keep the AC output voltage in spec.  To do this the inverter would take the excess voltage to charge batteries.  He then realized  that the charge was unregulated, so we proposed using the inverter's voltage controlled relay to switch the Sunny Boy off when the batteries were fully charged. 
     I first set the system up in my parking lot, with the pre-racked
modules laying flat on the pavement, and I tested the Sunny Boy and
Outback together, and it worked!!!
   I installed that first system, and it worked fine, but never did another one.  I did post this to wrenches at the time, but did not refer to it as AC  coupled.  Later, the Sunny Island came out and did this in a more sophisticated way.  I would recommend using the Sunny Island for AC coupled systems, but it will cost twice as much.

 Nick Soleil
Project Manager
Advanced Alternative Energy Solutions, LLC
PO Box 657
Petaluma, CA 94953
Cell:   707-321-2937
Office: 707-789-9537
Fax:    707-769-9037




________________________________
From: Jeff Yago <jryago at netscape.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Wed, October 21, 2009 7:53:11 PM
Subject: Re: [RE-wrenches] AC Coupled Re-visited

There have been some good advice related to battery bank sizing and a better description of how Sunny Islands work with SunnyBoys, but I am afraid we are getting away from my original post that started this and that was:

Since a battery based inverter has a battery charging section and all kinds of software control over the charging process when connected to the grid, why does the battery charging process "go wild" when the grid is down and the AC power is being backfed from a separate AC coupled inverter.  Also, why do we keep having to field wire and program an added power relay or a separate diversion load relay to prevent over-charging and destroying an AGM battery bank because the battery charging process suddenly has no clue how to charge the battery bank.

Yes, the Sunny Island varies frequency which will cause an AC coupled SunnyBoy to be faked out and drop off line to avoid overcharging the battery bank, but it seems to me this is all just software programming, so why can't any inverter manufacturer simply control battery charging no matter where the AC power is coming from?  Why can't grid tie inverters used in an AC coupled application have the software to recognize a full battery condition and stop charging without having to drop off line on an out of limit grid condition and then cycle through the 5 minute wait? 

I am not an electronics engineer, but I don't see where any hardware changes are required, just some added software to select during initial setup.  Many of you may not deal with battery based systems, but half of our systems are and AC coupled is a great way to avoid long low voltage DC wire run losses.   We select generator type from a setup menu, how about "Press 1 for standard inverter setup, press 2 for AC coupling".

Jeff Yago

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