[RE-wrenches] AC Coupled Re-visited

[boB Gudgel]

Jeff Yago wrote:
> 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.
>   

Because, people are using these inverters in ways they were not really 
meant to be used.   It can be made to work, but the inverter is not 
"aware" of the battery voltage in this mode
of operation....

Actually, another way to look at this is to put the onus on the grid tie 
inverter rather than the battery based inverter, providing the reference 
grid, to
be able to easily turn its power level down.   SMA just happens to have 
a way to do this in a pretty slick way.

  The regular battery based inverter, like the FX or Magnum, etc,  is 
"bidirectional".  When the grid-tie inverters voltage gets pushed up
because the battery based inverter has nowhere to put that energy, the 
AC voltage rises and so the DC battery voltage side must rise
as well.

That is, unless some inverter like the SI tells the grid tie inverter to 
lower its output power, the grid tie inverters' voltage will try to rise to
try and sell more power to the "grid".    But there is no "grid" when 
the batteries are full, so the grid tie inverters' AC voltage goes out
of UL range, and goes off line.

> 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? 
Other's could do this too, if they know what Grid-tie inverter is that 
they are tied to and how to control it (like varying frequency).   
SMOP.... Simple Matter
Of Programming as they say.   That is, ~IF~ the grid-tie inverter can be 
turned down.  This is the best way.   The battery based inverter can not 
use the energy.
It can't just keep the AC voltage at 120V and the battery voltage down 
if the grid tie inverter is trying to raise that AC voltage by trying to 
sell to it.

Otherwise, all the battery based inverter can do is to turn off.  Then, 
of course, the reference for the grid tie inverter goes off, and it must 
wait another
5 minutes after the battery based inverter starts inverting again.

>  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? 
>   

It's because it's not a charger at this point... It's a bi-directional 
"inverter".

> 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".
>   

It's kind of like when your stomach is full, you have to stop being fed, 
otherwise you get real sick.  (sort of)

boB





> Jeff Yago
>
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