[RE-wrenches] Charge Control in AC coupled system

[Kent Osterberg]

Doug,

In an ac coupled system with a Sunny Boy connected to the FX inverter's 
output or to the critical loads panel, a grid connected Outback inverter 
will work the same as it usually does - it is not affected by the Sunny 
Boy other than by the rise of the ac feeder voltage.  Just be careful to 
not overload the FX's internal transfer switch - everything works 
normally.  As long as there is grid power and the Outback inverter's 
internal transfer switch is closed, power (in excess of the critical 
loads) from the Sunny Boy simply passes through the FX to the grid. 

Problems arise when there isn't any grid power - that's when the system 
is a micro grid - because all of the power must go someplace locally.  
The FX inverter has no choice but to put all the energy from the Sunny 
Boy into the batteries, so the FX becomes an unregulated battery 
charger.  When the batteries are (nearly) full, the Sunny boy must be 
shut down.  A voltage controlled switch or the FX inverter's Aux output 
can open a relay to remove power from the Sunny Boy.

Kent Osterberg
Blue Mountain Solar


Doug Wells wrote:
>
> Wrenches,
>
> Howie, good to see you probing this issue.
> I have an upcoming system as well that is considering a micro-grid 
> approach.
> My question would be, is the Outback not able to sell back and charge 
> at the same time.  I understand that with power present on the ac 
> output terminal the inverter will try to charge the battery.  But 
> doesn't the unit also try to sell back on the AC output terminal.  
> Most Outback systems will sell down to the float voltage in a battery 
> back up situation that is grid connected.  Does all of the DC 
> connected array stop selling back with the Sunny Boys going full steam.
> For example.  Lets imagine we have 4 kW connected to an Outback dual 
> inverter grid connected battery back up and then add 4 kW connected to 
> a Sunny Boy in a micro-grid format.  When the sun is shining---and all 
> the loads in the critical panel are being met---that the Sunny Boy 
> overpowers the Outbacks and 1. Charges the batteries and 2.(my 
> question) overpowers the Outback's ability to sell to grid in its 
> traditional format?
> As always the guidance of the "List" is greatly appreciated.
> Thanks
>
>
> Doug Wells
> The Solar Specialists
> www.thesolarspecialists.com
> 802-498-5856
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