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Ac coupling further explained with references


Q:

Sean, sorry about not making myself more clear. My comment was about in a post of yours #1 and #2. A battery inverter keeps an grid-tied inverter runnin day and night with the grid-tied inverter charger the batteries during the day.

A:

Got it, 1 and 2 right here:

"An ac-coupled system is different from a bimodal/multimodal system, but both types of systems can work together.
Also, an ac coupled system can work off-grid or with the grid and batteries.
Lets look at the evolution of the ac-coupled system, starting with a normal battery based PV system.
1.    Visualize a battery based PV system that makes such a clean sine wave that you can hook up a grid-tied inverter to the ac side of your battery based off-grid inverter when there is no grid and the grid-tied inverter will think there is a grid and will push out current and power in phase with that off-grid battery inverter. This would be considered an ac-coupled system and also can be considered a micro-grid.
2.    Now visualize the grid tied inverter in 1 above pushing out more power onto the ac side of the off-grid inverter than your loads can use. What to do with that extra current? Trick out your battery based inverter to push the current backwards to charge the battery. This is extra tricky, because the battery inverter is using the battery to make a steady ac voltage and frequency while using the current from the grid-tied inverter to charge the battery."

Here is an image of an ac coupled system from HomePower:

http://www.homepower.com/articles/solar-electricity/design-installation/ac-coupling-methods

I like to explain it like it is a battery inverter/charger that makes a "micro-gird" and once that grid is made from the battery power, the interactive inverter can produce power from sunlight. When the interactive inverter is producing more energy from sunlight than the loads are using, the battery inverter/charger will send current from the ac micro-grid in the other direction to the batteries through the battery inverter/charger.

It is a difficult subject to grasp. We will always have the battery inverter charger keeping time and regulating the frequency and voltage on the ac side, however it can push current in either direction to charge the batteries when the sun is shining or to power loads at any time it is needed.

Also, here is a SolarPro article on the subject:

http://solarprofessional.com/articles/products-equipment/inverters/ac-coupling-in-utility-interactive-and-stand-alone

And here is a simple Schneider diagram for ac coupling on the Amerisolar distributor website:

http://www.amerescosolar.com/ac-coupled-battery-backup-systems

Thanks,
Sean White


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