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Ac-coupled PV system evolution and multimodal inverters


Q:
 
The textbook considers 'AC coupled systems' within the category of Off-grid systems. I think that there is an error here as 'AC coupled systems' are actually tied to the grid, even-though they have batteries for storage. Am I missing something here?
In the same chapter another category of system is 'Multimodal or bimodal PV systems', aren't we talking about the same here than 'AC coupled systems'?
Thanks in advance for clarifying the above.
 
A:
 
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.

3.     If you get 1 and 2, you can see that we have a micro-grid ac-coupled system that is not hooked up to the grid. Now we can take this extra special battery inverter and have an output that connects to the grid and there we have an ac-coupled system that connects to the grid.

4.     This ac-coupled inverter that connects to the grid, can also connect to a generator for extra backup when the grid is down and we need extra power.

5.     If the batteries are fully charged and there are no loads, there needs to be a way to slow down or turn off the grid-tied inverter, so we will have to be able to communicate with the inverter. One way this battery inverter can turn off the grid-tied inverter is by making the sine wave frequency outside of the limits of the grid tied inverter to make it turn off. For example, a normal USA grid-tied inverter will not work at 59 or 61 cycles per second (Hz), so it will anti-island (turn off) by using frequency control. The equipment in our house will not care if the frequency is off by 1 Hz.

6.     The proper name for a grid-tied inverter here is really an interactive inverter, but I have been calling it a grid-tied inverter to make a point. Interactive inverter is the NEC word and grid-tied inverter is the slang term that we are used to.
 
Multimodal inverters (formerly called bimodal inverters) are inverters that are battery inverters that have an output that can send extra power out to the grid. In the early days (1990s) before we took a liking to German string inverters, our grid-tied systems usually had to have batteries. Popular multimodal inverters of today are bought from Outback, Schneider and Midnight Solar.
 
A good way to look at a multimodal inverter is an inverter that can feed the grid as a diversion load. A diversion load is a load that you only feed when the batteries are fully charged.
 
Thanks,
 
Sean White



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