Using Grid Tie Inverters Off The Grid (Part 2 of 2)

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In this video, I continue where I left off in a previous video I made (Tricking an grid tie inverter with an off grid inverter), testing a theory I had.  Turns out that it does work.  I could have done a bit better in this video and shown with a clamp meter that there was indeed current running through the lines, but you will have to take my word for it that it works.

If you are going to do this on your own or at your own off grid system, do so at your own risk.  Keep in mind that with this setup, the batteries will not stop charging when they are over voltage.  It is only a good idea if your background loads are greater than your grid tie solar input, or if you have a diversion load.  In my case we have a hot tub with a heating element that turns on to absorb extra power, to make sure the batteries don't end up overcharged.

This works because the off grid inverter has an H bridge setup.  The inverter pulls from opposite corners of the H, essentially switching from positive to negative, 60 times per second (It actually does this in steps, to get close to the sine wave).  You essentially have 60HZ but a modulated square wave, at 48 volts (battery voltage).  In my case it is then run through a step up transformer within the inverter to achieve the 120/240v split phase.  This is then filtered and output of the inverter.

In my setup, the output of the inverter then goes into a load center, and is distributed to the various different loads.  In my case one of the feeds out of the main load center is "Panel L2", which is the panel in our garage/shop shipping container.  This is then fed through a 30 amp 2 pole breaker and runs to the microinverters.  These microinverters look at the voltage on the lines connected to them, and lock on to the same signal.  They then push power back through the same wires they collected the source signal from.  This means any loads in panel L2 and the main load panel are being powered in part by the micro inverters.

Any extra power or a higher net than what the load are using then goes back through the H bridge inside of the inverter.  This works because the grid tie inverters are still sync'd to the line frequency, so the transistors or MOSFETS inside of the inverter are opening at the same frequency as the microinverters are locked to....This means the micro inverters can push back through the MOSFETS inside of the inverter, and the current is essentially rectified back to DC and fed into the DC Bus.  If there is sufficient voltage, it will charge the batteries, but like I said before and I will caution again, it cannot tell when to stop charging because none of the electronics in the system actually know that this charging is happening.  Obviously if the voltage did get high enough, the solar charge controllers would see that the battery voltage went up and they would taper back on how much current they are putting out, but still has the chance for fault.

I can't say enough, if you are going to attempt something like this, do it AT YOUR OWN RISK.

Stay tuned for more videos, and as always, thank you for watching and making this channel sucessful enough to keep building cool stuff and putting it on the internet!!!