Using a grid tie inverter off grid

[karloucho]

Seems to me, to keep your GTI alive when the grid goes down, you need several things.
1. A relatively small Inverter with output protection against an in-fed voltage (if such a thing exists), a small battery to supply this Inverter, and a means of maintaining its charge.
2. Sufficient load to maintain the GTI output within the permissible voltage range programmed into the GTI. Ideally this would be in the form of a large battery, but arguably, the cheapest way to achieve it is by adding a big electric HWS tank with a wattage element approx. matched to the GTI max output.
3. Some form of controller to ensure that any power produced by the GTI goes into powering desired household needs, before being directed into the HWS.
4. A contactor to isolate the house from the grid.

If all this is even feasible, it's starting to sound far too complex, and ultimately, more complexity means more cost.
I have a far simpler solution, but not sure whether the necessary equipment even exists.
Install a Non GTI (I assume these operate as a voltage rather than current source), and a few contactors (incl. one to isolate the grid) to switch to an Off-grid configuration.
In my case, this would require a 5 kW Inverter capable of taking a 300VDC input (preferably dual MPPT). Is there such a beast, or any ideas as to alternatives? All I want is, if the grid goes down, the ability to use the power produced by my solar panels whilst the sun shines, without having to spend AU$14000+ to install a Powerwall plus controller.

 

[Ellcon123]

As trial I have been using a cheap 300W GTI in my system. Backfeeds thru my LF inverter. It's plugged into a smart plug connected to Home Assistant for control. With sunrise the plug turns on and with sunset it is turned off. It's also turned on/off depending on battery voltage. In effect the smart plug becomes a charge controller. Working well for a few months now.

 

[karloucho]

Not sure that I understand your setup - can you be more specific about this. Are you normally connected to the grid. When are you running on battery? What is your LF inverter rated at. In fact what is it exactly?
Sorry about all the questions, but as you've replied to my post, I'm guessing you have at least in part, worked out a solution to my question, and I'm very interested.

 

[Ellcon123]

I have a DIY off grid system to offset some loads, primarily the laundry. I also have my original panels on shed at the end of the yard but not connected.

I came across some YouTubes of people using cheap GTI's so thought I'll give it a try. A big plus is that the shed is a 30m cable run so with a GTI I can just connect with an extension lead. These people are generally plugging into their power points which isn't legal nor the safest thing to do. I use a low frequency (LF) inverter and a good feature of these is that they are bidirectional.

So a GTI doesn't care whether it is off grid or on grid, as long as there is a grid. You can't just plug it all together and walk away. A GTI is on or off and will supply all the power it can into what it thinks is an infinite grid. I'm only using a 300W GTI so not concerned about that. It won't overwhelm my system.

So if the GTI is connected to the AC side of my imverter it senses a grid and turns on. Whatever it produces either goes yo AC loads or back feeds thru the inverter to charge the batteries. And this is how it really works.

One problem is that the GTI is not like your charge controller and will not turn off while a grid is present. My GTI is plugged into a smart plug which is connected to my Home Assistant automation. I can now programme the plug so it works like a charge controller. When my batteries reach a certain voltage the plug turns off which on turn stops the GTI. Do the reverse at acertain lower voltage to turn back on. The smart plug is also a power meter so it provides all the data which is nice.

Hope this helps.

 

[zanydroid]

Seems to me, to keep your GTI alive when the grid goes down, you need several things.
1. A relatively small Inverter with output protection against an in-fed voltage (if such a thing exists), a small battery to supply this Inverter, and a means of maintaining its charge.
2. Sufficient load to maintain the GTI output within the permissible voltage range programmed into the GTI. Ideally this would be in the form of a large battery, but arguably, the cheapest way to achieve it is by adding a big electric HWS tank with a wattage element approx. matched to the GTI max output.
3. Some form of controller to ensure that any power produced by the GTI goes into powering desired household needs, before being directed into the HWS.
4. A contactor to isolate the house from the grid.

If all this is even feasible, it's starting to sound far too complex, and ultimately, more complexity means more cost.
I have a far simpler solution, but not sure whether the necessary equipment even exists.
Install a Non GTI (I assume these operate as a voltage rather than current source), and a few contactors (incl. one to isolate the grid) to switch to an Off-grid configuration.
In my case, this would require a 5 kW Inverter capable of taking a 300VDC input (preferably dual MPPT). Is there such a beast, or any ideas as to alternatives? All I want is, if the grid goes down, the ability to use the power produced by my solar panels whilst the sun shines, without having to spend AU$14000+ to install a Powerwall plus controller.

Did you search for AC coupling in the forum? That is the name of the commercial, regulator compatible feature to keep GTI up.

BTW it’s not just voltage range, there’s a variety of approved algorithms / circuits for anti-islanding. The hybrid inverters with AC coupling capability generally can convince many, but not all, GTI to stay powered up.

You can switch DC strings but note it requires a beefy contactor and/or intelligent enough PLC to switch as at a safe time.

 

[karloucho]

Thanks to Ellcon123 and Zanydroid for their insights. I've concluded that for 5kW system, I probably need to disconnect my GTI totally, and use a mix of AC and DC contactors to isolate grid and switch PV panels and house load across to a non GT system. I'm looking at the components I'd need and the cost, and starting to think I might be just better off saving for a Powerwall + Gateway. I could then sign up with an energy retailer offering VPP rates on exported power, which would reduce payback time, a bit.

 

[spoonhonda]

Thanks to you guys who actually put this to the test and gave us feedback here. I was trying to find the answer to this question on the internet and its not a very common thing to do or try to do. But sounds like its doable if setup correctly.

 

[zanydroid]

Thanks to Ellcon123 and Zanydroid for their insights. I've concluded that for 5kW system, I probably need to disconnect my GTI totally, and use a mix of AC and DC contactors to isolate grid and switch PV panels and house load across to a non GT system. I'm looking at the components I'd need and the cost, and starting to think I might be just better off saving for a Powerwall + Gateway. I could then sign up with an energy retailer offering VPP rates on exported power, which would reduce payback time, a bit.

You're going to have to sequence those contactors carefully to avoid disconnecting under load, especially if there are some high voltage DC strings involved. Feels like running your own ? y substation at that point. Fine if you're into that I guess

If you wait 6-12 months I bet lots of new options will come up, that are approved for grid tie. I think the EG4 18K is just the first of many.