Hi fellow BackShedders

I've been working on a Warpverter for some months now and tinkering with lots of possible configurations for the new virtual grid.  I've come up with what I think is a great compromise between being totally off-grid and also having the security of a grid for backup.

The essential parts of the solution are

1 - an automatic transfer switch with priority given to the Warpverter side of the circuit

2 - an old, pre-existing solar setup (panels and 3kW inverter) that remains grid connected 24 x 7

3 - new PV panels connected to a 10kW GT inverter

The thinking behind this design is to always have some buy-back solar going into the grid - that should cover the poles and wires base connection fees.

Any thoughts on why this is a good idea....or a very bad idea?

Looking at your diagram, the last sentence of the notes concerned me: you don't want the bms disconnecting the battery whilst the 10kw gti is backfeeding.
I see you have considered a current dump, but (potentially) 10kw is a lot of amps to dump at 48V - if you can manage it, that would be ok to keep the bms from disconnecting. A usefull AC dump would be an immersion heater, but they're typically only 3kw.
A simple solution would be to use a battery voltage sensing relay to disconnect the 10kw gti from the AC when the battery is close to full.

  mab1 said  

A simple solution would be to use a battery voltage sensing relay to disconnect the 10kw gti from the AC when the battery is close to full.

Thanks for the feedback @mab1. As you noticed, my design hasn't really worked out how to handle the'battery full' scenario. I'm sure it's not good but I don't really understand what would happen if the GTI was receiving PV input and the BMS shut off the charge MOSFETs. Would the GTI inverter blow up in some way?  I'd like to know the answer of anyone can explain. I realise that under 'normal' circumstances the GTI would be feeding back into the grid, but in this case that's not available, so what happens?
Regardless, I'm not planning to allow this so your suggestion to simply isolate the GTI based on battery charge level seems like something with pursuing.

I'm not sure myself what would happen, but with energy flowing from the gti, through the inverter, onto the dc bus, into the battery - if the battery suddenly 'disappears', that energy will just go into the inverter caps, pushing the voltage up until something blows up, or the inverter shuts down on dc overvoltage (if it is programmed to do so). Thing is, if the dc bus voltage spikes high enough, it won't just risk blowing up the inverter mosfets, it may blow up the bms mosfets, turning them short cct, and reconnecting the already full battery.

I don't know if there's a theoretical limit to the dc voltage from backfeeding in this circumstance:- certainly my Wiseguy inverter happily pushes the battery up to 60v (at which point the relay disconnects my gti's from the ac) even though the ac voltage on the primary is only 32v rms.

I certainly don't know what would happen with a warp inverter.

As Mab suggested using the anti islanding function of the GTI is the easiest and safest way to shut the GTI down when the batteries are full.
I have two GTI inverters on my off grid system, I use voltage sensitive relays boards to switch relays on the outputs of the GTI inverters.
So that when the batteries are fully charged the GTI inverters are disconnected from my battery inverter. They then see no grid and switch off.

Another way to load dump that much power would be on space heating. But you would need a fairly cold house to want to dump up to 10kw of heating into it. In summer that would not be feasible either.

pete

Alternately, change your car to an EV then use the excess solar to charge the car.
Better still get an EV that allows Vehicle to Grid connection and use the car battery instead of buying a new battery.

pete

Thanks for the feedback BSers.

I'll definitely implement a relay disconnect for the GTI - probably a 50A SSR should do the job.  I'll build a voltage detection circuit to trigger the relay, or maybe I can just utilise an existing alarm from the BMS.  Might be better to have something independent of the BMS though, just in case the latter fails.

I'm pleased the idea of keeping the existing solar system running independently has raised no eyebrows.  It's a good compromise (I think) between building a fully hybrid system and being totally off-grid.

I've redrawn the design showing the relay, and highlighting the old and new systems.

First of all I like the block diagram makes it easy , There have been quite a few off grid systems using smaller GTI's and setup the same way as others have suggested cutting off the GTI so it shuts down until required again, I haven't seen a system that relied entirely on a large 10Kw GTI to do all the battery charging most typically have a charge controller or 2 running as well, if there is please be nice to know.
It would be preferable that the BMS never has to shut off charge, this should always be handled first by the charger/system, same for any other battery chemistry there should never be an over-voltage event to your batteries.

The usual charge controllers have to act very fast to over voltage resulting from BMS shutoff or blown fuse etc, I have tested a Daly BMS in the shed with over voltage cutoff with inverter and charge controller running loads, it has tripped about 1000 times or more and no issues, Was messing around to see if I could kill it , however its only 3kw of panels,

Not sure how much power switching is going to occur when using a large GTI like that depends on the loads you have running and when the switch off and on, it may well play nice just have find out when its up and running.

There have been some GTI's regulated on their solar inputs and has been successful, don't know of any that are regulated on the output, so just an idea to ponder, the GTI naturally will keep pushing the voltage up, now if heavy-duty variable autotransformer was connected to its output before the inverter and be controlled ie, wound up as the battery bus voltage gets close to full the GTI will back off its output as it will see a higher voltage, when the batter voltage drops a little the Variac can be adjusted back, I will say it would be horribly expensive and wasteful  to have a 10Kw variac to do this job and not worth the effort.

Another way might be to have a few extra turns already on the inverter transformer and the feed from the GTI switched between windings with a control circuit to make this automatic and will make the GTI regulate much more like the charge controller, this is still clumsy and a PITA to do, perhaps someone can come up with a better, less mechanical way to achieve a better way to regulate the GTI.
Like I said just an Idea.


I might come back later to see where I messed this up
Edited 2026-08-18 15:23 by Revlac