I am sorry but you are missing the point entirely…
One last time…
The grid is huge and in this context, its voltage and frequency can not be influenced, significantly, by anything any appliance or equipment you have can do to it.
Unless the fault is so huge that it is taking out breakers or fuses.
I am not suggesting that the Inverter isn’t causing the voltage swings you are seeing, wherever you are measuring them, clearly it is.
I am saying that concluding it is faulty is not sound reasoning, because it would have to be drawing hundreds of amps, possibly many hundreds, depending on where you are measuring it, for it to affect the voltage that way. Assuming everything else is OK
It is just as likely that you have an issue elsewhere that makes relatively small load changes pull the voltage down, again wherever you are measuring it, and that this is causing the inverter to behave the way it is…
If you plugged a microwave into a long skinny extension and found that it reset itself every time you pressed start, would that mean that the microwave was faulty? or that way it was being used was inappropriate?
You will not fix your issue by taking one reading and jumping tom a conclusion.
You need to work through the problem and find where the voltage drop mis and how much load is required to cause that much voltage drop, as I have previously described…
With the inverter off…
If you can attach a 4kW load, circa 16 A at 240V, at the point where the inverter is connected to the supply, and you don’t see a voltage drop, then it would be reasonable to conclude that the inverter is somehow drawing massive current and is in fact faulty.
If your test load produces any significant voltage drop, which it absolutely shouldn’t, then you are looking for a fault somewhere else within your installation.
I do not know how to be any mire clear.
If you don’t test, logically, you will not find the issue.
Honestly, you may be better calling an electrician as you don’t seem to be grasping the problem.
I don’t mean any offence and I realise this could be a language thing, however it feels like you not understanding the problem, as opposed to not understanding the words.
If you don’t understand what I am asking you to test, or why, please get someone qualified to do the testing as that will be safer and more effective.
I can’t help any further until you, or an electrician, gathers more information.
be careful,
I hope you pin down the issue.
NB.
To be 100% clear here…
If I were testing this, I would be using different test equipment/methods to look for a voltage drop and or high impedance, probably without needing to add any test loads.
We seem to be limited to a single volt meter and no clear picture of what is connected where so a test load, a couple of heaters, seemed accessible and definitive!
It is also no more dangerous, in unqualified hands, than attaching the inverter in the first place.