FoxESS Community - Owners & Installers Forum

I am seeking to understand two items, one for my own benefit, the other to try and understand a friends issue with a different Inverter make

1) How does the Inverter detect grid down issues?
I previously guessed the Inverter may monitor the rising edge before comitting to provide current, but there a a few disparate comments that imply that the Inverter has a high and low voltage limit, which if it starts to generate beyond these limits, it will shut off
The understanding being,
a) if the grid fails near it's source, the Inverter voltage will be brought down, as it would not be able to supply current for the whole district, and the threshold would be reached
b) If the grid failed at the point of entry to the house, the Inverter voltage would go high and reach the threshold
Does anybody know and can give a proper answer, maybe a link to a good explanation

2) There are small imports and exports which would not normally be expected to occur (for instance grid import when a battery has a healthy state of charge). The discussion with a friend of mine, where his imports are more than the Fox Inverter goes along the lines of...
When a load starts, the Inverter is slower than the grid to respond to the new load, so the grid supplies the intial inrush, with the Inverter catching up
Likewise, when a load turns off suddenly, (using the battery, not enough solar) the Invreter will export current to the grid until it balances the load

My general understanding is that the Inverter controls it's voltage generation, by measuring the current at the grid interface, and knowing what it is giving itself
So if the battery needs charging and there is solar, the Inverter will satify the load first, and try to keep the grid current to 0Amps by raising a lowering the generated Voltage

I would appreciate anyone that knows, please confirming / correcting / providing further information to clarify this

Many thanks

I’ll have a quick go at explaining what I know,

Its a transformerless grid tied hybrid inverter, the AC interface is syncronised to the grid in terms of amplitude, frequency and phase - when it is generating it acts as a current controlled source (not a voltage controlled source) and sends current to the the grid (the act of doing so given the external cabling resistance raises the voltage at the inverter).
As a hybrid there is also a DC-DC conversion through a VBus and this is coupled to the AC interface often through a number of stages but essentially it is a PWM which synthesises a sinusoidal output which is phase locked to the incoming grid.

As the inverter is totally harmonised with the grid, should it fail i.e. either the voltage or the frequency going outside of set limits the inverter must by regulation detect this and immediately stop feeding the grid with power (known as anti-islanding protection).

In a more simplified description the inverter syncronises the grid and balances itself to not consume or send any power, however the balance is slightly biased, in self-use mode it is from the grid and in feedin it is biased towards the grid, this means you see a small amount of power being reported whilst it is running on battery.

The inverter attempts to output power to maintain a zero sum as measured by the CT, and because the house load is changing the inverter constantly follows it, but with large changes it is near impossible (given the L-CR stages) to adapt instantaneously and so you occasionally see power reported as being imported or exported but only for the fractions of a second that it takes to balance the load.

The data reported to the cloud comes from the various sensors that the inverter maintains, grid power, volts, freq, battery power, volts, freq etc.. and a snapshot ia taken to send to the fox cloud every 5 minutes - this does sometimes show these import/export power transients which appear to remain until the next 5 minute sample is taken, in reality they lasted only for a very short duration - this is why it is better to look at statistics to determine the real power situation.
In terms of references whilst I don’t have the precise schematics for the Fox inverter much of the technology is a ‘known-known’ and there are quite a few examples of transformer-less grid tied inverter designs on the internet.