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michal-kubina avatar image
michal-kubina asked

Limit battery oscilations with ESS

Hi.

I have a 3 phase MPII setup with AC coupling (fronius symo) on AC OUT, grid feed in is allowed

I use a small battery bank (I know, but a larger bank is not economical at this point.)


I need the system to use the grid for surge loads (e.g. switching on of induction cooker ) not the batteries!

It makes much more sense to draw 3kw for 10-30s from the grid than from the battery.

If I use scheduled charge during the day with a low SOC (when I expect the batteries to be charged from AC coupled PV) the surges still are drawn from the battery if minimal SOC is below current SOC, so this is a NO GO.

If I limit the inverter power to 0 during day, it wont invert surplus PV power from e.g. L1 to L2 if there is surplus.


My current idea is to use node-red and gradualy increase minimum SOC as the battery is beeing charged, so that it is always 2-3% above current SOC, this enables maximum inverter power between L1/L2/L3, disables battery discharge and enables battery charge from surplus AC coupled PV. And is AC coupled PV is not enough to power loads, grid will be used.

Then, at night, I will drop the minimum SOC to 40% and the battery will be gradualy discharged till morning with a constant draw of 300-500w.

Any other ideas?

Thank you





ESSAC PV Coupling3 phase
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Alexandra avatar image Alexandra ♦ commented ·
@Michal Kubina

Since you are trying to kix self consumption with not self consumption due to a small battery bank, I can tell you, it wont work or wont work well.

The only way to power assist from grid and not batteries is on a keep batteries charged system.

Otherwise it power assists from battery until PV catches up with load. And there is always a short delay there for ramp up. If you had DC PV it would be a bit more responsive.

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shaneyake avatar image
shaneyake answered ·

Unfortunately this is not how ESS is designed and why they have it repeated a few times in the manual that battery needs to support inverter full load.

Basically there is a Controller in the inverter that is trying to maintain a grid setpoint, if a big load kicks the inverter will ramp up and cover that load, the GX will send a new grid setpoint which will reduce the load on the inverter.

If you use the keep batteries charged option, then there is a separate mechanism called Overvoltage Feed-in that kicks in, this will hold the battery at certain voltage by exporting any extra power. This system will not use the battery for changes in load.

Are you loads on the output of the inverter? If they aren't then there is ways around this using NodeRed because the internal inverter control loop isn't in control.

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