question

Hi @colj,

In an off grid system the MultiPlus/Quattro generates a 230V AC sine wave and powers any load that is attached.

When a load is connected, it will draw on the voltage which will drop, and the multi will try to maintain the 230V and then current will flow from the higher voltage source of the Multi, to the lower voltage of the load. The bigger the load, the more the voltage will drop, and the more current that flows.

Introducing an AC PV inverter into an off grid system can modify the sine wave, voltage and current flow. The AC PV inverter synchronises to it, but the MultiPlus reference voltage is usually very stable, and any overshoot surplus (from a load fluctuation) will effectively be used to charge the battery.

There can still be 'overshoot' issues in off grid systems with AC PV inverters, which is why we have the 1:1 inverter sizing rule, and minimum battery sizing relative to AC PV, to give enough capacity to absorb these excesses without issues.

In an on-grid ESS system the 230V AC sinewave is formed by the grid, and the MultiPlus then tries to synchronise with it.

Because the MultiPlus sine wave is not drawn down by the load (as the grid voltage is providing the voltage stability), it needs to use another way to know how much power needs to be provided from the battery.

A CT (current transformer) or energy meter provides this information to the Victron system, indicating how large the load is by measuring the current that is flowing (instead of trying to maintain a fixed voltage stability like in off grid).

Depending on how much current and what direction it is flowing, the MultiPlus will raise or lower the AC voltage that it is generating.

If the MultiPlus raises its voltage above the grid voltage, current will flow out of the battery and into the grid line (and then to the load as it is the lowest voltage point in the system).

If the MultiPlus lowers it's voltage below the grid voltage, current will flow into the multi (itself becoming a load of the grid line) and then the battery will charge.

It is an ongoing effort at Victron to improve the response time between what happens on the grid, how that is sampled by the CT or energy meter, communicated to the MultiPlus and then adjusting it's voltage up and down to try and bring the grid set point to the target.

Some loads are very dynamic and they are not easy to keep balanced (given the delay described above)

Some grids are not very voltage stable and this will also affect the situation.

The good news is, generally speaking, while these overshoots and undershoots can appear to be large, they are often extremely short, so don't add up to lot of energy.

They also tend to be 'systemic' which is why the grid set point is adjustable. If you are seeing that even though the battery still has spare capacity, but you are still importing a bit of energy each day (this is on the VRM Dashboard), then adjust the grid set point so that over the 24 hours it is near zero (or your target).

If you haven't seen it yet, I would also suggest to watch this "What is ESS?" video that I made.

I hope this helps you to understand the system better :)