Double-conversion UPS with Victron

Q&A and troubleshooting
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1

Good morning.

I have a customer with a very unstable, poor-quality electricity supply. They experience voltage surges and drops, as well as frequency fluctuations, and I am considering what solution to offer them in order to provide a reliable, stable supply throughout the property.

Obviously, one of the options I have considered is installing a Victron MultiPlus-II with batteries and a charge controller. In addition, the customer already has a Fronius inverter, which I would retain using AC coupling.

The problem I see is that, in a typical installation, fluctuations in the grid voltage still reach the loads as long as they remain within the acceptance limits configured in the Victron. If I set those limits too narrowly in an attempt to protect the installation, the MultiPlus would connect to and disconnect from the grid very frequently due to the poor quality of the supply.

To avoid this, I had considered a different configuration: using a grid-connected inverter/charger whose sole function would be to charge the batteries when necessary, while having a separate Victron permanently supply the property from the batteries.

In this way, energy from the grid would always pass through an AC → DC → AC conversion before reaching the loads. In theory, fluctuations in the grid voltage and frequency would not directly affect the property or the equipment connected to it.

However, this configuration raises several questions for me.

Firstly, I am not sure how to manage the system correctly with a single Cerbo GX. Secondly, I would like to be able to export surplus photovoltaic energy to the grid, both from the DC side and from the Fronius connected on the AC side, in order to benefit from surplus export compensation.

The latter is also related to communication and system control. I do not know whether it would be possible to use two Cerbo GX units and have them communicate with each other, or whether there is another, more suitable way to manage an ESS of this kind.

Has anyone installed a similar system, or do you know of an architecture that would allow the loads to be completely isolated from a poor-quality grid while still retaining the ability to export surplus photovoltaic energy to the grid?

Thanks in advance.

4

There is no way to de couple an inverter system for clean power and feed in AC PV.

With node red combinations are possible for control.

This is not a system I have installed with feedback enabled as we eventually decided it was not work it, but it is one we have theorised a few times.

5

I do recall that a Multi RS inverter is in such bad grid conditions the best choice as is doesn’t need the grid as reference.

How much solar power is available?, Here I have a RS6000 and 6.9 kw of solar power (20 panels) , located in the Netherlands I am basically off-grid since 25 February 2026, only On grid because of ‘feed-in’ of surplus PV power.

6

But as long as it is connected to the grid, it will have to be synchronised with it; otherwise, it would not be able to feed power in, as this would cause a short circuit.

It has 3 kW with the Fronius, but the idea would be to install more for DC charging.

7

So, is it theoretically possible? Could I use Node-RED to communicate with the inverters? I understand it would be better to buy a couple of units and carry out some tests before placing the final order.

8

You could use nice red to communicate between systems to feed back excess from DC to AC to grid.

9

Not sure what your minimum load will be but on a smaller scale could use an ip43 48/13 and set up as a generator to run when soc drops below xx% soc? Turn off at yy%. So its more of a safety mechanism thats only active when theres no surplus being exported,