question

Alexandray, if what you say that "there must always be a neutral" would be true, then it would also not be possible to install a multiplus in mono-phase configuration in large parts of Belgium.

I explain why :

- if you have 3 phase delta grid-connection in Belgium, there is NO NEUTRAL
- all you have coming in is 3 wires.
- between each set of 2 wires of the 3 incoming wires, you measure 230V AC
- however, the 230V AC between wires 1 and 2, let us call this P1 (phase 1) is shifted 120 degrees compared to the 230V AC you measure between wires 2 and 3 (P2).
- P3 measured between wires 3 and 1 is again 120 degrees shifted compared to P2.
- So you have 3 phases P1, P2 and P3 shifted 120° between one another.
- And there is NO neutral...

Now if you would install a single multiplus, the AC in would have 2 wires coming in (aside from the ground) but none of them would be a NEUTRAL. If this works, which I presume it does and I read in this forum that you can have 2 multipluses in series that produces a 230V shifted 120° between them, then by itself the voltage measured over the 2 multipluses combined forms the 3d fase, correct?

Now, if you say that this cannot work as there is no NEUTRAL wire, does this then also mean that a single multiplus in Belgium cannot work? I have a sailboat with a single multiplus and it has worked in every port I visited till now so I find that hard to believe...

All input and comment very much appreciated!!!

Yes @Carl Dewever you are of course correct (my English cooker has 6 terminals at the back so it can be jumpered for either UK or Belgian grid).

If you have 2 x Multis at 120 deg then yes there will be 230V between the two output connections that are not shared, so voltage-wise this creates a phantom third phase. But the problem is likely to be that any current you draw from this is going to be 120deg out of phase with the voltage from both the Multis and they probably will not like that.

Also the installation instructions say the phase and neutral connections must not be reversed, that clearly doesn't mean anything on a Belgian supply as there is 127 volts AC between all three incoming connections and ground.

Another point is that since there is no neutral I am not sure what happens to the earth connection, especially if you want to have one Multi in backup mode for critical loads on one phase only. Maybe it requires an external 3-phase anti-islanding relay and then you can operate in island mode as a single phase T-N-S supply? Which raises another question, what is the classification of the earthing system in Belgium?

"But the problem is likely to be that any current you draw from this is going to be 120deg out of phase with the voltage from both the Multis and they probably will not like that."

No it will not be a problem, I mean : the way the voltage and the current are out of phase is determined by the load on the phase. If it is a resistive load, voltage and current will be in phase, if it is a capacitive load, they will be phase-shifted.
But suppose the load is resistive, then just as the 230V on the third (virtual) phase, the current will be in phase with the voltage and it will be the sum of the 2 vectors of the 2 phases coming from the multiplus. So no problem there.

Also, I don't think a multiplus has a problem with current and voltage being out of phase, otherwise only resistive loads would be allowed on a multiplus in stand-alone-configuration. Of course, the greater the angle between the current and the voltage, the less power (Watt) will be consumed for the same capacity (VA).

Another point is that since there is no neutral I am not sure what happens to the earth connection, especially if you want to have one Multi in backup mode for critical loads on one phase only. Maybe it requires an external 3-phase anti-islanding relay and then you can operate in island mode as a single phase T-N-S supply? Which raises another question, what is the classification of the earthing system in Belgium?

This is a paragraph I don't understand. Can you explain further, give an example

ps : not the whole belgian grid is delta, only part (guess: 30%) is like this

In backup mode the relays in the Multi will disconnect the AC in, and also connect the "neutral" and earth. But the "neutral" will be connected to one of the other incoming phases at the distribution board. So you would have to disconnect that incoming phase as well or there will be a big problem.

Not sure about your other remarks, the load current on the virtual phase will be 120 deg out of phase with the two (real) inverters so will appear as a mainly reactive load. Yes, the inverters may be able to cope with this but it will reduce the current available for the two real phases so not a free lunch.