question

1. Any parallel or three phase victron setup needs to be fed by the same battery bank. A system like that will synchronize its battery charging across all units, so they need to be on the same batteries. The negative pole of each inverter needs to be connected together permanently. Reason is, the communication cables would be an alternative path for the current if you would cut the negative to only one inverter while positive is still connected, leading potentially to fried cables, or even worse, fried inverters. You can use a switch, or fuse, in the negative path between the battery and a common busbar if necessary

2. I cant judge this

3. Victron can do both, but going off-grid means that you still need a power source. Would you supply the system by PV and MPPTs, or whats your understanding of "off-grid"

Also take note of the victron datasheets. A multiplus 48/5000 is rated at 5000VA not 5000W. There is an additional power derating depending on ambient temperature.

Further, using cranes in island operation, to me so far, was always a hassle. Ive only fed like three cranes by diesel gensets, but every time it was either an issue with the VFD cutting out due to frequency/voltage deviations, or it was an issue with the genset being fed reverse power from braking the load, or the security circuits cutting out. But it might work just fine when being fed by an inverter, i haven't had the opportunity to try that

Check for further information https://www.victronenergy.com/live/ve.bus:manual_parallel_and_three_phase_systems

Hey there,
I am so thankfull @Chrigu.

1. Battery Connection: Great! The design is simplified by using a single battery bank. I'm confident that with the help of the Lynx devices, I will be able to perform the installation correctly, protecting the inverters with the necessary fuses and also incorporating a switch to allow disconnection of the battery bank.

2. Minimum Battery Capacity: I will need to consult further on this requirement, although I understand that it will be necessary to size the capacity to cover the energy demand during power outages.

3. System Functionality: My question about whether the system should function as an off-grid or grid-tied system might have been poorly phrased. To clarify, the system is intended to power the crane bridge in case of a power outage and also assist the grid during normal operation. I understand that this is the PowerAssist functionality provided by the Multiplus II inverter/chargers.

   With this in mind, my plan is to connect the Multiplus II units (either 3kVA or 5kVA, depending on which is more efficient) to the grid, allowing them to be powered by the grid, and also install an MPPT solar charger on the DC side, so that the batteries and the system are powered during sunny hours, thereby reducing the energy consumption from the main grid. As I understand it, the MPPT charger can be configured to prevent energy from being injected into the grid.

   In Ecuador (where I plan to install this system), the legal approval of the system depends on whether or not there is energy injection into the grid. Therefore, if we opt not to inject energy into the grid, the system would operate without generating returns to the grid and would not require legalization with the utility company, correct?

   On another note, given that the VFD and the crane’s main motor utilize around 4kVA, it might be better to choose the Multiplus II 48/5000 inverter/chargers, correct? In any case, the system would draw energy from the grid but would not inject excess energy back into it. All energy would be supplied to the loads and/or the batteries, and the entire system would function as a backup for the grid and to charge the batteries.

   Finally, do you think this would be a robust enough solution for the crane bridge in question?

Thank you very much for your help

I'm confident that with the help of the Lynx devices

The lynx system is good, it provides positions for fuses in the positive path while providing a solid busbar for the negative

As I understand it, the MPPT charger can be configured to prevent energy from being injected into the grid.

Technically its not set on the MPPT, thats a setting in the master multiplus, the one generating L1. If no grid code is set in its config, then it will not feed back into the grid. The other two will follow the master unit.

Therefore, if we opt not to inject energy into the grid, the system would operate without generating returns to the grid and would not require legalization with the utility company, correct?

I cant say if this is accepted by the local authorities. For my country, connecting a system like this would already count as parallel to the grid, even without feed-out, needing already a lot of paperwork.

On another note, given that the VFD and the crane’s main motor utilize around 4kVA, it might be better to choose the Multiplus II 48/5000 inverter/chargers, correct?

It might be a good idea to get a load profile measurment. If the main motor is rated at 4kva, it does not mean that its pulling that all the time, but on the other hand, theres many other things on a crane that require energy, like gantry motors or just the general control panel full of relays and stuff.

Finally, do you think this would be a robust enough solution for the crane bridge in question?

The victron system in of itself, and its ability to form a three-phase grid, is robust in my opinion. But to be fair, im sure you would get the same answer in forums/communities of other manufacturers. Theres a certain pro-victron bias in here.
Whether or not 5kVA multiplus are capable of supporting the load as a whole, and if the island operation will be stable, i cant judge.