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s-forsberg avatar image
s-forsberg asked

3 phase AC coupled PV for backup or off grid without phase balancing?

Hello!

I am planning a three-phase installation with Three Multiplus II 5kVA with a SMA Tripower 8kW on AC out. The system will run in ESS with backup functionality.

When taking the three-phase Victron training I found this statement, which was new to me:

"A three-phase system can not aid in phase balancing during charging. Each unit will always equally charge the battery"

This really worries me regarding backup or off-grid use. Could this prevent AC PV from being useful for charging the batteries, or even working at all? I assume that three phase PV inverters will try to deliver equal power to all phases, but what happens if one phase (e.g. L1) is heavily loaded and the others are not?

I would have assumed this scenario as an example:

  • L1:Fully loaded, PV supplies 2 kW to load. Cannot charge battery.
  • L2:No load, PV supplies 2kW to battery
  • L3:No load, PV supplies 2kW to battery

But with the equal charge current rule it seems like the follwoing example is instead the case, where PV power cannot charge the battery during phase load imbalance:

  • L1:Fully loaded, no power left to charge the battery
  • L2:Cannot receive PV charge since L1 cannot charge
  • L3:Cannot receive PV charge since L1 cannot charge

This would seem reasonable enough, but what worries me in this example is that it also seems to mean that the SMA inverter is then expected to deliver power to three very differently loaded phases. I am not certain that it has the specifications to handle that.

It seems to me that to make the backup/off-grid use case viable, either the PV inverter or the three phase Multiplus system will need to do some phase balancing!


Am I right? How does this work in real life? Is it possible at all?

Multiplus-IIAC PV Coupling3 phase
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3 Answers
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @S_Forsberg

don't worry, the only limitation here is when feed-in is not allowed, because the AC-PV power cannot be limited per phase.

in ESS mode, power that is not used by loads will be used to charge the batteries

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s-forsberg avatar image
s-forsberg answered ·

Thanks @Daniël Boekel (Victron Energy Staff). I agree on what happens in ESS mode, but what is the case when the grid is not available?

I am also still trying to understand the statement from the Victron three phase training stating that each unit will equally charge the battery and that a three-phase system can not aid in phase balancing during charging. Can you please explain this?


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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Hi @S_Forsberg can you explain what exactly was stated?

in off-grid situations, also what energy is not used is charged into the batteries, this is per phase so it does not equally charge on all phases.

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s-forsberg avatar image s-forsberg Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Please see the slide from the Webinar with Margreet Leeftink "Three-phase and parallel system theory". Can you explain the last bullet?

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ s-forsberg commented ·

Hi @S_Forsberg

I've asked about it: this applies to charging with a generator, when loads are un-balanced on the output of the inverters, the system can not help to balance the generator loads (other than through power assist when a phase is loaded higher than the set maximum input current)


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s-forsberg avatar image s-forsberg Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Thank you for asking!

I still feel that one part of your previous answer is not consitent with the training. You write for an off-grid configuration that:

"in off-grid situations, also what energy is not used is charged into the batteries, this is per phase so it does not equally charge on all phases."

but the last bullet in the training states that:

"Each unit will always equally charge the battery."

Have I missed something that can make these statements both be true, somehow?

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s-forsberg avatar image
s-forsberg answered ·

Maybe @Margreet Leeftink (Victron Energy Staff) can help to sort this out? I have searched the Internet and this forum for answers but three weeks later I'm still stuck.

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