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Philipp Trenz avatar image
Philipp Trenz asked

3-phase + Genset: What about unbalanced loads?

Hey there,

I'm planing an ESS system for the site of an NGO in Peru with following specifications:

  • Unstable 3-phase grid (star type, 60Hz)
  • 3x Quattro-II in 3-phase config (5kVA, 48V)
  • 3-phase Genset (~15kW prime, 60Hz)
  • 8x Pylontech US3000C (~28kWh)
  • 1.7 kWp solar via MPPT (DC-sided)
  • (Planned for later: ~8-14kWp AC-Out1 sided solar via Fronius inverters, 3-phase)

I've read a lot through guides and this forum, but am still not sure about unbalanced loads at the generator. Our power metering shows a base load of about 0.3-0.5kW per phase, highest imbalance we've seen so far was a difference of about 3kW between phases. Therefore my following question:

To which limit will Quattros compensate a single-phase unbalanced load? Up to their 70A max. battery charge current in worst case of an empty battery? Would that mean 2*70A, so about 6.5kW, could be transferred via DC-side to one phase of load balancing? Only for a short amount of time, of course, as the Quattros are only rated for 4kW continuous power.

Additionally, can Quattros be programmed to disconnect the generator on a certain level of unbalanced load or is this up to the generator?

Thanks a lot!

MultiPlus Quattro Inverter ChargerGenerator
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1 Answer
Mike Dorsett avatar image
Mike Dorsett answered ·

As your generator is rated 15kW (or 15kVA?) it will run with 5kW on one phase and no load on the other 2 without any problems. However, the Quatro's will limit this up to their 4kW charging limit, possibly even higher if power assist is set up. If the biggest imbalance is 3kW, don't worry about it on a system of this size.

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Philipp Trenz avatar image Philipp Trenz ♦ commented ·

Hi @Mike Dorsett, thanks for your answer!

Actually, it's a 24kVA generator with pf=0.8. But as the site is located at 3000 meters above sea level, we only get ~80% of the rated power. It therefore should be something around 15kW.

If I get you right unbalanced load only is an issue if one phase exceeds 1/3 of the rated nominal power. So we're fine as long as we set the AC input current limit accordingly. In our case 20A should always be safe, right?

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Mike Dorsett avatar image Mike Dorsett Philipp Trenz ♦ commented ·
yes.The limits are due to the peak current capacity of the generator winding. Your altitude de-rating does not change this. Only issue to watch out for is how the AVR is connected for voltage sensing - this is normally cross phase 2 & 3 at a center tap. loading on phase 1 only may cause a slight volt drop that is not compensated for by the generator. However, the additional load balancing by the chargers in the multi's will compensate for this.
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Philipp Trenz avatar image Philipp Trenz ♦ Mike Dorsett commented ·
Awesome, thanks a lot @Mike Dorsett
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