minor-problem avatar image
minor-problem asked

Is it ok to run my wind turbine inverter during a power cut with a Multiplus II?


I've just fitted a Multiplus II ESS system at home with 4 off the Pylontech US5000 batteries and it's working beautifully. We have a Britwind 5kw wind turbine supplying power to the house via an SMA Windy Boy 5000 inverter. I have set this up on the critical loads AC1 out so it can supply power to the house even when we get a power cut which happens fairly often where we are in the Highlands of Scotland.

We have a power cut at the moment and I am a little concerned what will happen once the batteries have fully charged. Normally any excess electricity would just bleed back into the grid but as the grid is isolated I am unsure what will happen once the batteries get to 100%. Will the inverter cut it's output, will bad things happen?

I should add that the Windy Boy is around 10 years old and does not communicate to the Cerbo GX.

I can just isolate the Windy Boy manually from my system and then it will shut down as it used to in a power cut but obviously I may not always be on hand to do that in the future.

Can anyone let me know what happens if a 5kw inverter output has nowhere to go?

Thanks, Barry

Multiplus-IIHydro and Wind Power
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2 Answers
ojack avatar image
ojack answered ·

The Multiplus will increase the frequency on ACout1 to make the inverter shut down. If the windy boy supports this it will be ok. If not you should cut the windy boy manually.

In future and if windy boy can not cut of by increasing frequency you could place a Relais between windy boy and ACout1 and control it by the cerbo depending on SoC.

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minor-problem avatar image minor-problem commented ·
The power remained off for a few hours so I was able to see the Multiplus doing exactly that, when the output from the Windy Boy was more than the AC loads and charging requirement combined the WB has detected a frequency disturbance from the MP and shut down. A very useful feature.

Thanks for your help!

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sharpener avatar image
sharpener answered ·

If you have an 8kVA Multi or larger you are OK, if not search for "1:1 rule" and see also this thread.

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ojack avatar image ojack commented ·
Why Multi 8000? Windy boy 5000 is 5000VA = Multi 5000 is also 5000VA.

This should work i think.

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sharpener avatar image sharpener ojack commented ·

You are right, the advice here says

"Another question frequently asked is how can this factor be 1.0? Since the charger inside a 3000 VA Multi is not 3000 VA but closer to 2000 VA? The explanation lies in the fact that it will regulate. In other words: when there is too much power coming in, causing the charge current to exceed the limit, it will increase the output frequency again and will keep regulating the AC output frequency to charge with the limit."

And that is the official line. But I have never been very happy with it because unless the charger has an undocumented short-term overload capabiity, at the instant the grid trips there is nowhere for the excess AC power to go until the frequency control steps in, which will by definition take at least one cycle.

Also supposing the excess power gets to the DC side there is the following warning in the same document:

"Increasing this frequency will take a very short time, but during that time all power will be directed into the batteries as there is no other place for it to go. This causes the following:

  • When batteries are (nearly) full, the battery voltage will spike, possibly causing the Multi to switch off in DC over-voltage alarm.

  • The same spike will cause the AC output voltage of the Multi to spike, as these two are directly related, and when the spike on the battery voltage is high and fast enough, the Multi can never regulate its PWMs down fast enough to prevent the spike on AC. This spike can damage the PV inverter, the Multi and also any connected loads and other equipment."

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Related Resources

MultiPlus-II Product page 

MultiPlus-II Manual

MultiPlus-II 230V Datasheet 

VE.Bus Error codes

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