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Eliseu M avatar image

Multiplus AC output voltage dips on load peaks

Hello,

I'm having a small issue since the beginning of my (near) offgrid installation. Everytime the fridge compressor needs to start, it is very much noticeable on the lights. They flicker a little too much compared to what I wanted to be ideal.

Of course, this is expected to some degree. One must design system with a sufficient margin for the load peaks. My inverter was initially a 24/3000. I later added another one in parallel. They are fairly balanced, so I expect that now I have a system better or equal to an 24/5000. Further down the road, I improved the cabling to batteries from 50mm2 to 70mm2, and just about 1m lenght to busbar + 50cm to inverters. Even the fuse I passed from 250A to 300A to reduce resistance. Later on, I even changed to LiFEPO4 batteries (14kWh), with reduced internal resistance...


Well, even with all these improvements, although it got slightly better after each step, I never managed to remove the flicker on the lights to the extent I really wanted. In fact, I have a small impression (hope I'm wrong) that is getting slowly worse rather than better. Does anyone believe that is anything wrong with my system? Only about 3 years old. Could it be Multiplus capacitors or any other component is showing "tireness" due to the fridge constant startups all day long during the past years?


Below an example that makes the lights flicker. The voltage seems to go too low with too much ease for this kind of load. It's a ~150W fridge. I'm kinda worried. Or am I expecting too much?

1618561795892.png


MPPT - Solar Charge ControllerMultiPlus Quattro Inverter Chargerpeak
1618561795892.png (24.4 KiB)
5 comments
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To me its maybe all to do with the DC voltage and current, what are these graphs showing, and a diagram of your setup would help, and some pictures of the install.

I suspect DC voltage sage measured at the inverters when the load is applied.

please supply the DC voltage reading using a multi meter directly at the pos and neg terminals of each inverter, and we need the reading when the load is applied.


Plus the same readings at the battery bank


Yes, of course, that is the primary suspect. But I have done almost everything I could. VRM doesn't show any sag, but could be due to lack of resolution.

1618679220050.png

I have now 2x sets of 280Ah LiFEPO4 connected to busbar with ~50cm x 50mm2 + 600Ah Lead acid OpPzS with 1mx 70mm2 to busbar. From busbar to the 2 inverters in parallel I have 2x 50mm2 / 60cm lenght. Isn't this suposed to be enough? The Busbars are made of copper with cross-section of 120mm2.

The 2 sets of 280Ah have good cells with 0,13 ohm IR, measured by me. If my system has sag, then... almost every system should have...

But I do have on open mind. In the end, most likely it is the DC side. But I'm running out of ideas... changing the ANL fuse from 250 to 300A did improve the issue...

1618679220050.png (31.7 KiB)

@Elimac

I have one fridge that does this and it is smaller than the other. I think it is the design in the fridge compressor start up system.

It was on a 5kva and causes the light flicker you describe even with low loads. Moved it to the 3kva 24 to experiment.

We have a 95mm² cable on the battery as an experiment, seems better now also a 24v system. But 2x50mm² on each terminal will help, just make sure they are both exactly the same length though. Found also cable length affected it as well.

Related to the comment paulb made about voltage drop.

Yes, I have two fridges, only one causes noticeable flicker. But I don't understand one thing... in your case it improved changing from 5kVA to 3kVA inverter?

@Eliseu M It is a strange thing I agree. On the 3kva we had 95mm² shorter cable runs. Also larger ac out cable feeding the fridge. A second experiment with a long extension running smaller ac cable made the problem come back.

So the thinking is it is resistance related and the way the fridge compressor system is set up (not a soft start)

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