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2x Axpert MKS II - PV stability in SBU mode


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Hi Experts

I run 2x Axpert MKS II in parallel.  When they are running in Utility (aka grid or line) mode the PV watts is very stable.  When running in battery mode the PV stability isn't bad, but it's not as stable as grid.  In the attached image you will see the yellow line (PV) is very stable when in grid mode (where the brown grid watts is not 0).  When in battery mode (brown grid watts is 0) the PV fluctuates with about 10%.

Legend for attached image:

Brown: Grid
Yellow: PV
Blue: Load
Green: SOC

 

Screenshot from 2020-07-07 12-10-55.png

MQTT values.gif

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46 minutes ago, pierre. said:

When running in battery mode the PV stability isn't bad, but it's not as stable as grid.

So I guess your question is: why is that so?

It happens I answered a similar question just minutes ago, here. In short, I believe it's because in line mode, AC-in supplies the majority of the load power (and the loads are choppy). So nearly all the PV power goes to charging the battery, which is a smooth process, certainly compared to loads, which vary as people and thermostats turn things on and off. In battery mode, PV power is called on to supply the choppy loads as well as charge the battery.

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Yes that was my question :) Thank you for the answer.  Some additional info:

1) The first image attached to this post is with only one inverter running before lunch. After lunch both inverters are running in parallel again.  Parallel mode is less stable.

2) Just for interest sake, the parallel setup ran for 5 months without any issues.  Then one day it gave error 72 (current sharing fault).  After shutdown and restart they continued working fine again.

3) The second image is also the same setup where only one inverter is running, but with and double the amount of strings to it's panels, hence about half the voltage.  It's by far the most stable.

Screenshot from 2020-07-07 13-42-56.png

Screenshot from 2020-07-07 13-50-51.png

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Posted (edited)

@Coulomb Now even more interesting, according to the inverter, the load also becomes unstable once it's not on grid.  In the graphs below it switched to grid at 18:00 and then back to battery at 8:30:

image.png.139d69b43f5f8d40d969a074618330bf.png

Edited by pierre.
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4 hours ago, pierre. said:

according to the inverter, the load also becomes unstable once it's not on grid.

I have no idea what's going on there. Is the output voltage fluctuating wildly or something?

Or maybe the loads actually are steady, and it's a measurement error, and that error is triggering switches to grid, and back to battery again when the intermittent error goes away.

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@Coulomb you are correct. When in battery mode the output voltage is fluctuating.  I wasn't aware of it.

image.png.097f860e47310caf910cd25774674c84.png

In terms of switching to grid and battery: it's not unexpected.  When battery SOC goes below 40 it switches to grid and then back to battery when SOC is 50.

Any idea what would make my output voltage fluctuate and is it normal?

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18 hours ago, pierre. said:

Any idea what would make my output voltage fluctuate and is it normal?

It's not normal, and you should try to fix it soonish. All I can think of is loose wiring (AC-out or battery), or too thin wiring (AC-out should be 4 mm² or 6 mm²; battery cables should be at the very least 16 mm², preferably 25 mm² up to 50 mm²), or a problem with a breaker or fuse. A common problem with a battery fuse, if you use it as a battery isolator, is if you don't pre-charge properly, the fuse case can get nasty burn marks that don't conduct properly. It may be possible to clean up the marks or rotate the fuse to a clean area (wherever the fuse makes contact).

Check to see if the battery voltage fluctuates as well. It could be that you battery is undersized or high internal resistance.

Edited by Coulomb
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Hi @Coulomb, your feedback has been invaluable.  I've tested running each inverter on it's own up to 3kW, the output voltage remains stable.  When running together in parallel the voltage fluctuates with a 3kW load, then I increased the load to 6kW and the voltage started fluctuating all the way down to 210V.  I've checked that the current sharing and communication cable is plugged in correctly.  Also did a continuity test on the current sharing cable, didn't find any issues.

All cables are thick enough and confirmed that they make good contact.  Voltage on batteries (3x 3.5kWh Pylontechs) remains stable.

Still looking for the culprit...

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More information: it only happens when there is lots of PV available. I just tried both in parallel with PV disconnected. Added 6kW load which is provided from battery and output voltage stays constant at 230V...

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It sounds like the bus voltage is fluctuating, and the voltage control system can't react fast enough. 

Do you perhaps have more than 4500W of PV per inverter?

Or share PV between inverters?

Or separate batteries for each inverter?

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Hi @coulomb, I made 100% sure the PV isn't shared between inverters and that they run off the same battery bank.  I have 9 panels (3060W) PV per inverter.

I see you are helping another user with the same problem in a different thread.  I'll continue my investigation results there:

 

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  • 3 weeks later...
Posted (edited)

 

On 2020/07/14 at 2:41 AM, Coulomb said:

Check to see if the battery voltage fluctuates as well. It could be that you battery is undersized or high internal resistance.

Hi @Coulomb what is an undersized battery? I know the MKS II 48V manual says you need 200AH battery per inverter, but that is for lead acid right? With lithium the charge/discharge is much higher.  I currently have 3x PylonTech US3000 which comes down to 220AH.

I haven't sorted out the fluctuating output AC yet.  I've replaced the parallel cables and made sure all cables are same length.  In charts below, problem happens around 09:25 and again at 09:51.  Note that I can easily cause the system to become unstable by just turning on enough appliances to match (but not exceed) the PV generation.  Whats the chances a firmware update can fix this? I'm currently on 71.70 and can upgrade to 71.80 or your patched 71.80a.

image.png.046288edbd7f7e8afea15dac2ca12310.png

image.png.9c185204f9fcfcd32c2d6bcb967e2420.png

image.png.6229b11d8fd4c4a4c8677b5a92c2e013.png

 

Edited by pierre.
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3 hours ago, pierre. said:

 what is an undersized battery?

220 Ah of lithium should be OK for two inverters. I see that the battery voltage varies only about 1.5%, while the output voltage varies up to 8%.

I note that the PV voltage varies some 80% when the voltage goes unstable. This could be affecting the bus voltage, which will cause the inverters to struggle to maintain a steady output voltage.

3 hours ago, pierre. said:

Whats the chances a firmware update can fix this?

I'd be guessing, but I'll say the chances are slim. It would not hurt to try.

The problem may even be a side effect of the design. Though I'd expect more problems if your PV voltage was significantly above the magic 337 V needed to generate a proper 230 V sine wave (230 x √2 + 12 V), but yours seems to be just under that value. Your battery voltage times the presumed transformer turns ratio of 1:7 comes to some 348 V minimum.

My wild guess is that this means that your inverters have only a few tens of volts from the steady state PV voltage to the presumed bus voltage (around the battery voltage times 7), so there isn't much room for boosting the PV input before it raises the bus voltage. Raising the bus voltage is fine; it can go up to 450 V, but as it does, the output voltage will rise until the output voltage control system reigns it in.

 

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