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hamsolar avatar image
hamsolar asked

MP II 5kva x4 in parallel single phase backfeeding all the time with feed-in disabled.

Hi All,

I have an ESS comprising of:

ET340 connected to three phase grid but set to single phase and only using L1

Cerbo GX

4x MultiPlus-II in parallel single phase

2x BlueSolar MPPT 250/100

40kwh of battery connected to Lynx power in

The whole property is running off AC Out 1, I would normally not be using the ET340 in this case but I thought I would add it as the grid meter to fix the problem.

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My problem is that no matter what I do the system backfeeds power to the grid, I can make the Grid setpoint higher to counteract the back feeding (currently its set to 180w) but the amount of backfeeding is around 10 to 15% of the load so when there is a high load it overcomes my grid set point and starts back feeding again. It back feeds anytime of the day, solar available or not. When I initially setup single phase, the MP-IIs were set as the grid meter.

When 3 of these inverters were in 3 phase configuration the grid setpoint was at 30w and back feeding was not an issue, the system worked as expected. Unfortunately one of the phases became unstable and the MP-II would no longer accept the grid. The utility has been out twice to solve the problem but thus far we can't get a stable sync to the grid. Hence why I resorted to changing to single phase.

I suspect that one of the MP-IIs is somehow faulty, it seems to me like the internal CT is "leaking" the more current there is the bigger the leak becomes. I just wanted to confirm that this is possible before taking each MP-II out in turn to see if one is the culprit.

ESSess grid setpoint not reached
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2 Answers
nickdb avatar image
nickdb answered ·

Change multiphase regulation setting in ESS to individual phases.

If this does not resolve the issue then it is likely an imbalance across the inverters.

You need a clamp meter to check each one AC in/out to confirm all are working and they are evenly balanced (within a few percent).

Are they all the same age?

Is wiring symmetrical and of an appropriate gauge and length (as per the setup guide)?

Have all been setup correctly, did you check each one to make sure they are identically configured?

Many variables with so many inverters on one phase, easy to get it wrong.

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Alexandra avatar image Alexandra ♦ commented ·
Parrallel systems are infinitely more difficult to get right.
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Alexandra avatar image Alexandra ♦ commented ·
Parrallel systems are definitely more difficult to balance.


You had three for a three phase what made you move to 4 units? I have a feeling a mismatched unit in age etc may be part of the issue.

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hamsolar avatar image hamsolar Alexandra ♦ commented ·

All configs are identical (checked twice) but the AC wiring is not symmetrical, 4mm2 for AC in/out (furthest inverter from the change over DB has longest AC, 3m to last slave vs 2m of master). DC cables are symmetrical to Lynx busbar.


4th unit was purchased to troubleshoot the 3 phase being unable to connect to the grid. It's about 1.5 years newer than the original 3. The extra capacity will be useful for aircons in summer.


I will test with a clamp meter. Thank you for feedback.

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Duivert NL avatar image Duivert NL hamsolar commented ·
Part of the problem then is your non symmetrical ac cabling
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hamsolar avatar image
hamsolar answered ·

Initial results of clamp meter testing on AC1 out:

Load consistent at 3050w, grid setpoint 30w, back feeding at ~300w


 Master Slave1 Slave2 Slave3
Voltage 240v
Amps 3.6 3.3 2.9 2.8
AC wiring (m) 2 2.5 3 3.5

Fluke 302+ used to test.


After re-wire AC-Out:

Load consistent at 5150w, grid setpoint 30w. Grid still shows some fluctuations but mostly between 0w - 50w.


 Master Slave1 Slave2 Slave3
Voltage 240v
Amps 5.3 5.2 5.2 5.3
AC wiring (m) 3.33 3.33 3.33 ~3.33


Would it improve grid setpoint accuracy further if the AC-In cabling was also all the same length?

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nickdb avatar image nickdb ♦♦ commented ·
Yes. Length and gauge are critical. As are the age of the inverters, with the newest set as master.

What are the manufacture dates of all 4? It is the 4 digits in the serial number after the first 2 letters.


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hamsolar avatar image hamsolar nickdb ♦♦ commented ·
Device 1: 2250

Device 2: 2250
Device 3: 2252
Device 4: 2241 (this is the newest inverter I have purchased but it turns out to be the oldest, must have been old stock).

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nickdb avatar image nickdb ♦♦ hamsolar commented ·
Installation aside, those should work fine together.
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Duivert NL avatar image Duivert NL commented ·

ESS will constantly balance on grid, thats why it will always fluctuate around your gridsetpoint

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