Unbalanced load on Parallel 2x Multiplus-II 5000/48 system.

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oliverclaude asked Feb 21, '20

Unbalanced load on Parallel 2x Multiplus-II 5000/48 system.

HI All.

I'm hoping to get some assistance. I have been struggling to get my 2x multiplus-ii 5kw units in parallel setup correctly. Sorry. Let me phrase that again. They are set up precisely, in parallel, according to the user manual and is running, but the one always supplies all the load and goes into overload the moment I start pulling more than 5kw, the other unit does nothing the entire time.

When my system goes to pass-through mode, I can see that both AC cable runs thought the individual inverters are carrying the same amount of current. I have Volt and Amp sensors on both input and outputs of my multis to confirm this.

On the DC side, I have double-checked every connection, cable type and cable length, and places DC shunt current sensors inline with both inverters. The resistance through both DC runs should be very close to exactly balanced, but the one inverter will carry 80 to 100A (55V system) and the other inverter is sitting at 0.03A.

To make sure it wasn't anything to do with DC cables/system, I swapped the inverters around on the wall and kept everything else the same. The problem then moved with the inverter.

So my question, can anyone think of something else for me to test/configure/do? Or is it a faulty inverter.

Something I have noticed is that the inverter that never carries any load is disconnected from the shared AC bus, it fails with overload immediately, and then takes down the whole parallel system. It will not start again unless it is corrected to the shared AC bus, it just ends up in a overload loop and resets.

Lastly, I have upgraded my multis to the latest 469 firmware.

PS. Here is my .rvms file to have a look at my configured setup.

https://drive.google.com/file/d/1gJA8uLi4PELu5iGO1dQUiFhbKVDFKqLB/view?usp=sharing

Any assistance would be appreciated.

Regards,

Claude

Multiplus-II battery charging

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Answer 1 · 2020-02-21T11:39:23Z

Vihann answered · Feb 21, 2020 at 11:39 AM

Hi.

I Would start to look at the AC cable lenghts. Are the input cables the same lenght and the output cables the same length.

also make sure the inverter is running the latest firmware, as it has a parallel issue with 433.

let me know

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Answer 2 · 2020-02-21T16:55:50Z

Daniël Boekel (Victron Energy Staff) answered · Feb 21, 2020 at 04:55 PM

Hi @oliver.claude

It could be that one of the units is defective, try running them as stand-alone to see if one of them is causing this.

PS. with issues like this: when you are sure the wiring is ok, the fastest way to get things resolved is to contact your dealer / distributor.

Edit - please also do the parallel online training in Victron Professional.

And post a system wiring diagram, and some photos of the installation. It may help to see.

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Mike Dorsett commented · Jun 25, 2021 at 03:08 AM

@Daniël Boekel (Victron Energy Staff) I'm following this topic closely, as I'm running 2 Multiplus 1200/24 in parallel. The on line training video seems to have disappeared. I've been running my parallel system for 3 years with onlly one minor issue: The AC board in one of the inverters has stopped detecting the generator input. At the time I installed the system, I could not find any information as to whether to parallel the gen inputs, but as the generator capacity was less than that of a single multi, I decided not to. System worked fine for about a year, then the Ac board fault developed. I plugged the generator into the other Multi, and Lo the master / slave automatically swapped. System is still fine and power sharing is good. After reading several threads about this topic, I'm NOT going to update the firmware as this seems to be causing problems. Should the generator inputs be paralleled? Mike.

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Daniël Boekel (Victron Energy Staff) ♦♦ Mike Dorsett commented · Jun 25, 2021 at 10:02 AM

@Mike Dorsett are you saying you don't have the inputs paralleled? that is not how it works.... and master / slave don't swap magically also.

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Mike Dorsett Daniël Boekel (Victron Energy Staff) ♦♦ commented · Jun 25, 2021 at 06:18 PM

Hi Daniel,

The DC for the inverters is paralleled on bus bars, the AC output gores through 2 x 16A breakers, and is then paralleled into the RCCB of the EasySolar (some slight mods done to the internal wiring to permit this), BUT the Generator is only connected to one unit. Yes it does work - and yes the Master / Slave do automatically swap. (or they used to - before a gecko crawled down the back of an AC board and caused a fault...). This might not be how it was meant to work, but it does. Inverters are about 5 years old now, so firmware is probably well out of date. All I can say is that perhaps the information on designing parallel systems should be easier to find? I've just opened a professional account and still can't find this..

The Slave inverter appears to operate in invert/charging mode (i.e. taking power off the AC bus) while the generator is running, as the green LED stays blinking. The other inverter runs it's orange led to show normal charging mode.

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Answer 3 · 2020-02-22T11:29:19Z

oliverclaude answered · Feb 22, 2020 at 11:29 AM

Ok. So this morning I replaced all the wiring on the AC side. I ran overly long cables from my distribution board to my inverters. They are less than 1 meter apart, but my cables are all 3 meters in length. They are all exactly the same type and thickness and all circuit breakers are identical.

I cant believe it is a cable problem. As I have now done this on both AC and DC side.

Will see if I have time to swap the inverters around on the wall again tomorrow. This way I can check if the problem moves with the inverter or stays with the cabling/installation.

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Answer 4 · 2020-02-24T07:16:46Z

oliverclaude answered · Feb 24, 2020 at 07:16 AM

OK, I have made some progress.

I was busy testing all the inverters to see if they function as standalone units. (I expected one to be faulty.) And low and behold they both work 100% in standalone mode.

Afterwards, I was busy putting all the old configs back.

I put them in parallel and sent the config. Then I configured the master inverter and sent all the changes to all the inverters. (Without any assistants setup). I was shocked to see that the inverters were producing the same voltages, normally the slave inverter was always a good 20V below the master inverter. I then set up an ESS assistant on only the master, sent the config and they are still producing the same Voltage on AC1 out.

This is the first time I have got the two voltages on the outputs to be the same before bridging the outputs.

Question: Do I have to put another ESS assistant on the slave? At the moment I'm running without and seem to be doing fine!

Lastly, I then connected all the outputs to the house and started testing with large loads, and for the first time, the slave is actually sending some power. They are not close the balanced, but at least it is sending something. Will push the output higher tonight and see how close to balanced they are.

I have noticed that the current through the inverters, when in passthrough mode, are not perfectly balanced, so I will focus on getting them perfect first, then focus more on the DC side.

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Answer 5 · 2020-02-27T08:45:01Z

oliverclaude answered · Feb 27, 2020 at 08:45 AM

The system has now had a few days to stabilize and I found that the AC cables running through my inverters are carrying the same current during passthrough. Or withing my 95% tolerance criteria anyway. So happy with the AC cables.

The low inverter is still only carrying 25% of the total load, so next, I will redo/reconnect all the DC cables. Starting with the DC bolts and I will make sure to tighten them all to the same torque. Each inverter has 6 bolts/lug connections across the positive and negative cables.

If that doesn't balance the inverters better. I will consider adding lugs to my cables running into my fuse boxes to make the connections more uniform.

Will keep everyone updated.

PS. Thanks a million for everyone that has posted/messaged me ideas to hunt down.

Claude

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Answer 6 · 2021-06-14T18:22:45Z

Nicholas Herbst answered · Jun 14, 2021 at 06:22 PM

@oliver.claude have you managed to solve the problem? I am experiencing the same problem and have not been able to resolve it yet.

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Answer 7 · 2021-10-12T14:11:21Z

mike90 answered · Oct 12, 2021 at 02:11 PM

Experiencing a similar problem. 2 x MP II in parallel which overloads on grid failure and switches off. Load is usually between 3 and 4 kw when this happens. Have checked all wiring and connections...

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