julian-monedero avatar image
julian-monedero asked

Multiplus II having different frequencies in ACin and Acout1

I have the following setup:

1xMultipluss II 48/5000

1x 5kW grid tied PV inverter connected to the ACout 1 of the Multipluss II

1x BYD B-BOX 10 battery

Grid is conneted to the ACin of the Multipluss II and the grid tied PV inverter and loads are connected to the ACout 1. The Multiplus II is configured as an ESS system.

My problem is as follows: If I physically disconnect the grid from the ACin, the system works perfect. If I connect the grid to the ACin, what happens is that frequency in the ACout1 starts to change and the grid tied inverter stops inverting since frequency is out of range. This happens at any SOC of the battery and even at night when the PV invertir is not working. As long as I understand, ACin frequency and ACout1 frequency shoud be always the same, but this is not the case. Frequency variations goes from 48 Hz to 53 Hz (big changes in short time). I can also see the LEDs flikering sometimes. The main problem is that the PV inverter stops inverting due to changes in frequency. Does anyone have any idea why this is happening and how could I solve this problem. Thank you in advanced.

Multiplus-IIAC PV Couplingfrequency
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6 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @Julian Monedero,

It is not a known issue, so I would first look at troubleshooting the AC input connection and making sure that it sized, and secured correctly at every point and test voltage and frequency with a meter.

At grid voltages, that is the job of a qualified electrician.

The AC input and AC output are physically connected inside the Multi, the multi does not normally have the power to adjust the frequency of the grid, unless the grid connection is very weak, or improperly connected for some reason.

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julian-monedero avatar image julian-monedero commented ·

Thank you @Guy Stewart. I have checked the AC input and It is fine. As commented above I have found out that I have an issue at the instalación with voltage between the neutral and the earth. It is not the same at ACin and ACout (0V vs. 50V). Could this be the reason? Anyway, It should be near cero on both AC connections.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

Sounds familiar, we had this problem a while ago. The reason was that the AC OUT was connected to the AC IN, so the Multi tryed to sync his output to the input but can't get it.

Check the AC wiring.

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julian-monedero avatar image julian-monedero commented ·

Thank you @Matthias Lange. I have carefully checked the AC wiring and everything is fine. ACin and ACout are not connected between them. I have tested many things today but still did not find the problem. Just in case this is important, the Multipluss II is configured with Spanish grid code. In Spain grid is a TT system (earth from the house is not connected to neutral coming from the grid). Right now, voltage between neutral and earth at ACin is cero, but voltage between neutral and earth at ACout 1 is around 50V. Should I connect both neutrals to solve this problem?

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ julian-monedero commented ·

Hi @Julian Monedero

if there is a difference on the output between N and E on the output, something is wrong with your house wiring and/or ground (relay) of the unit.

Please have an experienced installer check your system.

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john-hagtharp avatar image
john-hagtharp answered ·

Hi @Julian Monedero,

Referring to the block diagram in Appendix B of the Multiplus II manual, you will see some internal relays that are important here:

1. A double pole relay that connects AC IN and AC OUT together. Upon detecting grid connected to AC IN, the Multiplus will first synchronise it's inverter to the grid and then close this relay. After that, it should be impossible to see any difference in frequency between input and output. Unless there's a hardware fault, the grid must either not be connected to AC IN or the Multiplus has not managed to synchronise to it.

2. A relay on the line of AC OUT 2. By default (can be changed with assistants), this relay will open when the grid is not connected and close again 2 minutes after it is reconnected.

3. A relay connecting AC OUT 1 Neutral to Earth. By default (can be changed in Multiplus configuration), this relay will open when the grid is connected and close when the grid is disconnected. The theory is that the connection between grid and neutral is normally upstream on the input side. When the input is disconnected, the Multiplus will provide that connection to enable downstream RCDs to operate as intended. It's also possible your grid code has some impact on this but I don't know any details about that.

It's possible that the varying output frequency you're observing is while it's attempting to synchronise with the grid. Varying the output frequency (when the grid is not connected) is also the standard way for the Multiplus to instruct PV inverters to throttle their output so this could also be what you are seeing. The size of this variation and the affect it has on PV inverter output require appropriate configuration on both the Multiplus and the PV inverter.

You haven't mentioned which PV inverter you have. There are a few (mainly Fronius) that have a more sophisticated way of communicating with ESS. You also haven't mentioned a device running Victron Venus OS (eg a CCGX) which is required to run ESS.

If it were me, I would start by disconnecting everything from the Multiplus II AC ouputs apart from maybe a test light and see if it operates as expected. If all ok, I would then try with your loads connected but the PV inverter disconnected. This will help narrow down the cause.

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Paul B avatar image
Paul B answered ·

Just to be clear here. once the Multi syncs and the main relay ac1 in to ac1 out closes, then the frequency is completely controlled by the main AC1 in device. the Inverter cannot override this

Now are you using Grid power or a generator.

Please measure the Frequency on AC1 in and on AC 1 out, I am sure you will find that they are always both the same.

on the colour control or Venus GX you can see both as per the below screen shot

if yours is different please show us a few screen shots

1589713479429.png (22.1 KiB)
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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @Julian Monedero

What you're describing sounds like the unit is trying to sync to grid frequency to connect to grid, but somehow it fails to do so.

The frequency swings are the unit trying to match input and output wave-form and voltage.

What assistants are you running? anything that ignores AC-in by any chance?

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trungt avatar image
trungt answered ·

Hi All...

I have been reading your threads and I have the same problem.....

My MP2 -5KVA/48V has been running fine for around 13 months (Im not grid tied. AC In from house to charge 25KWh lifepo4 batteries and AC Out ring back into the garage/house. Not running any Assistants). Happened like this - I turned off the AC In (from house) during battery charging cycle as I did not want full charge since it was going to be a sunny the day after. After that day/event, I was not able to charge batteries again as it was not able to latch on to the AC In supply. As you can see from the screenshot, the voltages are different. Inverting from batteries to AC Out is OK. Charging batteries from MPPT CC is also fine.


1) I have not tried to take it all apart and put it back together again. 2) The other thing that I was thinking was to disable the Loss of Mains (LOM) Detection which requires a password/code (not grid tied - so should be OK). Apart from that, I am snookered.

It has been sunny lately and so, the MPPT CC has been keeping the batteries full but, when the weather gets wetter/darker, I will need to get the charger working again. If I can't sort it out via the above 2 methods, it may have to go back to Victron.

Just a side line issues - I have been using a wifi plug/switch to control AC In to give me greater flexibility over an Assistant (charging programme assistant). Do you think the quick switching off of AC In power caused the issue? - If so, how would this be different if Assistants are used - surely, they also cut of the AC In power as well at the end of a charge cycle? Do Assistants have a more gradual power down function (rather On or Off through the wifi switch)?

Many thanks for your input, comments, ideas and etc..


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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·
251V on the input is pretty high, check your settings for the allowed input voltage range.
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JohnC avatar image JohnC ♦ commented ·


If you're not running Assistants, then you're not using ESS. So have no need for LOM or any of the protective systems. If you select 'None' as your grid code, the selection option for LOM will actually disappear, and it will advise that backfeed is disabled. So your Multi becomes just like any appliance in your home - an energy user, not a fiddler.

Grid supply is usually stable for most people, and accepted readily by the Multi. Gensets aren't so good, and if you still have issues there's this..

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