question

Julian Monedero avatar image

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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5 Answers
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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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?

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.

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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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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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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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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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.