Hello!
There is a completely offgrid system composed of 2 Inverter RS in parrallel configuration. For some reason the DC offset present in the mains is quite high, around 0.35V @ 240V AC. Measured at 0.5kW, 2kW, 4kW consumption, with resistive loads, it stays about the same.
I checked the official datasheet and users manual, could not find any information about normal DC offset values.
May I ask Victron for the official DC-component measurement and its documentation? @guystewart ?
Thank you!
If you have a noticable DC offset to the grid, then it’s likely your PV array has a ground leakage fault. Test this first.
If I remove all the solar connectors from the inverters, and then I measure again the DC component that is a good test, right?
There are any DC offset reference values for these inverters?
No Really.
In order to test the PV array for ground leak:
Both PV+ to ground and PV- to ground should be a smaller voltage then PV+ to PV-, but the voltage depends on the sensitivity / internal resistance of the meter.
E.G. for a good array: PV+ to PV- = 150V, PV+to ground = 20v, PV- to ground = 25V
for an array with ground leakage:
PV+/g +PV-/g will be approximately equal to PV+/PV- voltage.
e.g. PV+/PV- = 150V, PV+/g = 100V PV-/g = 50V
This shows that with 50V modules, 3 in series, the ground fault is 50/150 *3 from the PV-ve. i.e.between 1st and second module.
Be Careful as you may have high voltages on your array.
Well, thank you very much for the detailed explanation. I will definetly do this test.
Currently the weather is foggy, so I will wait for more sun. Will report back with the results.
However, to try to further isolate the problem, if I separate the 2 inverters, and check which one injects the DC, I could begin testing directly with panels attached to that inverter, this is correct?
Another strange thing, which could be related to this same problem:
We measured every panel individually with a solar tester multimeter, but strangely did not find significant difference between individual panels.
Thanks!
@Wanek this is an interesting question and not something I have considered in the past. May I ask what made you test the DC offset of a 240volt system? Given that you have a DC offset of 0.35 volts with an AC signal with an RMS value of 240 volts ( peak to peak of 678volts) is that really a large DC offset as it seems remarkably small to me. Also, what negative impact does a small DC offset have in practice?
I figure a DC offset is really an asymmetry between the positive half cycle and the negative half cycle. In other words a very slight wave shape distortion between positive and negative cycles causing a very slight offset in the RMS value of the positive cycle compared to the RMS value of the negative cycle compared to the centre of the peak to peak value of waveform.
I am trying to learn why this parameter is of concern.
Honestly, because my audio amplifiers toroidal transformer begin to hum quite loudly 
. Toroids are super sensitive to DC in the mains (DC saturates the ferrite core pretty soon). This is an expensive audio system, and such a transformer hum is not acceptable. It rises questions. Most of the time the core reason is DC, so this was the first thing I measured.
Well, the 0,35V itself is indeed small.
First of all, AFAIK, there are no concrete EU standards about how much DC is maximally allowed in the network, but anything above 0.1V is considered high and undesirable. If everything is working correctly in a grid, usually the DC should be around 0.05V (very rare in practice).
There can be several, from small annoyances to fatal deathly consequences:
Despite class B RCDs are already existing (which are immune to DC) since a long time, but they are NOT mandatory in EU, and in reality most of the buildings still use A or even worse, AC class RCDs in nearly every country. Class B devices are pretty expensive, and not mainstream (yet).
Generally DC presence usually indicates some faulty or poorly designed AC equipment in the system, so it always worth investigating (it can be just a phone charger, an EV charger or an inverter too).
Maybe a stupid question, but as Trevor has stated, 0.35V DC for a 678Vpp AC is about 0.05%…
How did you measured so precisely the DC component, filtering it from the AC part?
Which instrument is so precise? 0.05% error?
The advice of completely disconnecting the PV array(s) is good, to see if the inverter or the panels are the culprit.
In my Multi RS case (similar HW), with an meter with 0.03% DC accuracy, the DC offset is about 0.04V.
@Wanek thanks for that detailed and well considered response. I honestly had no idea about that issue so thanks for introducing something new to me.
Something basic: We are assuming you are not using ESS, seeing you are off-grid? I don’t know if that figure is acceptable even with the right conditions for ESS.
AFAIK, the DC offset is measurable with any standard DMM: just set to DC voltage, select a high resolution reading (low voltage) and measure across L and N. This is how I did it.
But for further record and investigation, Monday afternoon will do some new measurements with an oscilloscope. Then I can share some pics and see if there are any harmonics or PWM etc noise too.
Thanks for this, a huge help for reference, and the 0.04V is really close to the 0.05V which I read is an acceptable normal level generally in the grid.
However, I would like to see some official reference from Victron for these products. I’m a bit surprised that there is nothing in the datasheet about normal DC offset value.
This is indeed an offgrid setup, so I use solar panels, mppts, batteries and inverters.
Not sure what you mean exactly by ESS.
If I measure it like you said, I get about 0.015V DC offset.
For measuring in the same time the AC and the DC offset, the meter has to do what Trevor said, to measure the asymmetry between the positive and negative cycles. And then I will get that 0.040V DC offset.
Anyway, it can be said that another RS series is giving between 15mV and 40mV offset.
Your’s is 350mV which is too much if you are considering the your audio system as a reference.
Another thing to take into account is that I don’t have the panels’ frame grounded…
Exactly! Maybe the DMM measurement is not the most precise but it gives a quite good insight about it. Doesn’t really matters if it is 0.04V or 0.015V, but 0.35 is already way too much. The osciloscope images will give a more precise answer.
Please don’t forget to post here some images… Thanks! 
Dc offset of 0.35V /0.1 ohm can generate 3.5A.. (However, I don’t think it will get that high as source current will be limited if it’s leakage from a module.
I can’t send a screenshot from my system right now but ESS is NOT about the hardware you connect. It’s a specific software mode you enable on your GX device. It’s not recommended for use in off-grid situations. Details here: ESS - Start page [Victron Energy]
I understand now. There is no ESS for sure.
All inverters without a transformer have a DC component on the AC side. There is a Declaration of Conformity (DoC) for this. The so called 6mA (0.5%) limit. Without that DOC you need a B type RCD/RCBO anyway.
Next to that you’ll find a AC component between the PV- / PV+ and PE. Which is normal.
A steady voltage equal to the PV voltage is not. That is an error with Riso. Might be a connection, wire or a solar panel. Best thing is to actually measure that with proper gear and implement a fix for it.