Common Mode Voltage

[Fresno473]

Alright so to start off and introduce myself, I'm Josh. I did introduce myself in the introductions thread.


My problem and first ever question is (and I'm hoping some of you genius' out there can help) that I commonly have arrays that fault out my inverters for "High Common Mode Voltage" readings and it is typically happening during weather events (rain). The more I'm reading about this issue that can be common in solar systems, the more I'm venturing to say that the inverters must have some kind of design flaw. If I can provide any more information that you might need, please let me know! I really hope that someone out there either has a solution or can really just shed light on this subject so I can relay the information to my customer!

Thank you!

Josh

[Sunking]

Common Mode Voltage or noise means with respect to something usually ground. Differential Mode Voltage or Noise means between two points like the terminals on a battery post.

[inetdog]

In which case high common mode voltage could be caused by either leakage or capacitive coupling to ground from an otherwise unearthed DC circuit, or maybe an induced voltage on the whole loop from atmospheric electricity or lightning induced voltages.

[sensij]

There is some literature out there on common mode voltage problems in large three phase inverters, which sound like the type the OP may be using based on the intro post. From what I've quickly read, it looks like it is something of a design problem in multi-level inverters, with circuits to correct it as an active area of research. I suspect the best information for dealing with this particular problem will come from the inverter manufacturer.

[Fresno473]

There is some literature out there on common mode voltage problems in large three phase inverters, which sound like the type the OP may be using based on the intro post. From what I've quickly read, it looks like it is something of a design problem in multi-level inverters, with circuits to correct it as an active area of research. I suspect the best information for dealing with this particular problem will come from the inverter manufacturer.

That's exactly what I'm kinda thinking. It is commonly an issue with one brand of inverter I deal with. I haven't experienced this issue with any of the other brands we service. I understand that its a difference between positive and negative busses but that's about as far as I really understand from a theory side. Hence, knowledge is power. The better we understand what causes it and methods to fixing the issue the more likely we can actually figure out what solution might work best for us.

Inverter manufacturers are pretty limited to what information they want to divulge. I could ask the manufacturer of the inverters that never have this issue how they rectified it but most like would get laughed at. Even the manufacturer of the inverters that have this issue are likely to laugh as well.

Do you guys have some good information about how this all works? I found one article and tried reading it but it was so "technical" that it sounded like I was reading lawyers talking after the first few paragraphs.

Josh

[solarix]

Would help to have some details like inverter brand and type, but the term "common mode" is not "common" in the solar industry but probably refers to the voltage detected on the "minus" or "negative" array circuit depending on if it is a grounded or un-grounded type inverter.

[sensij]

Do you guys have some good information about how this all works? I found one article and tried reading it but it was so "technical" that it sounded like I was reading lawyers talking after the first few paragraphs.

Of the papers I scanned, this one was among the more accessible. It makes a point of mentioning that for inverter circuitry susceptible to this, the CMV will vary as a function of weather (or anything that can change the capacitance between the frame and earth, as Inetdog mentioned). Most members here (including myself) are more familiar with residential single phase inverters, and not likely to have run into this problem.

I'm not sure this can be boiled down into lay-speak, it really requires some technical knowledge to understand.

[Fresno473]

Of the papers I scanned, this one was among the more accessible. It makes a point of mentioning that for inverter circuitry susceptible to this, the CMV will vary as a function of weather (or anything that can change the capacitance between the frame and earth, as Inetdog mentioned). Most members here (including myself) are more familiar with residential single phase inverters, and not likely to have run into this problem.

I'm not sure this can be boiled down into lay-speak, it really requires some technical knowledge to understand.

Thank you all who were receptive to me being involved in this forum. One of your compadres, J.P.M. has some issues with me being a part of it. Especially given that he "Became an engineer because of solar in 1975. Been at it ever since," as stated on his member profile page, I'm not sure he has any room to be the forum bully that he has proven himself to be in the short half a day that I've been a part of this place. I'm pretty sure he is a wealth of knowledge but tends to use his childish side a bit too much. With that said, ADIOS!

[bcroe]

I am assuming the "common mode" voltage here is on the DC panel connection. That would be
impossible on inverters like my Fronius, which have ground on the negative panel feed (through
a 1A fuse). An inverter not using this ground certainly could have issues.

On the AC side, my inverters do monitor the (unused) neutral. But I would think an issue here
would be referred to as "unbalanced line". Bruce Roe