DC conductors interference on PV system

[AFerguson]

Rooftop PV system approximate size 2MW. Weather station had a lot of signal errors which were blamed on the DC cabling causing interference. Grounding was correct. Cable was installed in PVC, unshielded, and run with 120V power cable approximate distance 150 feet. Problem was solved by installing filters at the weather stations.

Any IEEE papers on distance to DC circuits causing interference? Would like to rule this out as a cause.

 

[dpc]

A 2 MW rooftop system??? Must be a very large roof.

I would suspect the inverters used to convert the solar cell dc output to ac a more likely source of interference. The dc conductors could be acting as a nice antenna system for radiation of RF noise originating from the inverters.

 

[magoo2]

I agree with dpc. The inverters produce harmonics and noise results from these odd frequencies. I don't believe the dc cabling has anything to do with this interference problem.

 

[VEBill]

...DC ...was run with 120V power cable approximate distance 150 feet...

Might be common mode noise on the DC wires.

Might be some noise on the AC power line.

The "DC circuits causing interference" is almost certainly a red herring.

 

[BrianG]

It seems like the stated solution has been applied backwards. EMI/RFI emission problems from the 'DC' cables seems to be a common problem in PV systems and is caused by the high power inverters putting h.f. components back on to the DC cables. This can affect broadcast radio, mobile phones, etc., over quite some distance, so it's not surprising that many filter companies make special filters to go between the inverter(s) and the PV grid's DC cabling.

A 2MW system has some serious DC current to handle at 120V (16,666A) and hopefully its not all down one cable! If it's distributed to more than one inverter then there are filters out there specially made for the purpose which can handle up to 2500A each.

 

[VEBill]

I was reading the OP that the 120V was a separate circuit.

 

[BrianG]

VE1BLL : OK, reading it again I think you are right. I wonder what is the actual DC voltage from the PV array to the inverter(s)?

 

[AFerguson]

Array voltage ranges 320-600Vdc.

 

[MagicSmoker]

I agree that the inverters are almost certainly using the DC feed as an antenna. Yes, each inverter should have an input filter to supposedly prevent this, but I suspect it is interaction between filters via the DC feed that is the root cause. That is to say, resonant current is sloshing back and forth between the filter networks of the inverters, using the DC bus as a transmission line. Fiendishly difficult stuff to model, much less solve, I'm afraid.

 

[VEBill]

No need to model it; just fix it. Common mode chokes aren't rocket science.

It should be solved as the radiated emissions propagation away from the cable are not likely to be permitted by spectrum authorities. Use a good quality AM or SW radio to confirm the *relative* magnitude.