Radio Frequency Interference

roberthend

My neighbour installed a 3-ph grid-tied 5kW solar system.  No optimisers just 17 x 300W panels and 3 phase inverter.  I wasn't home the day the system was commissioned but on day 2 I found the system generated significant radio interference for most of the day.  In character the interference was broad band similar in character to that produced by HV line insulators tracking.

I got hold of some DC input cable and AC output 5 core with a plan to make some common mode chokes.  Meantime the interference has reduced day by day to the point where with a yield of 40-50kWh/day it now makes only very occasional interference.

I am puzzled by the changes in behaviour and wonder whether someone with more solar experience might have an answer?    Do MPPT systems learn, initially going through a process of load variation under varying illumination conditions to generate a profile suited to array characteristics?  Or has something in the inverter been arcing and now stopped?

Thanks.

Bob, 5B4AGN

mcgivor

Is it possible work was being done on the system, checking voltages, current etcetera, if so perhaps there were open enclosures which would normally shield the RF noise and have since been replaced. Most equipment has to comply with some sort of regulatory standard such as the FCC in North America, as far as learning is concerned, I'm unaware but highly doubt it.

roberthend

Thank you for your response.  I appreciate it.

The unit was fully commissioned with all panels in place when the interference occurred.  Coincident with periods of significant RF interference the inverter also made more than usual audible noise.

The system comprises 17 x Sonnenstrom PERC 300 WP glass panels and a ZCS Azzurro 5.5KTL-X 3-ph inverter.

OK re my "learning" theory.  I have no prior experience with solar inverters hence no knowledge of the workings of MPPT.  I am just trying to understand the observed behaviour.

I guessed the inverter might measure array O/C voltage then make incremental adjustments to load value to detrmine the point of maximum power transfer, repeating the process across the full range of array O/C voltage.  My reference to "learning" assumed the O/C voltage & associated load value data might be written to a look up table so avoiding the need to repeat the more time consuming incremental approach each time the inverter powered up.

Then again, maybe not.

Estragon

My understanding of how the mppt works is the controller checks and adjusts voltage vs current at intervals of maybe a few minutes.  This lets it adapt/optimize for changing illumination, panel temp (voltage), etc.  

As for why the interference would diminish over time, I dunno.  Any loads (eg motors, flourescent lights) that might have changed?

NANOcontrol

In electronic motor drives, there is often a clause in the manual that states if the electronics has been stored for a period of time that the electrolytic capacitors have to be powered up without the motor running to reform the capacitors. It is stated that the capacitors can even explode if stored for a number of years without reforming due to excessive surge currents overheating them. Even new caps are not fully formed and have higher than normal leakage for a period of days. I have never heard this in regard to RFI, but it could be an explanation for things changing over several days with all other factors being the same.

BB.

As far as I know (and that is not very much in the grand scheme of things), there is no MPPT controller that learns over time (hours/days/weeks). They are very much an "in the moment" kind of device. And can retrain in fractions of a second and do a retraining in seconds to 5-10 minute cycles (5-10 minute cycles, typically for off grid / battery power system, not Grid Tied).

In general, RF noise has many sources... From the switching circuits that may give you "birdies" every 10-25 kHz or so, to broad band noise which can be current flow through diodes (as an example--Diodes are surprisingly "noisy").

Reduced emission over time... Variation in current flow (cloudy days, etc.), a poor conduit ground that somehow got "good" (bumped conduit, tightened conduit fittings, broke through paint on box hole, ran system without covers--Then buttoned after a day)... Something that was acting like a resonator or antenna (extension cord, tools stacked near inverters, etc.).

In general, when I did EMI design testing on computers, you could get within about 2 db of repeatability between setups... And frequently as much as 10 db variation between setups, labs, minor changes in wiring/emi control/etc...

What frequency range(s) you run, will have different sources.... Typically, conducted noise below 30 MHz, radiated from cables for 30-180 MHz, and >180 MHz were 1/4 Wave Length slot antenna in cases (doors, card slots, rivet/screws for enclosures, etc.).

-Bill "been a long time since I did that stuff" B.

roberthend

Guys

Thank you for your responses which are all much appreciated.  I will abandon thought of a learning MPPT system and assume some as yet unidentified cause of the RFI.

I have been told no changes were made to the electrical installation at the premises other than installation of the NM Solar system.  There had been no RFI before this event.  Perhaps the cause has somehow resolved itself.  However I will fabricate input and output common mode chokes as a precautionary measure, to have them ready should the RFI return.

Bob

BB.

Did it rain/dry during this time? (groundwater and soil conductivity changes?)

-Bill