Over the years, I have been asked whether or not PV systems emit significant levels of electromagnetic radiation or in industry parlance, electromagnetic interference or ‘EMI’. EMI is also called radio frequency interference or RFI.
Many electronic devices emit some levels of electromagnetic radiation; the amount and type is regulated by the FCC (Federal Communications Commission). One of the main sources of this radiation is from the power supplies incorporated into most electronic devices which convert AC, from the wall outlet, to DC voltage useable by the devices themselves. The main process by which this conversion occurs is called Pulse Width Modulation, an industry term for chopping current rapidly at variable rates. A by-product of this current chopping is that some of the energy gets radiated. This phenomenon is the means by which radio antennas broadcast radio waves.
The concerns expressed come typically from ham radio operators or people who have a sensitivity to EMI or are concerned that such radiation might cause harm.
Whether or not a PV system generates significant levels of EMI depends on a number of factors, the major ones being: quality of the inverter, use of metal conduit and proper grounding of all system components.
We recently had one of our systems tested by a potential client who, before making the investment in a solar system, wanted to be sure it would not create significant radio frequency interference with his ham radio operation. The system he tested is a 20kW SunPower ground mounted array with two SunPower inverters mounted on the house wall next to the electrical service.
After the testing was complete I asked a number of question to which the tester responded as follows:
1. Why are you concerned about RFI when considering a PV system?
I have been involved in “ham” radio for many years. Because operation in the High Frequency (HF) employs a very sensitive receiver, weak signals can be blocked by locally generated Radio Frequency Interference (RFI). Inverters used in some older systems have been significant generators of RFI. The U. S. Federal Communications Commission (FCC) sets the standards for how much electrical noise may be emitted from any electronic device. These regulated limits are not met in some equipment and the FCC has been slow to block the sale of products from non-compliant suppliers, so testing is the most expedient alternative.
2. What instrument did you use to test for RFI?
Full emissions testing is complex and expensive and must be done in a specially equipped laboratory setting. The easiest quick check is to use a portable communications radio and portable resonant antenna to monitor the frequency bands of interest.
3. What levels are you testing for and at what distance?
The distance from the inverter being tested to the communications antenna is a significant factor because any radiated interfering signal will drop off very rapidly with distance. In my tests, I was looking for the power level of the interference to be less than -11dBm (1.0 microvolt) using a resonant monopole receiving antenna within 20 feet distance from the inverter, solar panels, or interconnecting cables. The critical frequency bands tested were in the 7 MHz, 14 MHz, and 21 MHz bands.
4. Based on your tests, what do you have to say about the amount of RFI coming out of the PV system?
I was pleased that the site tested showed very low levels of RFI from the inverter and none detected from the cable path or the panels. No levels were detected in the frequency bands tested above -110dBm even within one foot distance to the inverter. This was a significantly lower level than was expected. By comparison, the old Trace inverter installed here in 1999 emits RFI at my threshold level at about 20 feet separation. The new inverters should not be a problem.
5. Was your testing comprehensive, i.e. did you look at every part of the PV system?
While I tested in the proximity of the panels, the cable path to the inverters and the inverters themselves, the testing at other frequencies was not done. I was interested in the RFI levels in the heart of the HF (shortwave) bands. I would expect that the RFI performance in other regions would be similarly good.
6. Was there any part that emitted more radiation than the others?
The only interference heard was very close to the inverter itself.
7. In your opinion, is the level of radiation of concern to home owners for one reason or another?
I would conclude that the inverter system tested is exceptionally clean relative to RFI in the HF bands tested. The RFI concerns are only relative to closely located sensitive communications receivers. The levels we are looking for are extremely low, a very small fraction of the levels generally everywhere in the environment.
Based on these findings in this limited test, I am very pleased to see that our system emits negligible electromagnetic radiation.