Keep in mind that the MPPT solar regulators/inverters are essentially a pulse width modulation supply. If the design engineer didn't properly design for RFI suppression, fixing it in the field can be challenging.
When you are trying to cure RFI problems, simply clipping on a ferrite or two may have limited results.
1.) Use a ferrite material that is suitable for the RFI frequencies involved. If it is an unknown ferrite material, expect unknown results. Sometimes two mixes of ferrite materials need to be applied to the leads to obtain sufficiently broad bandwidth coverage.
2.) Clip on ferrites only offer 20 -70 ohms of impedance to the common mode currents when the wires pass only once through the hole in the core. Wrap the pairs of wires 6-10 times through the clip on or use a suitably sized torroid form. With 10 turns, you get closer to a 2000 - 5000 ohm impedance to work against the common mode currents. It is a turns2 effect that is as effective as adding 100 clip on ferrites.
3.) Install the RFI chokes as close as possible to the offending device. Try putting RFI chokes on both ends of the cable.
4.) Use RFI chokes on all cables entering or leaving the offending device regardless of AC, DC, control, or network signals.
5.) Not all interference is common mode. You may also have differential RFI. This requires the installation of grounded brute force type filters. See an earlier post of mine in this section for an example.
6.) Some people report limited success by adding a Faraday cage around the device. If you try this, make certain that the RFI chokes are located inside the cage. The cage must completely enclose the device without significantly impeding airflow. That can make it a challenging construction project with the potential for limited results. In balance, this option should only be considered as a last resort and be prepared to be disappointed.
- Glenn W9IQ