As solar photovoltaic (pv) system installations are becoming more popular, solar pv radio frequency interference (RFI) is also becoming more evident and effective RFI suppression techniques are needed to ensure the performance of electronic devices connected to or in the presence of the pv system. Grid Tie and off the grid solar systems that connect solar arrays to charge controllers, batteries and inverters often generate RFI due to electrically resonant power cables acting as “antennas” radiating RFI into other devices causing audio and video interference as well as degradation of data rates in computing devices. In many cases “noisy” switching pulse width modulation charge controllers, inverters, optimizers, and micro-inverters inject broadband RFI signals into the interconnect cables which radiate the noise to victim devices.
How to cure RFI
RFI can be either radiated through the air or conducted over a wire or cable and requires an RFI SOURCE, an RFI VICTIM and a PATH between the SOURCE and the VICTIM. The best method to stop RFI is to eliminate the generation at the RFI SOURCE. The next best solution is to eliminate, or suppress the RFI PATH or PATHS between the SOURCE and the VICTIM. The last solution is to protect the VICTIM from RFI.
RFI can be stopped by suppressing the signal from the RFI (SOURCE) “transmitter”, its PATH “antenna” AND suppressing the RFI signal on the receiving “antenna” feeding the affected device (VICTIM).
With Palomar Engineers Solar RFI Victim suppression kits, you protect the victim devices by running all input and output “antenna” wires entering the electronic device/system through ferrite suppression filters installed close to the device.
With Palomar Engineers Solar RFI Source suppression kits, you suppress the RFI generated from Source devices by running all input and output “antenna” wires entering the electronic device/system through ferrite suppression filters installed close to the device.
Ferrites do not affect the signals going through the wires but they resist the passage of common mode RFI reducing the RFI signals on the “antenna” wires and suppressing the RFI from falsely triggering the effected device. Depending upon the RFI severity, some wires may require multiple ferrite rings or beads for additional RFI suppression. In all cases multiple wire turns through the center of ferrite RFI suppressors make a more effective RFI filter due to increased choking impedance which suppresses the RFI common mode current.
All solar components should carry a FCC certification of Part A for commercial installations or Part B for residential installations. Note that devices certified for type A only WILL cause RFI under 30 MHz and these devices should be avoided in residential installations. Devices without FCC certification should also be avoided – these devices are quite common on popular internet auction sites that sell goods direct from overseas locations not required to have FCC certification and testing.
Off The Grid systems
Much of the RFI suppression for solar systems concerns choking the AC/DC wires and cables connecting each device (solar array, charge controllers, inverters, batteries). Depending on the size of cable and length, snap on, ring or slip on ferrites should be chosen with the proper inside diameter to take 1 or more turns for each cable. If possible the +/- DC cables should be run together through the ferrite so that their electric fields cancel each other, otherwise ferrite “chattering” may occur with high DC current through the ferrites.
A minimum of 3-5 snap on ferrites (or 1 single multi-turn ring ferrite) should be at the end of each cable set (see examples below) to suppress the RFI and for long lines 3-5 additional should be at the center of each line or every quarter wavelength of the interfering frequency.
Typical simple off the grid solar system with ferrite locations shown:
Click on the RFI Solution link for the ferrite type needed:
Basic rules for systems using batteries:
The major RFI generators in a solar system are the charge controller and the inverter. They must both be isolated from the input and outputs cables attached to them. This means ferrite cores on the DC input cables from the solar array to the charge controller (at the controller end) and from the controller output to the battery (at the controller end). Likewise, the DC input to the inverter (at the inverter end) and the AC inverter output (at the inverter end) must also have ferrite cores. The DC cables are run TOGETHER through the ferrite cores and the AC output (hot, neutral, ground) are run TOGETHER through the ferrite cores. Use more than one turn through the center hole if possible. The ferrites selected should be effective at the switching frequencies of the charge controller or inverter NOT the harmonic frequency the interference symptom is heard or seen.
If you have multiple charge controllers controlled by a remote control device, then the remote control device control cables to each charge controller will also need to have snap on ferrites (choose a large hole diameter to have more than 1 turn if possible). Likewise if you have multiple battery banks and multiple inverters, ALL cables entering/exiting from these devices will need ferrite cores.
For snap on core installation use the FSB31-1 shown below:
Keep the DC leads from the charge controller to the battery as short as possible.
Keep the DC leads from the battery to the inverter as short as practicable.
Twist the DC leads together if possible. If not possible, keep them as close together as you can. The goal is to have the RFI magnetic energy from each lead cancel the RFI magnetic energy in the other. As was also pointed out, it may be helpful to run each DC leg in metal conduit and then GROUND the conduit to an earth ground – the shorter the better. Failure to ground the conduit will simply turn the conduit into another antenna. An RFI ground is separate from the earth “protective” ground. If you use the AC “ground”, it too becomes an antenna unless it’s kept short, and you’ve got a good connection to the grounding electrode conductor with highly conductive earth. It’s tough to achieve all three together, but it can be done in some locations.
Filter capacitors connected across the DC leads won’t hurt, but they are likely to be effective given the very low impedance of inverter input circuits.
Ferrite cores(slip on or snap on) may be slipped over the length of each cable (all wires in the cable go through the same ferrite core), and placed at the point where the AC cables exit the inverter and also where the DC cable enter the inverter (to prevent RFI generated in the inverter from back feeding into the battery lines). Toroid ring cores are very useful if you can get more than one turn of the AC cables through the center. The F240 rings with 1.4″ inside diameter can accommodate multiple turns (remember the RFI choking effectiveness is a function of the square of the number of turns – e.g. 2 ring turns = 4 snap on ferrites, 3 ring turns = 9 snap on ferrites). For single turn snap on or slip ferrite beads, you will need 6-12 inches of ferrite for each cable entering and exiting the inverter or one F240 of F400 (3″ hole) ring ferrite with 3-4 turns on the AC lines.
Here is a picture of ring installations:
Do not install ferrites at the battery end. Installing at the battery end, and leaving some cable exposed at the inverter allows the exposed conductors at the inverter to act as antennas.
Select the proper type of ferrite. Surprisingly, various formulations of ferrite react differently depending on the frequency range in which they’re used. Mix 31 comes in various shapes and also rings that are up to 3 inches in side diameter. This is the mix we recommend for RFI suppression from 1-300 Mhz. Use Mix 61 for 200-1000 Mhz.
AC EMI/RFI filters may also be helpful and may be installed on the AC output circuit at the inverter. These are made by Corcom, Tyco, and others. Select a unit rated for the output voltage AND current of the inverter. RFI filters will be UL/ETL/CSA recognized. If you find some that aren’t – don’t buy them.
Ground the inverter housing in accordance with the manufacturer’s instructions. All inverters today are required to meet certain levels of FCC interference criteria. Actions of internal RFI filtering circuits may be improved if the inverter is properly grounded.
For persistant RFI, you may have to construct a screen around the entire inverter, then connect the screen itself to earth ground. This screen should NOT come into contact with the inverter housing. To do so would defeat the purpose of the screen. However, properly filtered DC and AC leads may pass through it.
The screen cage is called a “Faraday shield”, which will keep interference inside. The cage can be constructed of ferrous or non-ferrous metal. Chicken wire or small mesh construction wire can be used. Build a “box” around the inverter, including the back of the inverter and keep the inverter enclosure from contacting the DC or AC cables. Connect the cage to its own “RFI” earth ground. This will be similar to a standard protective ground and is often a separate ground rod.
Grid Tie Systems
Ferrite cores (snap on) are snapped over the two (+/-) cables from the solar array module and the two wires hooked to the “string” connecting the micro-inverters together. Suppressing RFI on all wires in and out of the micro-inverters helps to reduce any broadband RFI “noise” generated in the SOURCE switching circuits (typically at 50-200KHz and harmonics thereof). Two ferrite cores are required for each micro-inverter/PV array – one ring for the PV DC cables and a snap on for the micro-inverter cable to the string bus – size to fit the trunk cable with 1 or 2 turns.
Application Note for Enphase M190/M215/M240/M250 series Micro-inverters (and similar other brands). For each PV array/micro-inverter use a RFI-PV-MI filter kit with a ring (ID=1.4″/36MM) ferrite for installation on PV module DC cables to micro-inverter and a snap on (ID=.4″/10mm) filter for the data/trunk cable output of the micro-inverter. At the connection of each junction box and the Engage cable install a RFI-PV-JB filter (ID=1″/26MM) with multiple turns through the center.
|Palomar Part #/Desc|
|RFI-PV-MI (1 ring+1 snap on for each PV Module/micro-inverter)|
|RFI-PV-JB (1 snap on per junction box)|
Example configuration: Site has 45 PV arrays in 3 strings of 15 arrays and 45 M215 micro-inverters and 3 junction boxes (one for each string).
Filters required are 45 RFI-PV-MI x $21 + 3 RFI-PV-JB x $19.50 = $1003.50 with free shipping in USA
APS YC500A microinverters with dual PV array cables require 2 RFI-PV-MI per microinverter (4 input/output cables)
Above examples are also applicable to ENECSYS, Aurora, Sunpower, Solaredge and many other manufactures with similar cable configurations.
Power Optimizer RFI
Ferrite cores (snap on) are snapped over the two (+/-) cables from the solar array module and the two wires hooked to the “string” connecting the optimizers together. Suppressing RFI on all wires in and out of the optimizers helps to reduce any broadband RFI “noise” generated in the SOURCE switching circuits (typically at 20-500 Khz and harmonics thereof). Two ferrite filters are required for each optimizer – we recommended Part # PV-RFI-PO (includes two ring filters). See typical installation diagram below:
|Palomar Part #/Desc|
|RFI-PV-PO (2 rings per power optimizer)|
|RFI-PV-JB (1 snap on per junction box)|
All strings entering the inverter (perhaps through one or more junction boxes should have a ferrite snap on Part# PV-RFI-JB, with a 1″ hole with two turns of the cable through the center hole). AC output should be in RFI metal shielded conduit or also run through PV-RFI-JB snap ons every 10 linear feet to suppress resonant “antenna” RFI.
All strings entering the combiner box should have a ferrite snap on Part # PV-RFI-JB with 1″ hole recommended with two turns. String lines exiting combiner box should a PV-RFI-JB ferrite snap on every 10 linear feet to suppress resonant “antenna” RFI or be contained in RFI proof metal conduit (grounded if needed).
String Cable RFI
Long string cables should have an RFI suppression ferrite every 10 feet with Part # PV-RFI-JB
Click on the RFI Solution link for the additional site specific ferrite type needed: