Noise & Radio Interference in PV Systems

Reducing EMI and RFI in Solar Electric Systems

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Reducing Electronic Interference in Solar Electric Systems

This information is mainly aimed at reducing or eliminating radio, TV, cell phone, and other electronic noise and interference in photovoltaic systems and from equipment used in PV systems, but much of it applies to anything or any equipment with EMI (electromagnetic Interference) or RFI (Radio Frequency Interference).

What is EMI and RFI?

EMI and RFI are similar and often have the same causes and solutions. RFI is interference or noise that is radiated - essentially, radio waves. EMI includes RFI but also includes non-radiated interference, such as line noise coming in from power or control lines. From here on we will use only EMI, as treatments are basically the same.

EMI can come from many sources. Almost everything in your home or car emits some EMI, including fluorescent lights, TV's, cordless phones, electric tools, auto ignition, etc. In solar and DC systems you often have additional sources, such as switching power supplies, charge controllers, DC light ballasts, and inverters (especially modified sine wave types).

FCC Part B

One of the major problems with solar and DC power equipment is that almost none of it meets the standards for FCC  Section 15, Part B. Nearly all appliances and electronic equipment sold today for consumer use in homes must comply with FCC part B - which regulates the maximum amount of EMI that devices (such as TV's) can radiate. That is why you don't get a lot of noise from your microwave and coffee grinder. But nearly all DC and solar equipment is exempt from Part B. Which means that they can put out a LOT more EMI and still be legal.

Main Sources of Noise

Any digital electronic equipment produces at least some noise. And nearly all equipment now used in PV systems is digital. The most common real problem equipment is charge controllers, DC lights, and some modified sine wave inverters. Nearly all charge controllers do not send a steady voltage/current to the batteries, but are pulsed. And high power digital pulses are one of the worst EMI sources.

How to Get Rid of EMI

The most common ways of reducing noise are:

  • Shielding
  • Cancellation
  • Filtering
  • Suppression

Shielding

Almost any metal will offer some shielding. A shield basically blocks the noise, just as the name implies. Metal enclosures are common for inverters and some other equipment. But metal conduit will also act as a shield. Shielding is effective but not always possible, and will not do much to stop any noise carried on the wiring to and from the device.

Cancellation

Cancellation might not be the best term. But it is very simple to do and pretty effective in some cases. It is basically just a matter of twisted wire pairs. Noise in twisted pairs tends to cancel itself out at each twist. It does not work in all cases, but is so simple, cheap, and usually easy to do, so that is often the first method to try.

We sell some cables - mainly for such things as shunt signal wires - that are shielded twisted pairs. That type cable is very effective in keeping noise in or out of wires. But shielded twisted pair wire in larger sizes can be hard to find and very expensive.

Filtering

Filtering has been around since electronics was invented. The most common method is to use capacitors across a signal line or wire to ground to get rid of the noise. Inductors are sometimes used also, but they have some frequency limits and can also get pretty bulky and expensive. One limitation of using capacitor filters is that you usually must have a good ground nearby for one side of the capacitor. If you have long leads between what you are filtering and ground, you could even make the problem worse.

Suppression

This is relatively new and often the most effective. The most common method uses Ferrite chokes, cores, and beads. Ferrites are powdered molded metal powders cast into various shapes and sizes. Ferrites are actually ceramics with . They are usually made of powdered Iron Oxide (Fe2O3), along with Zinc, Copper, Zinc, and other metal oxides.

We sell the snap-on chokes, which can be simply opened up and snapped around the wires or cables. You can stack as many as you want and stack different types if you have severe problems. You don't need one for each wire unless you have really fat wires - they work just as well if snapped onto a pair or bundle of wires. The are non-conductive so can be used almost anywhere, including on 115 volt power lines and battery or inverter cables.