What are the sources of noise for electricity in a power grid?

The noise sources can be divided into three groups:

• The utility: high-power converters, bringing resources into line and off
• The transmission: switching equipment going on and off, lightning, solar flares
• The users: million of loads turned on (inrush current) and off (kickback) all the time (especially inductive loads), arcs in brushed motors, switch-mode power supplies.

You have good answers from others. As I’m likely a bit older than many of the others, I’ll add a little historical perspective.

Back around 1970 or so, the AC power in the USA looked a lot more like a sine wave than it does today. That was before there were computers, variable frequency drives, switch mode power supplies, and other sophisticated electronic goodies connected to the grid. Over time these devices have appeared everywhere literally by the millions. Generally speaking, such devices are both victims of and sources of power line noise. Manufacturers do a pretty good job of “hardening” their own products against the noise, and a pretty bad job of preventing their products from transmitting noise back to the grid. Indeed there are regulations which limit this noise pollution, but even the allowed amounts become significant when multiplied by the millions of devices. Generally the higher the power switched by a device, the higher the electronic noise it produces. It’s almost impossible today to find AC power as clean as it was in the 1970s.

Here are a couple of personal examples of power line problems caused by consumer electronics.

First, about 25 years ago we had a house in a California suburb. It was on a cul-de-sac with four other houses, and a utility pole with a transformer that fed the five houses. I had set up our house with a 1970s-era home control system called X10. It was quite reliable in the good old days, but became less so as switch-mode power supplies crept into houses. One day our outdoor floodlights, which were controlled via X10, started switching on and off seemingly randomly. I spent hours trying to diagnose and solve this problem. Finally, I discovered that a kid in one of the other houses on our utility transformer had purchased a rechargeable electric toothbrush. When he picked the toothbrush up out of the charging stand all our floodlights would switch on. When he set the toothbrush back in the charger all of our floodlights would switch off. I tried everything I could think of to remedy this, with no success. I finally ripped out all the X10 devices in the house and replaced them with Insteon. Problem solved. (X10 is still on the market, but I don’t know anyone who uses it. Insteon went out of business about four years ago. I now use Apple HomeKit, which works fine and someday may not.)

Second, I have an aircraft communication band receiver in my office. (I’m a pilot so I’m into such things.) VHF aircraft comms is still analog AM, simple and reliable but not immune to noise. A few years ago we bought a new Miele washing machine. It’s fairly sophisticated by washing machine standards, with a variable frequency drive for the wash drum. I could hear a noise on my comms radio, varying in frequency as the wash drum changed speed. The washer and radio were supplied by the house power. With a little testing I confirmed that the noise came from the VFD in the washer, into the house power wiring, and through the radio’s wall wart power supply. I fixed it by building a new power supply with good noise filtration for the radio. But now…the radio is picking up noise from the power supply for LED lights in my office! When will this ever end?

Christophe, I wish you the best for overcoming your power supply noise challenges!

Maurice.