It's all about the harmonics

9 May 2021 | Research

By Chuck Newcombe

My first column in Fluke News Plus was about the tricks one might use to detect troublesome harmonic currents in neutral wiring in industrial and commercial buildings, using only a multimeter.

Big Guns

It now occurs to me that as utilities reclaim capacity by encouraging energy conservation, the many electronic energy saving devices they promote may eventually cause problems for their distribution systems. I say this because of something that happened at a small utility in Florida back in the mid-1990s.

Fluke had just introduced its first power harmonics analyzer - the model 41. A bright engineer for the Florida utility became what we call an "early adopter," because he suspected that harmonic currents were being fed back into his system somehow from one or more customers.

The problem was that a substation's ground fault relay was tripping regularly, yet the crews could detect no faults on their lines. When the current to the relay was measured using the model 41, the engineer discovered that most of the troublesome current was at a frequency of 180 Hz, the third harmonic of the 60 Hz distribution.

The engineer instituted a test program for his linemen using the Fluke harmonic analyzer to check the neutral current on the three phase feeds to their major customers. The problem turned out not to be their largest customer, an ice cream manufacturer. Rather, it was a golf course - that's right - a golf course. Further investigation determined that the problem was all the single phase golf cart battery chargers that were using energy in pulses because of the rectifiers in the chargers. We call those "non-linear loads."

In theory, the rectifiers distributed evenly on three phases should present a balanced load to the system with little or no neutral current - except for something called "Triplens." The third harmonic, and its odd multiples, in the current of each phase will add together as zero sequence currents in the neutral of a four wire three phase system - even if the system is perfectly balanced. In this case, it reached the threshold on the ground fault relay trip circuit.

A typical utility distribution circuit from substations to customers

The overhead line passing in front of my house is single phase - that is, only one of the three phases on the nearby boulevard is tapped to serve the homes on our loop. There is a "pole-ground" from the neutral at the pole serving my home, and others on selected poles down the street. This, in effect, allows current returning on the neutral, ultimately to the substation, to split between the resistance of the neutral conductor and a parallel ground path. So, theory says I should be able to measure a voltage drop due to the resistance of the earth between rods driven into the earth under the lines.

I put it to the test. Using the ground rods and an 80 ft roll of wire from a Fluke 1625 Earth Ground Test Kit, along with my trusty multimeter, I measured the ac voltage between two ground rods - one next to the pole with a pole-ground, the other 80 feet away under the line. I read about 56 mV, and the frequency was as expected - 60 Hz. There's nothing particularly remarkable about that.

The interesting part of the story came from my next test.

About a mile from our house on the boulevard, our three phase feed to the west joins a similar feed to the east, as they share poles another mile up a branch street to the substation. I parked near the substation at a pole with a pole ground and set up my test, only now, I was under two three phase circuits that ultimately serve customers in neighborhoods over a six or seven mile range.

The voltage I read was now considerably higher - 675 mV. And, the frequency was not 60 Hz, but rather the 180 Hz that I discussed above. The reason is that the three phase distribution circuits are experiencing "triplen" zero sequence currents on their shared neutral returns greater than the fundamental unbalance. Our utility is indeed seeing the results of all the harmonic producing electronics and energy saving devices that they are encouraging us to use to save energy.

Is it a problem for the utility? Probably not, unless they run into the situation that the Florida engineer experienced many years ago. In that case, they may have to readjust their fault trip currents at some point in the future. And that brings a thought to mind - what will happen as rechargeable electric cars become more popular.

The power thief in the night

The motors in our refrigerators and other appliances are linear in nature, as are the electric heaters and incandescent lightbulbs. This means that they don't produce harmonic currents. But, the new energy saving compact fluorescent lightbulbs, and the many electronic devices such as our tv sets and home theater systems are harmonic producing non-linear loads. And, many of them are drawing current, even though we have them turned off. It's a sure bet if you can turn it on and off from a remote - part of the circuit has to be live at all times to take your command.

The little ‘wall-warts' (the plug-in power supplies for gaming consoles and laptop computers) are also always on, so there is no shortage of energy guzzling, and potentially harmonic producing, loads in the average home.

So, if you want to save a few bucks, and help your utility's conservation efforts even more, consider unplugging them when not in use.

We can pretty safely conclude that power harmonics are now a part of our lives. Luckily, they are not likely to be a major problem to individual homeowners.

For those of you interested in how the utility grid is being changed, in part to handle these devices, read our new Smart Grid whitepaper.