While investigating a mysterious AFCI breaker tripping and a flickering light, we considered a few possible causes but eventually found the culprit: a defective bulb. Another potential source of the problem was forgotten until a separate investigation brought it back the forefront. The source? Supraharmonics.

When I entered into the world of power quality and the instruments to measure it back in the late 1970s, harmonics were becoming worth considering as a potential source of power quality problems. Early instruments used discrete filters to measure specific harmonic frequencies, such as 180 Hz for the 3rd harmonic for a 60-Hz fundamental, 300 Hz for the 5th and so on. It soon became apparent that this wouldn’t be a practical solution, as there were too many harmonic frequencies to consider for the voltage and current signals.

Using a mathematical process called the Fast Fourier Transform gained traction as instruments’ microprocessors became more powerful, especially the specialized digital signal processors (DSPs). DSPs were designed to efficiently perform the needed iterative multiplication steps.

Higher switching rates

With single-phase linear loads, such as the power supplies for most information technology equipment (computers, printers, modems, etc), calculating and monitoring the odd harmonics (3rd, 5th, 7th, 9th, etc.) up to the 19th was good enough in most applications. For three-phase applications, such as adjustable-speed drives, the switching frequency of the power supplies began to get faster, requiring higher-order harmonics to be included. 

For many years, the 50th harmonic was the de facto limit, being 3 kHz for 60-Hz electrical systems and 2 kHz for 50-Hz systems. As harmonic measurement and limit standards were developed, these limits were set in stone—that is, until the technology began to advance to higher switching rates.

Higher switching rates were used in power supplies because they reduced the size of the components and magnitude of the resulting harmonic currents. However, with the dominant electric load shifting from linear to nonlinear loads, harmonic distortion was found to be the cause of PQ problems, such as overheated neutral conductors and transformers, damage to motor windings and bearings, communication errors and false trips of protection devices. 

Signals in the 2–9 kHz frequency range and intraharmonics were measured. Intraharmonics are signals with frequencies between harmonic frequencies, usually caused by asynchronous conditions. This required instruments to compute 12 times more frequency bins.

Even higher frequency signals 

In the second decade of the 21st century, a few papers discussed even higher frequency signals. Interference from electric vehicle charging equipment (some with 50-kHz switching or chopping frequencies) was measured on the grid, as well as distributed energy resources (photovoltaic systems, wind turbines and battery storage systems). A CNC machine was found to emit signals affecting kitchen appliances, and solar panel farms caused portable phones and touch-dimmer lights to malfunction. 

This raised the bar of the frequencies of interest up to 150 kHz, and set the groups in motion to propose limits on supraharmonics, with phrases such as “levels having logarithmic drop-off with logarithmically increasing frequency.” Once again, we get limits before we agree on how to measure them.

In the previous article on the AFCI tripping mystery, the concept of Nyquist sampling theory was briefly reviewed, revealing that the instrument used in the investigation may not have had a fast enough analog-to-­digital conversion or sampling rate to accurately measure the defective bulb’s signals that caused the AFCI’s arc-detection algorithm to falsely trip. It turns out that pictures of the waveform distortion of supraharmonics looked very similar to those recorded from the defective bulb, except there was much more detail and a higher sampling rate.

An EV disruption

This brings us back to the beginning of this article and the quickly dismissed theory that the culprit was some source of interference from the neighborhood. With an auto body shop a block away, could it have been an arc welder? Since the residence was in the heart of Tesla territory, could a significant number of EVs charging after work result in interference as reported in one of the aforementioned studies?

Though some in the PQ industry dismiss supraharmonics as more of an academic study than a concern for electric utilities and their customers, how would we know if a substation protection system operates incorrectly and creates a blackout because of a condition no one is monitoring? 

Consider what the MRIs and CT scans did for imaging inside the body compared to X-rays. As the saying goes, just because you don’t see it with today’s tools doesn’t mean it’s not out there.

stock.adobe.com / Malchev