In the past, the harmonic frequencies we were most concerned with went up to the 50th at 60 hertz (Hz), which is 3,000 Hz, or the 40th at 50 Hz, which is 2,000 Hz. The PQ standards, such as IEEE 519 and IEC 61000-4-7, supported that range. There was mention of the 2–9 kilohertz (kHz) range, but there was not much interest in measuring them or setting limits. Now, the standards groups are considering frequencies up to 150 kHz, which would be the 2,500th harmonic. Why the gigantic leap, and what does this mean for PQ monitor instruments used for troubleshooting and benchmark audits?
First, a bit of history. The biggest contributors to harmonics used to be three-phase, full-wave rectified power supplies used for the power conversion of adjustable-speed drives or uninterruptible power supplies as well as single-phase, full-wave rectified power supplies used for IT equipment, such as PCs, printers, modems and faxes. The former were generally higher power devices, so the harmonic currents they created (5th, 7th, 11th, 13th, 17th, 19th) were larger than those lower powered IT equipment (3rd, 5th, 7th, 9th). Harmonic currents from the former were rarely above the 19th.
If any significant amplitudes were created, the current’s varying phase angles from changing loads would cancel out the signals as they summed together from multiple sources.
To accurately determine the harmonics, most PQ instruments utilize an analog-to-digital converter that must sample at least twice as fast as the highest frequency of interest, as stated in the Nyquist (or sampling) Theorem. For the 50th harmonics at 60 Hz (3,000 Hz), that would be 6,000 samples/cycle. Take that to 150 kHz, and it would require sampling engines to run 25 times faster. Speed doesn’t just kill; it costs money. It also costs more memory, as the methodology used to compute harmonics and interharmonics in IEEE 519 and IEC 61000-4-7 requires nearly 15,000 “bins” to store the data values used in the computations. Guess what 150 kHz would require? Instead, they proposed to change the methodology, which would require the instruments to have different methods for different signals. Complexity, besides confusing, also costs.
The standards changed the bins to 200 Hz, adding more complexity for the few instances where it was relevant.
Meanwhile, the designers of rectified input switch mode power supplies knew about the tight limits at the lower order harmonic ranges in the standards. They also knew they could reduce the physical size (and cost) of some of the power supplies’ components by operating at higher frequencies. As a result, significant power levels appeared above the standard harmonic range, which has limits imposed. In short, they pushed the problem above where it is regulated, sort of like driving at 1,000 mph to fool the police officer’s radar gun.
Sources associated with supraharmonics have been documented. Some documents refer to signals in the 9-to-150-kHz range. These sources included some of the newer industrial-size converters/inverters, such as those used in photovoltaic panels, street lamps, electric vehicle chargers, household devices and power line communication used in automatic metering reading systems. Occasional misoperation of LED lighting has been traced to these higher order signals. The filter capacitors used in nearly all electronic equipment to reduce their emissions below the present-day limits and reduce their susceptibility to these higher order signals are generally not designed to absorb energy in this higher range. Exposure to significant energy can reduce the life expectancy of the capacitors and, therefore, the equipment itself.
What does this mean for the typical PQ benchmark audit or troubleshooting application? We will ignore the aforementioned increased cost for instruments to measure this specialized area. Many of the rules of thumb that we used don’t really apply here, such as the summation of the triplen harmonics in the neutral conductor and eddy current losses in transformers and electric motors. Many people who investigate PQ problems have difficulty understanding how changes in harmonic impedance over the traditional lower frequency range results in varying harmonic power. It may be a more prudent approach for the standards groups and research institutes to get more facts about supraharmonics and learn when to look for these higher order harmonics as the culprit, rather than add more burden on the investigators.