After what seemed like an eternity for those of us working on the various power quality working groups and task forces of the Institute of Electrical and Electronics Engineers (IEEE) Power and Energy Society, a number of standards have been completed. These new standards will affect how and why we measure power quality phenomena or the associated parameters.
IEEE 519 1992, Recommended Practices for Harmonic Control in Electric Power Systems, had probably the longest gestation of any standard. It finally cleared the hurdles with significant revisions. Unlike the International Electrotechnical Commission (IEC) standards, which separate how to measure a particular parameter from what the limits should be, IEEE 519 has combined both. However, the measurement process is now harmonized with the long established IEC 61000-4-7, and the limits coordinate with the IEC 61000-3-6.
The new IEEE 519 document is still a recommended practice, as compared to the “thou shall” standards of the IEC. This includes the measurement and calculation of harmonics and interharmonics over either 10, 50-Hertz (Hz) cycles or 12, 60-Hz cycles (both equal 200 milliseconds in duration). The harmonic magnitudes are grouped by combining the magnitudes at the harmonic frequency along with the adjacent 5-Hz bins that result from the Fast Fourier Transform used. All of the other 5-Hz bins values are combined into the associated interharmonic.
Since the effect of harmonics is generally one of heating, having a high harmonic value for one cycle is usually meaningless, except if it causes multiple zero crossings of the voltage waveform and disrupts clock circuits. Instead, a short-duration harmonic value computed over 3 seconds (versus only the previous long term values over 10 minutes) and statistical values for each at the 95 and 99 percentile were added. This translates into the maximum value that can be expected 95 percent and 99 percent of the time—or only 5 percent and 1 percent of the time—will there be a value greater than those computed numbers.
The word “misuse” is apt for how some people try to apply IEEE 519. It is not a standard for harmonic emission limits for individual equipment or devices, such as the IEC 61000-3-2 and IEC 61000-3-12 standards. The IEEE has tried to create such a standard, but the various competing interested parties (utilities versus manufacturers) failed to reach a consensus. While it is usually the equipment within a facility that creates the harmonic currents measured at the point of common coupling where IEEE 519 places limits, it is not intended nor should it be applied to any individual piece of equipment. Since most manufacturers sell in a global market, compliance with the IEC standards is a requirement anyway.
Future revisions of the IEEE and IEC standards with regard to harmonics will likely address two areas of increasing concern. The average level of harmonic-voltage distortion is approaching and, in a number of cases, exceeding the recommended or mandatory limits on electric distribution systems. Though there aren’t reports of the catastrophic problems that some people predicted, some utilities are concerned that it will degrade the life expectancy of transformers and other equipment that are typically expected to last 30 or more years. Also, there is a reported increase in signals above the 2- or 3-kilohertz (kHz) measuring limit of the IEC and IEEE, respectively. Power converters operating between 2 and 150 kHz are blamed as the source of problems, and parts of Asia report the destruction of power supplies in electronic equipment as a result of high levels of harmonics. Hopefully, the standards groups will agree on how to measure harmonics before they impose limits.
IEEE 1564 Recommended Practice for the Establishment of Voltage Sag Indices is another recently published standard. It does not specify which indices should be used to measure performance of an electrical distribution (or transmission) system with regard to sags or dips. Rather, it tells how to calculate them consistently, so the performance of one site or system can be compared with another. Indices for single events, for a specific site or across an entire system are detailed, including the well-established SARFI-x, where the value is the System Average RMS-Variation Frequency Index that provides a count or rate of voltage sags, swells and/or interruptions for a system for number of sags below x percent of nominal. Other calculations include the voltage-sag energy index and voltage-sag severity (such as used in the SEMI F47 and ITIC curves) for single- and three-phase systems.
Soon, there should also be a new revision of the IEEE 1453 and 1453.1 standards, covering the measurement and limits for voltage fluctuations that result in light flicker. An application guide, IEEE 519A, will be released to help IEEE 519 users. Work is also progressing on other power-quality-related topics. As the chair of the IEEE PES Power Quality Subcommittee, I would appreciate any feedback on other areas that you feel should be addressed. You can contact me at firstname.lastname@example.org.