Having just come back from two weeks of working with different standards-making organizations, the NFPA and the Institute of Electrical and Electronics Engineers (IEEE), it seemed appropriate to share the good news with those who aren’t able to participate. Several important standards in the area of power quality will finally hit the public domain from the IEEE, covering power quality monitoring, limits on harmonics and interharmonics, an application guide for harmonic limits and voltage sag indices. All of these documents have either been recently balloted or are about to be balloted. Since they are not published yet, we cannot publish excerpts verbatim; however, there are some significant changes that will set the stage for the coming feature attractions.
The IEEE Std. 1159, Recommended Practice for Electric Power Quality Monitoring, has been unchanged since its origin in 1995. The power quality industry has obviously changed significantly since then in the monitoring equipment, the mitigation equipment, the susceptibility of equipment to power quality phenomena, and the contribution of equipment to power quality phenomena. The industry also has changed in those whose responsibility it is to deal with such. Electricians are forced to become more educated about these things, as the pressures from the global economy have contributed to a push for increasing productivity and reliability.
To this end, the new 1159 will provide significant new material on the phenomena, the instruments used to monitor it and how to safely monitor and interpret the data and information that was recorded—a more complete what, where, how and why of power quality monitoring. The often-cited Table 4.2 of the Categories and Typical Characteristics of Power System Electromagnetic Phenomena, which has been enhanced slightly, was one of the most significant contributions of the first edition of the document as it defined most of the power quality phenomena with a single name.
IEEE Std. 519, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, has been around longer than Std. 1159 and is one of the few power quality documents in the United States with limits. It has recommendations for steady-state harmonic and interharmonic values. A number of utilities and Public Utility Commissions have adopted the limits set out in the document.
Unfortunately, it is one of the most misapplied standards. The new version tries to eliminate some of the confusion that results from people applying the limits to very short duration harmonic values as well as to individual equipment, rather than steady state values at the point of common coupling. The measurement methods will be harmonized with the International Electrotechnical Commission methods, using the 200-millisecond window of 12 cycles at 60 Hz rolled into 3-second values and then into 10-minute values, along with utilization of statistical methods to compute the 95 and 99 percentile values to compare against the limits. This also is one of the few documents that gives voltage and current equal billing.
In seminars I taught in the mid-1990s, I spoke of the soon-to-be-forthcoming application guide for 519. Unfortunately, that has been going on for more than a decade, though IEEE Std. 519.1, Guide for Applying Harmonic Limits on Power Systems, is getting very close to press. As its purpose describes, it illustrates and provides examples of how to compute the key parameters used in the limit calculations, namely average maximum demand load current, short circuit ratio and total demand distortion. It also emphasizes how and where to apply the 519 standard and helps evaluate harmonic levels for existing and new customers as well as methods for harmonic control at the customer and utility system.
The final document waiting in the wings for its debut is IEEE P1564, Guide for the Establishment of Voltage Sag Indices. Another document with a significant gestation cycle, it has been generalized as a guide to include a number of methods to compute voltage sag indices. Sags remain the most common power quality disturbance at the majority of sites, so having an agreed-on method for counting them would seem almost elementary in its need, especially when legal matters arise from such. A single countable event can consist of several rms variations below the 90 percent of nominal limit if temporal aggregation is used, since the recloser operation during a fault condition can take several attempts to clear the fault. Rather than recommend a particular method, the document “assists in the choice of index and ensuring reproducibility of the results after a certain index has been chosen.”
All these documents will help further the understanding of power quality phenomena and how to prevent them from disrupting the processes that rely on a quality of electric supply that is compatible with the susceptibilities of the equipment (another IEEE 1346 standard that is the next revision to be tackled by the members). We’ll keep you posted.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.