Among the responsibilities of numerous working groups and task forces of the Power Quality Subcommittee under the auspices of the Transmission and Distribution Committee of the Power Engineering Society of the Institute of Electrical and Electronic Engineers (or more simply, the IEEE PES T&D PQSC) is a standard called IEEE Std 1346 Electrical Power System Compatibility with Electronic Process Equipment.
Unfortunately, this working group has not met in a number of years for a variety of reasons. First, the co-chairs are no longer active in the IEEE, having become victims of the restructuring and realignment of electric utilities that have reduced the number of PQ engineers that most utilities have undertaken. Second, the emphasis has shifted to matching the IEC standards, from flicker to harmonics to sags, rather than being concerned about what I think really matters most in power quality—can the equipment operate properly with the electricity that is supplied to it?
The purpose of the document summarizes what seems to me should be the driving force for power quality standards, if just one word was dropped from the first sentence to make it more encompassing:
“The purpose of this document is to recommend a standard methodology for the technical and economic analysis of compatibility of process equipment with the electric power system.
“This document does not intend to set performance limits for utility systems, power distribution systems, or electronic process equipment. Rather, it shows how the performance data for each of these entities can be analyzed to evaluate their compatibly in economic terms. The recommended methodology also provides standardization of methods, data, and performance of power systems and equipment in evaluating compatibility so that compatibility can be discussed from a common frame of reference.” (http://grouper.ieee.org/groups/1346/scope.html)
Whereas the focus of the document initially was process equipment, the concept is applicable to all equipment that is powered from electricity. It is like many other concepts in business—risk assessment and economic tradeoffs.
Cars could be made that would reduce vehicle-related fatalities and injuries significantly. But there is a cost associated with such that instead has set the standards (including the federally mandated ones) at a somewhat lower safety level that says, “this amount of risk is acceptable for this price of use.”
The document doesn’t try to set hard limits for the different power quality phenomena, such as IEEE 519 does for harmonics. In the real world, there are too many factors involved to say if the quality is above value X, then the process will run smoothly or, conversely, below that absolute value, and everyone is in trouble.
Different equipment has different susceptibilities. Different building wiring results in different disturbance levels from the same event. And sometimes, different members of the standards-forming committees come with different objectives. Though the consent process tries to be fair to all who would be impacted by such standards, the reality of human nature is that people tend to look out for themselves and their interests.
A good example of this is a recent power quality standard for a South American country. They have allowed the use of the concept of a sliding voltage reference, rather than a fixed nominal voltage, from which to determine whether a sag or swell has occurred. If the electrical grid is suffering from the effects of heavy loading, such as on a hot summer day, the voltage will tend to decrease as the current and the line losses increase.
Whereas a typical limit for a sag would be 90 percent of the nominal voltage, or 108V for a 120V circuit, the sliding reference concept allows the computation of changing thresholds over time; in this case, more than a one minute filtered value would be calculated. Though the user’s equipment will trip offline or have a malfunction at a fixed voltage level, let us say 100V in this example, the utility would not have to be held accountable for a sag to 99V if the sliding reference had drifted down over the course of the day to 109V, since 90 percent of that value is 98V.
So while one neglected standard worked to see that the electricity supplied and the equipment needs could come to a sort of economic equilibrium, other new standards work to set limits that would put more of the burden on one side of the economic equation only. But since most of us don’t have the opportunity to influence such standards, next month I will focus on how this neglected standard and similar methods would be helpful in solving day-to-day power quality problems and saving the consumers of electricity significant money through taking economically appropriate mitigation steps. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.