Part 8 Standardizing Reliability and Power Quality Metrics

Published by

  • John D. Kueck and Brendan J. Kirby, Oak Ridge National Laboratory
  • Philip N. Overholt, U.S. Department of Energy
  • Lawrence C. Markel, Sentech, Inc.

Published in Measurement Practices for Reliability and Power Quality: A Toolkit of Reliability Measurement Practices, 2004

Prepared by Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6285 managed by UT-BATTELLE, LLC for the U.S. Department of Energy under contract DE-AC05-00OR22725

Utilities and/or distribution companies are able to provide only a level of reliability and power quality commensurate with the designs of their distribution systems. These levels vary greatly among locations and among distribution system designs. Underground network systems with several redundant primary feeders like those used in major metropolitan areas can provide very high levels of reliability and power quality, while rural distribution systems with long overhead radial feeders often experience voltage sags. In some cases, feeders for industrial plants come directly from transmission substations and have an inherently higher level of reliability and power quality.

These design features and arrangements are not possible in every location. Assumptions regarding the level of reliability and power quality should not be developed in a vacuum, without considering constraints on the local utility. A number of utility measures can be taken to minimize interruptions. IEEE Std 1250-1995 provides two sets of measures, design measures and operating measures:

Design measuresOperating measures
General circuit layout and construction
Circuit exposure, length, and type
Protective coordination
Fault sectionalizing
Grounding considerations
Surge arrestor application
Equipment inspection and maintenance
Line inspection and maintenance
Right-of-way inspection and maintenance
Prompt identification of chronic problems
Analysis of interruption data
Line monitoring

In the future, a level of required reliability and power quality may be developed and specified for a broad range of industrial processes. This has already been done for the computer industry and the semiconductor manufacturing industry. Levels could be developed for residences, commercial establishments such as restaurants, critical facilities such as hospitals and communication networking centers, and industry.

A reasonable expected level of reliability and power quality could be developed for specific distribution locations. As discussed earlier, these levels would be a function of a number of variables including climate, design, and maintenance. It would be essential to keep index calculation methods uniform. For example, the method for acquiring restoration times after an outage would have to be identical, both in theory and practice, between utilities. Step restoration, where end times of interruption are measured in large blocks (for example, at the substation level), can lead to large discrepancies if other utilities are measuring time at the sublateral or even customer level. Ideally, the actual individual customer restoration time would be measured and used in the index calculation.

Development of such standards would require a computer-based system that is programmed and managed identically among the utilities to acquire data and calculate indices. Definitions of concepts such as a major event would also have to clearly defined and practiced uniformly. If this could be done, this common method of computer-based data acquisition and calculation would produce indices that were meaningful for comparison between utilities.

A drawback to such a standardized programmed and managed reliability database is that some utilities will inherently have better reliability indices than others due to simple differences in climate and geography. For example, factors such as the number of lightning strikes, the length of exposed feeders, a desert climate, and urban network system designs will have a significant impact on the reliability figures, independent of the actions of the utility to competently operate and maintain its system. As noted in the previous section, Pacific Power is presently using performance incentives based on reliability indices. Once the calculation method is well defined and understood by both the utility and the regulator, it provides a common ground for assessing performance. In New York State, performance metrics are quite well defined and are agreed upon by both utilities and the public service commission.

Published by PQTBlog

Electrical Engineer

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