Part 2 Reliability: Definition and Discussion

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

Reliability: Definition and Discussion

In brief, reliability has to do with total electric interruptions – complete loss of voltage, not just deformations of the electric sine wave. Reliability does not cover sags, swells, impulses or harmonics. Reliability indices typically consider such aspects as

  • the number of customers;
  • the connected load;
  • the duration of the interruption measured in seconds, minutes, hours, or days;
  • the amount of power (kVA) interrupted; and
  • the frequency of interruptions.

Power reliability can be defined as the degree to which the performance of the elements in a bulk system results in electricity being delivered to customers within accepted standards and in the amount desired. The degree of reliability may be measured by the frequency, duration, and magnitude of adverse effects on the electric supply.1

There are many indices for measuring reliability. The three most common are referred to as SAIFI, SAIDI, and CAIDI, defined in IEEE Standard 1366 (see Appendix A).

  • SAIFI, or system average interruption frequency index, is the average frequency of sustained interruptions per customer over a predefined area. It is the total number of customer interruptions divided by the total number of customers served.
  • SAIDI, or system average interruption duration index, is commonly referred to as customer minutes of interruption or customer hours, and is designed to provide information as to the average time the customers are interrupted. It is the sum of the restoration time for each interruption event times the number of interrupted customers for each interruption event divided by the total number of customers.
  • CAIDI, or customer average interruption duration index, is the average time needed to restore service to the average customer per sustained interruption. It is the sum of customer interruption durations divided by the total number of customer interruptions.

A reliability index that considers momentary interruptions is MAIFI, or momentary average interruption frequency index.

  • MAIFI is the total number of customer momentary interruptions divided by the total number of customers served. Momentary interruptions are defined in IEEE Std. 1366 as those that result from each single operation of an interrupting device such as a recloser.

The major drawback to reliability metrics is that there is a great deal of debate about comparing these indices from one geographic area to another and exactly how the input data is to be applied in making the calculations. This is discussed further in Chapter 6, “Pitfalls in Methods for Reliability Index Calculation.” In addition, there are concerns about how to “normalize” the indices for adverse weather. Many state public utility commissions require utilities to compute and track certain reliability indices, but comparing them from region to region and utility to utility has been problematic due to differences in how the data is applied, system designs, weather differences, and even differences in vegetation growth. Because of this, the indices are limited in their usefulness. If the calculation method is kept the same, they are useful within a specific geographic area in evaluating changes in reliability over time, perhaps as a measurement of the effectiveness of maintenance practices.


  1. Electric Power Research Institute, Dynamics of Interconnected Power Systems, A Tutorial for System Dispatchers and Plant Operators, prepared by Power Technologies, Inc., Schenectady, N.Y., for the Electric Power Research Institute, Palo Alto, Calif., May 1989.

Appendix A Terms and Definitions of Reliability

Major Sources for Terms and Definitions

The following is a list and brief synopsis of many of the major sources for terms and definitions of reliability and service quality. This list is not intended to capture every single definition, but rather, the important ones that are in common use or practice today.

IEEE Tutorial Course: “Probability Analysis of Power System Reliability,” Course Text 71 M30-PWR (1971)

Organization: The Institute of Electrical and Electronics Engineers (IEEE)
Targeted industry segment: Utility power system planners—bulk power and distribution systems

Limitations: This defines the terms and introduces the calculation techniques for power system reliability indices, but it does not address variations in methods for calculating the indices. As an older reference, it is not current on some reliability issues, such as demand-side management. Power quality is not addressed.

Strengths: It introduces concepts of power system reliability and provides consensus definitions of reliability terms and indices.

IEEE Std. 1366-1998: Trial Use Guide for Electric Power Distribution Reliability Indices

Organization: IEEE

Targeted industry segment: Utility distribution systems and substations and defined regions

Limitations: This standard deals primarily with interruptions over one minute and defines such indices as SAIFI (system average interruption frequency index), SAIDI (system average interruption duration index), and CAIDI (customer average interruption duration index). The indices are concerned with both the duration and frequency of interruption.

Strengths: These are the indices that are commonly reported in reliability surveys. The standard shows the mathematical definition of the indices and gives examples of their calculation.

Other: Although these are the most common indices, they do not include the voltage sag and dip disturbances that are so troubling to digital equipment. Also, there is a great deal of debate about comparing these indices from one geographic area to another, because rural areas, or areas with high lightning activity, are expected to have a higher number of outages than densely populated urban areas with network distribution systems, for example.

IEEE Std. 762: Definitions for Use in Reporting Electric Generating Unit Reliability, Availability and Productivity

Organization: IEEE

Targeted industry segment: Utility power system planners

Limitations: Does not treat intermittent generation, as from renewable sources (wind, solar), according to the generally accepted method.

Strengths: Defines and classifies parameters for use in generation availability reporting. Terms such as “maximum capacity,” “planned derating,” and “forced outage hours” are defined.

IEEE Std. 859-1987: Standard Terms for Reporting and Analyzing Outage Occurrences and Outage States of Electrical Transmission Facilities

Organization: IEEE

Targeted industry segment: Transmission system modeling and reliability evaluation

Limitations: The standard provides outage definitions and indices that are intended for use in system planning models, operations and maintenance planning, and system design. It is not intended to provide guidance on how to perform quantitative evaluations of system reliability, and it provides no guidance other than to give definitions for key indices.

Strengths: The standard provides the definitions of a number of common transmission indices such as “outage rate,” “failure rate,” and “mean time to outage.”

Other: The standard also provides definitions for terms such as “component availability” and “probability of failure to operate on command.” These indices are used by engineers involved in analyzing and predicting outages of transmission facilities.

IEEE Std. 493-1997: Recommended Practice for Design of Reliable Industrial and Commercial Power Systems (IEEE Gold Book)

Organization: IEEE

Targeted industry segment: Industrial and commercial electric power distribution systems. This standard could be used for microgrids.

Limitations: This standard is directed toward the designers and users of industrial power systems. It is not intended for utility distribution or transmission systems.

Strengths: The standard provides historical component reliability data that can be used in a quantitative evaluation of the industrial power system to compute load interruption frequency, expected duration of load interruption events, total expected interruption time per year, and system availability as measured at a specific load supply point.

Other: The method used for the evaluation is the minimal-cut-set method. System weak points can readily be identified.

Electricity Distribution Price Review—Reliability Service Standards, working document to benchmark Australian utility service quality

Organization: Office of the Regulator General, Australia

Targeted industry segment: Australian utilities and their customers

Limitations: This is an ongoing activity; therefore, the standards being developed are not yet available to the general public.

Strengths: The objective of this activity is to develop performance-based rates, including both reliability and service quality, by applying IEEE 1366 and monitoring the performance of distribution feeders. Observed levels of reliability and power quality will be used to establish benchmarks for feeders (by type of feeder and types of customers served) to set service quality targets. The utility’s incentive to meet or exceed the targets is through performance-based rates. This is an important attempt to apply indices to setting service quality and reliability standards, relating system performance to price (tariffs), and establishing benchmark levels for power system performance that are related to characteristics of the customers being served.

Other: The Electric Power Research Institute (EPRI) is developing a similar approach for U.S. utilities that goes beyond performance-based rates and IEEE 1366 to a QRA approach (see “Assessing and Evaluating Reliability,” in Chapter 2).

State of Illinois: Title 83 Public Utilities, Chapter 1: “Illinois Commerce Commission,”

Subchapter C: “Electric Utilities,” Part 411, “Electric Reliability”

Organization: Illinois Commerce Commission

Targeted industry segment: Illinois utilities

Limitations: Code language for the state of Illinois. Does not specify reliability levels: “design according to generally accepted engineering practices.” The utility is required to state what those practices are.

Strengths: Defines reliability terms and record-keeping requirements for Illinois utilities. Classifies outage causes, defines procedures and formats for annual reliability reports and assessments.

Other: Unique to one state’s requirements

Emerald Contract: power supply contract under the “Green Rate”

Organization: Electricite de France (EdF)

Targeted industry segment: Green Rate customers of EdF

Limitations: The topology of the French power system, particularly the distribution system, is unlike that of the U.S. power system. Also, EdF is a government-owned, vertically-integrated monopoly; it doesn’t operate in the same type of competitive environment as U.S. utilities. (However, European union rules on competition and third party access may change this.) This is a contract of a single utility.

Strengths: The contract contains an appendix that lists disturbances that may affect the quality of electric power, includes simplified definitions, and delineates EdF commitments and minimum acceptable levels of service quality and reliability. It also specifies the tolerances with which the customer should comply concerning disturbances generated by customer-owned equipment that could be injected into the EdF network.

Other: Provides an example of service quality commitments from the utility to the customer and requirements imposed upon the customer by the utility. Even if the numbers are not directly applicable to the United States, this approach could be applicable.

Quality of Supply Standards, User Specification, ESKOM, South Africa, NRS 048-1996

Organization: ESKOM and South Africa’s National Electricity Regulator

Targeted industry segment: Electric utility and customers

Limitations: This is one utility’s guideline, applied to a non-U.S. power system.

Strengths: This specification provides the South African electricity supply industry with a basis for evaluating the quality of supply delivered to customers by the industry, and with a means of determining whether utilities meet the minimum standards required by the National Electricity Regulator. The underlying principle is that, on a national basis, the combined cost of supply and usage of electricity be minimized. The specification recognizes that quality is affected by the users (as a result of the nature of the loads connected), as well as by the producers or suppliers. Customers are therefore essential partners with utilities in the effort to maintain the quality of supply while the supply networks are expanded and developed to allow electrification of South Africa to proceed effectively and economically. The specification deals with the voltage characteristics in statistical or probabilistic terms. NRS 048 provides an overview of standards and procedures for the management of the quality of supply in the electricity supply industry, with particular reference to the application of minimum standards to meet the requirements of the National Electricity Regulator. NRS 048 does not cover safety requirements, network design or equipment performance.

Reliability and Power Quality Performance-based Tariffs

Organization: DTE Energy (Detroit Edison Company)

Targeted industry segment: Three large industrial customers

Limitations: This is one utility’s strategy to retain customers by guaranteeing that the power supply system will meet performance criteria. Manufacturing facilities of the Big Three automakers had experienced higher-than-normal outages and voltage sags that had resulted in significant manufacturing losses. Faced with the possibility that these customers would turn to another energy supplier or self-generate, DTE offered a performance guarantee. The initial contract specified that DTE would pay stipulated “damage” costs to customers if they experienced more interruptions than specified in the contract. This performance guarantee was later expanded to include instances of voltage sags (i.e., power quality). DTE implemented measures to improve the reliability, and the resulting performance has almost completely met the reliability criteria. The power quality criterion has not been violated. The criteria were set based on historical performance, with an assumed year-by-year reliability improvement. The utility did a reliability assessment for each plant, and since existing problems or weak points were corrected, the plants’ reliability has been acceptable. The corrective actions seemed consistent with standard utility practices to provide acceptable quality service; it appears that the tariff and its penalty levels were designed more on a marketing basis than from a cost analysis. The tariff (1) retained the customers and protected DTE from competition for 5 years, (2) was used to economically justify reliability improvements that might have been within current best practices anyway, and (3) has not resulted in any significant performance payments to the customers.

Strengths: Despite its limitations, this is the first well-documented instance of service quality guarantees being included in customer agreements that recognized the utility’s obligation to meet at least a minimum level of reliability and power quality.

Other: The concept of performance-based rates and service quality guarantees seems to be gaining acceptance by utilities and regulators.

Other Sources for Terms and Definitions

Several sources define terms such as “sag,” “notch,” “undervoltage,” and “swell,” but not always consistently. Some of the more prominent references are these:

  • IEEE Std. 100-1988, IEEE Standard Dictionary of Electrical and Electronic Terms
  • IEEE Std. 1100-1999, IEEE Recommended Practice for Powering and Grounding Electronic Equipment
  • IEEE Std. 1159-1995, IEEE Recommended Practice for Monitoring Electrical Power Quality
  • American National Standards Institute/National Fire Protection Association, Standard 70, National Electric Code
  • B. Kennedy, Power Quality Primer, New York: McGraw Hill, 20

Organizations: Various organizations

Targeted industry segment: All in the power industry, including consumers, designers and electricians

Limitations: Terminology and definitions are still evolving, both within the United States and internationally, although there are attempts to make U.S. definitions consistent with international [International Electrotechnical Commission (IEC)] definitions. A major problem has been inconsistencies among utility power system designers, industrial power distribution designers, and end users. The rise in electronic loads (“the digital society”) means that some power system phenomena that previously were of no consequence are now critically important to the end users’ processes or can threaten personnel safety. There is not yet a single recognized standard for definitions and calculation techniques, but many industry groups are working together to remedy this situation.

Strengths: The references cited will provide a good foundation for power quality terminology. The Power Quality Primer provides an excellent overview of power quality concerns for both the provider and consumer.

Other: Industry groups, such as IEEE, are close to agreement on common terminology and definitions. Some of these standards (i.e., 1100, 1159) are listed again in the Power Quality Standards, as they provide requirements or guidelines in addition to definition

Published by PQTBlog

Electrical Engineer

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