scope:

A generating Unit generally includes all equipment from the resource supply system up to the high-voltage terminals of the generator step-up transformer and the station service transformers. Any event preventing the generating Unit from producing electricity at its maximum capacity is covered in the scope of this standard.

Reliability in this standard encompasses measures of the ability of generating Units to perform their intended function in a specified period. Availability measures are concerned with the fraction of time in which a Unit is capable of providing service and accounts for outage frequency and duration. Productivity measures compare the total power produced by a Unit to its potential power production. Productivity measures consider the magnitude of events as well as the frequency and duration of events.

Sometimes, unforeseen events render the generating Unit unable to provide the power required to the customer because of problems unrelated to the power plant equipment. Some examples of these "external events," called "Outside management control," are transmission system failures, labor disputes, catastrophic storms and lack of fuel or another needed energy resource. Other events deprive the Unit of a resource needed for it to generate electricity, in a way that was anticipated in the planning and design of the plant. Units of this type are referred to as Variable energy resource (VER) units.

Many of the performance indexes defined in Clause 9 are expressed as either outage rates or factors, and it is important to note the difference. A factor represents the percentage of time in the active state within a period of study that a Unit or Units occupied a given state, as in Availability factor (AF), Forced outage factor (FOF), or Service factor. A factor may also represent the percentage of a total outcome achieved. For example, Net capacity factor is the ratio of Net actual generation to Net maximum generation. Factors may be added to provide a total accounting for unit states during a given period.

By comparison, outage rates provide a measure of the probability, calculated from historical data, of the existence of an outage state any time in the future or under certain conditions. While the denominator of the Forced outage factor, for example, is Active hours, the denominator of the Forced outage rate excludes Planned outage hours, Maintenance outage hours, and Reserve shutdown hours.

Attaching the term "equivalent" to any rate or factor, as Equivalent availability factor (EAF) or Equivalent forced outage rate (EFOR), indicates that both full outages and deratings have been considered in the calculation.

The term "demand" applied to a rate, as in EFORd, indicates that the rate of occurrence has been calculated for periods when the Unit is in demand to generate. The term "Critical period" denotes an index calculated using only data from historical hours included in a specified Critical period, hours in which there was an elevated risk of shortage.

This standard includes indexes both for application at the unit level and for pooling the performance of a Group of Units. Group indexes can be either Weighted, indicating that data from each Unit influence the total in proportion to its capacity or other indicated weighting factor, or unweighted. See Clause 10.

This document standardizes terminology and indexes for reporting electric generating unit reliability, availability, and productivity performance measures that recognize the power industry's needs, including marketplace competition. This standard also includes consideration of VER units and Resource unavailability, and new indexes appropriate for that purpose. This document does not address common mode or dependent outages.

Purpose

This standard is intended to aid the electric power industry in reporting and evaluating electric generating unit reliability, availability, and productivity. It was originally developed to overcome difficulties in the interpretation of electric generating unit performance data from various systems and to facilitate comparisons among different systems. The standard also makes possible the exchange of meaningful data among systems in North America and throughout the world. It includes both indexes for unit performance analysis, which exclude outages or deratings owing to resource unavailability, and indexes for system reliability analysis, which include outages and deratings owing to Resource unavailability as well as equipment unavailability. The distinction recognizes that Resource unavailability affects the contribution of a Unit to system reliability, but is not something for which plant management should be held responsible.

The primary purpose of this standard is to define summary statistics of historical data, for reporting and analyzing outage occurrences, and to facilitate exchange of data between utilities. If indexes from Clause 9 or Clause 10 of this standard are used as inputs to predictive models, it is necessary to ensure that (a) the generators and the environment in which their performance was observed are similar to the system whose performance is to be predicted; (b) the period of observation over which the input data were collected is a sufficient multiple of the mean service time to outage to achieve a reasonable confidence interval for the indexes to be used in the predictive models, and (c) although this standard does not address common mode and dependent events, adequate consideration of such events has been given to avoid the fallacy of assuming that all outages are independent.