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Power System ReliabilityTable of Contents:
IntroductionThe main areas of an electric power system are generation, transmission, and distribution and the basic function of a modern electric power system is supplying its customers the costeffective electric energy with a high degree of reliability. In the context of an electric power system, reliability may simply be expressed as concern regarding the system’s ability for providing a satisfactory amount of electrical power [I]. The history of power system reliability goes back to the early years of 1930s when probability concepts were applied to electric power systemassociated problems [2,3]. Over the years, a large number of publications on the subject have appeared. Most of these publications are listed in Refs. [47]. This chapter presents various important aspects of power system reliability. Power System ReliabilityRelated Terms and DefinitionsThere are many terms and definitions used in the area of power system reliability. Some of these are as follows [811]: ^{•} Forced outage. This is when a unit or a piece of equipment has to be taken out of sendee because of damage or a component failure.
Power System Service Performance IndicesIn the electric power system area, generally various service performancerelated indices are calculated for the total system, a specific region or voltage level, designated feeders or different groups of customers, etc. [1]. Six of these indices are as follows [1,12]. 8.3.1 Index I This index is known as average service availability index and is defined by where is the average service availability. ct] is the customer hours of available sendee. Yi is the customer hours demanded. These hours are expressed by the 12 month average number of customers serviced times 8760 hours. 8.3.2 Index II This index is known as system average interruption duration index and is defined by where в_{2} is the system average interruption duration. a_{2} is the sum of customer interruption durations per year. Y2 is the total number of customers. 8.3.3 Index III This index is known as customer average interruption duration index and is defined by where 0_{i} is the customer average interruption duration. y_{}} is the total number of customer interruptions per year. «з is the sum of customer interruption durations per year. 8.3.4 Index IV This index is known as customer average interruption frequency index and is expressed by
where в_{4} is the customer average interruption frequency. a_{4} is the total number of customer interruptions per year. У л is the total number of customers affected. It is to be noted that the customers affected should only be counted once, irrespective of the number of interruptions they may have experienced throughout the year. 8.3.5 Index V This index is known as system average interruption frequency index and is expressed by
where в_{5} is the system average interruption frequency. a_{5} is the total number of customer interruptions per year. Уs is the total number of customers. 8.3.6 Index VI This index is concerned with measuring service quality (i.e., measuring the continuity of electricity supply to the customer) and is defined by [5,12]
where в_{6} is the mean number of annual down hours (i.e., service outage hours) per customer. (8760) is the total number of hours in one year. MTEI is the mean time of electricity interruption. MTTFis the mean time to failure (i.e., the average time between electricity interruptions). Example 8.1 Assume that the annual failure rate of the electricity supply is 0.8 and the mean time of electricity interruption is 5 hours. Calculate the mean number of annual down hours (i.e., service outage hours) per customer. In this case, mean time to failure (i.e., the average time between electricity interruptions) is
By substituting the calculated value and the specified values into Equation (8.6), we get
Thus, the mean number of annual down hours (i.e., service outage hours) per customer is 3.998 hours. 
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