28.1 Introduction

The primary function of a modern electric power system is to supply its customers with electrical energy as economically as possible and with an acceptable degree of reliability. Modern society, because of its pattern of social and working habits, has come to expect the supply to be continuously available on demand. This degree of expectation requires electric power utilities to provide an uninterrupted power supply to their customers. It is not possible to design a power system with 100% reliability. Power system managers and engineers therefore strive to obtain the highest possible system reliability within their socio-economic constraints. Consideration of the two important aspects of continuity and quality of supply, together with other important elements in the planning, design, control, operation and maintenance of an electric power system network, is usually designated as reliability assessment. In the power system context, reliability can therefore be defined as concern regarding the system s ability to provide an adequate supply of electrical energy. Many utilities quantitatively assess the past performance of their systems and utilize the resultant indices in a wide range of managerial activities and decisions. All utilities attempt to recognize reliability implications in system planning, design and operation through a wide range of techniques [1 12].

Reliability evaluation methods can generally be classified into two categories: deterministic and probabilistic. Deterministic techniques are slowly being replaced by probabilistic methods [1 12]. System reliability can be divided into the two distinct categories of system security and system adequacy. The...