Introduction

Due to the rapid increase in the demand for electricity, environmental constraints, and the competitive energy market scenario, the transmission and distribution systems are often being operated under heavily loaded conditions. In the earlier days, evaluation of the adequacy of the generation system and reliability of the transmission system were among major studies in the system planning. The recent experiences from blackouts indicate that, in many cases, the triggering events for such widespread failures took place in the distribution level [1]. Statistically, the majority of the service interruptions to the customers come from the distribution systems [2], [3]. Detailed reliability evaluation of the distribution system has, therefore, become very important in the planning and operating stage of a power system. For economic reasons, minimization of the losses in the distribution system should also be considered in a distribution system reconfiguration process.

The commonly used objectives for distribution system reconfiguration have been the minimization of the transmission loss and/or voltage deviations (from the nominal values) at the buses [4]–[۱۷]. An essential criterion for the reconfigured networks has been the preservation of the radial nature, mainly for the ease in protective relay coordination. The probabilistic reliability evaluation methodology described in this paper is, however, not limited to radial distribution networks only. Distribution reconfiguration is essentially a combinatorial optimization problem where the best possible combination of status (open or close) of the sectionalizing and tie-switches has to be found so that the objective function (such as the total real power loss) is minimized. The frequently used constraints in this optimization process have been the maximum-allowable voltage drop in the line, line current limits, transformer capacity limits, and any other possible network operational or planning constraints.