In this project to reach some goals such as minimizing energy losses and line loading and improving voltage stability in operation of distribution systems, in addition to voltage/VAr control and system reconfiguration, suitable output power of distributed generation are computed. For this purpose, an optimization problem is defined. For solving this combinatorial problem, a problem with a lot of discrete control variables, a solution algorithm based on tabu search is proposed. In addition, distribution system operation under abnormal conditions of distribution system or transmission system such as disconnection of one feeder section or one of active/reactive sources are studied. Operation of distribution system under conditions that voltage control buses are available in system and under unbalanced operation conditions are part of other conducted works in this project. The suggested operation techniques are used for 33 and 69 buses test networks.