Expert and managers have been interested in loss problem for a long time. From the beginning of electricity systems expansion; there have always been efforts to reduce the power losses. However, the amount of power losses is still high and there is always a considerable amount of loss in different parts of power system such as transmission and distribution lines and transformers present in annually reports. These reports represent the amount of attention and work the power system and distribution officials do regarding this issue. With further development and expansion of the power systems and shortage of the already running out natural fuels; efficient use of the power produced in the power plants is one of the most important issues at hand. There have been efforts to counter this issue such as increasing the amplitude of the line voltages in case of large power transmission in long distances. But increasing the voltage level has its own drawbacks of requiring higher isolation which makes it problematic to be used in distribution networks. The introduction of the reactive power consumption made the power engineers wonder that although the losses caused by power transmission can be decreased by increasing the conductors cross section has little economical justification; It is possible to reduce the amount of reactive power by using compensators so that the losses caused by reactive power transmission can be eliminated. Loss in distribution networks according to many reasons are the dominate part of power system losses. So, loss analysis and finding ways to reduce it became important. Optimal capacitor placement has an important role in power systems planning and operation. Installing shunt capacitors in distribution system, specifically for loss reduction, improving voltage profile in feeders, power factor correction and releasing feeder capacity, is known subject in effective operation of distribution systems. Of course do to not considering some of the operational limits such as unbalance system and nonlinear loads, by using shunt capacitors in distribution systems, the network loss may be significant. Unbalanced three phase lines, unbalanced loading and presence of single and two phase lines caused unbalanced conditions in distribution system. Vast nonlinear loads and power electronics elements usage, lead to harmonic current production and injection to power system. Capacitor placement in distribution system including location, type and optimal size determination is so to minimized losses and capacitor cost simultaneously. In large scale systems, solution of this problem is dependent of computer speed because of increasing bus numbers and system size. So presenting an algorithm which is capable to solve this problem in sufficient time and by acceptable accuracy Keywords: Capacitor Placement, Unbalanced Distribution System, Nonlinear Load, PSO Algorithm
