Power system single-phase models assume that generation, transmission and distribution systems are perfectly balanced. The great appeal of this type of model is its simplicity, for all the calculations become quite simple, when compared with the three-phase model, and the results obtained are well suited for most applications. There are cases, however, in which its results may not be accurate enough, as may happen with distribution system analysis (unbalanced loads), non-transposed transmission lines, large mutual impedances between lines, etc. These problems require a more accurate three-phase model. The paper extends decoupling theory in order to obtain a fast decoupled three-phase load flow which is suited for distribution power system analysis.1432188192