Symmetrical components method 1 is almost essential for the analysis of unbalance conditions in which the impedances of rotating machines play an important role. most of the problems in which the main regulatory factors are the impedances of transformer banks can; instead; be satisfactorily solved by combining the theory of the single transformer with the relations between voltages and currents in three-phase circuits. then; and as a review; the simplified theory of the transformer and the equations of the triphasic circuits are summarized.