Single-phase equivalent diagram
Linear power networks

When studying a perfectly balanced three-phase sinusoidal network, there's no need to study the behavior of the three phases, since all electrical quantities relating to phases 2 and 3 can be deduced from those relating to phase 1 by applying a simple time shift of T/3 and 2T/3, where T designates the signal period (20 ms at 50 Hz).

To limit the study of such a network to one phase only, it is first necessary to reduce all the couplings of all the electrical elements to star. If you keep some elements coupled in star and others in delta, you run the risk of mixing simple and compound voltages, or line and phase currents, which would not be coherent. All delta elements must therefore be reduced to an equivalent star, using the relationships...