Three Phase Transformer

THREE PHASE TRANSFORMER

Usually large scale generation of electrical power used 3 phase voltages of 13.3 kV or some what higher. Transmission is generally accomplished at higher voltages of 110, 132, 275, 400 and 750 kV for which purpose 3ϕ transformers are necessary to step up the generated voltage to that of the transmission line.

Like single phase transformers, the three phase transformers are also of core type or shell type. The basic principle of a 3ϕ transformer is illustrated in the fig., in which primary windings have been shown interconnected in star and put across 3-phase supply. The three cores are 120° apart and their empty keys are shown in contact with each other the centre leg formed by these three, carries the flux produced by the three phase currents I_R, I_Y, and I_B. As at any instant I_R+I_Y+I_B = 0, hence the sum of three fluxes is also zero.

Therefore, it will make no difference if the common leg is removed. In that case any two legs will act as the return for the third just as in a 3-phase system any two conductors act as the return for the current in the third conductor.