Phase Drop and Link to Live Phase

 

Phase Drop and Link to Live Phase in Distribution.

 When one phase is out to distribution transformer, secondary LV voltages are disturbed. One phase has normal phase voltage but other two phases have half or little more or less than half the phase voltage. But when drop phase is linked to one of live phase, secondary LV voltage pattern is different as under.

Consider one 11000/400 Volts Delta-Star distribution transformer. Out of three phase 11000 volts supply, one phase is open but linked to other phase as shown in the figure.

When one phase is out to distribution transformer, secondary LV voltages are disturbed. One phase has normal phase voltage but other two phases have half or little more or less than half the phase voltage. But when drop phase is linked to one of live phase, secondary LV voltage pattern is different as under.

 

Transformer is delta-star having three primary coils connected in delta. B phase is open but linked to Y phase.  There is no current in primary P3 as same phase is at both ends. Primary P1 and P2 have normal line voltage. Therefore magnetic flux is induced in corresponding core A and B only. Hence secondary S3 on limb C having no flux linkage has no induced voltage. Whereas secondary S1 and S2 have normal phase voltage but in opposite direction as in the diagram.  

Rated secondary line voltage is 400V

So rated phase voltage = 400/√3 = 230V

 Actual Phase Voltages

                Vr = 230 Volts

                Vy = 230 Volts

                Vb = 000 Volts

  Actual Line Volages

                Vrb = 230 Volts

                Vyb = 230 Volts

                Vry = 460 Volts

Normal phase voltage is available on two phase r and y. So single phase loads can be distributed on these two phases. Single phase loading condition is better than that of one phase drop case where all single phase loads have to be only on one phase having normal voltage.

Motor will not operate on this three phase supply as all voltages are in line and no phase shift to produce rotating magnetic field. On the contrary it is risky as motor is subject to (460/400) 115% voltage on one pair of phase.

This type of arrangements is adopted for load shedding on distribution feeders to suppress motive power load but provide essential single-phase supply for light, fan, refrigerators etc.