Teritiary winding?

For unbalanced single phase load, the star-star connection offers high reactance to flow of current. Any unbalanced laod current has three component viz positive, negative and zero sequence components. the zero sequence component on the secondary side can not be balanced by primary currents as zero sequence current can not flow in the isolated neutral of star connected primary. On the secondary side the zero sequence current sets up magnetic flux in the core. The iron path is available for this flux and the impedance offered to the zero sequence currents is very high. But the delta connected tertiary winding permits circulation of zero sequence currents in it. So impedance offered to the flow of zero sequence currents is lowered. For this purpose the tertiary winding is called stabilizing winding.

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