What is mean by interconnected star?

interconnected star or (Zig Zag connection) is a method of connecting the phase windings of a three phase transformer. normally, it's used for earthing of the the delta-connected low tension side of power Transformers. it has the benefit of eliminating third harmonic in the winding terminals, it also can be used as the auxialry transformer that provide the substation with it's own needs of electric power as following:

case 1: using the Z winding as earthing transformer: in this case the winding is stand alone, it means no primary or secondry side , simply , it's one three-phase winding, each phase winding is divided into tow equal peices, then these peices are interconnected in a manner that the first half of phase R is connected to the second half of pahse T, The first half of pahse S is connected to the second half of phase R, and the first half of phase T is connected to the second half of phase S: these connections are done but in reverse in order to eleminate third harmonic in the phase voltage , then reducing the neutral conductor size and also giving almost pure faundamental voltage to the load.

interconnected star or (Zig Zag connection) is a method of connecting the phase windings of a three phase transformer. normally, it's used for earthing of the the delta-connected low tension side of power transformers. it has the benefit of eliminating third harmonic in the winding terminals, it also can be used as the auxialry transformer that provide the substation with it's own needs of electric power as following:

case 1: using the Z winding as earthing transformer: in this case the winding is stand alone, it means no primary or secondry side , simply , it's one three-phase winding, each phase winding is divided into tow equal peices, then these peices are interconnected in a manner that the first half of phase R is connected to the second half of pahse T, The first half of pahse S is connected to the second half of phase R, and the first half of phase T is connected to the second half of phase S: these connections are done but in reverse in order to eleminate third harmonic in the phase voltage , then reducing the neutral conductor size and also giving almost pure faundamental voltage to the load.