The neutral formed by the zigzag connection is very stable. Impedance can be added after the secondary’s are summed (the 3I o path) Ensure the impedance is not too low for the desired fault limiting. With negligible current in the neutral under normal conditions, engineers typically elect to under size the transformer a short time rating is applied. Hence, the purpose of a zigzag transformer is to provide a return path for earth faults on delta connected systems. ( Using symmetrical components, this is I a0 = I b0 = I c0.) Zero sequence (earth fault) current exists between the transformers’ neutral to the faulting phase. If one phase, or more, faults to earth, the voltage applied to each phase of the transformer is no longer in balance fluxes in the windings no longer oppose. Under normal system operation the outer and inner coil winding’s magnetic flux will cancel each other and only negligible current will flow in the in the neutral of the zigzag transformer.ĭuring a phase to ground fault the zig-zag transformer’s coils magnetic flux are no longer equal in the faulted line. Therefore, the zigzag winding provides an easy path for in-phase currents but does not allow the flow of currents that are 120°out of phase with each other. If three currents, equal in magnitude but 120° out of phase with each other, are applied to the three terminals, the ampere-turns in the windings cannot cancel and the transformer restricts the current flow to the negligible level of magnetizing current. Therefore, the transformer allows the three in-phase currents to easily flow to neutral. If three currents, equal in magnitude and phase, are applied to the three terminals, the ampere-turns of the a2-N winding cancel the ampere-turns of the b1-b winding, the ampere-turns of the b2-N winding cancel the ampere turns of the c1-c winding, and the ampere-turns of the c2-N winding cancel the ampere turns of the a1-a winding. The outer coils are connected to phases a1, b1, c1 of the existing delta system.The inner coils are connected together to form the neutral and our tied to ground.The outer coil of phase c1-c is connected to the inner coil of phase b2-N.The outer coil of phase b1-b is connected to the inner coil of phase a2-N.The outer coil of phase a1-a is connected to the inner coil of phase c2-N.
The transformer coils are connected as follows:
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