A zigzag transformer is a special purpose transformer with a zigzag or 'interconnected star' winding connection. The most common zigzag transformer application is for the derivation of a neutral connection from an ungrounded 3-phase system and the grounding of that neutral to an earth reference point. Zigzag transformers are also used to control triplen (3rd, 9th, 15th, 21st, etc.) harmonic currents, to supply 3-phase power as an autotransformer (serving as the primary and secondary with no isolated circuits), and to supply non-standard phase-shifted 3-phase power.
Nine-winding three-phase transformers, typically having six identical secondary windings, can be used in zigzag winding connection as pictured. As with the conventional delta or wye winding configuration three-phase transformer, a standard stand-alone transformer containing only six windings on three cores can also be used in zigzag winding connection, such transformer sometimes being referred to as a zigzag bank. . In all cases, six or nine winding, the first coil on each zigzag winding core is connected contrariwise to the second coil on the next core. The second coils are then all tied together to form the neutral and the phases are connected to the primary coils. Each phase, therefore, couples with each other phase and the voltages cancel out. As such, there would be negligible current through the neutral point, which can be tied to ground.
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. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero sequence (earth fault) current exists between the transformer’s neutral to the faulting phase. Hence, the purpose of a zigzag transformer is to provide a return path for earth faults on delta-connected systems. With negligible current in the neutral under normal conditions, engineers typically elect to undersize the transformer; a short time rating is applied (i.e., the transformer can only carry full rated current for, say, 60 s). Ensure the impedance is not too low for the desired fault limiting. Impedance can be added after the secondaries are summed (the 3Io path).
An application example: occasionally engineers use a combination of Y (wye or star), delta, and zigzag windings to achieve a vector phase shift. For example, an electrical network may have a transmission network of 110 kV/33 kV star/star transformers, with 33 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required directly between the 110 kV/11 kV network the most obvious option is to use 110 kV/11 kV star/delta. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 110 kV/11 kV transformer provides the required earth reference point.
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