The Zig-zag Transformer generally has a ratio of 1:1. It consists of six windings, two for each phase – . The two windings for each phase(outer and inner windings) are wound on the same core but in opposite directions.
The windings are connected in a zig-zag fashion. For instance, the primary winding of R phase may be connected to the secondary of Y, the primary of Y phase would be connected to the secondary of B. In a perfectly balanced condition, the magnetic fluxes in the primary and the secondary are able to cancel each other; therefore, their magnetic fluxes cancel each other out. However, in a fault condition, the magnetic fluxes may not be equal and hence a fault current may flow through the neutral of the secondary winding.
Zig-zag Transformers are used to provide earthing for ungrounded systems. They are advantageous over delta-wye systems due to their low internal impedance and lower cost.
Apart from providing earthing to ungrounded systems, zig-zag transformers can be used to filter harmonic currents. Zig-zag transformers are connected close the loads that cause heavy harmonics. Since the transformer has opposing windings, the harmonic currents get cancelled.