<P> The delta winding allows third - harmonic currents to circulate within the transformer, and prevents third - harmonic currents from flowing in the supply line . Delta connected windings are not common for higher transmission voltages (138 kV and above) owing to the higher cost of insulation compared with a wye connection . </P> <P> Delta - wye transformers introduce a 30, 150, 270 or 330 degree phase shift . Thus they cannot be paralleled with wye - wye (or delta - delta) transformers . However, they can be paralleled with identical configurations and some different configurations of other delta - wye (or wye - delta with some attention) transformers . </P>

When the secondary of a transformer must be connected in delta