Phase Shifting Transformers to Cancel Harmonics

When there are several non-linear loads of equal ratings in a power systems, the use of phase shifting transformers could help cancel harmonics.

After harmonic filters, phase shifting transformers are one of the simplest solution to reduce harmonics in a power system.

To provide an example, let us assume that there are six equal size 6.6kV variable speed drives in an industrial plant supplied by a utility at 11kV.

To provide an effective harmonics cancellation, the loads need to grouped into two, each of which will be supplied by a transformer. The first transformer will be 11/6.6 kV, DYN11 (-30° phase shift), and the second will be 11/6.6kV, DYN1 (+30° phase shift).

As the two transformers have phase shifting opposite each other, any harmonics produced in the first group of load will be equal to the second group in magnitude but opposite in direction. The vector summation of the two will be, in theory, be a clean sine wave.

Phase shifting transformer solution to cancel harmonics is not 100% effective as the loads on each transformer need to be exactly the same.