An illustration of harmonic suppression due to transformer connection-a simplified approach

A basic simplified mathematical explanation is provided for the harmonic suppression that can occur when transformer pairs servicing power system converter or rectifier loads are connected delta-delta on one unit and delta-wye on the other unit, such that a 30° phase shift exists between transformer secondaries. The Fourier series is used to define the phasor quantities (relative magnitude) for each harmonic required to construct the current square wave. A relative phase angle is derived for each harmonic to complete the phasor. It is from these phasors (and the relationship of the connections for the transformer pair) that cancellation of certain harmonics does or does not occur