The DC component in the magnetizing current of a transformer is caused by the saturation of the core. When the core is saturated, the magnetization curve becomes non-linear, and the current required to produce a given flux density increases. This increase in current is due to the fact that the core material is no longer able to fully reverse its magnetization, and a residual DC field remains.In the case of a transformer with a Yy, Dy, or Yd winding connection, the third harmonic currents in the windings cancel each other out, but the DC component does not. This is because the third harmonic currents are in phase in the windings, while the DC component is not. As a result, the DC component adds up in the primary winding, and it can become significant if the core is heavily saturated.In the case of a transformer with a Dd winding connection, the third harmonic currents are out of phase in the windings, and they cancel each other out. This also cancels out the DC component, which is why the magnetizing current in a Dd connection has a lower DC component than in other winding connections.The shape of the magnetizing current waveform is also affected by the saturation of the core. When the core is saturated, the magnetization curve becomes more rounded, and the current waveform becomes more distorted. This is why the magnetizing current waveform in a transformer with a Yy, Dy, or Yd winding connection is more distorted than the waveform in a transformer with a Dd winding connection. Flag for review