A Y-Δ transformer bank CANNOT be paralleled with either a Y-Y or a Δ-Δ transformer bank because the phasor voltage difference between the two systems would be:

Three phase transformer connection

A Y-Δ (Wye-Delta) transformer introduces a 30-degree phase shift between the primary (Y-side) and secondary (Δ-side) voltages.

This means that if a Y-Δ transformer is connected in parallel with a Y-Y or Δ-Δ transformer (which do not introduce this phase shift), there will be a phase difference of 30° between the secondary voltages.

The resulting phasor voltage difference between the two transformer systems is given by:

\(V_{diff}=V_{secondary} \times sin(30)\)

\(V_{diff}=V_{secondary} \times 0.5\)

Thus, a Y-Δ transformer bank cannot be paralleled with a Y-Y or Δ-Δ bank because of this voltage difference, which would cause circulating currents and instability.