Question

Question: In per-unit equivalent circuits of practical three-phase transformers, under balanced three-phase operation, in which of the following connections would a phase-shifting transformer come up?

(a) $\mathit{Y}\mathbf{-}\mathit{Y}$

(b) $\mathit{Y}\mathbf{-}\mathbf{∆}$

(c) $\mathbf{∆}\mathbf{-}\mathbf{∆}$

Short answer

Answer

Therefore, the correct option is (b) $Y-∆$

Step-by-step solution

 Check for Y-Y and ∆-∆ connections.

In $\mathit{Y}\mathbf{-}\mathit{Y}$connection, the line voltages at both the primary and secondary sides are in phase and has different magnitudes.

Same condition is true for $\mathbf{∆}\mathbf{-}\mathbf{∆}$ connection.

Therefore, there is no phase shift in the $Y-Y$and $∆-∆$connection.

Check for y-∆ connection.

In $Y-∆$connection, the line voltages in Y connection leads the line voltage in $∆$connection by ${30}^{°}$.

Therefore, there is phase shift in the $Y-∆$connection.

Hence, correct option is (b) $Y-∆$.