Question

In either a$Y-∆$or$∆-Y$transformer, as per the American Standard notation, the negative-sequence phase shift is the reverse of the positive sequence phase shift.

(a) True

(b) False

Short answer

Therefore, the correct option is (a) True.

Step-by-step solution

Draw the positive and negative phase sequences of three-phase voltages.

Let $V_a$, $V_b$and $V_c$ be the phase voltages in primary side of the $Y-∆$transformer and $Y_a\angle {30}^{°}$, $Y_b\angle {30}^{°}$ and $Y_c\angle {30}^{°}$ be the phase voltages in $∆-Y$transformer.

Draw the positive and negative phase sequence of phase voltages in transformer:

Figure: 1

Draw the positive and negative phase sequence of phase voltages in $∆-Y$ transformer:

Figure: 2

Explanation for phase reverse of positive and negative sequence phase voltages 

From figure 1 and 2, it is clear that the positive sequence of phase voltages is ABC and negative sequence is ACB that is reverse of ABC.

Therefore, the correct option is (a) true.