Question

Consider the three single-phase two-winding transformers shown inFigure 3.37. The high-voltage windings are connected in Y. (a) For the low-voltage side, connect the windings in, place the polarity marks, andlabel the terminals a , b,and c in accordance with the American standard.(b) Relabel the terminals $\mathit{a}\mathbf{\text{'}}$, $\mathit{b}\mathbf{\text{'}}$,and$\mathit{c}\mathbf{\text{'}}$such that ${\mathit{V}}_{\mathbf{A}\mathbf{N}}$is $\mathit{90}\mathit{°}$out of phasewith ${\mathit{V}}_{\mathbf{a}\mathbf{\text{'}}\mathbf{n}}$for positive sequence.

Short answer

(a) The two-winding transformer with polarity marking is drawn below:

(b)

Draw the positive sequence phase shift for star connection.

Draw the positive sequence phase shift for delta connection.

Step-by-step solution

Determine the dot convention

Draw the given diagram.

According to a dot convention, when the current leaves the dot, the current direction is taken as negative.

When current enters the dot, the current direction is taken as positive.

Determine the given diagram with polarity marking

(a)

Assume that A, B and C are the three balanced phase system connection in the star, and N is a neutral point.

Also, a, b and c are the three-phases connection in the delta.

The diagram is drawn below with polarity marking according to American standards.

Draw the positive phase sequence diagram

(b)

Draw the positive sequence phase shift for star connection.

Draw the positive sequence phase shift for delta connection.

For positive sequence, $V_{AN}$leads$V_{an}$ by$90°$ .