Question

In per unit what is the maximum and minimum voltage regulation of transformer (approximately) respectively,

• A. $X_{p.u},Z_{p.u}$
• B. $Z_{p.u},-X_{p.u}$
• C. $X_{p.u.},-Z_{p.u}$
• D. $Z_{p.u.},X_{p.u.}$

Answer 1

The correct option is B $Z_{p.u},-X_{p.u}$

We know approximate voltage regulation formula,
$V.R.=Z_{p.u.}cos({\theta }_{eq}-\varphi )$

For maximum voltage regulation,
${\theta }_{eq}-\varphi =0$
$V.R.=Z_{p.u.}cos{0}^{\circ }$
$=Z_{p.u.}$

For minimum voltage regulation,
$\varphi =-{90}^{\circ }$
$V.R.=Z_{p.u.}cos({90}^{\circ }+{\theta }_{eq})$
$=-Z_{p.u.}\sin \left({\theta }_{eq}\right)$
$=-X_{p.u.}$