Question

In developing per-unit circuits of systems such as the one shown in Figure 3.10, When moving across a transformer, the voltage base is changed in proportion to the transformer voltage ratings.

(a) True (b) False

Short answer

Therefore, the correct option is (a): True.

Step-by-step solution

The per unit circuit diagram is redrawn as,

The conversion of the impedance from per unit impedance to ohm.

${\mathrm{Z}}_{\mathrm{base}}=\frac{{\mathrm{V}}_{\mathrm{base}}}{{\mathrm{S}}_{\mathrm{base}}}$

The base current can be calculated as

${\mathrm{I}}_{\mathrm{base}}=\frac{{\mathrm{S}}_{\mathrm{base}}}{{\mathrm{V}}_{\mathrm{base}}}$

The impedance of the line can be converted into per unit as,

${\mathrm{X}}_{\mathrm{L}.\mathrm{p}.\mathrm{u}}={\mathrm{X}}_{\mathrm{\Omega }}\frac{{\mathrm{S}}_{\mathrm{base}}}{{\mathrm{V}}_{\mathrm{base}}^2}$

Draw the per unit diagram,

Base voltage changes in the proportion to the voltage range.

Let ${\mathrm{V}}_{\mathrm{base}1}$ is the voltage at primary side, ${\mathrm{V}}_{\mathrm{base}2}$is the voltage at secondary side and $\frac{N_1}{N_2}$is the turns ratio, then the base value refers to the secondary side is given by,

Hence,moving across a transformer, the voltage base is changed in proportion to the transformer voltage ratings.