Question

Consider the adopted per-unit system for the transformers. Specify true or false for each of the following statements:

(a) For the entire power system of concern, the value of$S_{base}$is not the same.

(b) The ratio of the voltage bases on either side of a transformer is selected to be the same as the ratio of the transformer voltage ratings.

(c) Per-unit impedance remains unchanged when referred from one side

of a transformer to the other.

Short answer

The correct answers are:

(a) False

(b) True

(c) True

Step-by-step solution

Explain Sbase  of a power system.

(a)

$S_{base}$ is a base MVA power in a power system. It is used to find the per unit apparent power in a power system with original MVA power.

In a power system, it is not feasible to use different power bases, hence $S_{base}$ is same in entire power system.

Therefore, the answer is false.

Explain about voltage bases.

(b)

The base voltage on transformer, generator terminal, and transmission line is selected on the basis of transformer voltage rating.

Therefore, the answer is true.

Explain about per-unit impedance of transformer

(c)

While calculating a per-unit impedance in a transformer, the per phase diagram is considered in single line diagram with impedance referring to either primary side or secondary side.

Therefore, the per-unit impedance remains unchanged, when referred to one side from other.

Therefore, the answer is true.