Question

If the per unit leakage impedance for the primary of a transformer is ‘x’ on the given rated base value. If the voltage and volt-amperes are doubled, then the changed per unit impedance will be ___________

(a) 0.5x

(b) 2x

(c) 4x

(d) x

This question was posed to me in an interview for job.

This question is from Voltage Regulation of Transformer in division Transformers of Electrical Machines

Answer 1

Right answer is (a) 0.5x

To explain I would say: pu(new base) = (x)*(MVA(new)/MVA(old))*(kV(old)/kV(new))^2

                         = x*2*(1/4)

    = 0.5x.