Question

A (350 A/ 7A), 50 Hz current transformer has a primary bar. The secondary burden is a pure resistance of 1 Ω and it draws a current of 5 A. The magnetic core requires 350 AT for magnetization. Find the percentage ratio error.

(a) 10.56

(b) -28.57

(c) 11.80

(d) -11.80

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

Question is taken from Advanced Problems on C.T. and P.T. topic in division Extension of Instrument Ranges of Electrical Measurements

Answer 1

Correct option is (b) -28.57

To explain I would say: Im = 350/1 = 350 A

Ip = \(((nI_s^2)^2 + (I_m^2)^2)^{0.5}\) = 490.05

n = 350/7 = 50

∴ R = \(\frac{I_P}{I_S} = \frac{490.05}{7}\) = 70

∴ Percentage ratio error = \(\frac{50-70}{70}\) × 100 = -28.57%.