Question

A 200/1 Current Transformer (CT) is wound with 400 turns on the secondary on a toroidal core. When it carries a current of 180 A on the primary, the ratio is found to be -0.5%. If the number of secondary turns is reduced by 1, the new ratio error (in %) will be?

(a) 0.0

(b) -0.5

(c) -1.0

(d) -2.0

I had been asked this question in quiz.

Query is from Advanced Problems on C.T. and P.T. in portion Extension of Instrument Ranges of Electrical Measurements

Answer 1

Correct choice is (c) -1.0

To explain: Turn compensation only alters ratio error n=400

Ratio error = -0.5% = – \(\frac{0.5}{100}\) × 400 = -2

So, Actual ratio = R = n+1 = 401

Nominal Ratio KN = 400/1 = 400

Now, if the number of turns are reduced by one, n = 399, R = 400

Ratio error = \(\frac{K_N-R}{R} = \frac{200-200}{200}\) = 0.