A 200/1 Current transformer (CT) is wound with 200 turns

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A 200/1 Current transformer (CT) is wound with 200 turns on the secondary on a toroidal core. When it carries a current of 160 A on t he primary, the ratio and phase errors of the CT are found to be – 0.5% and 30 minutes respectively. If the number of secondary turns is reduced by 1 the new ratio error (%) and phase error (min) will be respectively

1. 0.0, 30
2. – 0.5, 35
3. – 1.0, 30
4. – 1.0, 25

Correct Option: A

Actual Ratio, R_{1_{= n₁}}		1 +	I_c
			I_P

= 200		1 +	I_c
			I_P

Where n₁ is the turn ratio,

n₁ =	N_S
	N_P

Nominal ratio, K_N =	200	= 200
	1

Ratio Error, R =	K_N - R₁	× 100
	R₁

Initially ratio error = – 0.5%

⇒ -0.5 =	200 - R₁	× 100
	R₁

⇒ R₁ ≅ 201

Now, n₂ (turn ratio) =	N'_S	= 199
	N_P

R₂ = 199		1 +	I_e		= 199 ×	201	≈ 200
			I_P			200

Then ratio, error =	K_n - R₂	=	200 - 200	= 0
	R₂		200

Phase angle error, θ =	180		I_m
	π		I_P

that is independent of turn ratio. Hence ratio error, reduces to zero, however phase angle error remains same.