Current Transformer – Construction, Phasor and Errors

This article describes the current transformer – Construction, Phasor, and Errors.  Current transformers are widely used for measuring the high current. The Current transformer steps down the high value of current to the low value, which the measuring instrument can read.

If 2000 Amperes current is to be read by a meter, it is not possible to read because a small shunt resistance of high power rating in parallel to the galvanometer is required to be used. However, using a small value shunt resistance of a high power rating to read high current directly by the meter is impractical. That is why a current transformer is used to lower the magnitude of the current so that the measuring instrument and protective relay can read the current.

Construction of Current Transformer(CT)

The current transformer has two windings called primary and secondary. The primary winding has fewer turns and is connected in series with the primary circuit. The entire circuit current passes through the primary winding, and it is desirable that the voltage drop across the primary winding must be as low as possible. The current transformer has fewer turns of large cross-sections, and thus, the voltage drop in the primary winding gets reduced to a great extent.

The secondary winding has more turns than the primary winding, which is connected by an ampere meter, energy meter, watt meter, transducer, and protection relay. The secondary winding of the CT must be connected with a low-impedance meter to keep the magnetic flux in the core up to its rated flux capacity.

The flux in the core above the rated capacity causes saturation of the magnetic core, which leads to the complete failure of the current transformer. The CT secondary must not be left open because, in this condition, the flux in the core increases manifold, and it causes CT core saturation.

Based on the construction, there are two types of current transformers- Live Tank CT and Dead Tank CT. In both types of CTs, the core and winding are enclosed in a porcelain structure, and this structure is filled with mineral-insulated oil,  which acts as cooling media and also provides the required electrical insulation.

Symbol of Current Transformer (CT)

Terminals P₁ and P_{2 show} the primary winding of the CT, and terminals S1 and S2 show the secondary winding of the CT. The CT ratio 2000/1 means the secondary current will be 1 ampere if the primary current through the CT is 2000 amperes.

Equivalent Circuit of Current Transformer(CT)

The CT equivalent circuit is given below.

The CT primary has few turns, and it has very little resistance. Rp and Xp, respectively, denote the primary winding resistance and reactance. The secondary winding resistance and reactance are denoted by R_s and X_s, respectively. The magnetization components of the current transformer are Rc and X_m. R_c is responsible for the core loss component, and X_m is the magnetization reactance of CT.

The other CT parameters are given below.

Phasor Diagram of Current Transformer(CT)

The equivalent circuit of the current transformer is similar to that of the power transformer and induction motor. The current transformer has two types of errors.

• Ratio Error
• Phase Angle Error

From the phasor diagram, the CT ratio error and phase angle error can be calculated.

CT Transformation Ratio

CT transformation ratio is calculated as follows. To find the transformation ratio, we need to calculate the primary current I_p, as per the definition,  and divide it by the secondary current Is.

Ratio Error of Current Transformer

The CT ratio Error is defined as the per unit deviation in transformation ratio from its nominal ratio. Ratio error is expressed in percentage.

CT Ratio Error Formula Derivation

Let us consider the part of the phasor of our importance for calculating I_p as shown below.

From the above phasor, the primary current Ip is the phasor sum of nI_s and I₀. The CT primary current Ip can be calculated using the vector addition formula.

The magnetizing current Io is very small compared to the primary current I_p. Therefore, the above expression can be simplified as follows.

From the above expression, it is clear that the transformation ratio is not equal to the turn ratio. The transformation ratio and turn ratio will be equal if α=0 and δ=0.This condition can be achieved if the core loss is equal to zero and the burden is purely resistive. This is an ideal condition. However, this condition is practically impossible.

Ratio Error of Current Transformer for Resistive Burden

Since the burden of the CT is generally resistive. Therefore, the power factor of the burden is unity, and hence δ=0

Phase Angle Error of the Current Transformer

The phase angle of the current transformer is defined as the angle between the primary current Ip and secondary current I_s. In the above phasor diagram, θ is the phase angle.

CT Phase Angle Error Formula Derivation

Consider the right angle triangle ABC in the above phasor diagram.

Phase Angle Error of Current Transformer For Ressistive Burden

The phase angle between the primary and secondary of the current transformer must be 180 degrees. The primary and secondary current must be out of phase. The phase angle error is the deviation in the phase angle of the primary and secondary current. The phase angle between the primary and secondary current of the CT is denoted by angle θ.

Since the burden of the CT is generally resistive. Therefore, the power factor of the burden is unity, and hence δ=0. The following expression gives the phase angle error of CT.

Since the burden of the CT is generally resistivity the power factor of the burden is unity and hence δ=0.

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