Getting to Know the Terms In Current Transformers – series1

Know the terms used in Current Transformer. 

In this post, We present some of the terms used in the current transformer. These terms are usually written on the nameplate of a current transformer.  We include an example of a current transformer nameplate as a reference in order to have better understand on how the term is included in a nameplate.

Rated Primary Current

Rated Primary current is the value of the primary current which is the base of the performance of a current transformer. The value of the rated primary current always written in the nameplate of a current transformer. In the example image, the magnitude of the rated primary current is 630A (indicated by the letter (A)).

Rated Secondary Current

Rated secondary current is the value of the secondary current which is the base of the performance of a current transformer. The value of the rated secondary current always written in the nameplate of a current transformer. In the example image, the magnitude of the rated secondary current is 1A (indicated by the letter (B)).

Rated Transformation Ratio

Rated transformation ratio is ratio of the rated primary current to the rated secondary current.

In the example image, the value of rated ratio is 630, that came from 630A divided by 1A.

In IEC60044-1 standard, rated ratio is expressed as Kn, as shown on below picture. 

For example, a current transformer is designed to work on the primary current of 630A to get 1A secondary current, base on the statement, we can interpret that its rated primary current is 630A and its rated secondary current is 1A .
Thus the rated transformation ratio is Kn = 630A/1A = 630.

Actual Transformation Ratio.

Actual transformation ratio is ratio of the actual primary current to the actual secondary current.

Current Error (Ratio Error)

Current error or ratio error is the error which a transformer introduces into the measurement of a current and which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio.

The current error expressed in per cent is given by the formula:

Where:

Kn is Rated Transformation Ratio.

Ip  is Actual Primary Current.

Is is Actual Secondary Current when Ip is flowing, under the condition of measurement.

Phase Displacement.

Phase displacement is the difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer.

The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually expressed in minutes or centiradians.

NOTE This definition is strictly correct for sinusoidal currents only.

The Following image is the sample of Esitas' current transformer nameplate.