The principle of current transformer is based on the principle of electromagnetic induction. The current transformer is composed of a closed core and winding. It's a little winding circle number, string in the current need to measure the line, so it often have all the current flows through the line, the secondary winding circle number is more, concatenated in measuring instrument and protection circuit, current transformer at work, it's two loop is always closed, so the measuring instrument and protection circuit in series with the coil impedance is very small, the working state of the current transformer is close to a short circuit.
Current transformer connection mode.
Rated ratio and error: the rated ratio of the transformer is the rated voltage ratio of the voltage transformer and the rated current ratio of the current transformer. The former is defined as the ratio of the rated voltage U1N of the original side winding and the rated voltage U2N of the side winding; The latter is the ratio of rated current I1N to I2N. That is, KN=U1N/U2N (for voltage transformer) KN=I1N/I2N (for current transformer)
Voltage (or current) transformer primary side voltage (or current) change within a certain range, the general rules for 0.85 ~ 1.15 U1N I1N (or 10 ~ 120%), deputy side voltage (or current) should be in proportion to change, and the original, deputy side voltage (or current) should be in phase. However, due to the internal impedance, excitation current and loss of the transformer, the ratio and phase error are called the ratio and the Angle difference.
Contrast is after conversion of the secondary voltage (or secondary current) with a voltage (or current) value of the latter is the difference between the size, the fU as the contrast of voltage transformer, fI contrast for current transformer. When KNU2>U1 (or KNI2>I1), the ratio is positive and vice versa.
For the voltage transformer without compensation, the difference is negative, the Angle difference is positive, the absolute value and the Angle difference decrease with the increase of voltage. When the core is saturated, the difference and Angle difference increase with the increase of voltage. For the current transformer without compensation, the difference is negative, the Angle difference is positive, and the absolute value and the Angle difference decrease with the increase of current. The compensation method can reduce the error of the transformer. Generally, it is compensated by winding additional winding or adding additional iron core on the transformer, as well as the corresponding resistance, inductance and capacitance element. The commonly used compensation method is the number of turns compensation, the fraction turn compensation, the small iron core compensation, the shunt capacitor compensation, etc

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