The rated transformation ratio KN of a current transformer refers to the rated voltage ratio of a voltage transformer and the rated current ratio of a current transformer.
The former is defined as the ratio of the rated voltage U1N of the primary winding to the rated voltage U2N of the secondary winding; the latter is the ratio of the rated current I1N to I2N. That is, KN = U1N/U2N (for voltage transformers) and KN = I1N/I2N (for current transformers). When the primary voltage (or current) of a voltage (or current) transformer varies within a certain range, it is generally specified to be 0.85～1.15U1N (or 10～120%I1N), and the secondary voltage (or current) should change proportionally, and the primary and secondary voltages (or currents) should be in phase.
However, due to factors such as internal impedance, excitation current, and losses in the transformer, errors in the ratio and phase may occur, which are respectively called ratio error and phase error. The ratio difference is the ratio of the difference between the converted secondary voltage (or secondary current) and the primary voltage (or primary current) to the latter, i.e., fU is the ratio difference of a voltage transformer, and fI is the ratio difference of a current transformer. The ratio difference is positive when KNU2 > U1 (or KNI2 > I1), and negative otherwise. The angle difference is the angle between the secondary voltage (or secondary current) phasor and the primary voltage (or primary current) phasor after a 180° rotation, expressed in fen (minutes). It is stipulated that the angle difference is positive when -妧2 (or -夒2) of the secondary side leads 妧1 (or 夒1), and negative otherwise. For voltage transformers without compensation measures, the ratio difference is negative, and the angle difference is generally positive. Both the absolute value of the ratio difference and the angle difference decrease with increasing voltage; when the core is saturated, both the ratio difference and the angle difference increase with increasing voltage. For current transformers without compensation, the ratio error is negative, and the phase angle error is positive. Both the absolute value of the ratio error and the phase angle error decrease as the current increases.
Compensation methods can reduce the error of the transformer. Compensation is generally achieved by adding additional windings or cores to the transformer, and by connecting appropriate resistors, inductors, and capacitors. Common compensation methods include turns compensation, fractional turns compensation, small core compensation, and parallel capacitor compensation.