Apparatus and method for improving the accuracy of instrument transformers, wherein one or more electrical parameters related to a phase of an electrical conductor are calculated. Signals related to one or more disturbing factors influencing the measurement of the electrical parameters - such as temperature, cross-talk effects- are also calculated. A corrective unit stores a plurality of corrective coefficients corresponding to different values of the calculated disturbing factors. Such coefficients are applied to the calculated values of the electrical parameters so as to improve the accuracy of measurement.