Use of a lock-in amplifier for calibrating an instrument current transformer

Errors of instrument current transformers (ICT) are mostly determined using a comparative method. The difference between the secondary currents (magnitude and phase displacement) of a standard and of the ICT under test is evaluated by means of special systems which display the errors of the ICT calibrated in relation to the standard. These systems can evaluate errors in a certain range of measured currents (mostly 5% - 120% of the rated value of the measured current) and only for certain transformation ratios of the standard and the ICTs under test. Measurements for smaller primary currents or for different ICT transformation ratios show a bigger error, or their accuracy is not generally determined. Due to increasing demands on measurement and energy saving, determining ICT errors at 1% of the primary current rated value or lower is now a topical issue. The use of a lock-in amplifier, as described in this paper, enables the evaluation of differences between a standard and an ICT calibrated at currents lower than 1% of the rated value, and also for atypical ICT transformation ratios.