A Calibration System for Electronic Current Transformers with Analogue Outputs

摘要

A high-accuracy system for calibration of electronic current transformers (ECTs) with analogue outputs is described. Standard calibration method given in IEC/EN 60044-8 concerns a lock-in amplifier to compare the two voltage signals; one comes from ECT through an integrator while the other signal obtained from a current shunt connected to the secondary terminals of a current transformer. Accuracy of such calibration system directly depends on the accuracy of lock-in amplifier and it is considered as the lack of the system. However, the calibration system presented in this paper is based on a specially designed transconductance amplifier, and based on comparing two current signals on a current transformer test set. Transconductance amplifier has in-phase and quadrature output options to amplify the output signals of relevant current transducers. In-phase accuracy of the transconductance amplifier has been evaluated with a non-inductive squirrel-cage current shunt, while its quadrature accuracy with a magnetically shielded reference Rogowski coil. Performance of the calibration system has also been evaluated. Its estimated relative uncertainty is within 100ppm for magnitude and 100urad for phase.