Investigation of Sensor Component Properties for Application in DC Current Transformer

Direct-current transformers (DCCTs) rely on small toroidal cores made of soft magnetic materials with high permeability, whose magnetic properties affect the sensitivity and linearity of the sensor. Reliable characterization of such small cores and of the resulting sensor component requires measurements with stated uncertainties, which are rarely reported together. This paper applies and integrates established measurement methods, namely, dynamic hysteresis loop measurements on ring specimens and dc current detection from the asymmetry of the saturation current peaks, into a laboratory test setup for the characterization of DCCT sensor components. Permalloy and nanocrystalline toroidal cores are characterized at 50 Hz. The nanocrystalline cores exhibit a maximum relative permeability of approximately 300 × 103 and a coercivity of approximately 1.1 A/m, compared with approximately 100 × 103 and 4.2–4.9 A/m for the permalloy cores. The sensor component with the tested core is calibrated against a reference current shunt for both current polarities, showing a linearity error of 0.59% FS in the range from −4 A to +4 A. The expanded uncertainty of the reference current measurement is ± 926 µA/A according to the GUM and ±906 µA/A using the Monte Carlo method (k = 2). The results show that the tested nanocrystalline cores are better suited for sensitive dc current detection and provide a basis for core selection and DCCT design.