INDUCTIVE CURRENT SENSOR BASED ON NANOCRYSTALLINE ALLOYS
In this paper are described the design and application of an inductive magnetic sensor based on nanocrystalline alloys. The sensor is adjusted for measurement and monitoration of currents with intensities from some microamperes to miliamperes. Its functional behaviour, performance and a specific application are described. One of the biggest advantages of this current sensor is its non-invasive characteristic, it means, is not required any direct contact with the circuit that is being monitored. It becomes appropriated for diverse applications where this requirement is necessary. An example of this sensor application is the metal oxide surge arresters leakage current monitoring.
The ferromagnetic nanocrystalline alloys are, in general, magnetically soft materials; it means, possess high permeability μ, low coercivity HC and small hysteresis loss, when compared with other conventional ferromagnetic alloys. This type of material is composed of a physical structure of nanocrystals with 10 nanometers of diameter, also, possess a constant magnetic permeability on a wide amplitude scale of the applied field. One another advantage of the nanocrystalline alloys is its superior induction saturation when compared with others conventional soft magnetic alloys, allowing a significant reduction of the core dimensions, Such properties permit the design of current transformers appropriated for low intensity signals monitoring using nanocrystalline alloys. In addition of the high permeability values, the alloys electric resistivity does not suffer a big variation with the temperature. Its high magnetic permeability facilitates the core magnetization process, requiring a relatively small magnetization current of the primary circuit, this characteristic allow the use of this material in current transforms applied to measurements with the motivation to reduce the turns ratio and phase errors when compared with the utilization of classic ferromagnetic materials generally used in commercial current transformers. Considering the low leakage current intensity of the ZnO surge arrester, it was developed a toroidal inductive current sensor with magnetic permeability μ and high sensibility and linearity. However, it’s necessary to observe the operation limits in the hysteresis and saturation curves with the purpose to avoid non-linearity problems. In Figure 1 is presented the operation region necessary to obtain a linear ratio between the voltage V in the current-voltage converter output and the leakage current IT in the current sensor operation. Linear region of the magnetization curve. One of the motivation for this research is the utilization of this current sensor in the ZnO surge arresters, because they lead a small leakage current when they are submitted to the nominal operation voltage. The total leakage current amplitude increase with the time, and this current it’s one of the parameters used to monitoring the surge arrester degradation level. The ZnO surge arrester leakage current can be divided in two components: resistive and capacitive. The third harmonic component of the resistive current is the most sensible parameter used in the monitoring of the surge arrester degradation level. Consequently the sensor must have a high linearity, sensibility and resolution degree to allow an accurate diagnostic of the ZnO surge arrester degradation.
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