This paper presents an investigation of the temperature dependence characteristics specific to cryogenic planar Multi-Layer Inductors (MLIs). This paper establishes that the inductance of a planar MLI at a specific frequency varies with temperature when the sensor is cooled down to 4.2 K while providing a detailed analysis of various possible factors that might contribute to the variation in the sensor performance, such as the thermal deformation and the variation in the properties of sensor materials, using a combination of experiments and simulations. By calculating the interlayer capacitance, we have attempted to adopt a novel approach in the investigation of the effects of thermal deformation on the sensor. In order to arrive at that, the relative permittivity of the base material (G10CR-FR4) at cryogenic temperatures was obtained through experiments. The ANSYS static structural package was used for modeling thermally induced deformations, after which the deformed capacitance and inductance were obtained using Ansoft MAXWELL. From the analysis, we have concluded that the variation in the inductance of the sensor has a direct correlation with the electrical resistivity (hence the residual resistivity ratio) of the coil material. The number of inductor layers and the area of the component layer will also determine the temperature dependence phenomenon. These conclusions are not obvious from the established inductance models.
High Temperature Superconducting (HTS) cables have remarkable electric power transmission characteristics compared to conventional power cables. Thus, HTS cables are suitable for the sustainable electrical grids of the future. Electric faults of various origins and durations are inevitable in a commercial electric power transmission network. The integration of HTS cables to these networks requires reliable cable operation under fault conditions. However, it was found that HTS cables require a long recovery interval after the fault and subsequent quench. It is primarily attributed to the high thermal resistance of the cable dielectric layer. An innovative dielectric design is proposed in this article to improve the thermal performance of HTS cables and the results are compared with that of a conventional HTS cable. Transient thermal analysis was carried out to determine the recovery interval and the electric insulation characteristics were studied using an electrostatic analysis. Both studies were performed using Finite Element Analysis (FEA). It was found that a reduction in the recovery interval is possible without deterioration in the electric insulation level.
Electrical conductivity measurement using an eddy current sensor requires measuring the absolute change in the impedance of the sensing coil. There have been many different attempts at this by using phase sensing circuits, utilizing impedance analyzers etc. This work utilizes two different oscillator circuit (LC oscillator using an unbuffered inverter and Relaxation Oscillator) to determine the change in impedance of the coil. The LC oscillator is designed in such a way that the circuit is insensitive to change in the series resistance of the sensing coil and hence, only a function of the inductance of the coil. Once the inductance of the coil is determined, a relaxation oscillator is used in an operating region in which the output frequency is a strong function of the series resistance of the coil. This switched oscillator circuit technique is used to determine the impedance of a cryogenic sensing coil at 295 K and 77 K with a Niobium (Nb) target kept at specific distance. Experimental data is compared with that of an impedance analyzer circuit and measurement errors are presented.
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