Non-linear Resistive Material Research Articles

Surface potential measurement of model stator bar with stress grading system by field sensor

This paper proposes a novel method for measuring the potential distribution along the stress grading system of a model stator bar whose voltage is over several tens kV. The technique uses a Pockels field vector sensor by introducing an inverse calculation process from the measured field profile to the potential profile on the measured stator bar. With this system, a potential distribution along a model bar under 50 kVp, 50 Hz voltage is successfully measured. In principle, it is possible to estimate the potential either by measuring the horizontal or vertical component of the field. The utilization of the vertical component is preferable in measuring a model bar due to robustness against optical alignment errors. Through the potential distribution measurement of the model bar, although the maximum value of the electric field increases slightly with the applied voltage, it is confirmed that the electric field grading function was working due to the non-linear resistive material.

Nonlinear resistivity in epoxy composites containing Mn‐based ferrite magnetic particles

ABSTRACTIn this study, magnetic particles were employed as nonlinear resistive materials in electric stress grading composites. Epoxy resin films containing either manganese‐based ferrite magnetic (Mn‐FM) particles or manganese‐magnesium‐based ferrite magnetic particles were prepared, and the threshold voltages required for these films to exhibit nonlinear resistivity were determined. The sample containing Mn‐FM particles having irregular shapes [Mn‐FM(i)] at 20 vol % showed the highest nonlinear resistivity. Assessments of the magnetic characteristics of these materials demonstrated a relationship between magnetic flux density and nonlinear resistivity. The magnetic flux density of the Mn‐FM(i) composite was the highest among the various specimens, indicating that magnetic flux density may affect nonlinear resistivity. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48229.

Linear Subspace Reduction for Quasistatic Field Simulations to Accelerate Repeated Computations

The design of electrical machines and high-voltage devices requires the simulation of non-linear low frequency electromagnetic problems in time domain. Typically magneto- and electro-quasistatic problems in magnetic vector/electric scalar potential formulation lead after space discretization to differential-algebraic or ordinary differential equations, respectively. Therefore, huge systems of equations have to be solved in both cases. Typically a large number of degrees of freedom (DoF) is in domains with constant material parameters, i.e., linear subdomains, e.g., air or vacuum in exterior domains. In this paper, we present a method for low frequency simulations based on the proper orthogonal decomposition to reduce the DoF in these linear subspaces. The application of the method will be shown for a simple transformer model and within a global sensitivity analysis (uncertainty quantification) of the switching point in a non-linear resistive material used in a 11 kV standard insulator model.

Nonlinear resistive electric field grading Part 2: Materials and applications

The article emphasizes the ability of new nonlinear resistive materials to be tuned for specific applications, which opens up new possibilities in designing electric field control of products for medium and high voltage applications.

Power conversion of energy fluctuations

An analysis of the theoretical efficiency of converting the nonlinear equilibrium thermal fluctuations of a system to a useful form of energy is made based on equilibrium distributions of statistical mechanics. For the model considered, electric energy fluctuations, as in a resistive material, are converted to a useful form of energy by combining these fluctuations with those from a nonlinear resistive material at another temperature. The system selected for the computed results uses two Alkemade diode models, each in contact with a heat reservoir, as the resistive materials. The resulting rectified current is calculated by deriving a master equation giving the probability of discrete electron jumps across the potential barriers of the diodes. From this, the efficiency is computed of the direct power conversion of thermal to electric energy and of electric to thermal energy as a function of the output or input voltage of the system. These results for the examples are in agreement with fluctuation theory and show that the conversion efficiency increases as the system size decreases to make the fluctuation energy a significant fraction of the total energy. The results also show that there are no theoretical limits on the assumed model in approaching the efficiency limitations of the Carnot cycle. Finally, the limitation on the available fluctuation power that could be provided from a unit area of a resistive film to an assemblage of small circuits was considered. It was determined that this very high limitation was of the order of ${10}^{8}$ ${\mathrm{W}/\mathrm{m}}^{2}$. This indicated the potential that, if the material and fabrication problems could be solved, this technique could yield practical amounts of power at high efficiencies over a wide range of temperatures.