Study On Interfacial Polarization Characteristics Of Double Layer Nonlinear Dielectrics

Non-linear insulation materials are widely used in insulation of cable terminals and motor stator windings due to the functions of grading the electric field.With the development and application of nonlinear insulating materials,a composite insulation structure consisting of double or multiple layers of nonlinear insulation dielectric appears in power system,such as motor insulation and cable termination,etc.However,the composite insulation structure composed of double insulating dielectrics with different properties leads to interfacial polarization.And thus,research on mechanism of the interfacial polarization of nonlinear dielectric will provide the necessary theoretical basis for design of the insulation structure.Therefore,it is of great theoretical and engineering significance to study the transient and steady-state characteristics of interfacial polarization in double-layer nonlinear dielectrics.In the existing interface polarization theory,a single-layer linear insulating dielectric is equivalent to a circuit formed by parallel connection of a linear capacitor and a linear conductance.The equivalent circuit model does not consider the relaxation polarization behavior of a single-layer dielectric,which does not match the reality.In this paper,a new equivalent circuit model was proposed,which adds a relaxed polarization branch composed of a nonlinear capacitor and a nonlinear conductance in series based on the equivalent circuit in the existing theory and changes linear elements to nonlinear ones,and thereby,a parallel equivalent circuit with three branches consisting of non-linear elements was established.In this equivalent circuit,each nonlinear equivalent capacitance and equivalent conductance are mathematically described in terms of voltage contolled polynomials,and then,a simulation model was constructed using Simulink software according to the principle of current continuity,loop voltage law,and function of nonlinear components.Then,using the signal generator and the linear high-voltage amplifier,the response current waveform and data were obtained through experiments under the triangular wave and the trapezoidal wave excitation voltage with different frequencies and amplitudes,and then the total current waveform and data obtained by the simulation were compared with those obtained through the experiments,whose results showed that the two were close.Therefore,the equivalent circuit proposed in this paper is more in line with the reality.Finally,according to the validated equivalent circuit model,a two-layer nonlinear dielectric simulation circuit model was constructed.The parameters in the simulation circuit model are determined based on 0.2mm thick polyethylene samples.The influence of the field strength on the interfacial polarization was investigated under several step voltages with different amplitudes,whose results showed that the higher the applied voltage amplitude was,the slower the interfacial polarization was established and the slower the depolarization process was.The effect of different material properties on the interfacial polarization was explored through changing the ratio of CS1 and GS1 in the upper layer and the ratio of CS2 and GS2 in the lower layer,whose results showed that the polarization and depolarization times were shorter when the ratio of CS to GS in the upper and lower layers was close,that is to say,the polarization and depolarization times of a two-layer structure consisted by two similar dielectrics were shorter.Finally,simulation results were verified through experiments.