Modelling electrical properties of composite materials

Abstract: Composite field grading materials are used to avoid stressconcentrations in high voltage applications such as cableaccessories and generator or motor end windings. The compositematerials consist of an insulating matrix filled with suitableconducting or semi-conducting particles. Silicon carbide (SiC)powder is one such filler that is being employed. The compositematerials display complex electrical characteristics that aredependent on filler properties, particle concentration,frequency and electric field. Optimization of the stressgrading properties would be facilitated if the characteristicof a specified material mixture could be calculatedapproximately.In this thesis a microscopic model for the local behavior atthe SiC grain contacts as well as a macroscopic model for theglobal performance of the composite material are presented. Thedc and ac characteristics of different SiC powders and variouscomposite materials have been studied by experiments andsimulations. The electrical properties of ethylenepropylene-diene monomer (EPDM) rubber filled with the SiC grains havebeen characterized by several time and frequency domainmeasurement techniques.It is shown that the SiC grain contacts can be modeled bySchottky-like barriers. The SiC powders are heavily doped andthe dominating conduction mechanismin the major part of thefield range is tunneling by field emission, amplified bypre-avalanche multiplication. The frequency dependentproperties are governed both by the interfacial barrier regionsand by the surrounding dielectric.A three-dimensional electrical network model for describingthe frequency dependent electrical properties of the compositematerials has been developed. Accounting for different types ofcontacts between the filler grains is fundamental for theresulting characteristics. The distribution of the conductingparticles in the matrix also affects the electrical propertiesand a well dispersed, and not only random, arrangement is morerealistic. Non-linearity is incorporated in an amended version,which treats the timedependent case.The model has been implemented in a MATLAB® program andthe calculations have been compared to measurements on EPDMrubber filled with SiC grains. It is demonstrated that thenetwork simulations reproduce the general characteristics ofrelevant concentration, frequency and field dependentexperimental results.Keywords:field grading; composite materials; SiC;rubber; electrical properties; nonlinear; time-dependent;model; network; conduction mechanisms; Schottky-barrier;