High-voltage low-frequency dielectric-spectroscopy used for characterization of nonlinear insulation materials

Common small-amplitude dielectric spectroscopy (DS) is restricted to linear response. We discuss here two concepts of DS at high voltages for characterizing nonlinear insulation behavior relevant for HVDC equipment. The first method uses the higher harmonics of the current, which are naturally generated by nonlinearities. The second method measures small-signal linear response with a superimposed high steady-state voltage, which yields the differential admittance. After a brief introduction to the theoretical prerequisites, experimental results for two illustrative material types are discussed, a polymer-filler composite used for electrical field-grading and a ZnO varistor ceramics used in surge arrestors. Particular focus is on the reconstruction of the current-voltage characteristics. General experimental feasibility of the methods is proven by showing consistency of the different measurements. Additionally, materials with non-robust electrical behavior or very long relaxation times can be identified from inconsistencies, which also provide the limits of the methods.