Nonlinear Impedance Spectroscopy - Using higher harmonics response and differential impedance for electrical characterization of DC insulation materials

Abstract

With the growing relevance of high-voltage direct current (HVDC) technologies comes an increased need of reliable electrical characterization of insulation materials. Electrical conduction mechanisms in insulation materials under DC voltages are rather complicated and appropriate characterization tools need to be further developed. This thesis investigates the potential of using nonlinear impedance spectroscopy(IS) for electrical characterization of insulation materials at DC fields, or at low frequency AC fields. A theoretical background is provided where it is shown for different models how nonlinear current-field dependence give rise to currents that contain amplitudes at higher frequencies than that of the exciting voltage (the fundamental frequency). Experimentally, different material systems were studied with two advanced nonlinear IS measurement techniques; higher harmonics measurements at high fields and differential conductivity measurements with small amplitude AC spectroscopy at large superimposed DC. The main results are presented in form of steady state current-field characteristics. We therefore conclude that nonlinear IS is in principle useful for electrical characterization of nonlinear insulation materials, particularly field grading materials. Limitations of the methods are related to non-robust material properties and problems with reaching a quasi-steady state at reasonable frequencies.