Electrical Characterization of Partial Discharge Resistant Enamel Insulation

Licentiate thesis, 2014

Adjustable speed drives for rotating machines have become increasingly popular as they provide possibilities of smooth and accurate process control as well as for energy savings. In such systems, due to the fact that the voltage applied at terminals of motor windings is no longer purely sinusoidal but characterized by a high content of harmonics, the appearing electrical and thermal stresses yield premature failures of the winding insulation, mainly because of an increased partial discharge (PD) activity. To tackle the problem effectively, PD-resistant enamels have been developed, by introducing various inorganic fillers into the base polymer of the wire insulation, that exhibit an increased resistance to PD activity. The effect seems, in most of the cases, to be strongly dependent on the dielectric properties of the filler used and the degree of interactions with the host materials. In this respect, a recently developed enamel insulation, in which the top layer of the multilayer coating is filled with micro-particles of chromium (III) oxide (Cr2O3), has demonstrated exceptional properties. To elucidate the effect of material properties on the PD behavior, the presented in this thesis study concentrates on electrical characterizations of the new enamel insulation by measuring and analyzing its electric conductivity and complex dielectric permittivity and comparing these parameters with of the properties of enamel without the filler. In addition, the decay of surface potential induced by deposition on the enamel wire surfaces electric charges from a corona source is examined. The performed analyses show that the addition of Cr2O3 filler results in a highly conductive and dispersive material, as compared to the base enamel. As a consequence, the top coating layer of the enamel wire insulation activates suppression of PD activity, while the insulating properties of the bulk material are kept unchanged. The presence of chromium oxide results in enhanced interfacial polarization and low frequency dispersion (hopping conduction) of the enamel. The material conductivity is increased because of enhancement of charge carrier mobility as well as a shift of the maximum of trap density distributions towards lower energy levels.. The mechanism of the surface potential decay is analyzed by means of a model in which the combination of charge injection, dipolar polarization and intrinsic conduction appear to be the most important contributors to the observed surface charge dynamics.