Electrical Aging Tests on Conventional and Nanofilled Impregnation Resins

The main insulating system components of stator windings of rotating electrical machines are organic enamel and impregnating resin. When power electronic converters supply motors, the stator insulation system is subjected to voltage waveforms such as PWM composed by trains of pulses. Due to reflection phenomena, overvoltage can be associated to pulsed waveforms and partial discharge (PD) activity can incepts leading to the insulating system failure in short time. In this work two types of conventional and nanofilled impregnating resins, based on unsaturated polyester and solvented alkydic resin matrices, have been compared. Suitable twisted pair samples prepared by means of wires insulated with conventional enamel and by two impregnating resins have been tested. The twisted pair samples have been subjected to sinusoidal voltage waveforms, having amplitudes slightly higher than the Partial Discharge Inception Voltage. Aging tests have been performed until the insulating system breakdown, which is assumed as end-point criterion: during the tests, the PD activity has been monitored by means of a PRPDA system. For each sample, starting from the PD signals post processing, the apparent current of positive discharges has been considered: this derived quantity showed a linear behavior with the degradation time. In order to define a life model, the slopes of the straight lines have been put in relation with the related lifetimes by means of the inverse power law.