Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms

It has been observed that voltage waveforms generated by power electronic converters may affect significantly the reliability of electric motor insulation. Since partial discharges are considered to be the main cause of the reliability loss, new enamel insulations for magnet wires are being developed in order to withstand better stress amplification. The electrical characterization of these insulating materials is often carried out through aging tests which may provide estimation of life under different stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation between aging factors and stress conditions. This paper deals with this topic, showing experimental evidences of relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated with charge accumulation, which can be directly evaluated through space charge measurements. Characterization of insulating materials and comparison of materials candidate for application in power electronic waveform environment can be carried out resorting to the methodology proposed here. This approach can provide, therefore, a useful feedback to wire manufacturers regarding, e.g., the choice of additive nature and enamel components for magnet wires in power-electronic controlled motors. Index Terms--Square waveforms, magnet wire enamel degradation, space charge trap depth, partial discharge inception voltage, enamel surface conductivity.