Electrical aging test on twisted pair specimens under different environmental conditions

Low voltage stator windings of electrical machines are often realized by means of wires insulated by organic enamel. This kind of enamel can be subjected to partial discharges (PD) activity that could lead to breakdown condition the electrical insulating system so that the life time could be shorter than the expected one. In this research work, twisted pair specimens insulated by non corona resistant polyamide-imide enamel have been subjected to 50 Hz sinusoidal waveforms having different amplitudes. The electrical stress has been chosen so that in all the test conditions the applied voltage was higher than the partial discharge inception voltage (PDTV) value. In this way the surface aging has been inducted by the partial discharge activity. Aging tests have been carried out into an environmental chamber in order to control the relative humidity (RH) value so to analyze the effect of this parameter on the specimen life times; the total breakdown condition was chosen as end-point criterion and the relevant life time values have been recorded. The obtained results show that the PD activity is strongly influenced by the relative humidity levels and the change of its intensity considerably affects the time to breakdown values. Furthermore, the obtained data allowed to compute the life curves relevant to the different considered RH levels evidencing the relationship between the environmental conditions and the life time values.