Effects of ambient humidity and thermal aging on properties of Nomex insulation in mining dry‐type transformer

Temperature and humidity are the main factors that cause the decomposition of Nomex paper widely used as turn‐to‐turn insulation material of mining dry‐type transformers. In this study, for understanding the effect of ambient humidity and thermal aging on the properties of Nomex paper, specimens with the initial moisture content of 7% were aged in the oven for 168, 504, 672 h at 180°C, respectively. After thermal aging, corona discharge experiments of unaged and aged specimens with different aging degrees were carried out at different relative humidity (RH) levels from 10 to 70%. The characteristic parameters of corona discharge, such as the average discharge amplitude and the number of discharges, were measured. Then, the surface conductivity and the surface potential decay rate were measured. The morphology and attenuated total internal reflectance Fourier transform infrared spectroscopy were also analysed for explaining the difference of properties be-tween unaged and aged specimens. The experiment results indicate that the average discharge amplitude decreases with the increase of RH or the deepening of the aging degree of specimens, but the number of discharges shows the reversal tendency. The changes in the physical structure and chemical composition, which is induced by the hydrolysis reaction of Nomex paper, accelerate the surface potential decay and increase the surface conductivity.

Development and pattern identification of end-winding discharge under effect of relative humidity and temperature for HV motors

During operation of high voltage(HV)motors,different types of discharges,e.g.surface discharge,corona discharge and bar-to-bar discharge can occur at the same time,increasing the difficulty of partial discharge(PD)sources determination and PDs pattern identification.In this study,the end-winding of a 10 kV motor coil was artificially aged and the related PD was measured.The initiation and variation of different PDs under multi-factor stresses were studied.The localised fingerprints coexisting with multiple PDs were identified and analysed.The results confirm that the end-winding discharge process was significantly influenced by the experiment relative humidity(RH)and temperature.The bar-to-bar discharge was easily recognised at a lower voltage since the identification of bar-to-bar discharge pattern would be affected by the corona discharge pattern at a higher voltage.It is shown that it is more difficult for the corona discharge to be detected when RH exceeded 80%,while the surface discharge dominated at the higher RH.In addition,the PDs were more easily identified with the rise of the temperature.This study can provide a reference in PD identification test and be useful for the PD online monitoring of HV motors.