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

Abstract

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 between 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.

An improved second‐order kinetic model for degradation analysis of transformer paper insulation under non‐uniform thermal field

“…The fluctuation of moisture with time at each winding height is reflected in the form of error bars. Eventually, the mathematical relationship between normalised moisture content and winding height can be expressed as Equation (4).…”

“…The main component of insulating paper is cellulose, which is a polymer of glucose monomers. It is recognised that the degree of polymerisation (DP) is an intrinsic parameter that characterises the ageing condition of insulating paper [4,5].…”

A Comparative Analysis Applied to the Partial Discharges Identification in Dry-Type Transformers by Hall and Acoustic Emission Sensors

“…Before a complete transformer insulation failure, it is common to detect low-energy ionization processes in an insulation system. This phenomenon, known as partial discharge (PD), can be considered an important indication of the operational condition of the dry transformer [25][26][27].…”

“…In addition, several works have also evaluated the effect of discharges on insulating paper. Li et al (2020) [25] presented a model to assess the degradation of insulating papers caused by partial discharges. Mishra et al (2020) [29] proposed a method for evaluating the condition of insulating paper based on frequency domain spectroscopy (FDS) under temperature variations.…”

“…More specifically, in the case of transformers with dry insulation, the failures can occur in the epoxy resin [24], which constitutes the device's external insulation, and it is directly in contact with the environment. On the other hand, PDs can start in the paper that physically insulates the two transformer's windings [25]. In this scenario, the development of methodologies that result in the differentiation of discharges in Nomex paper and epoxy resin becomes essential for improving the capabilities of the monitoring system since each type of material requires different maintenance actions.…”