The Application Of Isothermal Relaxation Current Method In Generator Stator Bar Detection

The condition of stator bar insulation, which is generator's main insulation, largely determines a generator's remaining life. Therefore, detection of stator bar insulation is particularly important. An insulation detection and data analysis method based on isothermal relaxation current is used as primary investigation tool to measure stator bars with different structure and aging methods. For thermocycling aging which has a different phenomenon in stator bar measurement, standard test aluminum bars divided into two groups as with and without anti-corona layer are used for further investigation. Isothermal relaxation current measurement is carried out as well as partial discharge, dielectric loss and AC breakdown test.After measuring every aging stage of different kinds of stator bars and aluminum bars, three analyzing method are used, which are: fitting slope of depolarization current under log-log coordinate, aging factor A based on isothermal relaxation theory, and dielectric properties in frequency domain using Fourier transform. The results show that: with the aging process, the fitting slope n's absolute value will decrease, which can diagnose insulation status effectively. The amplitude of spectrum curve also generally decrease with aging, and in some of the analysis results, even a spectrum peak's movement can be observed. For electrical and thermal united aging and electrical , the increasing of aging factor A can reflect insulation deterioration, but for thermocycling aging, stator bar and aluminum bar with anti-corona layer's' aging factor A shows an obvious downward trend while aluminum bar without anti-corona layer' aging factor A remain almost unchanged. This difference can basically reflect that the main result of thermocycling aging is: the interface of main insulation and anti-corona layer is aged and stratified, while the main insulation is rarely affected. The post curing phenomenon can also be observed in thermocycling aging's analysis result. And compared to thermocycling, the aging of main insulation is more significant under electrical stress.