Diagnosis of generator stator winding insulation based on dissipation factor measurement

Two groups of fresh full size stator bars of an 18 kV/180 MW generator were subjected to accelerated thermomechanical and electrical-thermal combined stress aging respectively to investigate the deterioration of groundwall insulation. The dissipation factor measurement was performed during the thermomechanical stress aging together with tap testing and dimension measurement. In addition, the dissipation parameters were also measured during the electrical-thermal combination aging. The correlation analysis between the dissipation parameters and the dimension of coil straight section, the insulation delamination were made respectively. It was found that the high voltage dissipation factor measured at room temperature, the dissipation factor tip-up and the capacitance tip-up are strongly correlated with the degree of insulation delamination, which indicates that the three parameters are sensitive indicators of the aging condition of stator bar insulation. It was also found that among all of the dissipation parameters, the high voltage dissipation factor measured at high temperature has the strongest correlation relationship with the change in the dimension of coil straight section. It is inferred that the parameter can reflect more chemical and structural changes within the insulation in addition to insulation delamination. The comparative analysis on the voltage dependency of dissipation factor implies that the characteristic crest on the dissipation factor curve could be taken as a fingerprint parameter to distinguish the insulation delamination type.

I. Introduction

A lot of research has been done on the condition assessment of large generator insulation in the last decade [1]–[3]. It was reported that there was a week correlation relationship between the breakdown strength and the traditional nondestructive parameters, such as partial discharge quantity, capacitance, dissipation factor, and resistance. The maximum correlation coefficient between those parameters and the breakdown voltage was only 0.54 [4], [5].