Ageing-condition assessment of generator transformer bushings by means of dielectric simulation models

Diagnosis and ageing-condition assessment of electrical equipment are getting more and more important, as many of the units in use are close to the end of their designed service life or have even exceeded it already. There are several tools for diagnosis which are well known and proven for transformer monitoring and assessment. In principle, the same tools can also be used for bushing diagnosis. The most common bushing diagnosis method is the capacity and dissipation factor measurement at power frequency and ambient temperature. It can be used to detect partial breakdowns prior to a bushing failure, but it is insensitive for the detection of ageing processes. At ambient temperature, dielectric analyses like PDC (polarization-depolarization current) measurement is significantly better suited to get information about the aging condition related to water or conductive ageing products inside the dielectric (see also [1]). To evaluate PDC measurements, it is very important to have the possibility for comparison of the results with either older measurements of comparable units or with simulation results. For bushing simulation, a special model was developed and presented in [2]. This tool was now adapted to 400 kV OIP (oil-impregnated paper) bushings. Further it was improved to perform more detailed simulations. The simulation model then has been used for comparison with measurement data of service aged bushings as well as for investigation of the influence of temperature and water content by the use of different parameter settings gained from laboratory sample measurements.