A practical investigation on the correlation between aging and the dissipation factor value of mica insulated generator windings

The dissipation factor tan delta is a well known parameter for the diagnosis of electrical high voltage insulation. It is used according to standards for the evaluation of new mica insulated stator bars of high voltage machines now for decades. It is also commonly used as parameter for the estimation of aging conditions and operation reliability of power generators. While the methods for the measurement of partial discharge have been in an evolutionary process over the last 30 years, the principal method of tan delta measurements has not changed. So it is nowadays possible to observe the tan delta value over a long period for certain machines. This work shows the experimental results of an investigation which started more than 20 years ago. Tan delta values of several generators have been collected over a period of about 20 years. The tan delta was measured from 0.2 of the nominal voltage up to nominal voltage in steps of 10 % at power frequency of 50 Hz. Besides of the electrical measurement, environmental parameters such as humidity and temperature were determined because of their influence on the tan delta values. Unfortunately, the environmental conditions were not constant at on-site tests. Results of this investigation shows the dependence of tan delta on the operating time. Depending on the type of the insulation system, different progression of the tan delta can be observed.