Experimental investigations on the behavior of partial discharges in dry type transformers as a function of temperature and voltage stress

Summary

Dry type transformers are widely used in power transmission and distribution systems due to their environmentally friendly properties. Partial discharges (PD) are known as one of the early sources of insulation failures in transformers. Therefore, PD measurements are used as the main quality assessment of power transformers. Dry type transformers are compact enough to fit inside the nacelle cubicle or tower of wind turbines. Due to the different environment conditions and the importance of these transformers as a kind of new generator step up transformer within the green energy approach, these kind of dry transformers should be tested more strictly. Therefore, in terms of lifetime of the transformer, more information about performance of this type of transformers especially at higher temperatures are needed. In addition to increase the possibility of failure detection by production, lightning impulse (LI) tests could be also considered as routine tests. The temperature also could change frequently due to the different environmental conditions and load powers in wind turbines. In order to do a laboratory investigation, a new coil of a dry type transformer with internal voids has been prepared. This sample has been used in order to investigate the effect of LI as well as temperature on PDs. X-Ray photography was done in order to localize the voids inside the coil. The localized voids have been investigated with special arrangement to produce internal PD activities inside the insulation materials of the coil. Finite element method has been used in order to simulate the electric field inside the sample. A shielded thermal cabin has been used in order to control the temperature of the test object during the experiments, which increases up to 60 °C. Partial discharge activities at different temperatures as well as LI effect on the partial discharge behaviour have been investigated. The results show the dependency of partial discharge inception and extinction voltage on the temperature. The partial discharge activities also have been affected by lightning impulse, which is discussed in more details.

Additional informations

Authors

S.A.M. NAJAFI, H. MOVAGHARNEJAD, U. SCHIEFELBEIN, P. WERLE

Keywords

Partial discharge, Dry type transformer, Lightning Impulse, Temperature