Dependence of the Impulse Strength of Typical Insulation Structures in Oil-immersed Transformer on Waveshapes

Though the ultrahigh voltage-class (UHV-class) transformers have been increasingly used, the increased entrance capacitance always makes it difficult to generate the standard lightning impulse (SLI) in the lightning impulse withstand voltage test. To verify the equivalence of other impulse voltage waveforms and provide basis for the revision of corresponding standards, the dependence of the dielectric strength of typical insulation structures in oil-immersed transformer on impulse voltage waveform is studied. The turn-to-turn insulation model and the oil-pressboard insulation model were utilized, characterized as the common insulation structures. The front time and the tail time of impulses varied from 0.07 to 2.6 mu s and 3 to 300 mu s, respectively. Breakdown characteristics of these two insulation models under different impulse voltages were acquired through experiments. The results show that, no matter what the insulation model used, an increase either in front time or tail time decreases the 50% breakdown voltage. On the basis of the experimental results, discussions are made on the time related breakdown processes and the fungibility of SLI.