The effect of wavefront on the breakdown characteristics of transformer oil under positive impulse voltages

Abstract

Larger entrance capacitance of ultrahigh voltage transformers makes it more difficult to generate the standard lightening impulses in the on-site tests, especially for the front time. To analyze the rationality of substituting test impulse with longer wavefront for standard lightening impulse, this paper studied the effect of wavefront on the breakdown characteristics of transformer oil under positive impulses in the quasi-uniform and non-uniform electrical field. The results showed that the 50% breakdown voltage under quasi-uniform field was almost independent of front time, but the minimum breakdown voltage increased as front time increased. However, both the 50% breakdown voltage and the minimum breakdown voltage under non-uniform field were positively related to the front time. The recorded light signals of positive streamers indicated that the initiation processes of positive streamers dominate the breakdown under positive impulses. The initiation voltages of streamers were determined through experiments and it was found that the relationship between the initiation voltage and front time was consistent with that between the 50% breakdown voltage and front time. Based on experimental results, some discussions were made on impulse withstand tests of oil-insulated equipment.