Investigation on the partial discharge of the cavity in Nomex insulation for mining dry type transformer

Cavity discharge is one of the main reasons caused the deterioration of Nomex insulation in dry type transformer. Different from the oil-paper insulation, Nomex paper is completely exposed to the air, that is to say, two types of partial discharge (PD) will be simultaneously generated in the cavity embedded in Nomex insulation and in the micro-cavity of interlayer insulation, which discharge characteristics are significantly different from that in oil-paper insulation. Therefore, it is significant to carry out the research on the discharge characteristics of the cavity in Nomex insulation so as to conduct the condition evaluation and fault diagnosis of mining dry type transformer. This paper mainly doses the following researches with the object of Nomex insulation in mining dry-type transformer. Combined with the actual structure of the dry-type transformer winding, the cavity discharge simulation model was built on the COMSOL Multiphysics simulation platform, and the mechanism of cavity discharge is analyzed from two aspects: electric field strength and space charge distribution. In the shield room environment, the model of cavity defects in Nomex insulation is established, based on this model, using conventional pulse current method, the PD test is carried out under the voltage stress increases with the step. The characteristics which can effectively reflect the change law of PD are extracted from the experimental data, and the cavity discharge law under different voltage is analyzed. The results show that: with the increase of voltage, the number of PDs increases, the phase of initial discharge moves forward, the phase distribution is widened, the discharge pulse amplitude increases gradually, and the shape of the discharge spectrum changes from hill to rabbit ears. Briefly, this paper provides a theoretical basis for the condition evaluation and fault diagnosis of Nomex insulation in mining dry type transformer.