Dynamic Behavior of Micro-droplets in Liquid Dielectric and Its Effect on Partial Discharge

Moisture has a great influence on power transformers, because high moisture will seriously degrade the insulation performance, especially when the moisture is suspended in the oil in an emulsified state. In this paper, a multi-physics simulation model coupled the flow field, electric field and phase field was established to investigate the effect of the dynamic behavior of water droplets on the insulation properties of liquid dielectrics. The results showed that the suspended micro-droplets under electric field exhibited tensile deformation along the direction of the electric field. The degree of deformation increased with the increase of electric field strength and the droplet radius. The deformation ratio can be expressed as a function only related to the dimensionless electric capillary number Boe. When the electric field intensity exceeded a certain value, the micro-droplets would be unstable and the head would become pointed, forming the Taylor cone. When the electric field was further enhanced, the tip of the water droplet emitted micro-jets. According to the existing theory, the critical electric field strength required for the instability of water droplets of different radius was calculated. The influence of the dynamic behavior of water droplets on the breakdown characteristics of the liquid was discussed. When the water droplet was unstable, the electric field near its head was severely distorted, which would easily lead to partial discharge and vaporization of the water drop. The micro-jets emitted from the tips and the arrangement of the micro droplets along the electric field direction would easily form a water bridge, which could induce breakdown of liquid dielectric.