The bubble effect in bushings – investigations on models

This paper refers to the bubble effect in transformer bushings. The bubble effect consists in releasing water from moistened cellulose insulation after exceeding a critical temperature. The most dangerous result is a pressure increase that leads to an explosion and fire. Research was conducted using laboratory models of OIP (Oil- Impregnated Paper) and RBP (Resin-Bonded Paper) types of bushings immersed in mineral oil and synthetic ester. For all of the investigated systems we determined the dynamics of water adsorption upon contact of the insulating material with air and the temperature characteristics of bubble effect initiation depending on water content in cellulose. In the experiment we investigated the dynamics of changes of water content in dielectric liquid, water content in air over the surface of the dielectric liquid, and air pressure over the surface of the dielectric liquid. In the future, the technical possibilities will be analyzed of using the dynamic changes of the three quantities investigated here to detect the bubble effect in bushings.