Electrical stress and pre-breakdown in HVDC transformer oil : Modelling and simulating the relationship between electric charge and cavity formation in a dielectric liquid.

Sammanfattning

This study examines cavity growth in transformer oil under HVDC electrical stress using MATLAB and COMSOL Multiphysics. A pressure-balance model was implemented in MATLAB to calculate the cavity radius as a function of applied charge, while a COMSOL model with moving mesh was developed to simulate cavity expansion. Since the models are based on different governing equations, both were validated separately against experimental data from Hitachi Energy under atmospheric and reduced pressure conditions. The MATLAB model gave the lowest radius errors in most regions, especially when the cavity growth was stable and close to the spherical quasi-static assumptions. The COMSOL simulation showed larger deviations, mainly at reduced pressure and higher charge levels, but still followed the expected physical behaviour under the tested conditions. The results showed that increasing electric field strength increases cavity growth, while higher surrounding pressure suppresses it. The largest cavity expansion occurred at high electric field strength and low pressure. Overall, the results show that the models can describe the main relation between electrical stress, pressure and cavity growth, but more detailed experimental data and less simplified assumptions are needed to reduce the error margins under unstable cavity conditions.