328 / 2022-03-15 14:51:44

Dynamic Behavior of Bubbles under Electric Field and Its Effect on the Breakdown Characteristics of Transformer Oil

transformer oil, gas bubbles, dynamic behavior, discharge, breakdown

Oil-immersed transformers may cause a large number of bubbles in cases of high temperature, moisture intrusion, partial discharge, vibration cavitation, and poor degassing. It has been qualitatively assumed that the bubbles degrade the insulation performance, but little is known about how this happens. This study is dedicated to obtain the dynamic behavior of bubbles under uniform and non-uniform electric fields and their effect on the breakdown characteristics of transformer oil. We built an experimental platform consisting of three parts: the bubble generation unit, the high voltage supply unit, and the high-speed image recording unit. A capillary tube was used to generate the specified size and number of bubbles; a power frequency test transformer (100 kV/20 kVA) was used to provide the required high voltage; a high-speed camera was used to record the dynamic behavior of the bubbles in a time-resolved manner; and the step-up method was adopted to determine the breakdown voltage of the transformer oil under different experimental conditions. The experimental results show that the bubble will be stretched and deformed in the direction of the electric field under uniform electric field, and its shape shows periodic variation with the applied AC voltage. The shape change was lagged behind the voltage change due to the fluid inertia. The bubble dynamic behavior under non-uniform electric field was migration, and the bubble will be repelled by the electric field force to the region with lower electric field strength. The presence of bubbles under uniform electric field greatly reduces the breakdown voltage of transformer oil and often triggers the breakdown of the entire oil gap as soon as the bubbles are discharged internally; while the presence of bubbles under non-uniform electric field has almost no effect on the breakdown voltage of the oil gap. Therefore, the threat of bubbles to the insulation system is related to the form of the electric field, which is due to the fact that the electric field affects the dynamic behavior of the bubbles. The study contributes to an in-depth understanding of bubble-induced insulation failure accidents and breakdown phenomena of liquid dielectrics.