Migration And Accumulation Characteristics Of Bubbles In Oil-immersed Transformer Under Coupled Effect Of Electric Field And Mechanical Vibration

Monitoring and maintaining transformers extends their life expectancy and reduces costs.Power transformers are at the heart of power generation,transmission,and distribution systems.Its decay can lead to precious losses.In power transformers,bubbles are considered one of the leading causes of insulation degradation.H owever,bubble formation during transformer operation is normal and almost unavoidable.Existing stresses in power transformers,such as thermal,electric,and vibration,generate bubbles.Bubbles are a significant source of insulation degradation and failure.Understanding bubble creation,migration,and accumulation mechanisms in an insulation system remains challenging.Therefore,the problems of the bubble’s actions require further explanation.Moreover,research has recently examined bubble dynamics in non-electric fields.Further investigation of bubble behaviour and mechanisms in high voltage fields is necessary,especially under the combined mechanical vibration and electric field stresses.The current research provides a scientific perspective on the migration and dispersion of bubbles and their electrical effects on power transformers.A series of experiments were conducted to observe existing bubble phenomena in an active power transformer.The experiments aimed to investigate the mechanics of a single gas bubble in oil-immersed equipment,interactions between gas bubbles or clusters of bubbles subjected to vibration,and interactions with electric fields.Experiments in an electric field and under vibration revealed the impact of an electric field on bubbles.The investigation showed that bubbles at the two-phase interface created a polarization charge.The superposition of the electric field and the applied electric field produced by the polarization charge produces non-uniform electric field distortion.In a nonuniform electric field,the bubble’s trajectory changes.The bubble was always pushed away toward the uniform low electric field area.An increase in field strength increases the migration of bubbles.Small bubbles were more susceptible to uneven electric field forces than big ones.The periodic,consistent distortion of the bubble under electric field force was observed.While moving,bubbles stretched and oscillated.Under the action of a high electric field,when the bubbles contact the uneven electric field,the bubble speed decreases first.Due to inertia,the two bubbles collide,sticking together and migrating upward.A strong electric field changes the charge at the bubble-liquid interface and shreds large bubbles into smaller ones.When consecutive bubbles form,the electric field distribution becomes non-uniform,and small bubbles rise zigzagging.The bubble rising rate gradually decreases with an increase in vibration frequency,while the distortion coefficient slightly increases with an increase in vibration frequency.Moreover,numerical investigations were carried out to observe the dynamic behaviors of a monodisperse bubble swelling in an oil transformer exposed to vibrations and an electric field to clarify and verified the observed phenomenon.In this study a two-dimensional model was used to solve the Navier-Stokes,vibrational,and electric field equations,with the level-set approach adopted to track the trajectory and shape of the bubble.The research scrutinized the effect of the electric field,vibration frequency,amplitude,and bubble size on bubble motion and deformation.The results showed that the electric field inside the bubble was relatively uniform.The field strength enhancement coefficient was about 1.44.Bubbles accumulation analysis showed that when two bubbles are vertically distributed,the electric field between them is strengthened and the internal field strength of the bubbles is uniform.However,when the distance between them is small,the field strength inside the bubbles cannot be distorted and the field strength development coefficient is range of 1.3-1.4.Small bubbles,on the other hand,have a higher enhancement coefficient than large bubbles.Considering vibration effect,the pressure inside the bubble changes with the frequency imposed,while the field strength inside the bubble increases with the vibration frequency.The small vibration amplitude leads to the peak migration rate.The vibration stress contributes to the detaching frequency and migrating speed of bubbles.The bubble rising rate gradually decreases with vibration frequency,and the distortion coefficient increases slightly.The increased frequency or amplitude of the vibrations affects the bubbles,which become oval or flat.An electric field and vibration stress combined aggravated bubble distortion,making bubbles accumulate easily.Due to the motion of the upper prime bubble,the lag bubble was subjected to slight drag,and vibration amplitude increased coalesces of the bubble,which is ahead of the stream.Before the cluster forms,bubbles from the two adjacent sources move toward their axis.The given analytical theories is justified by the observed phenomena in experiments.The bubbling on the paper surface,surface flashover,and bubble features created next to surface flashover were investigated.An analysis of the competing forces shows that,for a certain bubble size,the electric gradient force dominates buoyancy close to the top of the electrodes.The cellulose bubbles’ bridging activity is controlled by dielectrophoretic strength,and vertically positioned electrodes require an electric field intensity greater than 2×104 V/cm to break bubble.The flashover occurs along the uppermost part of the Pressboard.and the discharge appeared and declined 28%of flashover voltage under both paper and oil,whereas it was less declined in the oily condition.The migration of bubbles on the paper’s surface has little impact on the discharge and flashover compared to those embedded and accumulated between the paper and electrodes.Cushing tiny bubbles implanted between coatings or electrodes creates extreme energy and sparks.These sparks may ignite a flashover.The postaction of PD and flashover led to microbubble accumulation in oil.This research provides a deeper insight into the bubble’s action under a combination of vibrations and electric fields beneath a power transformer.It provides key techniques and parameter analysis for the investigation of bubble problems.