Investigations On Motion Behaviors And PD Characteristics Of Free Charged Metallic Particles In The Oil Channel Of Converter Transformers

Converter transformers are one of the most critical and expensive power apparatuses in a high-voltage direct current(HVDC)transmission system.Their safe and reliable operation play a vital role in the safety and stability of the entire transmission system.However,during the manufacture,transportation,installation,operation and maintenance,transformer oil will inevitably be contaminated by free metallic particles,which come from the material processing,mechanical vibration,oil pump wear,and other processes.Plenty of free metallic particles in the oil severely reduce the insulation performance of the oil,easily cause partial discharges(PD)in the early pollution stage,and induce insulation breakdown when the pollution is severe.The pollution has become the main problem in the safe operation of converter transformers,which is highly concerned by domestic and foreign scholars and engineers.However,the current related research mainly focuses on the influence of free metallic particles on the insulation performance of static transformers oil under alternate current(AC)or DC voltages when the oil channel surface is bare.The investigations on the charging process when the particles in forced-oil-circulation oil contact the solid insulating dielectrics on the surface of the valve-side oil channel,the motion characteristics of the particles under the flow and AC-DC composite electric fields,the discharge characteristics caused by the particles,and the related mechanism of the process and characteristics are scarce.Obviously,the effects of those charged particles on the insulation performance of the oil will be even more complicated and have become an intricate problem that needs further study.Therefore,a circulation flow device of transformer oil and an observation system of particle motion were established in this paper.The charge quantity of free metallic particles was measured when the surface of parallel plate electrodes covered with insulation paper.Moreover,the PD characteristics of transformer oil contained the particles were systematically studied for different oil flow speeds,DC components in applied voltages,and paper-covered cases of electrodes.Furthermore,a laminar flow solid-liquid two-phase flow model coupled with electric fields was constructed,and the particle movement was observed to verify the model.Based on the particle movement motion,the mechanism on the PDs for different factors was clarified.The main research work and results obtained are as follows:(1)Based on the oil channel in the actual converter transformer,an experimental discharge platform that could simulate various experimental conditions and a motion observation system for micron-level free metallic particles were established.which lay an experimental foundation for investigations of the charging characteristics,PD characteristics,and motion characteristics of free metallic particles in transformer oil.(2)The kinetic equilibrium equation was used to measure the charge quantity of free metallic particles when parallel plate electrodes were covered with insulation paper.The charge quantity of particles after contacting the bare electrode agreed with the theoretical value,and the particles did not settle on the electrode surface.The particles would temporarily settle on the electrode surface when they touched a paper-covered electrode.The particles could leave a paper-covered electrode without settling after applied voltages exceeded PD inception voltages,but the probability of occurrence was much lower than that of the settling.When a single electrode was paper-covered,as applied voltages increased,the settling time decreased,and the charge quantity of particles after contacting the paper-covered electrode increased.The charge quantity was about half of that for the bare electrode.The settling time and charge quantity for double paper-covered electrodes were longer and lower than those for the single paper-covered electrode case.The particles mainly charged through the conduction of insulation paper,which caused the settling time and reduced the charge quantity.(3)PD experiments of free metallic particles in transformer oil under different flow speeds,DC components in applied voltages,and paper-covered cases of electrodes were conducted.Feature parameters,such as PD inception voltages,discharge repetition rates,mean discharge magnitude,and accumulation discharge magnitude per unit time were obtained,and discharge patterns were also constructed.It was found that the faster the flow speed was,the weaker the PD intensity was.The discharge patterns under AC voltages were symmetrical,and the PD intensity was the strongest.The discharge patterns were asymmetric when applied voltages contained DC components,and the PD intensity became strong as the DC components increased.When a single electrode was paper-covered,PDs occurred in the rising phase intervals,and the PD intensity was weaker than that for the double bare electrodes case.For the double paper-covered case,PDs occurred in the declining phase intervals,and the PD intensity was weaker than that for the single paper-covered case.(4)The movement of free metallic particles under different factors was observed.A laminar solid-liquid two-phase flow model coupled with electric fields was constructed to simulate particle trajectories.Both experimental and simulation results agreed that under DC voltages,uncharged particles fell towards the lower electrode and became charged,the charged particles moved back and forth between the parallel electrodes while moving with the oil flow,and collided with electrodes.The faster the flow speed was,the longer the horizontal displacement of the sedimentation was,the larger the landing-point spacing was,and the landing-point time interval unchanged.The more the DC components in applied voltages were,the closer the particle trajectories were to that under DC voltages.As the DC components increased,the landing-point spacing and time interval became shorter.Moreover,the charged particles under AC voltages only moved back and forth near the lower electrode,and meanwhile moved horizontally with the oil flow.Both the landing-point spacing and time intervals were the shortest.When a single electrode was paper-covered,the particles temporarily settled on the paper-covered surface and then moved to the opposite electrode while moving with the oil flow.The landing-point spacing and time intervals were longer than those for the double bare case but shorter than those for the double paper-covered case.(5)Based on the validated simulation model,the collision frequency between particles and electrodes,the electric field in the gap between particles and electrodes,and the particle collision densities at different phases were calculated and extracted.Based on them,the influence of different factors on PD characteristics was clarified.The results showed that the faster the flow speed was,the higher the collision frequency was,and the gap electric field unchanged,causing that the PD intensity decreased with the flow speed.The collision frequency under AC voltages was the highest,and the gap electric field was also relatively stronger,so the PD intensity was the highest.The more the DC components in applied voltages were,the higher the collision frequency was,and the stronger the gap electric field was,leading to a stronger PD intensity.When a single electrode was paper-covered,the collision frequency and the gap electric field were lower than those for the double bare case and higher than those for the double paper-covered case.This explained the reason why PD varied with different paper-covered cases.When the applied voltage contained AC components,PDs easily occurred at the phase interval with high collision density,so the phase distribution of collision density mainly determined the discharge patterns.