Effect Of Temperature On Discharge Characteristics Of Metal Particles In Flowing Oil

In the large power transformer,the engineering pure insulating oil is commonly used as its insulating medium.The insulating oil will be inevitably contaminated by particles in the assembly,operation and other aspects of transformer,which will seriously affect the performance of insulating oil.The discharge can be explained by the small bridge theory in the insulating oil containing metal particles.However,the insulating oil will flow because of the cooling mode of forced oil circulation and the temperature difference of the large power transformer,thereby making the discharge characteristics change compared with the static state.The temperature of the insulating oil will change due to the difference of the temperature and the load carried by the transformer.And the oil temperature is one of the main factors that affect the performance of the insulating oil.At present,the research on temperature affecting the performance of insulating oil is mainly focused on the static oil.Therefore,the simulation and experimental studies on the effect of temperature on discharge characteristics in the flowing insulating oil containing particles are conducted in this paper.In this paper,the experimental platform for simulating the flow channel of transformer is established by equipments in the laboratory,and impurity defect model of the metal particle is determined.The partial discharge(PD)collected by the Ultra High Frequency(UHF)method and the breakdown voltage of metal particles in the flowing insulating oil at different temperatures are obtained.Considering the interaction of three fields of flow field,electric field and particle motion,trajectories of metal particles in the flowing oil at different temperatures and voltages are simulated by the finite element method.The trajectories are used to explain the discharge characteristics of metal particles in the flowing oil.The main work and achievements are as follows:1)The three-dimensional simulation model of the movement of metal particles by the electric field force and oil drag force is constructed in the flowing oil.Trajectories of metal particles are obtained at different temperatures and different voltages by the finite element method.It is found that metal particles are oscillating in the direction of the electric field and move forward in the direction of the oil flow.With the increase of temperature and voltage applied to the plate,amplitude of metal particles motion is bigger,and metal particles are easier to reach the bottom plate,and the rebound height after collision is also higher and higher.2)The two-dimensional simulation model is established combining trajectories of metal particles.The distortion of the electric field is obtained when metal particles are in different positions,different aggregation degrees and different amounts of charge.It can be resulted that if metal particles are closer to the plate,and the gather of metal particles in the direction of the electric field in the oil is more intensive,the distortion of the electric field in the oil will be more serious.When the voltage applied to the plate is greater,the induced charge is more varying the increase of the electric field in the oil,which is worse for the distortion of the electric field in the oil.3)Based on the simulation analysis and the type of discharge caused by metal particles in the flowing oil,the PD and breakdown process of metal particles are analyzed in the flowing oil.It is concluded that the influences of temperature on breakdown and PD are related to trajectories of metal particles at different temperatures and voltages.And the breakdown of metal particles in the flowing oil is also related to bubbles produced by PD.4)The PD is obtained at different temperatures by experiment.The Φ-U-N spectrum,Φ-U scatter plot and Φ-N spectrum are drawn for the PD.And the characteristic parameters of PD severity are extracted,such as the amplitude and discharge frequency of PD.It shows that the phase of PD is mostly taken place in the vicinity of 90 ° and 270 °,and no apparent polarity effect.With the increase of temperature,the PD is weakened.When the temperature rises to 80 ℃,the PD begins to increase.These are closely related to trajectories of metal particles in the flowing oil at different temperatures.The results are analyzed by combining the trajectories of metal particles at different temperatures.5)The breakdown voltage is gotten at different temperatures by experiment.The results exhibit that the breakdown voltage increases with the increase of temperature.When the temperature rises to 65 ℃,the breakdown voltage begins to decrease with the temperature continuing to increase.These are related to trajectories of metal particles in the flowing oil,the bubbles generated by PD and the gas solubility.