Research On Verification And Optimization Of UHV Transformer Main Insulation

The UHV transformer plays a very important role in China's UHV grid,and it is difficult to be designed and manufactured because of high insulation level.The electrical strength of oil is crucial to UHV transformer's insulation,and the various impurities existed in the oil is the major reason causing dielectric strength reduction of transformer main insulation.At present the manufacturing technology and material quality of UHV outlet device imported form oversea is more advanced than that made in China,and the differences should be considered in the independent design and insulation margin control of the products.However,the influences of fiber impurities on the oil gap discharge characteristics and UHV transformer main insulation margin checking method have few reports so far.In this paper,variety of research methods such as theoretical analysis,simulation calculation and model experiment were integrated used to investigate the influences of fiber impurities on the oil gap discharge characteristics and optimization of UHV transformer main insulation verification.The main contents are as follows:(1)The sources and kinds of impurities in the transformer oil were introduced,and the quantity and distribution of impurities,whose diameter were above 5?m,contained in the UHV transformer oil in service were measured by light scattering method.Then,the distrubution law of impurities in the oil was revealed,and the parameters were given in the formular,describing the size and distribution of impurities in the oil.(2)The oil gap discharge characteristic test system at power frequency was established,and the partial discharge inception voltage of transformer oil under four different types electrodes which were parallel disks electrodes,mushroom shaped electrodes,needle-plate electrodes and column-plate electrodes,were investigated through adding micrometer scale microcrystalline cellulose powder.The effect of fiber impurity content on the transformer oil discharge characteristic was explored,and the formular for correcting partial discharge inception voltage of the transformer oil was proposed.(3)The dielectrophoresis force on fiber impurity in a fluid dielectric is calculated through analyzing the particle polarization effect in non uniform electric field.The motion equation of fiber impurity in transformer oil was derived from force analysis within multi-physics field,based on which the motion model of impurities in the transformer oil was built.The motion trace and aggregation processes of impurities were simulated by direct coupling multi-physics analysis software"COMSOL-Multiphysics",and the impact of impurity size and position on motion trajectories was found,which explained the experimental results the effect of fiber contents on the discharge characteristics of transformer oil.(4)Through the finite element numerical calculation method,the electrical fields of critical region in 1000kV autotransformer main insulation were analized,including the intermedium region between HV windings and MV windings,upper terminal region between HV windings and MV windings,intermedium region between HV windings and tank or yoke,the connecting area by bushing and HV of MV lead wire.The insulation margin of the first oil gap structure was tested through the model experiment,and was compared to the calculated value by WEIDMANN method.Some suggestions about internal control factor were put forward to domestic UHV transformer main insulation design and domestic UHV outlet device design.(5)The domestic 1000kV outlet device structure was introduced,and the electric field distribution in different parts was obtained by using the two-dimensional and three-dimensional calculation software.The partial discharge inception field strength and insulation margin of first oil gaps in the UHV outlet device critical insulation area were analized,and were compared with the same foreign products.(6)The UHV outlet device insulation margin test system was designed and manufactured,on which the domestic 1000kV outlet device insulation margin were tested.The test scheme was established to examine the outlet device insulation margin.According to the V-t characteristics of oil gap discharge,the judging criterion on long time insulation performance of UHV outlet device at power frequency was ascertained.The partial discharge measurement circuit was analized,and the tail shielding was improved accrding to the electric-ultrasonic signal combined fault location after partial discharge test.All the insulation tests were successfully completed on the improved system,which could meet the requirements of verifing the long time insulation performance and reliability of domestic UHV outlet device.