Research On Spatial Electric Field Characteristics In Oil Of Valve Side Outlet Model Of Converter Transformer Under Various Voltage Stresses

The reliable insulation design of converter transformer is of great significance to the safe operation of HVDC system.The valve-side outlet device is located between the body of converter and the bushing,which not only bears the AC voltage,operation overvoltage and lightning impulse,but also bears the DC,AC composed DC,polarity reversal voltage.Recently the research on the space electric field of oil-pressboard insulation structure is only limited to the simple and equivalent plate electrode model,and the electric field characteristics of different sizes are not clear and there is no research on the electric field characteristics of large-scale complex structure.In this paper,a planar-array photoelectric detector is developed.The photoelectric conversion coefficient is 0.442A/W,the spatial resolution is 1.4mm2,and the measurement accuracy is up to 98%.The electric field of the±800kV valve-side outlet is measured by using the detector.Compared with the simulation results,the distribution characteristics of electric field and charge in different oil gaps are obtained finally..The electric field is distributed according to the dielectric constant of the medium under the AC voltage.The electric field is mainly distributed in the oil of transformer.The electric field in different oil gaps keeps steady with time.The electric field in the inner,middle and outer oil gap is 1.710kV/mm,1.620kV/mm and 1.495kV/mm respectively.The maximum error between the simulation value and the measured value is 3.18%.The electric field is distributed according to the resistivity of the medium under DC voltage,and the electric field is mainly distributed in the pressboard.the electric field in the inner oil gap gradually increases for a period of time and then enters the steady state over time.The electric field in the middle and outer oil gap decreases firstly,and finally tends to be stable.The electric field in the inner oil gap is 2.86 times of that in the outer oil gap.The closer the inner oil gaps is to the high voltage electrode,the higher the electric field in the oil is.The transition process of space electric field in oil under negative voltage is similar to that under positive voltage,but the steady electric field of three oil gaps under negative voltage is higher than that under positive voltage.The zero-crossing time of electric field in different oil gaps is inconsistent with that of applied voltage under polarity reversal voltage,that is,the electric field in the inner oil gap is later than that of applied voltage,the electric field in the middle oil gap is almost the same time as that of applied voltage,while the electric field in the outer oil gap is earlier than that of applied voltage.Different polarity reversal time experiments show that the shorter the polarity reversal time,the higher the electric field after polarity reversal in the oil.Under impulse voltage,the electric field in oil increases with the increase of voltage amplitude,and decreases slowly with the increase of wave front time.The closer the space position in the same oil gap is to the high voltage electrode,the larger the electric field in oil is,with a maximum deviation of 1.92%.