Simulation Research On Space Charge Characteristics Of Epoxy Composites For Converter Transformer Valve Side Bushing And Distribution Of Bushing Electric Field

Converter transformer valve side bushing is the key power equipment in UHVDC transmission project,which plays an important role in the safe and stable operation of converter station.As the main insulating material of bushing,epoxy composites has prominent interface effect and obvious space charge accumulation under the action of high temperature,high field and polarity reversal voltage.The distortion of electric field and aging of insulating materials will easily lead to insulation failure of bushing.At present,there is little research on numerical simulation of space charge of epoxy composites under extreme conditions,and the mechanism of space charge effect on bushing electric field is not clear.In the thesis,the space charge characteristics of epoxy composites is studied from the perspective of space charge.At the same time,the electric field distribution mechanism of converter transformer valve side bushing is studied on the basis of space charge effect.It is of great significance to improve the design and manufacture of main insulation of converter transformer valve side bushing.The main research are as follows:1)Schottky effect is used to characterize the extracting effect of temperature and electric field on space charge,and the tunneling effect under high field is equivalent by logical judgment.A finite element model of bipolar charge transport coupled with electrostatic field and mass transfer field is established based on the above improvements.Space charge characteristics of single-layer epoxy material under high temperature,high field and polarity inversion are simulated.And the migration and accumulation of carriers and the corresponding electric field distribution are obtained.According to dynamic characteristics of charges in the dielectric,the mechanism of space charge accumulation in epoxy material under extreme conditions is systematically explained by combining the theories of hopping conductance,limited carriers extraction,thermionic emission and field tunneling.2)The single-layer epoxy material is extended to the three-layer epoxy-paper-epoxy composites material and the interfacial polarization theoretical model under the three-layer medium is deduced.A two-dimensional finite element model based on interfacial polarization is constructed by its microstructure.The space charge characteristics of single-layer epoxy material under high temperature,high field and polarity inversion are simulated.The distribution of space charge and electric field at the interface is obtained by simulation,and it is found that the charge density of the two interfaces is asymmetric and the concave and convex parts of the interface are larger.A finite element model of multi-layer dielectric model is established by defining the interfacial electric flux based on bipolar charge transport model,and the dynamic space charge distribution is obtained.Space charge distribution law is explained from the perspective of interface bidirectional barrier and carrier mobility difference.The equivalent relative dielectric constant of epoxy composite at different temperatures is obtained by synthesizing the two models,which provides a parameter source for the subsequent calculation of electric field distribution of bushing considering space charge effect.3)In the thesis,the finite element model of ±800k V converter transformer valve side bushing is established,and the temperature field and electric field distribution of bushing under electrothermal coupling are studied.The results show that the establishment of temperature gradient leads to significant differences in electrical conductivity of each insulating layer.Positive charge accumulate in the insulation under the action of interfacial polarization,which weakens the electric field at the side of current-carrying guide rod.The electric field distribution of bushing under polarity reversal voltage is simulated on the basis of temperature gradient and compared with that under isothermal condition.It is found that the electric field distortion is greater under the temperature gradient.Because of the space charge effect,the maximum electric field migrates to the current-carrying guide rod side and is smaller than the maximum field strength before inversion.And the electric field distribution in different parts shows that the space charge has the great influence on the electric field of the capacitor core.