Electric Field Distribution And Regulation Methods Of Valve-side Bushing Insulation For A Converter Transformer

The valve-side bushing of a converter transformer is a key HV equipment serving as an interconnection between the AC and DC grid,and its performance is closely associated with the safety and reliability of the HVDC system.However,field grading within the bushing insulation presents a major challenge for DC applications.The electric field distribution with DC component is mainly determined by the conductivity of insulation that is nonlinear with the temperature.Temperature gradients in the insulation hence will lead to variable gradients of material properties and subsequent an unexpected field distortion.On the other hand,since the operating voltages and power ratings have seen a continuous increase,polymeric insulating materials used in bushing will be exposed to serious electrothermal stresses and prone to injection and accumulation of the space charge under DC voltage.The undefined field migration and accumulated space charge will cause the modification of local field strength leading to partial discharge and breakdown.Therefore,the regulation of the electric field distribution within the bushing insulation is of great significance for the development and reliability of the HVDC system.In this thesis,the epoxy resin used in the dry-type bushing is taken as the research object,and an electric field regulation method based on material modification is proposed and investigated.FEM simulations are performed to clarify the variation of electrothermal characteristics within the bushing.Nanoparticles of graphene oxide(GO)and fullerene(C60)with low weight fractions are doped into the resin to improve the temperature coefficient of conductivity.Space charge behaviors and breakdown strengths are tested to assess the internal electric field distribution and the insulation performance of the prepared nanocomposites.The main conclusions have been drawn as follows:·Eletrothermal coupling simulation results,of an established ±800 kV valve-side bushing model,show that there is a gradient of temperature along both radial and axial directions within the bushing core,which plays an important role in the electric field distribution.Obvious field migration is observed when the rated current flows in the conductor due to the highly dependence of the conductivity of insulation on the temperature.The maximum electric field strength is displaced to the lower temperature region located in the outermost shield layer inside the bushing core,and reaches 9.3kV/mm under the operating voltage 800 kV with a current of 4500 A.The varying load current and ambient conditions cause the temperature distribution as well as the electric field distribution undefined and variable,making the structural optimization of the condenser core harder.The investigation of the effects of various influence factors on the electrothermal characteristics shows that increased load current,elevated valve hall temperature and reduced transformer oil temperature magnify the migration magnitude of the electric field and should be paid more attentions during the operation.It is found that the reduced temperature coefficient of conductivity of the insulation improves the field distribution effectively.The material with the activation energy for conduction lower than 0.84 e V could limit the maximum field strength,in the bushing designed in this thesis,below the allowable value of 8 kV/mm.·EP/GO and EP/C60 nanocomposites with various filler loading are successfully fabricated using the two-phase extraction method.Conductivity measurement results show that the activation energy for conduction of the nanocomposites decreases with the increase of the filler content,indicating the reduction of the temperature coefficient of conductivity and the improvement of the electric field distribution.Trap distribution characteristics show that the density of the shallower traps is increased by the doped filler.However,the density of the deeper traps is reduced by the incorporation of the GO filler,while both the level and density of which are slightly increased by the C60 filler.According to the analysis of the charge transport mechanism with dual discrete energy levels,it is believed that the conductivity at lower temperature is dominated by the deeper traps while that at higher temperature is determined by the shallower traps.Increased density of the shallower traps may be responsible for the reduced activation energy for conduction of the nanocomposites.·Results from the space charge measurement with the PEA method show that there is heterocharge accumulation in the vicinity of the electrodes at lower temperature for the EP/GO nanocomposites and it is attributed to the introduced impurities as evidenced by the reductions of the glass transition temperature and the DC breakdown strength.However,the amount of the accumulated space charge is mitigated by the doped GO filler with proper content(0.1 wt %),at higher temperature.More recombination centers are provided by the GO filler due to its large surface area.Meanwhile,more scattering centers are also introduced by the distributed filler leading to the improvement of the AC breakdown strength of the EP/GO-0.05 and EP/GO-0.1.Space charge injection is effectively suppressed by the inclusion of the C60 filler.Quantum chemical calculation results show that there is an energy difference of 1.23 eV of the LUMO and HOMO between the C60 particle and the resin,indicating that the charges trapped in the particles should overcome this barrier to join the charge transport.It is concluded that more potential wells are introduced by the doped C60 filler acting as trap sites to capture the injected charges.A homocharge shielding layer could be formed and the barrier height is increased for the EP/C60 nanocomposites as evidenced by the improvements of the space charge distribution even at higher temperature(90 ?)and the DC breakdown strength.Epoxy-based nanocomposites show a great potential for applications in HV bushing insulation system,and the field grading based on material modification has been proven to be a promising route to regulate the electric field distribution.The work presented here is aimed at providing groundwork on the way to the improvement of the field distribution within the bushing core and theoretical support for the design and operation of the bushing.