| In the converter valve hall of ultra-high voltage direct current(UHVDC)transmission,film capacitors play important roles such as the damping buffer and voltage support,whose safety and reliability depend greatly on the dielectric performances.Under operating conditions,capacitors are subjected to combined effects of electrical and thermal fields,resulting in the insulation breakdown or even failure of the whole machine.With the increasing voltage level and the development trend of the equipment miniaturization,more stringent requirements have been placed on the dielectric properties of capacitors.Therefore,based on the nano doping,crystal morphology control and molecular reconstruction,the relationship between micro structures and macroscopic properties of polypropylene(PP)films is studied in this thesis.The dielectric properties are comprehensively controlled,in order to provide an experimental basis and theoretical support for the optimal design of DC capacitors.(1)A modification method of PP films doped with organic-inorganic hybrid nanoparticles is proposed.Studies have shown that the ultrafine needle-like structures of carboxylated cellulose nanocrystals(C-CNCs)contribute to the promotion of heterogeneous nucleation.Combined with the crystalline morphology and dielectric properties,a physical interaction model among the trap distribution,charge transport and dielectric breakdown is established.It is confirmed that the nano polyhedral oligomeric silsesquioxane(POSS)has a weak ability to adjust the crystalline structures of PP,and enhances the breakdown strength by the carrier trapping ability of the nanoparticle itself,whose mechanism is different from that of C-CNCs.(2)A morphology regulation method based on the micro-doping of organic additives is adopted to further optimize the aggregated structures of PP films.The influence of the nucleating agent content and cooling process on the microscopic parameters is studied.Besides,the trap parameters are extracted by surface potential decay tests.According to the epiphytic crystallization theory,the intrinsic mechanism of performance improvement of PP films is revealed.On the basis of reducing the crystal size,the spherulites are optimized into spindle shape by the organic crystallization accelerator.At 25 °C,the dielectric loss of the modified film is reduced to within 0.05 %,and the DC breakdown strength increases by ～16.99 %.(3)A comprehensive design method of molecular and crystal structures is adopted to prepare linear/long-chain branched PP composite films with different ratios.At the molecular scale,it is confirmed by quantum chemical calculations that long-chain branched structures have the ability of carrier capture and trap regulation.At the aggregate state scale,it is confirmed by multiphysics simulations that the crystal structures with suitable size and uniform distribution can help to suppress the local field distortion inside the film,thereby improving the DC breakdown strength of the modified material.(4)According to the operating conditions in the UHV converter valve hall,the insulation performance tests considering electrical and thermal multi-stress are conducted.The effects of the thermal aging treatment,ambient temperature,DC superimposed harmonics or pulse voltage on the breakdown strength of PP films are obtained.Moreover,the insulation failure mechanism under different stresses is discussed.Among all samples,the breakdown elevation of composite films based on the micro-filling of organic additives is the most significant,which is proved to be an effective means to improve the insulation performance of PP films.In summary,based on the optimized design of micro structures,the coordinated regulation of the dielectric loss,breakdown strength and charge transport process are realized in this thesis,which is expected to be applied to improve dielectric performances of PP films for HVDC converter valve capacitors. |
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