Influence Of Nanoparticles On DC Breakdown Strength Of Oil-paper Composite Insulation

Converter transformer is one of the key equipments in High Voltage Direct Current transmission system.The oil-paper composite insulation(OPCI)in the valve winding of converter transformer could withstand the DC voltage at runtime.However,the electric field distribution and transition process of DC voltage are complex,as a result,the DC breakdown strength of OPCI is weakened and thus there are still many insulation faults.Hence it is very significant to enhance the breakdown strength of OPCI under DC voltage.There are many studies focusing on the property improvement of OPCI by nanoparticles.However,most of these studies just singly study either oil or oil-impregnated paper,only few studies take them as a whole and focus on the effect of nanoparticles on multi-layer OPCI,which greatly limits the application of nanoparticles in converter transformer.Therefore,the nano-modification methods for DC breakdown strength of OPCI was studied in this paper,then the influence law and mechanism of nanoparticles were revealed to provide a reference for the practical application of nanomaterial in converter transformer improvement.The OPCI was modified by TiO2 nanoparticles,and the DC breakdown voltages of OPCIs were measured under different nanoparticles concentrations,different structures and different nanoparticles morphologies.The results show that the breakdown strength of OPCI increases first and then decreases with the increase of the concentration of spherical nanoparticles,For the OPCI without modfication,the breakdown voltage under the structure of paper+oil+paper is significantly higher than that under the structure of oil+double-layer-paper,while the breakdown strengths of modified OPCI have little difference under different structures.At high concentration,compared with the breakdown strength of OPCI modified by spherical nanoparticles,that of OPCI modified by hollow spherical nanoparticles was remarkablely increased.Meanwhile,the breakdown voltages under positive and negative polarity are different,and the polarity effect changes with particle concentration,structure and particle morphology.The results of dielectric spectrum and polarization current tests show that the addition of spherical nanoparticles slightly increase the power frequency dielectric constant and the conduction current of samples,while significantly enhance the dielectric constant in the low frequency band and the absorption current.The effect of hollow spherical particles on the polarization current is similar but obviously smaller than that of spherical particles.In addition,nano-addition can also significantly increase the absorption current in OPCI.In view of the limitations of traditional RC model,a double-layer extended Debye model is proposed to analyze the interlayer polarization process in OPCI modified by nanoparticles.The voltage distribution and breakdown process of different OPCIs are studied with the proposed model.The results show that the distribution voltage in oil decreases while the voltage in paper increases with the addition of nanoparticles.The effect of hollow spherical particles is significantly smaller than that of spherical particles at the same concentration.As a result,for the OPCI without modfication,the OPCI modified by low concentration of spherical nanoparticles and the OPCI modified by hollow spherical nanoparticles,the breakdown occurs first in the oil gap,while the breakdown of the OPCI modified by high concentration of spherical nanoparticles starts first in oil-impregnated paper.When the polarity effect is not considered,the breakdown voltages calculated based on the voltage distribution are basically consistent with the measured values.The results of interface charge analysis show that due to the influence of the addition of nanoparticles on relaxation polarization,the polarity and accumulation of interface charge of OPCI are different with different nanoparticles contents and morphologies,which is the fundamental reason for the difference in voltage distribution and breakdown voltage.At the same time,the adsorption of cellulose on negative charge and different breakdown process of OPCI are the main reasons for the polarity effect of DC breakdown.