Electrical Performance And Modification Mechanism Of Transformer Oil Based Al₂O₃ Nanofluid/pressboard Insulation

With the commencement of high voltage alternating current?HVAC?and high voltage direct current?HVDC?,the demands on insulating reliability of power transformer is getting more and more imperative.The mineral oil in conjunction with paper is the main insulation components in oil-immersed transformers;their insulation properties play a significant role in the safe and stable operation of power transformers.To strengthen the insulation level of ultra-high voltage transformer and to reduce its size and weight,it is imminent to enhance the insulating performance of transformer oil and oil-impregnated cellulose.Recently,a unique novel effort of suspension of nanoparticles?NPs?into mineral oil?MO?has been carried out and the results have exhibited improved insulation characteristics of transformer oil.Nine types of nanoparticles?Al₂O₃,Fe₃O₄,TiO₂,ZrO₂,Si₃N₄,AlN,SiC,ZnO,SnO₂?bought from a company which are conductive,semi-conductive and insulative in characteristics with same concentration?0.1g/L?and oleic acid modification,were used to modify the transformer oil.The effects of nanoparticle types on insulating features of nanofluid?NF?under lightning impulse?LI?voltage were studied.The prepared NFs manifested the enhancement in positive LI breakdown voltages as compared to MO.The Al₂O₃ NFs showed the maximum improvement?10%?in the positive lightning impulse breakdown voltages followed by the Fe₃O₄ NFs?7%?.The Al₂O₃,Fe₃O₄,TiO₂ were selected from these nine nanoparticles to study the effect of concentrations?0.05 g/L,0.1g/L,0.2 g/L,0.4 g/L?,surface modifications?oleic acid,stearic acid?of these three nanoparticles on dielectric performance of MO under positive and negative lightning impulse.The Al₂O₃ NFs manifested approximately 1.11 times enhancement in positive LI breakdown strength at 0.2g/L optimal concentrations followed by Fe₃O₄ NFs 1.07 at0.01g/L optimal concentration.It was found with Al₂O₃ and Fe₃O₄ NPs modification,the positive LI breakdown voltage and chop time of MO were enhanced whereas the negative LI breakdown voltage and chop time of MO was reduced.On the other hand TiO₂ NPs has little effects on insulation performance of MO.The dielectric properties?permittivity,conductivity?measurement of the prepared three NFs showed that the Fe₃O₄ and TiO₂ NFs manifested high dielectric loss and distinct dielectric constant from pure oil which restricts their practical applications as transformer liquid insulation.The alumina NFs presented low dielectric loss and permittivity close to the mineral oil but they were highly unstable for long term and there breakdown performance was adversely affected by agglomeration.The Al₂O₃ nanoparticles with favorable features were prepared in our lab to develop transformer oil/paper insulation system with better insulation performance.The NFs were prepared with different shapes?nanorods,nanoplates?,surface modifications(oleic acid,C₂,C₁₈)and concentrations?0.6 g/L,0.8 g/L,1.2 g/L?of Al₂O₃ NPs.The positive LI BD voltages of MO were improved by 1.11 times with nanorod shape,oleic acid?OA?surface modification and 0.8 g/L concentration of NPs.The effects of Al₂O₃nanoparticle with different gap distances on insulating properties of nanofluid under LI voltage were examined.It was noted that Al₂O₃ NPs suspension,the positive LI breakdown voltage of transformer oil were enhanced but the negative LI breakdown voltage of were decreased.Surface creeping discharge at oil/liquid interface is believed as one the major reasons for oil-immersed transformers collapse.In order to curtail the surface creeping discharge,the effects of Al₂O₃ NPs on creeping discharge and flashover properties of the oil/pressboard interface under AC and LI voltages.The creeping flashover and partial discharge tests of oil-impregnated pressboards?OIPs?and nanofluid-impregnated pressboard?NIPs?were conducted.The outcomes manifested that Al₂O₃ NPs can enhance both partial discharge inception voltage?PDIV?and creeping flashover strength of OIPs.The positive LI creeping flashover voltage?FOV?of NIPs was 15%higher than those OIPs and the chop time of NIPs was also prolonged 26.4%of that of OIPs with gap distance of 30 mm.The negative LI creeping flashover voltage?FOV?of NIPs was 14%higher than those of OIPs whereas the chop time was 35%higher than those of OIPs with30mm gap distance.Moreover,AC creeping FOV and PDIV of NIPs was higher than OIPs.Thermally stimulated depolarization current?TSDC?and Pulse electroacoustic?PEA?test techniques were used to measure space charge behaviors of oil/pressboard pre and post modification of Al₂O₃ NPs.It indicated that NIPs has a more uniform internal electric field and higher decay rate as compared to OIPs,which is caused by the rise of shallow trap density in NPs related to the difference of internal structures,based on the test results of TSC.It was found that NPs can enhance the shallow trap desity.The generated electrons would lose their kinetic energy through repeatedly trapping and de-trapping process,leading to decline in elcetron mobility.The higher trap density of NPs,lower average energy,and lower mobility of electrons are obtained.The electrons with lower energy are convient to be captured by those molecules with high electron affinity,and be converted into negative space charges.These negative space charge decrease the electrical field strength at the front of positive streamer and therefore reduce the propagating velocity of positive streamer.The streamer velocity in NIPs was higher initially and tends to decline with time and ultimately it was reduced to half as compared to OIPs at particular time.Additionally,the prepared insulation system samples of liquid?MO and NFs?and solid?OIPs,NIPs?were thermally aged at 55?and 130?for 30 days.The effect of alumina NPs on thermal aging of transformer oil and pressboard were also investigated.The results showed that Al₂O₃ NPs can slow down the aging of oil and pressboard.The average AC and positive LI BDV of aged NFs was 11%higher than aged MOs.The average LI creeping FOVs of NIPs were 6%higher than OIPs.Additionally,average AC creeping FOVs of NIPs were 8%higher than that of OIPs.