| Converter transformer is the key equipment in the DC transmission system,and its safe and reliable operation is of vital importance to power grid and ultra-high voltage transmission.The operating conditions of the valve side of the converter transformer are different from that of traditional power transformers.There are AC superimposed DC and polarity reversal voltage electric field.Higher requirements for oil-paper insulation performance are putting forwards because complex voltage types.In order to ensure the safe and reliable operation of the converter transformer.On the one hand,we need to have a deeper understanding of the insulation breakdown characteristics of the oil-paper insulation under AC superimposed DC and polarity reversal voltage.On the other hand,we need to improve the insulation performance of oil-paper insulation.Based on previous study of nano-Al2O3 doping modification in cellulose paper,insulation properties of nano-Al2O3 modified samples under AC superimposed DC and polarity reversal voltage,and the influence of thermal aging effect is analyzed.In this paper,the preparation of insulation paper,the surface modification process of nano-Al2O3 and the doping process of nano-Al2O3 into insulation paper are explored with the laboratory platform.Then the nano-Al2O3 modified insulation paper and conventional insulation paper whose insulation properties fulfill the GB requirements were prepared,afterwards accelerated thermal aging test under 130?.The main research results are as follows:?1?The electrical and physicochemical properties of nano-Al2O3 modified paper and insulation oil at different aging stages are investigated.It is found that the compatibility of nano-Al2O3 insulation paper and insulation oil is good under the long-term thermal aging.The addition of nano-Al2O3 can improve the insulation performance of insulation oil while improving the performance of insulation paper.?2?The effects of thermal aging on AC superimposed DC breakdown field strength of nano-Al2O3 modified paper and ordinary paper were investigated.In the influence of compound field on the breakdown characteristics of oil-paper insulation,the DC field plays a leading role.And the higher the DC content,the more obvious decline of the breakdown field with aging degree.Nano-Al2O3 doping has effect on the increase of the polarity reversal voltage,but the improvement is mainly confined to the electric field with low AC content.When the AC content is less than 50%,the breakdown field strength can be increased by about 5%to 10%.For the composite electric field with high AC content,it is still necessary to enhance the performance of insulation oil to enhance the AC superimposed DC breakdown field strength of oil-paper insulation.?3?The change regulation of the polarity reversal breakdown field strength and the influence of the space charge in the polarity reversal process were investigated.The nano modified paper has the effect of inhibiting the accumulation of space charge.The total amount of space charge accumulated is 21.7%lower than that of ordinary paper,53.56%lower than ordinary paper at the end of aging,and the distortion degree of electric field is always lower than that of ordinary paper.The breakdown strength of nano modified paper at various aging stages is always higher than that of ordinary paper6%-12%.The effect of lifting can be maintained during the aging cycle.Nano-Al2O3 modified paper has the effect of inhibits the accumulation of space charge,which causes less electric field distortion.And promotion effect can be maintained in the whole aging cycle.For ordinary paper,there are mainly shallow traps in the initial stage of aging.As the aging process is carried out,the trap density of each trap level is increased.And a large number of deep energy level traps are introduced,leading to deep traps in the late stage of aging.For nano-Al2O3 modified paper,the peak of trap energy density increases gradually as the aging degree deepens,and the trap energy level corresponding to the peak decreases first and then increases.During polarity inversion process,the interfacial polarization charge and capacitive distribution electric field paper all have impact on insulation breakdown characteristics.However,these effects are closely related to the time of polarity reversal and voltage amplification,and further studies are needed to investigate quantitative analysis. |
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