| The main insulation of valve-side winding in converter transformer is subjected to AC-DC combined voltage consisting of AC and DC voltage,and harmonic voltage components.The electric field distribution of its vale-side insulation is different from that of traditional power transformer.With the increase of running time,the insulation defects in converter transformer may happen and cause partial discharge.The creeping discharge which is one of the most dangerous discharge type develops along the pressboard to the iron yoke,the core,and the tank wall and finally leads to insulation failure.Creeping discharge properties as well as electrical aging fauilre properties of oil-paper insulation are closely related to the type of applied voltage.Thus,there are great differences in properties of creeping discharge and electrical aging fauilre of oil-paper insulation between converter transformers and traditional power transformers.Considering the special condition of main insulation of valve-side winding in converter transformer,the thesis has studied the creeping discharge properties and electrical aging failure model of oil-paper insulation under AC-DC combined voltage systematically.The main contents of the thesis are as follows:(1)The basic properties and phenomena of creeping discharge of oil-paper insulation under AC-DC combined voltage are experimentally studied.The electric field distribution characteristics at the insulating structure of valve-side winding in converter transformer where creeping discharge may occur are analyzed.The experiment of creeping discharge of oil-pressboard insulation in highly and slightly non-uniform electric field under AC-DC combined voltage are carried out by using needle-plate and column-plate electrode separately.The influence rule and mechanism of DC component on phase distribution and statistical characteristics of discharge and the white mark occurrence are analyzed.The experimental results show that DC component has a significant effect on discharge phase distribution and white mark occurrence under highly non-uniform electric field,but little effect on those under slightly non-uniform electric field.The white marks are essentially gas channels and significantly increase the discharge intensity and development speed,and its occurrence is influenced by electric field distribution and geometric structure of electrodes and the moisture in pressboard.(2)An electrothermal physical model for creeping discharge of oil-paper insulation under highly non-uniform electric field at AC-DC combined voltage based on the electrohydrodynamic model of liquid dielectric and heat transfer equation is established.The corresponding relationship between the current pulse waveform of single discharge and the generation,movement and disappearance of charged particles is studied by simulation.The temperature distribution of discharge channel is analyzed and correlated with the macro phenomenon,such as white mark and carbon mark on pressboard surface.The single discharge process under AC-DC combined voltage and AC voltage are compared,thus the influences of DC component on the physical process of single discharge and the temperature distribution of discharge channel are revealed.The simulation results show that the rising edge of current pulse is closely related to the field ionization intensity of transformer oil molecules,while the falling edge is mainly controlled by electron adsorption and the composite process of charged particles in the channel.DC component significantly increases the development speed and stop length of streamer at the oil-paper interface,and reduces the electron density in the highest temperature region of discharge channel on the oil-paper interface,thus the highest temperature in this region is decreased,which is an important reason why it is difficult for white mark occurrence on the pressboard near needle tip under AC-DC combined voltage.(3)The failure process of oil-paper insulation at creeping discharge under AC-DC combined voltage and AC voltage are studied,and the white mark development and discharge statistical characteristics during the discharge development under the two types of voltage are compared,thus the influence mechanism of DC component on oil-paper insulation failure process is revealed.The experimental results show that the influence of DC component on the failure process of oil-paper insulation is related to the development path of white marks on pressboard.DC component makes it difficult for the white marks developing along the pressboard surface to develop forward,but it has little effect on the white marks developing into pressboard.Therefore,it is more difficult for the creeping discharge to cause failure of oil-paper insulation under highly non-uniform electric field at AC-DC combined voltage,and the failure process of oil-paper insulation and the characteristics of discharge pattern under slightly non-uniform electric field at AC-DC combined voltage is similar to those at AC voltage.Under highly non-uniform electric field,it is found that surface flashover in the discharge process is more likely to occur under AC-DC combined voltage than under AC voltage when the insulation margin is same,and the reason and path of surface flashover are analyzed.Preventive measures for preventing surface flashover are put forward.The change law of the characteristics of discharge patterns under slightly non-uniform electric field is analyzed,and the corresponding division method of discharge development stages is established.(4)The electrical aging test of oil-paper insulation by creeping discharge under slightly non-uniform electric field is carried out,and a large number of electrical aging failure data of oil-paper insulation are obtained.Weibull distribution test of the failure data and the characteristic parameter extraction are carried out.The influence of AC and DC components on characteristic electrical life of oil-paper insulation is analyzed,and an improved inverse power life model of oil-paper insulation considering AC RMS value and AC content is established.By combining the improved inverse power life model and Weibull distribution,the oil-paper insulation failure model considering AC RMS value,AC content and failure time is established.The experimental results show both AC and DC components have significant effects on the characteristic electrical life of oil-paper insulation,and the influence of AC component is larger than that of DC component;moreover,the influence of AC and DC components on characteristic electrical life has significant interaction effect.Hypothesis test results show that the proposed failure model of oil-paper insulation has good accuracy and adaptability.The research content of this thesis is helpful to further understand the rule and mechanism of creeping discharge of the main insulation of valve-side winding in converter transformer from generation,development to insulation failure,and it also provides theoretical supports and experimental basis for fault diagnosis and insulation reliability assessment of oil-paper insulation in converter transformer. |
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