Research On Dielectric Properties And Partial Discharge Characteristics Of Electrical Insulation Pressboard

At present,insulating pressboards used in large transformers still rely on imports from foreign manufacturers.In recent years,with the continuous optimization of raw materials and production processes of domestic electrical insulating pressboard,the gap between the domestic production level of insulating pressboard and the same industry abroad is gradually narrowing.However,due to the lack of systematic and in-depth research on the insulation characteristics of electrical pressboard foundation in China,the problem of "whether to use and how to use" of domestic pressboard needs to be solved urgently.Dielectric characteristic and partial discharge characteristic are important performance indexes of electrical pressboard.The dielectric characteristic directly determines the distribution of electric field in the transformer and is the basic parameter of transformer insulation design;Partial discharge can effectively reflect the insulation properties of the material and is the necessary basis for the determination of allowable field strength of electrical pressboard.Therefore,in this paper,the dielectric characteristics and partial discharge characteristics of electrical insulation board were studied,analyzes the difference of dielectric characteristics and partial discharge characteristics between imported and domestic insulating pressboard,and gives some Suggestions on the selection of electrical insulating pressboard,and provides a basic database for structural design and optimization of oil-pressboard insulating equipment.Therefore,according to the current domestic and foreign electrical insulation pressboard manufacturing and application,18 batches with totally 270 samples of electrical insulating pressboard produced by 6 typical domestic and foreign electrical insulating pressboard manufacturers(foreign manufacturers A,B,domestic manufacturers C,D,E,F)are selected,where all electrical insulating pressboards used by large power transformers.are included.research on the measurement of the dielectric properties at different temperature and the partial discharge characteristics at room temperature are conducted,comparative study and analysis of the dielectric properties and partial discharge initiation characteristics of different batches of the same manufacturer and between different manufacturers were carried out.With the results:1)The dielectric properties of insulating pressboard at different temperatures between 20℃ and 80℃ are explored,and the results show that,as the temperature increases from 20℃ to 80℃,the volume resistivity of insulating pressboard decreases from 1014 Ω·m to 1012 Ω·m,the surface resistivity decreases from 1016 Ω·m to 1015 Ω·m and the relative dielectric constant has a small increase by about 10%.This is because as the temperature increases,the directional mobility of the impurity ions inside and on the surface of the pressboard increases,and the relaxation time decreases,resulting in a decrease in resistivity and an increase in relative dielectric constant.And with the increase of temperature,the resistivity of insulating pressboard decreases,and the performance of resistivity becomes more unstable.The test results of dielectric properties of insulating pressboard at different thicknesses show that as the thickness of the insulating pressboard increases,its volume resistivity and surface resistivity show a downward trend,while the relative dielectric constant increases.Especially under high temperature conditions,when the test temperature is 80℃,the volume resistivity of the domestic manufacturer F’s pressboard decreases from 5.00×1013 Ω·m to 3.50×1013 Ω·m,with the decrease range of 30.0%;The surface resistivity decreased from 7.70×1015 Ω to 3.60×1015 Ω,with the decrease range of 53.2%;And the relative dielectric constant increased from 3.398 to 4.300,with the increase range of 26.5%.2)Partial discharge characteristics of different insulating pressboards are obtained.It is found that the initial field strength of partial discharge of different electrical insulating pressboard samples are between 14.7 kV/mm and 20.1 kV/mm,the standard deviation are between 0.91 kV/mm and 2.28 kV/mm,the data dispersion is good;The initial discharges of different electrical insulating pressboard samples are between 3.7 pC and 4.6 pC,and the standard deviation are between 0.84 pC and 1.76 pC.3)Comparing the dielectric properties of different batches of insulating pressboard from the same manufacturer,it is found that for manufacturers B,C and D which have 2 batches,the standard deviations of the volume resistivity,surface resistivity and relative dielectric constant of the products of manufacturer D at 80 0 C are 1.34×1012 Ω·m,2.20×1014 Ω and 0.06.The quality stability of its different batches is better than that of manufacturers B and C.For manufacturer A,which has 6 batches of pressboard,the range of average volume resistivity,surface resistivity and relative dielectric constant of the pressboard at 80℃ reach to 3.26×1013 Ω·m,6.46×1015 Ωand 0.10 respectively.For manufacturer E,which has 3 batches of pressboard,the range of average volume resistivity,surface resistivity and relative dielectric constant of the pressboard at 80℃ reach to 1.28×1013 Ω·m,4.66×1015 Ω and 0.22 respectively.Manufacturer E has poor quality control in terms of dielectric constant.Comparing the dielectric properties of insulating pressboard from different manufacturers,the volume resistivity and surface resistivity of the insulating pressboard of manufacturer A at 80℃ reached 1.14×1013 Ω·m and 4.14×1015 Ω,which superior to other manufacturers obviously.While manufacturer B showes advantages in surface resistivity and the relative dielectric constant,which are 4.42×1015 Ω and 3.39 respectively.Insulation pressboards of domestic manufacturers still have gaps with foreign manufacturers in terms of dielectric properties.4)Comparing the partial discharge initiation characteristics of different batches of insulating pressboard from the same manufacturer,The average standard deviation of the initial partial field strengths of 6 batches of insulating pressboard from foreign manufacturer A is 1.07 kV/mm,The difference between the different batches is small,and the range is only 1.70 kV/mm.In the case when the number of batches of manufacturers B,C,D,and E is less than that of manufacturer A,the standard deviation of the average initial field strength of the partial discharge of the insulated pressboard are 1.26 kV/mm,1.66 kV/mm and 1.62 kV/mm respectively.Foreign manufacturer A’s insulation board is significantly better than other manufacturers in quality control of the initial field strength of partial discharge.Comparing the initial characteristics of partial discharge of different manufacturers,the initial field strengths of partial discharges of foreign manufacturers A and B reached 18.47 kV/mm and 18.89 kV/mm,respectively,which were higher than those of three domestic manufacturers.Considering that the initial field strengths of the partial discharge are higher,it is reasonable that the initial discharge of manufacturers A and B is slightly higher than that of other manufacturers.5)Comparison and analysis of dielectric and partial discharge characteristics of electrical insulating pressboard.Although insulating pressboard of some domestic manufacturers can achieve the leading volume resistivity,surface resistivity and relative dielectric constant at lower test temperatures such as 20℃ and 40℃,under the condition of 80℃ close to the normal operation of the transformer,its dielectric characteristics and partial discharge initial characteristics still have gaps-with those of foreign manufacturers,which means,at present,it is difficult for domestic pressboard to completely replace the imported products.