| The distributions of power demands and power generation resources are imbalance in China. More than 2/3 of the proven amount of the coal resource is in north and northwest China, and more than 3/4 of the exploitable capacity of hydro resource is in southwest region. However, more than 2/3 energy demand is in the relative developed central and eastern region. In order to satisfy the needs of load center and optimize the allocation of the resources, the large blocks of power and long distance power transmission is needed. The power grid backbones consisting of 500 kV AC (330 kV AC in northwest China) and±500 kV DC transmission lines now in China can not satisfy the power transmission requirements for the problems of insufficient ability of power transmission, insufficient line corridors and substation sites, short circuit current beyond standards at heavy loaded regions. In order to guarantee the strategy of transmitting power from west to east, exchange between south and north, and nationwide interconnection, the EHV/UHV transmission and transformation projects are under construction actively, hence, there will be more EHV/UHV transmission lines across the polluted, ice covered and high altitude regions.For EHV/UHV transmission lines, the insulator strings to satisfy the electrical insulation and mechanism performance under all kinds operating conditions is one of the main factors to guarantee the safe and reliable operation of power system. If the selection of insulator strings is not appropriate, pollution and icing flashovers are easy to happen, the influenced area is wider and economy loss is more under the condition of nationwide interconnection. The insulator type and string length also determine the tower height and window dimensions. Consequently, the insulation strength design and insulator selection are important problems in the AC transmission line design, and the pollution withstand voltage characteristics, icing withstand voltage characteristics, potential and electric field distributions of the insulator strings and grading rings should be considered.The 50% pollution withstand voltage data of 1000 kV AC long insulator string is not sufficient yet, and the icing flashover voltage data of EHV long insulator string is hot enough, especially there is almost no UHV long insulator string icing flashover voltage data for the insulation selection of EHV/UHV transmission lines in China. There are open boundary, floating conductors, geometry dimension complex and with multi-medium material problems of the potential and electric field distributions numerical analysis of the EHV/UHV transmission lines insulator strings.The pollution flashover characteristics of UHV AC long insulator strings and insulator string selection, icing flashover characteristics of EHV AC long insulator strings, coupling finite element method and boundary element method (FEM-BEM) and sub-region approximation FEM numerical analysis method, potential and electric field distributions along EHV insulator strings before icing experiments in the large scale artificial climate laboratory, potential and electric field distributions of UHV AC typical composite insulator strings and grading rings are studied in this thesis.(1) The pollution flashover characteristics of UHV AC long insulator strings are studied in the large scale fog chamber, conclusions can be drawn that:1) The relationship between pollution withstand voltage of typical insulators andρSDD,ρNSDD is power function and the exponents are negative. The ESDD influence characteristics exponents of typical insulators are 0.1703～0.2059 when PNSDD=0.5 mg/cm2; the NSDD influence characteristic exponents of typical insulators are 0.0910-0.1028 whenρSDD=0.1 mg/cm2.2) The pollution withstand performance and effective creepage distance of double and triple sheds aerodynamic insulators are better than normal type insulators.3) The single I, double I and single V FXBW-1000/300 insulator can withstand at least 698.6 kV voltage whenρSDD/ρNSDD=0.25/1.0 mg/cm2.(2) The 150 UHV AC insulator strings selection researches, considering the design aim flashover probability Pn=2% and 5%, indicate that:1) Considering the CaSO4 correction, the insulator string can reduce 2～11 units for typical pollution levels and insulators.2) The length of insulator string should be 12.090-14.235 m for CA887-EZ insulator under the conditions of Pn=2% andⅠ,Ⅱpollution levels, that is 11.310～12.870 m for XSP-300 insulator.3) Compared with Uamax=768.5 kV, the insulator string can reduce 5-7 units under the conditions of Uamax=689.6 kV andⅠ,Ⅱpollution levels, and the insulator string is 11.115～12.870 m; the XSP-300 insulator string can reduce 5-6 units, and the string length is 10.140～11.700 m.4) Compared with Pn=2%, CA887-EZ and XSP-300 insulator strings can reduce 2-3 units under the conditions of Pn=5% andⅠ,Ⅱpollution levels.5) XSP-300 insulator strings can reduce 12～14 units according to the recommended k1=1.04.6) Compared with double I insulator string, single I string can reduce 0-1 units when the string distance is 600 mm, CA887-EZ single V string can reduce 7-11 units under the conditions ofⅠ,Ⅱ,Ⅲ,Ⅳpollution levels, that is 7-10 units for XSP-300.7) Composite insulator can effectively reduce the UHV AC insulator length,9.750 m and 10.530 m spacing height composite insulators can be used in III, IV pollution level regions, forρSDD/ρNSDD=0.25/0.5 mg/cm2, string length can reduce 35.06% by using 9.750 m composite insulator instead of using XSP-300 insulator, forρSDD/ρNSDD=0.35/0.5 mg/cm2, string length can reduce 33.33% by using 10.530 m composite insulator instead of using XSP-300 insulator.(3) The icing flashover characteristics of energized EHV transmission line insulator strings are studied in the large scale artificial climate laboratory, conclusions can be drawn that:l) Equipped with the given grading ring and simulative conductor, under theρSDD=0.1,0.05, 0.025 mg/cm2 and heavy icing conditions, the flashover voltages of 28 units XWP2-160 insulator string are 20.5% lower,13.1% lower and 2.3% higher than the 500 kV transmission line rated phase to earth voltage respectively, those are 18.7% lower,12.0% lower and 4.3% higher for the FXBW4-500/160 composite insulator respectively.2) The equipped grading ring may reduce the effective dry arcing distance of the insulator string, under theρSDD=0.1 mg/cm2 and heavy icing conditions, the flashover voltage without grading ring and simulative conductor is 11.6% higher than that with grading ring and simulative conductor.3) Under theρSDD=0.1 mg/cm2 and the heavy icing conditions, the icing flashover voltage of 28 units XWP2-160 insulator is linear with the string length, the effect of pollution to pollution flashover voltage is higher than that of pollution to icing flashover voltage.4) The icing flashover voltage of 3+1 alternate sheds insulator string is about 6.1%(243.3 kV) higher than 28 units XWP2-160 insulator string, both with grading ring and simulative conductor, while the voltage is still lower about 15.7% than the rated phase to earth voltage.(4) The three dimensional FEM-BEM method and sub-region approximation FEM are realized on the CEMLAB platform. The effects of these two methods have been proven, the three dimensional FEM-BEM can be used to analyze the problems with floating voltage conductors and infinite open boundary, sub-region approximation FEM can be used to accurately analyze the potential and electric field distributions for the interest regions by sub-region approximation the interest regions step by step and with fine mesh. These two methods can be used together to deal with the three dimensional electrostatic problems including complex geometry structure, with multi-medium, floating voltage conductors, infinite open boundary. It is also found that the potential distribution of one phase insulator string is influenced less by the other two phase conductors for the cat shaped and cup shaped tower windows.(5) The three dimensional FEM-BEM method and sub-region approximation FEM are used to analyze potential and electric field distributions of EHV insulator strings before icing in the icing flashover experiments. With and without tank wall, grading ring and simulative conductor are considered, conclusions can be drawn as follows:1) Grading ring and simulative conductor can improve the high voltage end potential distribution of the insulator strings, and the simulative crossarm can improve the low voltage end potential distribution of the insulator strings.2) If the tank wall dimension decreases, the withstand voltage at high voltage end of the insulator string increases, and the withstand voltage decreases at low-voltage end.3) The maximum withstand voltage percentage along insulator strings at the conditions with tank wall, open boundary and practicial line equipment are 20.08%,20.64% and 17.96% respectively.(6) The potential and electric field distributions of 1000 kV AC cup-shaped tower, composite insulator single I, double I and single V strings are analyzed by using three dimensional FEM-BEM and sub-region approximation FEM.The maximum withstand voltages pencentage along the insulator of single I, double I and single V strings are on the No.3 unit, and the values are 7.22%,7.58% and 9.16% respectively. The maximum electric fields along the insulator axis are at 0.7875 m insulation distance to the high voltage end, and the values are 2.20 kV/cm,2.27 kV/cm and 2.14 kV/cm for singleⅠ, doubleⅠand singleⅤstrings respectively. |
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