Research On Switching Overvoltage Mechanism And Insulation Coordination Of UHVDC Converter Stations

For the unbalance distribution of energy sources and Loads in China, the West to East and the North to South power policy which means that transmitting power from energy bases in the West and North China to the load centers in the East and South China are imposed. The UHVDC(Ultra High Voltage Direct Currunt) is first choice in this strategic plan for its superiority that being fit for huge and long distance power transmission. The stable operation of the UHVDC system is very important for the safety and stabilization of power system in China. The researches on switching overvoltage and insulation coordination are important part in the reasearch on UHVDC transmis-sion. A perfect overvoltage protection and suitable insulation design would ensure the safety and stabilization of UHVDC system with reducing the project construction and equipments manufac-ture cost. The main works in this thesis are oragnized as follows:(1) Taking Xiangjiaba-Shanghai ±800kV UHVDC power transmission project as an example, the main equipments and their parameters are introduced in this thesis. The calculation methods of main circuit parameters are provided. The computational formulaes and parameters values ac-cordingly are listed. The main circuit parameters are calculated. The simulation conditions for simulating the most severe overvoltage are presented. And the arrester scheme configuration and parameters are provided at last.(2) The idea of establishing the UHVDC power transmission system switching overvoltage simulation model are elaborated in this thesis. Based on the idea, the specializations of modules which constitute the simulation model and hierarchical structures are listed. The funtion and param-eters of key segments are presented and the cooperative relationship and logical structures between the main controller in UHVDC control system is provided at last.(3) According to the different switching overvoltage conditions on the equipments in UHVDC converter stations, the switching overvoltages are divided into AC bus overvoltage, overvoltage in AC filter, valve overvoltage, converter bus overvoltage, DC bus overvoltage, DC neutral bus over-voltage and overvoltage in DC filter. They are all simulated, analyzed and researched in detail in this thesis. The amplitudes of these overvoltages and the energy absorbed by the arresters which are used to limiting these overvoltages are calculated. The switching overvoltage mechanism, devel-oping process and characteristics are analyzed. And the most severe switching overvoltage factors for each overvoltage are presented. Based on the results of switching overvoltage mechanism, the methods and application of ideas on control strategy for limiting the overvoltage and are presented and discussed.(4) Taking Xiangjiaba-Shanghai and Yunnan-Guangdong ±800kV UHVDC power transmis-sion projects as examples, the arrester scheme configurations and insulation coordination designs are introduced, and the insulation coordination results of these two kinds of typical projects are dis-cussed. Based on the discussion and insulation coordination actual parameters of these two projects, two key principles for UHVDC transmission project are presented:the insulation level of equip-ments which are operating under higer voltage condition should be reduced as lower as it can be. That of equipments which are operating under lower voltage condition could be setted higher for significantly enhancing the stabilization and safety of these equipments and arresters with a little increasing on the manufacture cost.(5) Considering the higer DC voltage level UHVDC power transmission project which is under research, the switching overvoltage and insulation coordination in ±1100kV UHVDC system are ayslyzed and discussed. Based on the switching overvoltage mechanism researched in the former chapters, the differences between ±800kV and ±1100kV UHVDC system switching overvoltages are researched. The regulation of the switching overvoltage changing with the DC voltage level are presented. By comparing the differences between the insulation coordination designs under these DC two voltage levels, and taking the key principles analyzation presented in the former chapter into account, the conclusion that key points under these two DC voltage levels are different is presented. That is in ±800kV UHVDC system, decreasing the cost of arresters and valve hall space is the most important points in insulation coordination design, but in ±1100kV UHVDC system, decreasing the insulation levels of equipments is more important than that in ±800kV UHVDC system. The principles of insulation coordination in UHVDC converter station are summarized at last.