The Influence Of Controlled Switching Technology On Insulation Level Of Ultra High Voltage(UHV)System

With the development of Ultra High Voltage(UHV)system in China,the solar energy and wind energy in the north and the water resources in the south can be smoothly transported to the southeast coastal cities.In order to transport energy more safely and efficiently,the insulation level of UHV system has become the most important research.One of the main factors affecting the insulation level is the switching overvoltage.In consideration of safety,technology and economy,controlled switching can be taken to limit the switching overvoltage of UHV system.Then controlled switching can decrease the insulation level of UHV system.However,the influence of pre-breakdown characteristics and mechanical dispersion of SF6 circuit breaker,line coupling effect on the optimal closing time is still lack of detailed research at present.In addition,there are few researches on the quantitative analysis of the influence of controlled switching on line risk of failure and controlled reclosing strategy of UHV line.The purpose of this dissertation is to study the influence of the nonlinearity of rate of decrease of dielectric strength(RDDS)of UHV circuit breaker on the optimal closing time and pre-breakdown time,the quantitative mapping relationship between the pre breakdown characteristics and mechanical dispersion of the phasing controlled circuit breaker and the risk of failure of UHV line,so as to solve the technical problems of the application of controlled switching technology in the UHV system.This dissertation proposes a calculation method of pre-breakdown voltage curve based on SF6 gas breakdown criterion of the steamer theory.Aiming at the problem of the uneven electric field strength in the arc extinguishing chamber of UHV circuit breaker during the closing process,RDDS is nonlinear when the circuit breaker is closed.Then the pre-breakdown time under different relative value k is calculated.Combined with mechanical dispersion,the optimal closing instants of controlled closing different load are determined.In order to verify the suppressing effect of controlled closing on overvoltage,the 1000 kV Changzhi-Nanyang-Jingmen demonstration project of China is simulated.The model is based on ATP-EMTP for simulation calculation.The simulation results show that for no-load transmission line,the influence of pre-breakdown instants on overvoltage is greater than that of mechanical dispersion,and more attention should be paid to the pre-breakdown process.For different residual magnetism of UHV transformer,three controlled closing strategies are adopted:rapid,delay and simultaneous closing.Simulation results show that the combination of the three closing strategies can suppress overvoltage and inrush current.The technology of controlled switching can obviously reduce the overvoltage of UHV system,and then reduce the switching impulse insulation level.Due to the coupling effect between the phases of the transmission line,induced voltage will be generated on the second phase and the third phase after the first phase is closed.Controlled closing at the voltage zero point will generate overvoltage.Therefore,based on the modal analysis of traveling waves and Laplace transform theory,the calculation method of the optimal closing instants of each phase breaker is proposed.The line models of different topological structures are established by ATP.The simulation results verify that the optimized controlled switching strategy is effective for untransposed and transposed transmission lines,and the effect is more obvious for double-circuit lines.In principle,controlled switching can reduce the switching overvoltage that determines the insulation level of UHV system.However,this qualitative understanding is too macro and fuzzy.In order to apply controlled switching in UHV transmission system,it is necessary to enlarge this process,clearly give the details of each link,and carry out quantitative calibration.Therefore,the model of risk of failure of closing UHV transmission line is established.The model represents the mapping relationship between closing performance of circuit breaker,line topology and risk of failure,and solves the problem that a small parameter change requires to repeat simulation and deal with a large number of results.According to this model,the risk of failure of closing UHV line can be forecasted.In other words,the user can predict the risk of failureof line with controlled switching on the premise that the closing performance of circuit breaker and line parameters are known.On the contrary,when the risk of failure of line need be considered,the circuit breaker manufacturer can adjust the closing performance parameters according to the user requirements.With the continuous development of UHV system,the safety and reliability of system operation become more and more important.Once a fault occurs,it will have a significant impact.When UHV system is reclosed in permanent fault,large current will be generated.The normal operation current and fault current are calculated according to the two-source system model.Then the optimal reclosing angle of UHV is 0° or 180°.Finally,the simulation results show that the controlled reclosing strategy is feasible when reclosing angle is 0° or 180° of voltage.The method can reduce the peak value of reclosing current to less than 33%of the maximum current that may occur in random reclosing.Based on the theoretical analysis and simulation in this dissertation,the influence of the closing performance of circuit breakers and line topological structure on the optimal closing time is studied.The mapping model between the closing performance of circuit breakers,line topological structure and risk of failure of line is established.The strategy of UHV controlled reclosing is put forward.The quantitative analysis of the influence of controlled switching on the insulation of UHV system is realized.The results provide a certain reference of application of controlled switching in UHV system.