HVDC Breakdown Properties Of Liquid Nitrogen In High Temperature Superconducting Fault Current Limiters

China's rapid economic development makes the rapid growth of electricity demand in various regions.China's power grid structure is constantly strengthened to ensure economic development and the distribution of power resources and demand is uneven,makes the grid short-circuit current level rise further,also gradually beyond the level of open circuit breaker.Therefore,in order to ensure the continuous development of the power grid to meet the increasing demand for electricity in China,how to limitation the short-circuit current has become an urgent problem for development of the power grid.At present,there are many techniques to limit the short-circuit current.However,the traditional current limiter has many disadvantages,but the superconducting current limiter developed by using the properties of superconducting materials can solve these disadvantages,and can design the ideal fault limiter.The advantages of superconducting fault limiter include low loss,fast response and self-recovery.With the continuous construction of the HV-network and UHV-network,the working voltage level of the superconducting current limiter continues to improve,and one of the first problems that the superconducting current limiter working under high voltage needs to face is the insulation problem.To be applied in the power grid,the safety and reliability of power equipment are the key factors to be considered in the first place.The insulation problem of the superconducting current limiter is more complicated because its superconducting winding works in the low-temperature environment of liquid nitrogen.The insulation problems in the superconducting current limiter are between the superconducting windings and the Dewar cans,namely the main insulation,the insulation between the superconducting tape and the superconducting tape,and the insulation between the superconducting tape and the adjacent superconducting winding disks.This thesis mainly studies the main insulation of superconducting current limiter.Firstly,in order to study this problem,a COMSOL simulation model is designed to simulate the electric field distribution of the main insulation of 160kV superconducting current limiter by referring to the structure of 160kV superconducting current limiter in the actual project.Secondly according to the simulation results to design breakdown test of liquid nitrogen under HVDC using needle-plate and ball-ball electrode.Combined with the needle-plate and the ball-ball electrode simulation to study the HVDC breakdown properties of liquid nitrogen.In the test under needle plate electrode,the distance between electrodes increases from 5 mm to 180 mm,and the highest breakdown voltage reaches 500 kV.In the test under the ball-ball electrode,the electrode spacing was increased from 5 mm to 30 mm,and the highest breakdown voltage reached 500 kV.The HVDC breakdown test at this amplitude has not been studied before.Then analyzing the electric field distribution obtained by simulation of the needle-plate electrode,the ball-ball electrode and the main insulation of SFCL and the results of the previous liquid nitrogen breakdown experiment.Drawing the curves of voltage and field intensity,and getting the basis for judging whether the breakdown of liquid nitrogen is possible under HVDC.At last,this thesis makes use of the above analysis results to design the main insulation of the superconducting current limiter so that it can have better insulation performance.Under the condition that the main insulation of SFCL has enough insulation.Reducing the size of the SFCL to improve the economy.