Study On Disconnector Repeated Arc And High Frequency Transient Characteristics Of The Generated Overvoltage

Disconnector is a significant component of gas insulated switchgear(GIS).There will be dozens of times breakdowns repeatedly between connecters when the disconnector switches off and on a short bus with no load.It will produce the very fast transient overvoltage(VFTO),which has high amplitude(up to 3 p.u.),steep wave front(3?100 ns)and high frequency(up to 100 MHZ).VFTO is an important standard to measure the insulation level of GIS and its adjacent electrical equipment,and it is the key to the design scheme of switch structures and substation layout as well.Breakdown characteristics of gas medium in disconnectors,arc discharge characteristics at the breakdown moment,and residual charge distribution characteristics of short no-load bus are all the key factors to the transient amplitude-frequency characteristics of VFTO.However,the current mathematical arc models cannot fully reflect the multiple restrike arc characteristics,which is rooted in:1.The micro particle dynamics equation of arc discharge model is limited by mathematical algorithm,so the solution of centimeter gap has a large amount of calculation and is difficult to make the calculation convergence;2.The dynamic breakdown voltage curve of disconnectors is inaccurate;3.The arc model cannot combine the microphysical process of discharge with clearance structure of gas chamber,operation characteristics and transient overvoltage characteristics in circuit.In view of this,this paper propose FDM-FEM-FCT hybrid algorithm to implement the simulation of the dynamic characteristics of charged particles in the centimeter gap.The control experiments are set to verify the scientific nature and validity of the simulation method.The streamer model of the initiation and development process is established when the contactors restrike by studying the microphysical process of positive and negative streamer discharge.The influence of application voltage's amplitude and polarity,electric field non-uniformity coefficient,initial electron density and gas temperature are analyzed,which can provide a theoretical basis for the research on the restrike arc characteristics of disconnectors.The dissertation mainly conducts following several aspects:(1)The characteristic mechanism of SF₆ streamer discharge in centimeter gap was studied.FDM and FEM are respectively used to discretization the spatial physical quantity and time quantity in discharge process.The mapping relationship between the two kinds of grids is established by means of coordinate transformation and linear interpolation to simulate the discharge process in the centimeter-level gap.The physical process during the streamer initiation in uniform and non-uniform electric fields is analyzed,and the simulation model from the electron swarm to streamer is established to study the initial characteristics with different clearance lengths.Development processes of positive and negative streamers are simulated,and the change law of the charged particles number density are obtained during them which take the distortion effect of space charges on electric field into consideration.The discharge current,streamer development speed,streamer radius and other microscopic parameters are calculated.The experimental platform about gas discharge characteristics and emission spectrum are built to verify the correctness of simulation analysis method,which provides a theoretical basis for further research on the variation law of arc resistance.(2)The physical process of streamer initiation in non-uniform electric field is analyzed.2D numerical simulation models of the secondary electron avalanche in positive and negative streamers are built respectively containing the photoionization and light emission.The microphysical development characteristics of the positive and negative streamers are analyzed,including space distribution of the charged particle and electric field,channel radius,development speed,discharge current and so on.The influence of clearance distance,amplitude and polarity of applied voltage and electric field non-uniformity coefficient on the streamer characteristics are calculated and studies.A numerical method for calculating the initial corona voltage and breakdown voltage is proposed considering the electrode structure.Typical electrodes are selected as the examples to carry out the test about corona discharge and breakdown characteristics.The formation mechanism and variation rule of gas discharge polarity are explained both by experiments and simulations and the accuracy of simulation method is verified at the same time.(3)Effect of gas temperature and the initial particle density on discharge characteristics are analyzed.A two-term approximation of the Boltzmann equation is used to calculated electron energy distribution function of hot SF₆,getting the reaction coefficient of collision ionization adsorption process in different reduced electric field intensity.Based on the total reduced ionization and adsorption coefficient of SF₆ gas with different temperatures,the reduced breakdown intensity can be gain.The streamer discharge characteristics of 300K to3000K SF₆ gas is analyzed,which provides a theoretical basis for revealing the SF₆ arc breakdown mechanism.(4)UHV GIS disconnector is selected as an example,and the electric field distribution characteristics of gas chamber are analyzed.On the basis of the streamer initial physical process,the dynamic breakdown voltage curve of disconnectors is calculated.The streamer dynamics simulation is combined with line electromagnetic transient simulation,the arc multi-restrike mathematical model of disconnector is established.The high-frequency transient characteristic parameters of line are analyzed with arc repeated burning.Aiming at the actual UHV AC VFTO test loop,simulation platform is built.Disconnector arc reigniting mechanism under different switching on and off speed is analyzed as well as the VFTO high-frequency transient characteristics.On the basis of the VFTO experimental data,the influence of the operating speed on VFTO amplitude,restrike times and duration are counted and studied.The relationship between the operating speed and the residual voltage of the short bus on load is statistical analyzed,as well as the relationship between the operating speed and VFTO largest amplitude.