Measurement And Influence Of Space Charge Distribution In Corona Cage Based On Sound Pulse Method

The existence of space charge in the gas will change the corona discharge characteristics of the transmission line and the distribution of the surrounding ion flow field,which will affect the engineering design of the UHV transmission line,and is also an important influencing factor for the evaluation of the electromagnetic environment around the UHV transmission line.The numerical calculation model is currently the most widely used method to study the space charge density distribution.However,due to the various assumptions in the algorithm,the calculation result of the space charge density distribution differs from the actual distribution.Under complex operating conditions and extreme environmental factors The performance is particularly prominent.Therefore,the accurate measurement of the space charge density distribution in the gas under corona discharge is helpful to analyze the shortcomings of the assumption,which can correct the numerical calculation model and improve the accuracy and reliability of the calculation model.The improvement of space charge density measurement and calculation accuracy has important engineering significance for the design of transmission line corridors and the evaluation of transmission line electromagnetic environment.In addition,the accurate measurement of the space charge density distribution in the gas is also helpful for further study of the corona discharge characteristics and the improvement of the corona discharge mechanism.In this paper,the space charge density measurement and discharge initiation characteristics/ surface field strength in corona cage under DC negative corona are carried out.Based on the principle of space charge measurement by the acoustic pulse method,a method of inversion calculation of the space charge density distribution based on the physical relationship between the acoustic wave signal,space charge,and acoustic excitation electric field is proposed,and a space charge density distribution measurement system is built.Then based on the calculation model and experimental platform,the distribution characteristics of space charge density under the influence of multiple factors are analyzed?The functional relationship between space charge density and different environmental factors is obtained,and the quantitative prediction of space charge density is established by using multiple linear regression method.Finally,according to the streamer model,the finite element method and the maximum charge density ratio model,the initiation characteristics of corona discharge and variation of surface electric field study considering the influence of space charge distribution in the gas is analyzed.The main innovative achievements of the thesis are as follows:(1)An improved inversion calculation model of space charge density and acoustic excitation electric field is established,and an acoustic pulse measurement system of gas space charge density distribution in corona cage is constructed.In the measurement analysis,the method of superposition average method and adaptive filtering combined denoising is proposed,which can control the measurement repeatability error within±2%(50 measurements).The the space charge density of wire with 1.5 mm radius at the high voltage end were compared with the Townsend calculation method based on the Kaptzov assumption and the finite element iteration method based on the empirical formula of surface electric field strength.The results showed that the relative error of the space charge density at the high voltage end was within 7.9%,which proved the effectiveness of the space charge density measurement system.(2)The effects of wire voltage,wire size,gas temperature,gas pressure and gas relative humidity on the distribution of space charge density are revealed,and a space charge density fitting calculation model based on multiple linear regression method is established.With the increase of voltage and temperature and wire size,the space charge density increases linearly.The space charge density decreases linearly with the increase of air pressure.The influence of relative humidity on space charge distribution presents a v-shaped curve.On this basis,a multiple linear regression model of the space charge density distribution in the corona cage under the combined action of multiple factors was established.Based on this model,the comprehensive influence of the above five factors on the space charge density distribution was analyzed.Through a large number of space charge density measurement experiments,the space charge density database was established,and it was found that the influence degree on the space charge density distribution was wire size>relative humidity>applied voltage>temperature>air pressure.(3)A flow model considering the effect of space charge on the initial characteristics of corona was established.Through the calculation of the flow model,it was found that after considering the influence of space charge,the corona initial voltage and the corona initial electric field of three groups of wires with different radii(1.0 mm,1.5 mm,2.0mm)in the corona cage all decreased by more than 8%,and the decreasing range gradually decreased with the increase of wire size.At the same time,the space charge density is used as a new parameter to study the initial characteristic of corona,which is consistent with the corona voltage value determined by the traditional corona current and photon number,the relative error is 1.2%?1.8%.(4)The analysis based on the finite element calculation method and the maximum charge density ratio(the ratio of the calculated value of the space charge density at the high-voltage end to the experimentally measured value)reveals the change of the electric field intensity on the surface of the wire after the corona inception in the corona cage.The results show that the electric field intensity on the surface of the wire decreases with the increase of the corona discharge intensity after the corona discharge occurs.Therefore,it is considered that the Kaptzov hypothesis has limitations and a correction method for the boundary conditions of the Kaptzov hypothesis in the finite element calculation method is proposed.The correction method is used to calculate the space charge density for wire with radius of 1.5 mm at the distance of 0.1 cm?0.2 cm?0.5 cm?0.7 cm?1.0 cm?1.5 cm from conductor surface.Compared with the space charge density calculated by Kaptzov assumption and measured by experiment,the results show that the relative error between correction method and experiment is smaller(compared with the relative error 15%?22% between the measured value and calculated value based on Kaptzov assumption),which is 2%?14%.Therefore,it is preliminarily believed that the law of the surface electric field intensity of the wires in the corona cage gradually decreases with the increase of the corona discharge intensity is accurate.