| Overhead transmission line is an important energy transmission channel of a power system and it is an important part of a power grid.Shielding failure and back flashover will cause the line insulator flashover,which will lead to switch trip and eventually cause large area power blackouts,which will seriously affect the security and stability of power system.In addition,lightning overvoltage invade into the substation along the transmission line after lightning stroke happens.Equipment in the substation suffers excessive voltage and lead to insulation breakdown,resulting in power failure and reducing the reliability of the system.Therefore,it is of great significance to carry out the research on lightning protection.The acquisition of lightning parameters of transmission lines is the basis of lightning protection research,which can provide data support for the classification and identification of fault types and the “differentiation” lightning protection strategy of transmission lines.When lightning strokes to the transmission lines,due to the complex electromagnetic environment,lightning protection research lacks measured lightning parameters.Existing monitoring devices usually installed in the substations,due to the complex wiring of the substation,lightning overvoltage is subjected to complex reflection,and so the actual lightning parameters cannot be obtained accurately.Meanwhile,researchers are mostly focused on a single characteristic quantity on the feature extraction of lightning fault characteristic quantity,such as lightning overvoltage or lightning current on a specific phase wire.Lightning fault is a complex physical process,and it is difficult to fully reflect its characteristics only by a single physical quantity.In addition,impulse corona generated when lightning strokes to the transmission line is beneficial to the lightning protection.The insulation design tends to be conservative,if impulse corona is not considered in the lightning transient calculation,resulting in the sunk cost of large insulation margin.The development of impulse corona is closely related to the generation and dissipation of the space charge.Current research on impulse corona lacks the analysis of the quantity change law and the migration law of the space charge.Therefore,it is of great significance to study the micro process of impulse corona discharge in order to explain the physical essence of impulse corona and the lightning transient calculation.Aiming at the above problems,a set of lightning multi-characteristic quantity monitoring system for transmission lines is established in this paper,which can monitor the voltage of the insulator string and the lightning grounding current of the transmission tower after lightning strokes to the transmission line.Six kinds of direct lightning strokes can be identified based on the polarity discrimination method considering the lightning stroke point and the lightning current polarity.The influence of impulse corona on propagation characteristics of lightning overvoltage wave through the transmission line is studied by means of the monitoring system.The influence of space charge on corona characteristics of transmission lines is studied by building a corona discharge space charge measurement platform in the laboratory.Based on the analysis of space charge characteristics,a hybrid impulse corona numerical model is established which considered the effects of the change in the number of microscopic particles and their migration during corona discharge,then an impulse corona circuit model which can be used in simulation software is established.The influence of impulse corona on the propagation characteristics of lightning voltage wave is also studied.The main theses of this research are as follows:(1)A kind of transmission line overvoltage sensor is developed.Based on the principle of air coupling stray capacitance,the sensor realizes the passive non-contact measurement of overvoltage,the high-voltage arm capacitance of the sensor is calculated by the wire-plate model and the series method,and the calculation formula of wire-plate capacitance is obtained for the preliminary guidance of engineering design,and the calculation formula is verified by Ansoft simulation and actual measurement.Aiming at the strong electromagnetic environment of 110 k V transmission line under lightning stroke,the electromagnetic shielding structure of the sensor is designed and the low-voltage arm circuit of the sensor is improved.The performance of the sensor is verified by power frequency response experiment and impulse response experiment in the laboratory.A wideband and wide lightning current amplitude range sensor for lightning current measurement in the field is developed.Based on the parallel connection method of multiple outer integral Rogowski coils,the lightning current flowing through the tower is monitored.Through quantitative analysis and Saber simulation,the reasonable matching of the structure,winding turns and other parameters are realized.An on-line lightning multiple parameters monitoring system for transmission lines is developed.By setting up sensors on multi-base tower,combining with data acquisition and transmission unit,power supply unit and user unit to form a monitoring system,then online monitoring of transmission line lightning parameters outside an 110 k V station can be realized.(2)A method is proposed to identify different lightning stroke types by signal polarity variation.A transmission line lightning fault simulation model is established by EMTP-ATP,and different lightning fault condition is simulated through the model.Combined with the simulation results,based on time-domain and wavelet transform modulus maxima(WTMM)analyses,the polarity of insulator voltage,the polarity of tower current,and the mutation polarity of tower current when insulators flashover are extracted as characteristic quantities of polarity discrimination.Hence,a lightning fault recognition method is established based on polarity identification.Simulation and measured data verify the feasibility of the method.(3)Based on corona cage,an experimental method of measuring space charge through the sampling capacitance is proposed,and the influence of space charge on the corona characteristics of transmission lines is studied.The impulse corona discharge on the wire generates a large amount of space charge,which changes the electrical environment of the line and affects the distribution of the electric field in the space around the line.Due to the complexity of outdoor line running environment,impulse voltage frequency is too high,the analysis system needs extremely high accuracy,the experiment of space charge for transmission line is not easy to operation,by simplifying the circuit structure of corona discharge,based on the transmission line corona discharge corona cage,a space charge analysis platform is constructed in the laboratory to study the characteristics of power frequency corona,high frequency ac corona and single polarity high frequency attenuation oscillation corona,then the influence of space charge on corona characteristics of transmission lines is analyzed.It can be used to explore the characteristics of space charge in the early stage for the subsequent impulse corona model based on the change of space charge quantity and the analysis of the law of motion and migration.(4)An impulse corona equivalent circuit model considering the effect of space charge is proposed.The particle density distribution,field intensity distribution,corona current and charge loss around the transmission line under the influence of space charge are simulated and analyzed.Based on the simulation analysis,the Q-V curve is obtained,and the model parameters of the desired impulse corona circuit model can be obtained through parameter fitting,and then the impulse corona circuit model can be applied to the lightning transient analysis. |
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