Synthetical Modeling And Simulation Algorithm Research Of Lightning Transients On Transmission Lines

The basic purpose of lightning protection of transmission line is to reduce lightning accident times and decrease the losses caused by lightning. The overvoltage protection and insulation coordination of transmission lines depend in great degree on the magnitude and waveform of the overvoltage. The insulation level of transmission line and the insulation coordination of different divices such as insulator, transformer and breaker, must endure the possible overvoltage. So doing research on lightning overvoltage of transmission line is propitious to the design of the insulaton coordination of transmission line and electrical devices and to the ecomic optimization of the system.Attenuation and distortion may occur on lighitning overvoltage when travling along the line and the main causations are corona effect and frequency dependent effect of transmission line in lighting transient situations. The two aspects are studied in this paper.When the voltage on line exceeds corona inception voltage, corona occurs. In order to reduce calculation errors, the electric field is devided into two parts, one is the nominal field with no charge and the other is the field by space charge. When solving the equation, simulation charge method is used to solve the nominal field and finite element method to the space charge field. When solving the space charge field, the given initial value of charge density is applied to calculate the space electric field, and the calculated total field is then used to calculate the space charge density reversely. The Q-V curve and the dynamic capacitance of the line at corona can be obtained by iterative calculation. For the corona effects of other points on the line, the thinking of lookup is used in this paper, that is, when the voltage on some point reaches a certain value, the corona charge and dynamic capacitance are obtained by lookuping the Q-V curve on the lightning stroke point.When considering the frequency dependent parameters of the line, its radial parameters are described by series and parallel of resistance and inductance. The calculation results verify that the model not only can describe the frequency dependent characteristics of the line in a large range of frequency, but also need not to do transformation between frequency domain and time domain which can save the calcualtiong time and the more important is that it can be connected to the corona model convinently. The paramenters of each RL module are evalued by Newton-type iteration method whose convergence rate is more rapid. If it is needed to consider the frequency dependent characteristics and the corona effects of the line synchronously, the capacitance in the frequency dependent model is changed to the dynamic capacitance.In order to make the models established in this paper more complete, the general vertical conductor model to express the tower is replaced by the inclined conductor tower model, and the more accurate multiwave impedance tower model is proposed to analyze the lightning transients of transmission tower. The tower model combined with the insulator flash model can be used to analyze the total process that when the tower is stroked by lightning, the voltages on it are rised, and then the insulator flashes and lightning overvoltages are produced on the line. For the transient circuit model, the method of transient equivalent calculation circuit is applied, which converts the complex network composed of resistances, inductances and capacitances into the circuit made up of only parallel resistances and current sources. Node voltage method is used to solve the voltage on each point of the line.According to models and calculation methods proposed in this paper, the corona effect under double exponential lightning waveform, damped oscillation waveform and fast surge are calculated, the physical process of each branch on the Q-V curve is described, and on the basis the characteristics of minor loop curve under damped oscillatory waves are also analyzed. A simulation experiment is made to verify the effectivemess of the model proposed in this paper. In addition, the situations of only considering impulse corona effect, only considering frequency dependent effect of line parameters and considering corona effects and frequency dependent characteristics synchronously are analyzed and compared with each other. The calculation results indicate that the attenuation and distortion of corona effects on transmission line are larger than those by frequency dependent effects, but it is more conforming to reality considering the two aspects synchronously. The influences of different aspects on lighting overvoltage are also compared in this paper, for example, it is more obvious for the attenuation and distortion functions of lighitning waveform with short wave fronts and wavetails on the travelling waves. Software for analyzing lightning overvoltages on transmission line is programmed by matlab, and the calculation method and results proposed in this paper can provide guidelines for the lightining protection and insulation design of practice lines.