CALCULATION OF LIGHTNING SURGES IN TRANSMISSION LINES USING ELECTRIC CIRCUITS CONSIDERING SOIL FREQUENCY CHARACTERISTICS
A lightning strike causes voltage surges in a transmission line, the spread of which is dangerous for the electrical equipment of electrical substations. The transmission line theory is usually used to simulate wave processes, but for the adequate consideration of the impulse corona and the earth-return impedance, it is necessary to split the line into elements so that an alternative approach using multi-link electric circuits becomes competitive. The impulse corona has a nonlinear characteristic, so calculations of wave processes are carried out in the time domain. The problem is that the earth-return impedance is defined in the frequency domain (according to the Carson formula), therefore at the moment it has to be simulated using complex equivalent circuits. This paper presents a time-discrete circuit of frequency-dependent earth-return impedance, which simplifies the solution. According to the recommendations of CIGRE, calculations of grounding systems for impulse effects are carried out in a soil model with frequency-dependent parameters. This recommendation was applied to the earth-return impedance, which allowed us to detect the effect of reducing the delay in wave propagation in high-resistance soil caused by an increase of the soil electrical conductivity at the first moment of the wave arrival. The reliability of the method is confirmed by comparison with experimental data, calculations by the Fourier method in the transmission line model, and calculations in the EMTP program.