| Electromagnetic fields generated by cloud-to-ground lightning play real threat to power systems and especially the distribution lines because of its sharp steepness, high amplitude, wide spreading range and the complex coupling path. Hence the lightning induced over-voltage is an important problem should be solved for lightning protection of distribution network. For lightning electromagnetic fields, there are a lot of research results of the near field, but little achievement on the wide-area features and the spreading characteristics within field region has been reported at present. For lightning induced over-voltage on overhead line, the calculation formula used in power system specification can just estimate a less accurate peak value of the induced voltage only, which can hardly guarantee the accuracy and cannot get the voltage waveform. Study on the characteristics of lightning electric fields and inductive coupling on overhead lines has important theoretical and practical significance for establishing cloud-to-ground lightning electromagnetic calculation theory and method and lightning protection of overhead lines and modern electrical equipments.Hence, theoretical and experimental researches in three aspects had been carried out this paper as the study on the characteristics of lightning electromagnetic fields through observation and theoretical calculation and study on the characteristics of the lightning induced voltage on overhead lines through theoretical calculation, and progress had been made as follows:Aim to difference limitations such as low computational accuracy and weak computational performance for wide-area lightning electromagnetic fields, a new non-uniform grid FDTD algorithm is proposed for large space distant conditions based on traditional uniform grid FDTD algorithm. Wide-area electric fields generated by cloud-to-ground lightning were calculated. A non-uniform grid multiple order FDTD method which can solving the calculation of lightning induced voltage for long-distance overhead line and long-distance return-stroke have been proposed combining the second-order FDTD method for solving Agrawal model equations with the non-uniform grid FDTD method. Induced voltage waveforms were calculated by this method. The correctness of the two proposed methods is verified by comparing with the experimental data.By 9 stations distributed VLF/LF lightning observation system, this paper also carried out a systematic study on digital observation of electromagnetic field wave. In order to evaluate the sensitivity and response characteristics on different detection distances of exploring antenna, a calibration factor was defined. And then, field region boundary division of cloud-to-ground lightning was analyzed from the angle of measurement based on the physical concepts of field division at present and the relationship between calibration factor and the return-stroke distance (the distance between the observation point and the return-stroke channel). The distance characteristics of natural cloud-to-ground lightning field region were clearly revealed by this work. In order to make a statistical analysis of the 3625757 cases cloud-to-ground lightning data from Foshan and surrounding areas of Guangdong Province in 2014, the radiation field statistical characteristics within 20～300 km with considering the influence of the return-stroke distance and the return-stroke current amplitude were obtained.Based on summaring common values range of characteristics parameters about lightning electromagnetic fields calculation and, effect of the earth conductivity, relative permittivity of the earth, channel length, return-stroke velocity, amplitude of the base current, rise time of the base current, the equivalent thickness of the ground and return-stroke model on the vertical electric field and the horizontal electric field of the cloud-to-ground lightning was studied by using the non-uniform grid FDTD method. The calculation results revealed the typical waveform variation laws of the electric field generated by cloud-to-ground lightning changed along the return-stroke distance.Influence of factors such as the return-stroke location, return-stroke dstance, earth conductivity, height of conductor, length of conductor, termination resistance and amplitude of channel base current on lightning induced voltage of single wire was studied by using the non-uniform grid multi order FDTD.Moreover, influence of the factors such as the return-stroke location, return-stroke dstance, earth conductivity, way of wire arrangement, amplitude of channel base current and length of line on lightning induced voltage characteristics of lOkV single and double circuit overhead line were also studied by using the same method. The characteristics of induced voltage waveforms in time domain and the induced voltage peak distribution on line were obtained. Work in this paper can provid theoretical basis for understanding the coupling physical mechanism of lightning induced voltage on overhead lines. |
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