Research On Simulation And Detection Technology Of Lightning Return Stroke Electromagnetic Field

The lightning is one of the most dangerous nature disasters. Our country has placedgreat importance on lightning protection. But those protections devised under existinglighting basic characters can not fully satisfy our production and life because of thelimitation of the recognition about lightning process and the lack of measurementtechnology. In order to investigate lightning deeply and basically, protect and predictlightning and the following dangers, and accelerate the lightning location process, we needto establish more convenient and reliable models about lightning and to detect lightningmore effectively and accurately. Based on these, the lightning modeling methods anddetecting technology have been studied deeply: at first the lightning current andelectromagnetic field models have been analyzed and simulated precisely, then acloud-to-ground(CG) lightning detecting system has been developed and the rationality hasbeen tested, at last a recognition method has been proposed to classify first and subsequentreturn stroke. The main contents and the creative results are below:1. The whole physical discharge process and the characteristics of lightning have beenanalyzed explicitly. The reason why lightning currents and electromagnetic fields aredifferent corresponding to different return strokes is investigated based on this theoreticallystudy.2. The lightning channel-base current models have been simulated, and a new lightningchannel-base current piecewise-function model has been proposed to solve the problemswhich are general among those common channel-base current models, such as the peakfields and peak field time are uncertain, the model functions are complicate and theparameters are difficult to adjust, and some models have no integral and derivative. Withthis piecewise-function model, the peak field value and peak field time can be decideddirectly getting rid of the amplitude correction factor, the parameters are easy to adjust, andthe rising part and the decaying part of the current waveform are independent with eachother which are dependent in other models. Then the first return stroke, subsequent returnstroke and trigged lighting stroke were simulated based on the experiments of naturelightning and trigged lightning in recent years, the characters of the current waveforms arereasonably agreeable with those measurement results. At last, combined with lightningreturn stroke engineering models, the validity of this model was proved according thecalculation of the lightning electric and magnetic fields. The results indicate that the piecewise-function model is a desired model which can be convenient used in further study.3. Considering the different discharge process of first and subsequent return strokes, afractal model of lightning return stroke channel based on the diffusion limited aggregationmodel has been proposed to improve the usual vertical channel for calculating lightningelectric and magnetic fields. This fractal channel model conforms to the natural lightningchannel. The box counting method is used to calculate and compare the fractal dimensionsof natural channel and fractal lightning channel, and the fractal parameters are adjustedaccording to the fractal dimension variation range of first and subsequent return strokes.Besides, by using a dipole method, the vertical electric fields and the horizontal magneticfields at different distance on ground are derived. The simulated results are correspondingto the basic characteristics of experimental lightning electromagnetic fields, which indicatethe validation of the fractal model, and also show that the tortuous channel and branchesbring more high frequency components in electromagnetic fields, especially first returnstroke.4. The configuration of lightning CG lightning detecting system is presented in thisarticle, including the detailed hardware and software solutions of different functionmodules. The test has been made and the results show the reliability of this system.5. Because first and subsequent return strokes cannot be distinguished in those existinglightning detection systems, the Laplace wavelet has been used to analyze the electric andmagnetic field waveforms of first and subsequent return strokes. The characteristics of fieldwaveforms in time and frequency domains are presented. The precise time of arrival of theinitial peak field can be used to improve the lightning location accuracy. And the domainfrequencies of the initial peak field of first return strokes are higher than that of subsequentreturn strokes, which can be used to distinguish and classify first and subsequent returnstrokes in lightning detection system. The power spectrums are expressed to indicate thepower distribution in different frequency of the lighting electromagnetic field. Besides, thefrequency and power spectrum of other lightning events such as preliminary breakdownpulses and leaders are also analyzed The results show that the Laplace wavelet is aneffective tool and can be used to determine time and frequency of the lightningelectromagnetic field with greater accuracy.