Spatial Distribution Of Electromagnetic Field In Lightning Return Stroke

Lightning electromagnetic pulse(LEMP) is a random global high powerradiation source. It can cause interference, failure or damage in electrical andelectronic equipments, transmission lines and communication cables. For mostequipments, cables and sensors which related to human daily life are arranged nearthe ground, the published researches on the attributes of lightning electromagneticfields are confined to the discrete points near the ground. But with more and moreapplications of Ultra-High Voltage(UHV) transmission network, the low-altitudeaircraft, giant buildings and underground cable, it is necessary to make the scope ofthe study on LEMP expand to the entire space. On the other hand,for the hugeamount of computation and storage, the published studies on LEMP are confined tothe rough distribution by comparing diffferent waveforms on discrete points. Itobviously does not provide strong support for LEMP protection design andstandards formulation.For these reasons, the LEMP induced by lightning return stroke is selected tobe the object of this research. The spatial distribution of characteristic parameterswhich extract from LEMP are calculated in entire space by an originally improvedfinite difference time domain (FDTD) method. The spatial distributions of LEMPunder the condition of different parameters and their change trend are also given inthis paper. And all these distributions constitute the spatial distribution rules ofLEMP systematically and completely.First, a "processing window" program is added between the time steps ofFDTD method in this research. This novel approach can be called PWFDTD(processing-window finite difference time domain). For this method avoids thehuge amount of calculation and storage when computing the distribution of LEMPparameters, it can get the LEMP and their spatial distribution fast and conveniently.After comparing the results get from different method with the data from PWFDTDapproach in different positions and different parameters, the precision and accuracyof the new algorithm is demonstrated.Second, the characteristic parameters of LEMP at each grid in a few kilometersrange are calculated in this paper with PWFDTD method. For the first time, the exact spatial distribution maps of these LEMP parameters are achieved. thesecharacteristic parameters includes the maximum LEMP, the peak value of LEMPtime derivatives, the maximum power density of LEMP, the energy density, therise-time T10%-90%(The time from10%peak to90%peak)of LEMP, the T10%-90%andhalf-width T50%-50%(The time from50%peak to50%peak again) of LEMP timederivatives. Based on sampling and analyzing to the distribution of theseparameters, the basic distribution rules of LEMP are summarized from differentperspectives.Third, the change trend of LEMP parameters are analyzed for the condition ofdifferent model of lightning channel, different base current, different velocity ofcurrent, different conductivity of ground, the horizontal and vertical stratificationground and the uneven feature of terrain. The influence of spatial distribution mapsof these LEMP parameters for different conditions is summarized. Since the basicdistribution rules of LEMP does not change in different conditions, its universalityand practical value is confirmed.At last, the LEMP radiated by lightning strikes to tall structures are calculatedat each grid of entire space. The distribution rules of LEMP are summarized in thiscase. And the change trend of distribution are analyzed for the condition ofdifferent height of tower and reflections at the top and bottom of tower. Theinfluence of spatial distribution maps of LEMP for different conditions issummarized.All these spatial distribution maps and rules of LEMP were not seen in thedomestic or foreign research. Compared to the existing research, which areconfined to the rough distribution on discrete points, these distribution maps andrules are more accurate, complete and intuitive. They are not only useful to betterunderstand the process and mechanism of lightning, but also can be principles ofshielding design for electrical and electronic equipments, transmission lines andcommunication cables. They could be used as protection design and layout rules inmany aspects, and can also provide important help to the observation of LEMP andlightning location.