Study On Lightning Activity Regularity And Risk Assessment Based On LLS In Chongqing

Lightning disaster is one of the ten most severe natural disasters defined by UN in its document "Ten Years of International Disaster Reductions" and is called "one major public hazard in electronic age" by China Commission for Conformity Certification of Electrical Equipment. Chongqing region is one of the regions in China hit most frequently by thunders and lightning, where lightning causes dozens of casualties and economic losses worth of at least one hundred million Yuan each year. So thunder and lightning disasters prevention is now very urgent and important. But such prevention is based on mastering of laws of thunder and lightning activities and risks from thunder and lightning related disasters. So author of this article has studied such laws and made evaluations of the risks in Chongqing region by utilizing data from monitoring of Lightning Location System in Chongqing region (1999-2008), data about thunderstorm days (1951-2009) from34meteorological stations in Chongqing and related materials about thunder and lightning disasters(1954-2008). He also adopted many diagnosis and simulation methods, such as lightning current magnitude and lightning current wave head steepness probability mode recommended by IEEE and DL/T620, mathematical statistics,(rotated) empirical orthogonal function, and logarithmic normal distribution in this study. He has drawn the following major conclusions:1. From1999to2008, negative lightning frequency in Chongqing region was2,719.964, which accounts for94.6%of all lightning and far higher than lightning (155,415, accounting for5.4%of the total). But average of positive lightning current magnitude was56.49kA, which is much higher than that of negative ground lightning (39.74kA). Positive lightning thunder and lightning current magnitude probability density mode is F(x+)=(50.22/x+)e(x+-4.165)2/0.4431+(211.4/x+)e(x+-2.912)2/1.209(The two peaks are18.4kA and64.4kA respectively), which conforms to double logarithmic normal distribution and superposition. Negative lightning thunder current magnitude probability density mode is F(x-)=(6143/x-)e(x--3.079)2/.9291(single peak is21.73kA), which is compliant with logarithmic normal distribution. IEEE mode for lightning current magnitude's cumulative probability can reflect lightning distribution characteristics in lightning statistics more objectively than DL/T620mode. Based on IEEE mode, the cumulative probability modes for positive lightning, negative lightning and total lightning current magnitudes in Chongqing region, which are (P+(>IP)=1/[1+(IP/44.49)2.224],P-(>IP)=1/[1+(IP/32.62)2.728and P+(>IP)=1/[1+(IP/33.04)2.668), and the theoretical probability mode for lightning current wave steepness there, which is (P+(>a)=1/[1+(a/26.87)2.224].P-(>a)=1/[1+(a/19.7)2.728]和P(>a)=1/[1+(a/19.95)2.668) have been obtained.2. Lightning occurs to Chongqing region all the year round, which mainly concentrates on April to October and is caused by robust convection current weather in flood season. Lightning frequency is higher then and daily lightning distribution is the one dominated by night lightning (accounting for over60%). It is characterized with double peaks and double valleys. During14:00-18:00and22:00-03:00, lightning occurs at a higher rate, which fully demonstrates the regional weather characteristics of "Rain always comes at night in Chongqing region". During1999-2008, number of lightning was in a growing trend. Lightning increases at a climatic tendency of443times/month and at a faster pace in spring and summer, and alternates from longer cycles of12months,20months and29months to shorter cycles of3months and6months. The major abnormal space distribution characteristics are uniformity in whole Chongqing city(lightning with more or few thunders) and reverse directions in west and northeast part of the city. Areas covered by abnormal lightning is divided into four parts (west part, middle part, southeast part and northeast par of Chongqing city). On the basis of comprehensive lightning positioning system and manual observations of thunderstorm days, a mode for relationship between manually observed thunderstorm days and lightning ground strike densities and observation systems, and a thunderstorm day mode for reflecting lightning ground strike densities have been established in all administrative areas of the city's districts. Based on this, divisions were made for the whole city. The conclusion is that Fudu,Dianj iang and areas to the west of Pengshui are regions with higher Iightning frequencies and the rest are regions with medium Iightning frequencies.3.Disclosed the1aw that lightning parameters change with elevations and geographic latitudes and developed a related mode.In which the mode of lightning current magnitude(total lightning,positive lightning and negative lightning)changing with elevations is1g(Imean)=10-4H+1.5081,lg(Imean)=10-4H+1.6492and Ig(Imean)=9×10-5H+1.5014.The mode of positive lightning percentage changing with elevations is y=2×10-80x2-5×10-5x+0.0665.The mode of low magnimde(0-100kA) lightning percentage changing with elevation is as100=-4x10-5H+0.9905and The mode of low magnitude(100-200kA)lightning percentage changing with elevation is ar100=3×10-5H+0.0091.The mode pf lightning current magnitudes (total lightning,positive lightning and negative lightning)changing with latitudes is lg(Imean)=0.031λ+0.6509.Ig(Imean)=0.0298λ+0.86and Ig(Imean)=0.0287λ+0.7191,The mode of densities(total lightning,positive lightning and negative lightning)changing with latitudes is Nr=0.2676λ3-24.598λ2+752.45λ-7656.2, NP=0.0148λ3-1.3611λ2+41.771λ-426.54, NN=0.2528λ3-23.237λ2+710.68λ-7229.7.The mode of positive lightning changing with latitude is α=0.010λ-0.263. The modes of lightning current magnitudes(total lightning,positive lightning and negative lightning) changing with latitude are lg(Imean)=0.0056ψ2-1.2154ψ+67.774, lg(Imean)=-0.008ψ3+2.6023ψ2-282.38ψ+10214and lg(Imean)=0.0041ψ2-0.9025ψ+50.992respectively.The mode of ground lightning(total lightning,positive lightning and negative lightning) densities changing with latitudes is Nr=-0.0603ψ4+26.153ψ3-4256.7ψ2+307896ψ-8×106. NP=-0.0087ψ4+3.7533ψ4-608.37ψ2+4382ψ-1×106and NN=-0.0516ψ4+22.4ψ3-3648.3ψ2+264072ψ-7×106. The mode of low magnitude (0-100kA)lightning percentage changing with1atitudes is αs100-0.0051ψ+0.4137and the mode of high magnitude(100-200kA)lightning changing with latitudes is αr100=-0.005ψ+0.58014.Big differences in lightning time and occurrence space have occurred to Chongqing region in the early time stage(1999-2003)and later time stage (2004-2008). Lightning densities in the late time stage are221%higher than in the early time stage, esp. in negative lightning (from1.43times/km2.a to4.80times/km2.a). Ground lightning frequencies increase remarkably in most time points in later stage. In November, positive polar and negative polar ground lightning frequencies increase to the highest proportions, which are as high as4,726%and518%respectively. Positive polar lightning density in the eastern region is at a higher increase rate than in the western region, while the change in the western region is not so obvious. Negative polar ground lightning densities in both eastern and western regions increase clearly. In the later stage, both positive polar and negative polar lightning current magnitudes are higher than those in the early stage, with positive polar one at a higher rate (38%). Positive and negative polar (except for Jan.) lightning current magnitudes are higher in the later time stage than in the early one. Daily change in positive polar lightning current magnitude in the later time stage is very different, which is "double peaks and double valleys" compared with "one peak and one valley" in the early time stage. Positive polar lightning current magnitude in the later stage is not so clear in the trend of "higher in the western region and weaker in the eastern" as in the early stage. In the later stage, negative polar lightning current magnitude is higher than in the early stage in most areas of western region, while it is lower in the eastern region. For positive lightning, proportions of high magnitudes in the early and later stages increase clearly from west to east, while high magnitude lightning in the later stage (excluding Zhongxian County) increases compared with the early stage. Proportion of high magnitude negative lightning is lower in the middle and western regions and higher in the eastern region in the early stage, while it is higher in the middle and western regions and lower in the eastern region.5. Proposed a method for effective application of lightning positioning data based lightning parameters and modes in risk evaluations and developed a mode for making categorized evaluations of lightning disaster risks.(1) Regional risk evaluation and division. A regional risk evaluation method for regions where buildings are with or without lightning protection devices, a regional risk evaluation method for regions where all kinds of buildings are with lightning protection devices installed as per required codes, and a method for distinguishing them have been proposed.(2) Lightning risk evaluation for projects involving long-span electric lines. The article proposes to make risk analyses and divisions according to line-passing corridors and identify sections where lightning easily strikes, and evaluate lightning prevention and disaster reduction benefits by analyzing and comparing effects before and after lightning prevention devices.(3) Lightning disaster risk evaluations for building groups. A method for evaluating risks from impacts of permanent, temporary and highrise buildings on surrounding areas has been developed. For permanent buildings, the article proposes evaluation items such as coverage of risks from explosive hazard areas, areas under lightning electromagnetic impulse affects and disaster resistant capability, apart from proposing items to be evaluated according to IEC62305. A mode for quick judgment of lightning risks involving temporary buildings and houses used in construction sites and emergency disaster rescues has been developed to provide theoretical basis for making decisions about emergency rescues. For high rise buildings, method and contents for evaluating their impacts on surrounding environment has been provided. For evaluation of disaster resistant capability, the article has mainly analyzed the mechanism for the IEC proposed and experience-based mode of0.1%lightning strike probability for areas protected with lightning rods and protective angle method with Lulun law and made theoretical analyses of lightning prevention effects with the largest protective angle (including front and side lightning strikes), which has provided scientific and theoretical bases for the protective angle method proposed by IEC.