| Yan’an is located in the Loess Plateau,and is the high incidence of lightning in Shaanxi Province.In order to study the characteristics of the cloud-to-ground lightning in Yan’an area, this paper analyses the lightning data in Yan’an area with the lightning location monitoring system in Shaanxi province. It is concluded that the characteristics of space-time distribution of lightning activity in the area. The changing rules of the lightning are also analyzed with the knowledge of meteorology. It compares and analyses various calculation formulas of lightning current amplitude cumulative probability to give a further analysis to the characteristics. Also the lightning current probability density formula is introduced, and give examples between the curves made according to cumulative probability formulas and the measured values with various cumulative probability calculation formulas of lightning current amplitude in order to compare the size of error.It gives a more direct display of the difficulties that errors brought to engineering calculation. At last, least squares curve fitting method of CFTOOL in MATLAB is applied to deduce a more accurate cumulative probability formula of the lightning current amplitude in the region. The formula, of course, has been testified to get the current amplitude characteristic equation of the cloud-to-ground lightning. The result shows that:(1) The overwhelming negative lightning accounts for 96.34%, and the positive only takes up 3.66%; The average current amplitude of the positive lightning(strength, similarly hereinafter) is 66.18 kA, which is significantly greater than that of the negative,39.43 kA; the positive lightning frequency of current amplitude between 40-50 kA is the highest, up to 691 times, accounting for 14.84% of all the positive lightning, and the negative frequency of current amplitude between 30 ~ 40 kA is the highest, up to 41760 times, accounting for 32.06% of the total negative frequency lightning.(2) The occurrence of the ground lightning displays obvious diurnal variations. That is, negative lightning shows bimodal distribution, and the main peak appears between 15:00 and 16:00, while the secondary peak appears between 2:00-03:00, and the valley value occurs between 09:00-10:00. The maximum is about 9.67 times of the minimum; from 10:00 to 16:00, the number of the negative lightning keeps increasing, but from 16:00 to 01:00, it is on decline, and at 15:00-20:00, it maintains at a higher value. Compared with that of the negative lightning, the daily variation of the positive is relatively slow, which also shows a bimodal distribution, whose main peak appears at 16:00-17:00, and the secondary peak appears at 05:00-06:00, while the valley value appears at 11:00-12:00. The maximum is 8.9 times of the minimum. The variation characteristics of the total ground lightning is similar to that of the negative lightning.(3) The occurrence of the ground lightning also shows an obvious monthly change. The monthly change peak of the positive lightning is a month earlier than that of the total and negative lighting. According to the data analysis, it can get that negative lightning in August is the most, and from April to October, it changes as a parabola shape, and the total lightning is of the same changing rule, in which the positive is the most in July and from April to October changes as a parabola shape. It is convincing to know that July and August the period for lightning. The total and negative lightning in August accounts for 40.11% and 40.85% respectively of those of the year. And the positive lightning in July takes up 26.46% of that of the year.(4) Huang long is the region of the most ground lightning while Wu qi is the region of least ground lightning. The high incidence areas of negative and positive lightning are almost the same.The areas concentrate on An sai, Yan’an and Huang long,the direction is the northwest-southeast.(5) Through a comparative study of the cumulative probability formulas of the lightning current amplitude, it finds that the regulation method will bring lots of errors to the calculation, similar for IEEE Std as well as CIGRE. However, IEEE Std and CIGRE is more accurate than the regulation method, especially for the lightning current of small value, there are smaller errors.(6) The paper fits the formula for calculating the probability of lightning current amplitude in the region, when the unknown coefficients of α is 36.04 and β is 4.349,the minimum fitting error, the best fitting effect.Thus the most precise cumulative probability distribution formula of the lightning current amplitude is calculated. At the same time it is found that when the lightning current Ic is in 0-150 kA, the fitting error varies in the range of 0.025-0.018; and when Ic is 35 kA, the fitting error comes the greatest, equals to 0.025,when Ic is greater than 150 kA, the fitting error gets close to zero.(7) The accuracy of the deduced formula is verified with the lightning current amplitude data of 2013 in Yan’an area.The result shows that the deduced formula is more accurate to reflect the distribution characteristics of lightning current amplitude cumulative probability than other formula such as IEEE Std and CIGRE formula. The curve of the fitting with the deduced formula is the most similar to the curve with the the measured data. It is concluded that the deduced formula has good accuracy and practicability. |
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