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CARTOGRAPHIC REPRESENTATION OF THE VENEZUELAN KERAUNIC ACTIVITY
J. TARAZONA, C. FERRO A.J. URDANETA* CVG Electrificación del Caroní, C.A. Universidad Simón Bolívar
(Venezuela)
SUMMARY The results of the initial four (4) years of the implementation of the computerized monitoring system for the detection, location and characterization of the Venezuelan atmospheric lightning discharge activity are presented. These results correspond to years 2000-2003 and are used to construct the first chart of the Venezuelan keraunic activity obtained by means of this data, showing the resultant traditional curves of isodensities of number of discharges to ground per square kilometer per year. The lightning detection system consists of twelve (12) geographically distributed sensors that use the azimuth and the arrival time as well as the intensity to detect and locate the cloud to ground lightning discharges. On the other hand, as it is well known, the lightning discharges affect the behavior of the aerial electrical lines, and the design of electrical transmission lines to assure an adequate number of outages per year, requires not only the information related to the number of discharges per km2 per year in the zone affected by the line project, but also needs probabilistic information related to the electric current of the lightning discharges that take place, which is usually not provided by the charts. The information about the lightning currents available from the computerized data can be used to perform specific, tailor made, calculations to determine the probabilistic behavior of the lightning currents for the geographic zone affected by the transmission line. A complementary graph is proposed in this work to present this statistical information in a usable fashion. In particular, a family of curves is created using the registered data, each of these curves showing the cumulative probability of the peak or maximum of the lightning currents for each of the density levels defined in the map of ground flash density. The resultant curves are compared with the traditional Anderson-Eriksson lightning discharge current probability curve. It is observed that these curves shift down towards smaller accumulated probability values for the same discharge currents as the density of lightning discharges per year increases. With the proposed complementary graph the standard calculation method for the determination of the expected number of outages per year of the transmission line project is directly applicable using the information provided by the two charts. Finally, sensitivities of the outage rate of a 400kV transmission line were calculated with respect to different parameters and some conclusions are derived. This article has been derived from of the information presented in [1]. KEYWORDS Lightning detection and location, Transmission line outage rate, Keraunic activity.
21, rue d’Artois, F-75008 PARIS B2-206 CIGRE 2006http : //www.cigre.org
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INTRODUCTION The characterization of the keraunic activity is a very important part of the description of the atmospheric activity and in general of the characteristics of the climate of any region. In particular, lightning discharges strongly affect the behavior of the aerial electrical lines. The design of electrical transmission lines to assure the adequate performance of the circuits, i.e. a desired minimum number of outages per year, requires not only the information related to the number of discharges per km2 per year in the area involved by the electric line project, but also needs probabilistic information related to the electric current of the lightning discharges that take place. The use of lightning location systems permits a complete characterization of the keraunic activity, allowing a more precise calculation of the electric line design parameters. The Venezuelan Lightning location system is in service since the end of 1999. The information provided by this system is used for the first time to construct the isokeraunic charts of the region, presented in this article. The information about the lightning currents is obviously available from the computerized data provided by the modern detection systems, and the tendency is to use this data to perform specific, tailor made, calculations to determine the probabilistic behavior of the lightning currents in the geographic zone involved in the transmission line project. The use of a complementary graph or table to present this statistical information is proposed in this work in order explore a simple approach two overcome this limitation of the use of maps or charts. .- Venezuelan lightning location system The Venezuelan lightning location system covers the area served by the national generation and transmission system with an efficiency of detection of electrical discharges of 90%. It comprises twelve sensors that use the azimuth, the arrival time as well as the intensity to detect and locate the cloud to ground lightning discharges. The location of the discharge is reached with accuracy of 500 meters and is necessary that at least two sensors take part in the detection, with the overlapping of the areas of influence of each sensor. All the sensors send online information to the control center located in the Center of Hydro-meteorological Prognosis of CVG EDELCA, where the information that comes Fig. 1. Lightning detection sensors in Venezuela. from each sensor is processed and the lightning parameters computed. The distribution of the sensors is illustrated by Fig. 1. .- Venezuelan keraunic activity Before the implementation of the lightning location system, the available information of the Venezuelan keraunic activity was very limited. It basically consisted of two maps. The first one, depicted in Fig. 2, has been in use for more than 30 years and pictures the number of thunderstorm days per year, [2]. The number of lightning discharges to ground may be derived using the traditional empirical mathematical expression that relates the number of thunderstorms with the number of lightning discharges to ground: [3]
Ng=0.12·Td (1) Although improvements to this equation have been suggested,[4] this expression, known and used worldwide, has been traditionally used in Venezuela.
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More recently, a second chart was proposed, shown in Fig.3. [5] It was elaborated using satellite information. It represents an interesting effort to describe the atmospheric activity with the available information, although the gross linear approximations that were used to construct the chart result in unrealistic sharp corners in the drawings of the curves of equal thunderstorm days per year.
Fig. 2. Number of thunderstorm days/year [2] Fig. 3. Chart constructed using satellite information [5] Information about the probability distribution of lightning peak currents was not available in Venezuela, so the Anderson-Eriksson curve [3] was usually assumed valid for the country. RESULTS The recorded data obtained for the years 2000-2004 was processed to construct the ground flash density (GFD) chart presented in Fig.4, 5 and 6. The plotting was performed using a bicubic interpolation process and a grid size with squares of 100 km side, correspondent to 0,909 geographical degrees, size recommended for the study of big territorial areas. [6] Fig. 4. Physical map and Ground Flash Density of Venezuela. Years 2000-2003. (Number of Flashes/km2/year)
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Fig. 5. Ground Flash Density of Venezuela. Years 2000-2003. (Number of Flashes/km2/year)
Fig.6. 3D Chart of Ground Flash Density of Venezuela. Years 2000-2003. (Number of Flashes/km2/year) The national average GFD resulted in 4.08 discharges/km2/yr. Five areas with a higher atmospheric activity are observed in these charts. One of these areas is a well known exceptional zone with extremely intense lightning activity is characterized with densities of more than 55 discharges to ground per km2 per year. Although it is not registered by the lightning location system, the frequency
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of cloud to cloud discharges, usually of very low intensity, is known to be much higher in this region. This phenomena, is known as the “Relámpago del Catatumbo” –may be translated as “The Catatumbo Flash”- being the Catatumbo a local river that flows into the Maracaibo lake. [7] The number of thunderstorm days per year was also calculated using the registered data and equation (1). The results are presented in Fig.7. Although it is clear that the use of equation (1) has several limitations, especially in those areas with very high lightning activity, this chart is a reference that permits to compare the resultant levels with the previous available information (Fig. 2 and 3), in order to identify important differences in the characteristics of the atmospheric activity that are brought to light with the new information provided by the lightning location system.
Fig.7. Number of Thunderstorms days per year resultant from the measurements and form the application of equation (1) .- Characteristics of the lightning currents On the other hand, as it is well known, the lightning discharges affect the behavior of the aerial electrical lines, and a proper design of electrical transmission lines in order to assure the adequate performance of the circuits, i.e. number of outages per year, requires not only the information related to the number of discharges per km2 per year in the zone crossed by the line project, but also needs probabilistic information related to the electric current of the lightning discharges that take place. The probability distribution curve of the current of the detected lightning discharges, which resulted from the measurements for all the Venezuelan territory, is presented in Fig.8, where it can be compared with the well known Anderson-Eriksson curve [3]. It can be observed that only 1% of the lightning discharges have a current of more than 100 kA and that currents of more than 200 kA have a practically null probability. Although typically it is estimated that 90% of the discharges are of negative polarity, [3] an important difference was detected since 23% of the registered events were of positive polarity.
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Figura 8. Probability distribution of lightning discharge currents
.- Peak Current Probability Complementary Chart The information about the lightning currents is obviously available from the computerized data, and the tendency is to use it to perform specific, tailor made, calculations to determine the probabilistic behavior of the lightning currents for the specific geographical zone involved by the transmission line project. In order explore how to overcome this limitation for the use of the maps or charts, a complementary graph is proposed in this work to present this statistical information. In particular, a family of curves is created using the registered data, each of these curves showing the cumulative probability of the peak or maximum of the lightning currents for each of the density levels or steps defined in the map of lightning discharge. See Fig.9. It is observed that the resultant curves obtained by a minimum squared deviations fitting procedure shift down towards smaller accumulated probability values for the same discharge currents as the density of lightning discharges per year increases. As the frequency of lightning discharges to ground increases, the phenomena is associated to smaller clouds, or also to clouds that are closer to the ground and therefore associated to smaller lightning currents. With the proposed peak current probability complementary graph, the standard calculation method [3] for the determination of the expected number of outages per year of the transmission line project is directly applicable using the information provided by the two charts. SENSITIVITY STUDY OF THE LIGHTNING FORCED OUTAGE RATE OF A LINE In order to compare the traditional method for the evaluation of the lightning performance of a transmission line with the method proposed here, a sensitivity study was conducted upon the forced outage rate of a transmission line due to lightning, by varying both the ground discharge density and the probability distribution of the lightning peak current, and also by varying the cell size used for averaging the lightning parameters. The studied line, located in the eastern region of Venezuela, was a
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Fig.9. Peak Current Probability for Different GFD Levels (Complementary Chart) 400 kV, 2 conductors per phase, 169,4 km line, with two ground wires which provide a shielding angle of 16,5˚. The eletrogeometrical method [3] showed that the shielding was effective over the whole line, which resulted in the lightning forced outage rate having only the component due to backflashover. The minimum peak current for backflashover as well as the forced outage rate was evaluated using the method proposed in [3]. Only the main results are presented in the following. It is worth mentioning that the interest here is in the relative values of the outage rate obtained by the various methods, rather than the absolute values, therefore the discussion will be restricted to the relative comparison of the results. .- Sensitivity of outage rate to variations in GFD and in probability distribution of current peak. The objective of this type of analysis was to share some light on the identification of better methods of calculation of line outage rate when data from lightning location systems is available, in comparison to traditional methods which recur to past statistics, measurements and assumptions about the keraunic behaviour of the region where the line is located. Since the keraunic activity in a region has a probabilistic behaviour and therefore changes along the line, one approach is to divide the line in square cells of certain size and take the average lightning parameters (GFD, and peak current probability distribution (PCPD)) in each cell to make the calculation. Another approach is to use the characterization proposed in this article for the keraunic activity, which evaluates the average GFD in each cell, but uses the peak current probability distribution that corresponds to the GFD in the cell, instead of the PCPD in the cell itself. This was done for square cells with sides which ranged from 2 km to 100 km. In Fig. 10 the results are shown, where the difference in the forced outage rate obtained using the proposed characterization and the tailor made study approach, using the specific PCPD for each cell mat be appreciated. It can be seen from this figure that when the PCPD associated to each cell is used in the calculation, the outage rate is highly dependant on the cell size as it decreases below 50 km while remaining approximately constant for cell sizes above that level. It is worth noting that for small cell sizes, below 10 km, the PCPD looses statistical validity due to the low number of flashes within the cell, and perhaps this is the reason for
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the sharp decrease observed in the graph. However, when the proposed characterization is used the outage rate is approximately constant for cell sizes below 40 km, with a moderate increase above that size. The proposed characterization does not suffer from the statistical limitation of the other approach for cell sizes below 10 km, since the PCPD curves that are used correspond to all the geographical areas of the country with the same GFD. On the other hand, the results of both methods get closer as the cell size increases towards 100 km, being lower the values provided by the proposed characterization. Fig.10. Sensitivity of the Outage Rate CONCLUSIONS A first cartographic characterization of the Venezuelan keraunic activity using the information provided by the National Lightning Location System (years 2000-2003) has been performed. Important differences were detected among the different regions of the country and with respect to the previously available information, and in particular with the chart of thunderstorm days per year. The resultant probability distribution curve of the peak lightning currents turns out to be under the traditional Anderson-Ericksson curve, with smaller probabilities for the high peak currents. The traditional chart illustrating the isodensities of lightning discharges to ground was presented. Also, a family of probability curves of the peak currents is obtained and presented for each of the keraunic levels of the chart, clearly showing smaller probabilities for higher ground flash density levels. The sensitivity of the outage rate of a 400 kV transmission line shows important variations after considering the information provided by the lightning location system. The outage rate of the line depends on the size of the cells of the grid used for the tailor made computations of the GFD and on the method used to treat the probabilistic behavior of the peak currents. A cartographic characterization of the keraunic activity is proposed, consisting of the traditional ground flash density chart complemented with a family of probability curves of the peak currents associated to each level of ground flash density. This method allows the calculation of the transmission line outage rate by means of simple charts without continuously recurring to sophisticated computer calculations, reflecting a realistic description of the atmospheric activity. The results were compared with those obtained by the standard approach and suggest that the proposed method is promising and worth of additional research efforts. BIBLIOGRAPHY [1] C. Ferro. “Caracterización Cartográfica De La Actividad Ceráunica Venezolana y Su Uso En El
Diseño De Líneas Aéreas” (Universidad Simón Bolívar, Caracas, July 2004). [2] Servicio Meteorológico - Fuerza Aérea Venezolana, “Atlas Climatológico 1971-1970”, 1984. [3] J. G. Anderson. “Transmission Line Reference Book 345 kV and Above” (Chapter 12, Second
Edition, EPRI, California, USA, 2001) [4] Task Force 33.01.02, Lightning Location Systems, CIGRE, “Characterization of lightning for
applications in electric power systems” (CIGRE Publication No. 172, December 2000). [5] M. Martínez, et al, “Actividad de Rayos en Venezuela Utilizando la Data del Sensor Óptico
(LIS) del proyecto TRMM de la NASA” (Revista Científica, Facultad de Ingeniería Universidad del Zulia, No. 2, August 2003).
[6] H. Torres, “Variation of Lightning Parameter Magnitudes Whitin Space and Time” (24th International Conference on Lightning Protection (ICLP), Birmingham, UK, September, 1998).
[7] L.F. García, “Tormentas en el Lago de Maracaibo”, (IV Jornadas Profesionales, CVG EDELCA, Puerto Ordaz, May 1998).
