ASSESSMENT SOFTWARE OF THE RISK OF DAMAGE DUE TO LIGHTNING

Transcription

1 ASSESSMENT SOFTWARE OF THE RISK OF DAMAGE DUE TO LIGHTNING Carlos A. Avendaño A Henry F. Ibáñez O. Helmuth E. Ortiz S. Electrical Protection Research Group - GIPUD Distrital University Francisco José de Caldas Abstract -This paper describes the procedure for the assessment of the risk in low voltage electric systems, due to the effects of lightning. The purpose is to determine the degree of vulnerability of the electric system, and in this way to specify the design parameters for the implementation of the integral lightning protection system. The Electric Protection Research Group of the Distrital University (GIPUD) develop a program, the EVAL I, to evaluate the assessment of the risk of in low voltage electric systems due to lightning, in constructions of common use. For the assessment of the risk we took as a reference the procedures recommended by the international standard IEC-61662, Assessment of the risk of due to lightning, pubished in 1995 by the International Electrotechnical Commission (IEC). Index Terms: Risk assessment, lightning protection, overvoltage protection. 1 INTRODUCTION The transitory surges of atmospheric origin are the biggest cause of faults of electric systems and especially of the destruction of sensitive electronic equipment. To attenuate these effects different techniques have been implemented, such as: Design of the Lightning Protection System (LPS); use of Surge Protection Devices (SPDs); use of screens in power cables, as well as in communication lines; earthing systems and equipotencial bonding; etc. To do the design of the protection systems, the initial step is the one of evaluating the level of risk of the facilities before the lightnings. This level of risk gives design clues which guarantee an acceptable degree of reliability at a smaller cost. Important information from this evaluation is for example: Maximum and minimum magnitudes of the parameters of the lightning in the implementation of the Electrogeometric method; current magnitude to determine: down conductors for the LPS, SPDs, and distances of security, as well as protection measures for touch and step voltages. The evaluation of the risk can save costs substantially in facilities with low level of risk, which would not require of an integral system of protection against lightnings, and on the other hand, to specify effective and efficiently the necessary measures for a highly exposed system. 2 GENERAL RELATIONS The risk of in an electric system depends on the following variables: Ground Flash Density. Lightning current parameters. Equipment strength against overvoltages. Applied protection measures. These variables present a probabilistic behavior, and this is why its interaction with the system to evaluate, should be studied under the criteria of this type of analysis. If the probability that a lightning impact in a structure is correlated with the probability that it produces, it is possible to demonstrate that the annual risk of can be expressed by equation (1). N P t R = (1 e ) δ (1) Where: N: Expected average annual number of lightning flashes to the structure. P: Probability of on the structure. t: Time, in years. δ: Coefficient to take into account the economical or social consequences of the, or malfunctioning, of the system.

2 The product N*P represents the average number of annual faults of the system for the condition P, as is indicated by equation (2). F = N P (2) If the time of observation is t = 1 year, and in the case that N P<<1, the preceding formula may be simplified as follows (3). R = F δ (3a) 3 ASSESSMENT OF THE RISK To carry out the evaluation of the risk, the different types of faults which can be originated as a consequence of an lightning flash must be known, in cases the impact is direct or near to the construction, or to the supply service systems. Five types of faults have been classified, and for each of them the respective risks should be evaluated independently. These fault types are: 1. Injury or loss of human life 2. Unacceptable loss of service to the public. 3. Loss of irreplaceable cultural heritage. 4. Losses not involving human, cultural and social values. 5. As type 4 but not involving sensitive equipment. Due to these facts, equation (3a) can be expressed as: R j = F j δ j (3b) Where j representes each of the fault types to be evaluated. The sources that can produce the fault types mentioned previously are: S1: Touch and step voltage by direct lightning flashes 1. S2: Fire, explosion, mechanical and chemical effects by direct lightning flashes S3: Overvoltages on equipment by direct lightning. S4: Overvoltages on equipment by indirect lightning. 2. S5: Fire, explosion, mechanical and chemical effects by indirect lightning flashes 1 Lightning that impacts directly on a construction or at a distance not bigger that 3 of the height of the same one. 2 Lightning that impacts in the proximities from the structure to a bigger distance that 3 of the height of the same, and smaller than 500m. Each fault type can be caused by different sources of. To the existent relationship between the different fault risks and the possible sources of is denominated Matrix of Vulnerability, this matrix is presented in the Table 1. Table 1. Types and sources of Source of Type of Direct Indirect lightning lightning S1 S2 S3 S4 S Each risk of can be expressed as the sum of three different components, as it is presented in equation (4). where: Rd = Nd P δ Ri = Nn P δ R Rd + Ri + Ro = (4) (5) Component related with direct impacts (6) Component related with impacts near to the system Ro = Nk P δ (7) Component related with impacts where: Nd : Nn: Nk: on supply of service systems Average annual number of direct flashes to the structure. Average annual number of flashes striking to ground nearby to the structure. Average annual number of flashes affecting an incoming service. To evaluate the level of risk of each one in the possible five fault ways it should be determined the parameters: Nd, Nn, Nk, F and d. However, it is not necessary to determine the coefficients Rd, Ri and Ro, because they are implicitly contemplated in the procedure that is explained next. 3.1 Annual frequency of direct flashes (N d ). The annual average of direct impacts of lightning on the structure is giving by equation (8). where: Ng: Ae: Nd = Ng Ae (8) Density of lightnings towards earth, in the place where the construction is located.. Effective area of the construction

3 The effective area is determined as indicated in Figure 1 (IEC , 2000, p 26). 3.3 Frequency of lightning flashes affecting an incoming service (N k ) The annual average of impacts in the service supply systems which originates potential increments to the interior of the construction, is giving by equation (10). Where: Ak: Nk = Ng Ak (10) Area of influence of the service incoming to the structure (narrow area surrounding the ongoing of the service supply system) 3.4 Calculation of frequency of per year according to the fault type (F j) Figure 1. Effective Area (Ae). 3.2 Nearby lightning flash frequency (N n ) The annual average of nearby impacts to the structure that originates increments of the earth potential, which can affect both the structure and its service supply systems, is giving by equation (9). Where: Ag: A e Ae = ab + 6h (a + b) + 9 π h 2 Nn = Ng Ag (9) Surrounding area to the construction. The surrounding area is determined as indicated in Figure 2, where the interior area is Ae and the external radius (expressed in meters) is equal to the magnitude of the resistividad of the terrain ρ (expressed in Ω m) and not bigger than 500m. Ae Ag Figure 2. Surrounding Area (Ag). ρ To determine the number of s per year that may be present in a fault condition, the frequencies of per year due to each one of the fault sources are added, as is presented in the matrix shown in the Table 2 (IEC 61662, 1995). The 1 indicate that the component should be taken into account to calculate the frequency of for the determined fault type. Table 2. Matrix of the frequency of. Components of the frequency of Type of Direct lightning Indirect lightning H A D B C E G Where. H: Frequency of due to step and touch voltages A: Frequency of due to fire, explosion, by direct lightning flashes B: Frequency of due to nearby lightning flashes C: Frequency of due to fire, explosion, by the lightning flashes affecting the incoming services. D: Frequency of due direct lightning flashes. E: Frequency of due to overvoltages by nearby lightning flashes. G: Frequency of due to overvoltages by the lightning flashes affecting the incoming services. The factors H, A, B, C, D, E, G are calculated as shown by equations (11) to (17).

4 H = Nd Ph (11) A = Nd Pt ( P1 + P2 + P3 + P4) (12) B = Nn Pt P3 (13) C = Nk Pt P3 (14) D = Nd ( P2 + P3) (15) E = Nn P3 (16) G = Nk P3 (17) Where P h: Pt: Probability of due to step and touch voltages by direct lightning Probability of dangerous sparking triggering fire or explosions. P1: Probability of dangerous sparking on metal installations. P2: Probability of dangerous sparking on electrical installations internal to the structure P3: Probability of dangerous sparking on incoming services. P4: Probability of dangerous sparking on incoming external conductive parts The value of each one of the previous probabilities depends on variables such as: Characteristics and content of the structure, type of construction, type of surface covering the ground, type of external installations, protection measures, etc. Finally the frequency of per year(f) it is calculated as they indicated by equations (18) to (20). Fd = H + A + D (18) Frequency of due to direct lightning. Fi = B + C + E + G (19) Frequency of due to indirect lightning.. F j = Fd + Fi (20) Frequency of. According to Table 2, the frequency of due to each one of the fault types, is calculated as it is indicated in Table 3. Table 3. Frequency of per type of Type of frequency of. (Fj) 1 F1 = H + A + B + C 2 F2 = A + D + B + C + E + G 3 F3 = A + B + C 4 F4 = A + D + B + C + E + G 5 F5 = A + B + C 3.5 Calculation of the factor of the allowed number of faults (d j) The acceptable number of faults caused by effects of the lightning, must be calculated for each one of the fault types. This factor depends on: Number of people and time of exposition to the fault risk. Type and importance of the public service. Value of the involved goods. The factors δj for each fault type is calculated as shown in Table 4. Type of Table 4. Factor δ δ j 1 δ1 = 1- (1 - t / 8760) n 2 δ2 = (n * t ) / (n t *8760) 3 δ3 = C i / C t 4 δ 4,5 = Cm / Cv Calculation of the risk The risk for each one of the fault types is calculated according to equation (3b). Once calculated the level of risk, this is compared with the value of acceptable risk of fault (Ra), which is shown in the Table 5 (IEC , 1995). Table 5. Values of acceptable risk. Type of R a Description Loss of human life Innacceptable loss of service to the public Loss of cultural heritage 4 y 5 * Losses not involving human, cultural, social values and sensitive equipment. * According to the criteria of the designer If the calculated risk is bigger than that of the Table 5 (Rj>Ra), a system of integral protection should be implemented to reduces the risk at a sure level. This protection system should have a superior efficiency to the one calculated with the equation (21). In the event that (Rj<Ra), the construction doesn't require additional measures of protection. Ra E = 1 (21) R j

5 This value of the efficiency of the system must be approached to the normalized values, which are shown in the Table 6 (IEC , 2000, p 25). Table 6. Values of efficiency in function of the protection level. Protection level EFFICIENCY E I 0.98 II 0.95 III 0.90 IV 0.80 It doesn't require <0.80 protection measures The determination of the protection level required for each one of the evaluated risks, allows to fix the design parameters specified by the norms IEC Protection of structures against Lightning and IEC Protection against Lightning Electromagnetic Impulse. 4 PROGRAM FOR THE EVALUATION OF THE RISK EVAL I The previosly described methodology was implemented in a computer program through the development of a graduation project in the Faculty of Technology of the Distrital University Francisco José de Caldas, by the students Carlos Alberto Ospina Espejo, Franki Camargo Tamayo, and Gabriel Araque Grosso. The software not only generates the value of the level of risk, but also allows to set the design parameters specified by the norms IEC Protection of structures against Lightning and IEC Protection against Lightning Electromagnetic Impulse. These parameters are: Radious of the rolling sphere for the application of the electrogeometric method. Materials and dimensions for the terminals of the External Protection System - EPS. Average distance between down conductors of the EPS. Materials and dimensions for the down conductors, and for the electrodes of the earthing system. Materials and dimensions for the equipotential conductors. Distance of security between the EPS and structures, and metallic ducts not grounded. Magnitude of the maximum current of direct impact of lightning in wave form 10/350 us for the selection of SPDs. Some of the windows of presentation of the program are shown next. 4.1 Study Case Figure 3. Initial display to execute EVAL I Making use of the program EVAL I to evalualte the risk of, or loss of human lives for a structure with the following characteristics: Dimensions of the building: height 18m, wide 60m, length 100m, distant from other constructions. Steel reinforced concrete structure. 200 people are normally present for a time of aprox hours per year. The ground flash density of the territory is 1 flash per km 2 per year. A marble surface layer of soil outside the building. The incoming mains supply is a low voltage underground cable. The incoming telephone line is an underground cable. There are small fire fighting equipment. Common materials are involved as the content of the structure. Resistivity of the soil is 100 Ω m. This initial information is loaded to the software through displays on which the program requests for these basic parameters. Once the program is executed successfully, as a result of the evaluation of risk, there are clues of design for an integral protection system against ligthnings, in case the system so requieres. Otherwise the program informs the system does not requier any protection. Some of the displays to enter information, and the way the results come from, are shown in Figures 4 and 5, respectively. For the case studied, some of the results given by the program are: Eficiency of the required system: E=0.92 Required protection level: IV

6 The Colombian Institute of Technical Standars - ICONTEC has been working in the normalization of the design of the protection systems against lightnings, because of the importance that this represents for the insurance and industrial sectors of the country, and in this order of ideas, this work is a significant contribution for this activity. 6 REFERENCES - Avendaño, C. Ibáñez, H y Ortiz, H (2002). Evaluación del riesgo de daño en sistemas eléctricos de baja tensión por efecto de los lightnings. Mundo Eléctrico Colombiano, Vol. 16, No 47, Bogotá, Colombia. Figure 4. Display to enter basic information. - Flisowski, Z. y Mazzetti, C. (1999). Risk assessment method for the protection of electronic systems against lightning overvoltages, Proceedings V International Symposium on Lightning Protection, Sao Paulo, Brasil. - International Electrotechnical Commission (1995). Standard IEC Assessment of the risk of due to lightning. Geneva, Switzerland. - International Electrotechnical Commission (1997). Standard IEC Protection against Lightning Electromagnetic Impulse. Geneva, Switzerland. Figure 5. Display showing results. Radious of the rolling sphere: 60m Average distance between down conductors: 20m Lightning current: 100 ka, 10/350 us - International Electrotechnical Commission (2000). Standard IEC Protection of structures against Lightning. Guide A: Selection of protection levels for lightning protection systems. Geneva, Switzerland. - Ospina, C. Araque, G y Camargo, F (2002). Diseño de software para la evaluación de riesgo contra descargas atmosféricas. Trabajo de grado, Universidad Distrital Francisco José de Caldas. Bogotá, Colombia. 5 CONCLUSIONS The use of the software EVAL I simplifies the obtention of the level of risk, since it integrates all the variables and procedures described by the international norm IEC An advantage of the presented methodology, is that it allows to evaluate protection systems against existing lightnings, calculating its efficiency and allowing to see its possible strengths or weaknesses, and the danger to the human beings that occupy the protected constructions. The success of an integral system of protection against lightnings depends on the quality of the information that one has on the construction to protect, since the evaluation of the risk is carried out with this information. AUTHORS E.E. Carlos Alberto Avendaño Avendaño: calitoave@hotmail.com Electrical Engineer of National University of Colombia (1998), High Voltage Specialist of National University of Colombia (2001). Professor of Distrital University Francisco José de Caldas, GIPUD Director. E:E. Henry Felipe Ibáñez Olaya: hibanez@multi.net.co Electrical Engineer of National University of Colombia (1994), Control and Automation Specialist. Professor of Distrital University Francisco José de Caldas, GIPUD Researcher. E.E. Helmuth Edgardo Ortiz Suárez: helmuthos@ieee.org Electrical Engineer of National University of Colombia (1998). Professor of Distrital University Francisco José de Caldas, GIPUD Researcher.