DESIGN CHALLENGES FOR DISTRIBUTION OVERHEAD LINES SUBJECT TO HIGH IMPACT LOW PROBABILITY EVENTS

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1 Paer 1353 DESIGN CALLENGES FOR DISTRIBUTION OVEREAD LINES SUBJECT TO IG IMPACT LOW PROBABILITY EVENTS Alessandro P. DADAM Fernando. MOLINA Sergio L. S. CABRAL CELESC Distribuição - Brazil CELESC Distribuição Brazil Matrix Eng. em Energia Brazil alessandrod@celesc.com.br fernandohm@celesc.com.br sergio.cabral@matrixenergia.com.br Walter PINEIRO TAG Inovação Brazil walter.inheiro@gmail.com.br Wilson IRAKAWA TAG Inovação Brazil whirakaw@terra.com.br ABSTRACT The severity and frequency of weather events occurrence in Brazil, such as storms, windstorms and tornadoes, has increased in the ast few years and resented imortant effects on the reliability indicators of main Brazilian utilities, directly imacting their oeration and maintenance costs. This aer resents the studies and efforts of CELESC D, a utility in southern Brazil, in the ursuit to reduce the imact of occurrences originated from high imact low robability weather events, in order to reduce the time for electrical system restoration, imrovements in service to its customers and the consequent reduction in reliability indicators. INTRODUCTION The consequences for the distribution lines in the occurrence of large magnitude meteorological henomena are high mechanical stress and vibration that imair their oeration, reflecting the erformance of SAIDI and SAIFI continuity indicators. The most common causes of occurrences in medium voltage lines, in times of storm, are caused by falling trees and objects (lates, tiles, structures or tree branches) launched on conductors, causing the ruture of then, short circuits and, in some cases, the break of oles. Celesc D to minimize the effects of atmosheric henomena on their distribution networks, reduce the number of outages and to reduce recovery time, and yet, considering the necessity imosed by the regulator to imrove the reliability indicators of its electricity distribution lines, decided to invest in a R&D Project, whose objective is the redefinition of the: a) values of meteorological arameters; b) methodology for calculating the mechanical stresses acting onto the conductors and overhead lines structures, considering the effects of extreme winds, characterized as winds of high intensity and short duration burst; c) mechanical stresses acting on the conductors and structures in the occurrence of falling trees onto the conductors through simulations by finite element method; d) use of mechanical fuses to romote mechanical coordination of networks in the occurrence of tree falling, rotecting the ost and conductors. The results of these studies allowed Celesc to develo new standards for overhead distribution networks, mechanically coordinated in order to avoid or minimize the imact of occurrences during storms. CLIMATE PARAMETERS At this stage of research studies have been conducted to characterize the major weather events that take lace in Santa Catarina and made statistical treatment of the wind seed data measurements at meteorological stations to obtain the maing of the concession area of Celesc and setting values of meteorological arameters to be used in mechanical calculations. Figures 1 and show on the ma of the state of Santa Catarina the wind lines of same seed referred a 10- minute and 3 seconds winds, resectively, to wind return eriod of 50 years, determined by the methodology for forecasts of annual wind seed limits based the statistical distribution of extreme values Gumbel. Figure 1 Winds(km/h) - 10 min/return eriod of 50s Figure Winds (km/h) - 3s return eriod of 50 years CIRED 015 1/5

2 Paer 1353 Table 1 resents a summary of the characteristic values of 10 minutes wind seeds (maximum wind) and 3 seconds winds (wind gust) reresenting the Santa Catarina State, calculated for return eriods of 50, 100 and 150 years. Table 1 Maximal Wind Seed in Santa Catarina Return Period Wind Seed (10 minutes) Wind Seed (3 seconds) 50 years 105 km/h 170 km/h 100 years 110 km/h 180 km/h 150 years 10 km/h 190 km/h NEW MECANICAL CALCULATIONS METODOLOGY CONSIDERING TE EFFECTS OF EXTREME WINDS At this stage was develoed a new methodology for the mechanical calculations of distribution overhead lines considering the results of studies erformed to redefine the maximum wind roject seed and ressures acting on structures and conductors, in addition, a methodology was included to determine the stresses caused by the high intensity and short duration wind (wind gusts). A. Discussion about Meteorological Parameters The definition of roject wind seeds was made with emhasis on statistical aroach, more realistic and widely used in transmission line rojects to relace the deterministic treatment currently used in distribution network rojects. The new methodology rooses that overhead distribution lines must be designed to withstand, without failure, the mechanical loads roduced by winds with maximum seeds defined by statistical rocessing of measurement data of meteorological stations from nearby regions and robabilistic characterization to return eriod 50, 100, or 150 years. The variables involved in the definition of roject wind seed for the mechanical calculations of conductors and structures are: - Return Period: corresonds to the time estimated that a maximum wind seed is equaled or exceeded at least once, measured in years. The choice of return eriod (50, 100 or 150 years) to be adoted in the rojects of distribution networks direct imacts the reliability level and construction costs. - Integration time: corresonds to the time interval over which the wind seed is calculated. Thus, a 10 minute wind is the average seed in a measurement interval of 10 minutes obtained from an anemograh. The use of the integration time in the calculations of structures is directly connected to the inertia of the obstacle and its dimensions. For examle, a structure such as a ole or a tower, mechanically resonds to long term wind (10 minute) and the short gusts (3 seconds) because it has a dimension comatible with the wind wavefront. The cables, which have much smaller size, take longer to resond to the wavefront, therefore efforts for the conductors are used only with 10 minute integration time wind. All these values are arameterized in ABNT [] and IEC 6086 [3]. B. New Methodology to Calculate the s Due to Wind Considering a Statistical Aroach of Meteorological Parameters In the roject of structures, to calculate the mechanical load due to wind action should be considered the two conditions of maximum winds below, being determinant the one with the highest value of mechanical load: mechanical loads roduced by maximum winds with 10 minutes integration time acting on the structures and cables; mechanical loads roduced by high intensity winds, with integration time of 3 seconds, erforming only on the structures, to reresent the gusty winds during storms. Load Calculation for 10 min winds (maximum wind) As described above, a wavefront 10 minutes winds acting in an overhead distribution network structure causes horizontal forces in the conductors and oles. The calculations of wind loads on each comonent will be made according to the following criteria: a) Wind ressure - 10 minutes 1 q10min v 10min (1) q 10 min wind dynamic ressure for 10 minutes (N/m ) - air secific mass (kg/m 3 ) - correction factor of in function of temerature associated to the maximum wind and altitude v 10 min 10 minute roject wind seed (m/s) b) Wind ressure on conductors vc q 10 C () min xc vc roject wind ressure on conductor (N/m ) q 10 min dynamic reference ressure for 10 minutes wind (N/m ) C xc - drag coefficient of the conductor, equal to 1.0 for cylindrical elements G c - combined factor of the wind for conductors, deending on the height and terrain category G L ga factor c) Wind load on conductors c 10min d v (3) vc c c 10 min horizontal load on conductor due to 10 minutes wind vc - roject wind ressure on conductor (N/m ) d c - conductor diameter (m) v m - average ga or wind ga structure (m) c m L CIRED 015 /5

3 Paer 1353 d) Wind ressure on ole v q 10 C (4) min x v roject wind ressure on the ole (N/m ) q 10 min reference dynamic ressure for 10 min wind (N/m ) C x - drag coefficient of the ole, equal to: 1.0 for cylindrical elements (circular ost).0 for flat surfaces (rectangular ole) G t - combined wind factor obtained deending on the height of the center of gravity of the structure relative to the ground. e) Wind mechanical load on the ole 10min A (5) v 10 min horizontal load on the ole due to 10 minutes wind v roject wind ressure on the ole (N/m ) A ole surface area exosed to wind (m ) Obs.: For the flat oles, 10 minutes wind loading should be calculated on the surface erendicular to the incidence wind direction. Load calculation for 3 seconds winds (gusts) The gust of wind stress is caused by localized bursts, ie if they extend to an overhead distribution line, only the ost will be affected. The conductors, for having small size, have a low resonse to dynamic ressure. The calculation of mechanical loads resulting follows: a) Pressure for 3 seconds winds 1 q3 s v 3 s (6) q 3 s reference dynamic wind ressure for 3 sec (N/m ) - air secific mass (kg/m 3 ) - correction factor of in function of temerature associated to the maximum wind and altitude v 3 s 3 seconds roject wind seed (m/s) b) Wind ressure on ole v q 3 C (7) s x v roject wind ressure on the ole (N/m ) q 3 s reference dynamic ressure for 3 sec winds (N/m ) C x - drag coefficient of the ole, equal to: 1.0 for cylindrical elements (circular ost).0 for flat surfaces (rectangular ole) c) Wind mechanical load on ole 3 s A (8) v 3 s horizontal load due to wind on the ole v roject wind ressure on the ole (N/m ) A ole surface area exosed to wind (m ) Obs.: For the flat oles, 3 seconds wind loading should be calculated on the surface with greater wind incidence area. d) Calculation of the resulting wind loads t To calculate the resulting effort must be added the horizontal loads of each comonent and transferred to 10 cm from the to. Resulting in the structure due to the wind 10 minutes (maximum wind): 10min c10 min 10 min (9) Resulting in the structure due to 3 seconds wind: 3 s 3 s (10) To determine the wind load to be considered in the mechanical dimensioning should be adoted which the highest value between 10 min e 3s. C. Wind Load Calculation Examles It can be observed in Table 4 the results of simulations erformed to illustrate the use of the roosed methodology to calculate efforts due to wind in rural overhead distribution structures, with distance of 80m between structures. The cable used is a 336,4 MCM and covered conductor (3 x 185 mm Aluminum): Circular section concrete ole (11 m x 300 dan) Rectangular ole (11 m x 300 dan). Were calculated wind loads for the wind hyothesis with maximum return eriod of 50, 100 and 150 years on the structure and cables, and the extreme case of wind gust acting only in the structure. Table Resulting from wind loads in rural networks Return eriod (years) Wind tye Wind load on oles Bare conductor Covered conductor Rectangular Circular Rectangular ole Circular ole ole ole A B A B Max. Wind 460, ,36 645, ,83 Wind gust 133,0 59,59 196,91 133,0 59,59 196,91 Max. Wind 504, ,44 708, ,99 Wind gust 149,13 91,03 0,75 149,13 91,03 0,75 Max. Wind 600, ,44 843, ,69 Wind gust 166,16 34,7 45,96 166,16 34,7 45,96 Note: are indicated in bold, loads values that exceed the secified for excetional load limit of the oles, that are 40% more than the nominal load. DETERMINATION OF EFFORTS INVOLVED IN TREE FALL ON TE NETWORK To determine the mechanical stresses in the network due to the imact caused by a falling tree on the cables, were used standard sacer cable structures modeled by the finite element method, a through dynamic transient analysis using ANSYS software LS-Dyna. A. Network configurations For the tree fall simulations on comact networks were considered the following networks configurations and tree ositions on the ost of central san: Urban lines (sace between structures of 30 m) and tree in the middle of the central san and the 3 m to the ole; Rural lines (sace between structures of 50 m) and CIRED 015 3/5

4 Paer 1353 tree in the middle of the central san and 3 m to the ole. B. Comments about model The model was considered with linear behavior and was not included any failure criteria for any of the comonents; With the excetion of cables, all items of the analysis were considered rigid (* MAT_RIGID). To the cylinder reresenting the tree, the density was adjusted to be equal to the mass of kg. A node at the base of the cylinder was set so as to create a bearing allowing tilting of the tree. All other hard items were considered fixed C. Results of tree falls simulations As an examle are resented below the results for one of the cases of simulation erformed with rural lines (san of 50 m) and tree falling in the middle of the central san. In Figure 3 is shown schematically the basic configuration of the simulated sacer cable line and in articular the tree fall osition. Figure 3 Structures and initial osition of the tree on the line As an examle is shown below the effort grah roduced in anchorage 1 due to fall from the tree on the messenger cable. For other items and structures is resented only a summary in table form. a) Anchorage 1 Figure 4 Result of tree fall simulation on the network Table 3 - Resulting stresses in the anchorage structure 1 m Longitudinal on the messenger V1 Vertical on messenger fixing Longitudinal Conductor Fase Longitudinal Conductor Fase Longitudinal Conductor Fase V Vertical in the fixing bracket b) Passage 1 Table 4 - Resulting stresses in the assage structure 1 1 Longitudinal on the messenger V1 Vertical on messenger fixing Transverse on messenger fixing c) Passage Table 5 - Resulting stresses in the assage structure 1 Longitudinal on the messenger V1 Vertical on messenger fixing Transverse on messenger fixing d) Anchorage Table 6 - Resulting stresses in the anchorage structure m Longitudinal on the messenger V1 Vertical on messenger fixing Longitudinal Conductor Fase Longitudinal Conductor Fase Longitudinal Conductor Fase V Vertical in the fixing bracket D. Discussion about the obtained results As seen in the resented simulations, the values of the stresses roduced by the imact of large tree falling on the sacer cable line make it imossible, technically and economically, the roject of structures to suort these levels of effort. The recommended alternative to avoid that tree fall imacts and other large objects on the network cause further damage, is to romote the mechanical coordination with the use of mechanical fuse devices, being the least resistance comonent in the structures and CIRED 015 4/5

5 Paer 1353 rojected to oerate (break) when subjected to a certain load, before the ruture of cables or oles. MECANICAL FUSES DEFINITION To reserve the integrity of the ole, messenger and cables in case of occurrence of events in the distribution lines, such as falling trees and heavy objects, the element or weaker comonent (mechanical fuse) to be inserted in the structure will break when subjected to a equivalent dynamic effort, with value lower than the ole, messenger cable and conductors nominal ruture load. A. Comonents mechanical characteristics of sacer cable lines Find below an analysis of the main tyes of sacer cable lines structures, to identify the alied loads and the oints where can be introduced the mechanical fuse elements. a) Structures mechanical rotection Passage structure Anchorage structure b) Concrete ole Table 9 shows the oles excetional loads and ruture loads secified in Brazilian standards, and, in the last column, the values suggested for the ruture of mechanical fuses, considering the objective to break with a force equivalent to 80% of the ole breaking load. Table 7 Dimensioning of the mechanical fuses for ole rotection Nominal load Excecional load (1,4xRn) Ruture load (R=*Rn) Mechanical fuse (0,8xR) CONCLUSIONS The results of the statistical analysis of weather stations measurements data allowed obtaining, for the Celesc concession area, a maing occurrence of winds with maximum seeds exected with robabilistic characterization of occurrence. What enabled revise the current mechanical structures calculation methodology for overhead distribution networks, according to the characteristics of each region and robabilistic characterization to return eriod (50, 100 or 150 years), including efforts due to high intensity bursts and short duration winds. The results of studies to estimate the mechanical forces acting on conductors and structures, when trees falls onto the conductors, through simulations by finite element method, indicated the need for the use of mechanical fuses to minimize the risk of cables and oles ruture in the case of falling trees or heavier objects on the overhead distribution line. The results of these studies allowed Celesc to develo new standards on overhead networks, mechanically coordinates, which are being subjected to field trials (exerimental lines). REFERENCES [1] Molina Fernando, Dadam P Dadam, Pinheiro Walter, irakawa Wilson; Climate Change In Brazil And Its Reflections In The Reliability Of Celesc Distribution Overhead Lines - nd International Conference on Electricity Distribution CIRED - Stockholm, June 013 [] ABRADEE (RTD-6), Trações e Flechas de Cabos Condutores, (em Português), Rio de Janeiro, s.d. [3] IEC 6086:003 - Design criteria of overhead transmission lines CIRED 015 5/5