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TB 767 B2- Vegetation fire characteristics and the potential impacts on overhead.

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内容提示: Overhead linesB2Vegetation fire characteristics and the potential impacts onoverhead line performanceReference: 767June 2019 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance Members H.F. VOSLOO, Convenor ZA H. HAWES AU A. BRITTEN ZA P. FROST ZA F. LIRIOS AU M. LEE AU R. NEL ZA J. CALITZ ZA J. FERNANDES BR H. VALENTE PT Reviewers V. NAIDOO NO P. DULHUNTY AU WG B2.45 Copyright © 2019 “All rights to this Technical Brochure are retained by CI...

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Overhead linesB2Vegetation fire characteristics and the potential impacts onoverhead line performanceReference: 767June 2019 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance Members H.F. VOSLOO, Convenor ZA H. HAWES AU A. BRITTEN ZA P. FROST ZA F. LIRIOS AU M. LEE AU R. NEL ZA J. CALITZ ZA J. FERNANDES BR H. VALENTE PT Reviewers V. NAIDOO NO P. DULHUNTY AU WG B2.45 Copyright © 2019 “All rights to this Technical Brochure are retained by CIGRE. It is strictly prohibited to reproduce or provide this publication in any form or by any means to any third party. Only CIGRE Collective Members companies are allowed to store their copy on their internal intranet or other company network provided access is restricted to their own employees. No part of this publication may be reproduced or utilized without permission from CIGRE”. Disclaimer notice “CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”. Vegetation fire characteristics and the potential impacts on overhead line performance ISBN : 978-2-85873-469-6 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance ISBN : 978-2-85873-469-6 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 5 Executive summary The subject of fires under power lines proved to be a wide one. It covers many related areas in an attempt to reach a wide readership with an interest in this subject. Chapter 1 introduces the subject of fires. The question of why electric utilities should be concerned about fires under their lines is addressed in chapter 2 where the costs associated with fires are discussed. This not only deals with the cost of fire to society, but also focusses on those costs to the electric utility stemming from fires. Because by its very nature, fires are dangerous, and that danger increases when they burn close to live conductors, chapter 3 is dedicated to understanding the risks and indicate ways in which these risks may be reduced. The subject of the insulation breakdown of gasses during a fire has been the subject of many studies, spanning a number of decades. In chapter 4 the history of work that was done is reviewed, and further expanded by looking at work that was done outside of the classic electrical engineering fraternity by adding the biochemical processes which occur during the combustion of vegetation material under power lines. The theory of insulation breakdown and conductivity of fires is discussed in annexure A. In order to find a model to describe the breakdown process in engineering terminology, one of the correspondents, Mr Tony Britten, developed a model for this purpose and also tests the assumptions with some real data. This is dealt with in chapter 5. The terminology used with fire is an important first step in understanding how fires burn. In 6, the “anatomy” of fire is explained to the reader as a precursor to 7. In any study of fire, fire behaviour must be considered. In 7 this concept is defined for the reader and the aspects that influence this behaviour are discussed. This work has largely been done by institutions and persons involved in the prevention and suppression of fire as well as students of pastoral science, where the effects of fire on vegetation are of concern. This knowledge will not only prove important in the planning of vegetation management strategies but may also be used in the design and placement of towers for new lines in fire-prone areas. As will be seen in 8, climate and weather have a profound effect on the occurrence and behaviour of fires. Because of these influences, it is of course also possible to predict the onset of fires by prediction of the weather. This has been proven as a valuable aid to system operators. It was not possible to deal with every subject in detail; however, the Technical Brochure will raise the awareness of the reader to those aspects that needs to be considered when dealing with a fire under power lines. Some of the mathematical treatments appear in the text. More detailed work is taken up in the annexure. TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 6 Table of Content Executive summary ....................................................................................................5 1. Introduction and Background .............................................................................9 1.1 Background ................................................................................................................................... 9 2. Costs associated with fires under power lines ................................................ 11 2.1. Societal costs of fires ................................................................................................................. 11 2.2 Costs associated with loss of supply ....................................................................................... 12 2.3 The effect of fires on overhead conductors ............................................................................. 13 2.4 The effect of fires on Tower structures (steel and wood) ....................................................... 15 2.5 Fire damage to other equipment on lines ................................................................................. 18 2.6 Fire effects on substations,transformers and switchgear ...................................................... 21 2.7 Conclusion .................................................................................................................................. 22 3. Danger of fires under OHL to firemen and the public ...................................... 23 3.1 Introduction ................................................................................................................................. 23 3.2 Conditions under which fire induced flashover are likely to occur ....................................... 23 3.3 Step and touch potential ............................................................................................................ 25 3.4 Minimum safe working distance from overhead line ............................................................... 28 3.5 Calculating the horizontal distance to a point of discharge under windy conditions .......... 28 3.6 Resistance to ground of the Arcing Point ................................................................................ 30 3.7 Magnitude of the fault current ................................................................................................... 31 3.8 The generated step voltage ....................................................................................................... 33 3.9 Safe distance between a fireman and the point of discharge ................................................. 33 3.10 The extent of the hazardous zone ............................................................................................. 36 3.11 An approximate solution ............................................................................................................ 36 3.12 Conclusion .................................................................................................................................. 38 4. Insulation breakdown during fires under overhead lines ................................ 41 4.1 Introduction ................................................................................................................................. 41 4.2 The electrical breakdown of a gas ............................................................................................ 41 4.3 Overview and conclusions of fire experiments ....................................................................... 42 4.4 The reduced air density theory ................................................................................................. 50 4.5 Particle initiated flashover theory ............................................................................................. 51 4.6 Chemistry in flames and combustion ....................................................................................... 53 4.7 Temperatures of flames ............................................................................................................. 56 4.8 Flame conductivity ..................................................................................................................... 57 5. Proposed model for the breakdown mechanism in a fire ................................ 59 5.1 Introduction ................................................................................................................................. 59 5.2 Basis and rationale for the model used to predict the likelihood of flashover ..................... 59 5.3 The DC Case. .............................................................................................................................. 61 5.4 Application of the DC (direct current) model to fire flashovers ............................................. 68 5.5 The AC (alternating current) case ............................................................................................. 69 5.6 Measurement of AC voltages and currents in fire conditions ................................................ 73 5.7 Measurements of fire-induced corona in transmission lines ................................................. 76 5.8 Predicted performance of transmission lines under cane fire conditions ............................ 78 5.9 Comparison of Eskom results with Lanoie & Mercure (1997) ................................................ 81 5.10 Discussion .................................................................................................................................. 83 5.11 Concluding remarks ................................................................................................................... 84 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 7 6. Fire ...................................................................................................................... 85 6.1 Introduction ................................................................................................................................. 85 6.2 Fire initiation ............................................................................................................................... 87 6.3 The anatomy of a fire .................................................................................................................. 90 7. Fire behaviour .................................................................................................... 93 7.1 Factors influencing fire behaviour ............................................................................................ 94 7.2 Available heat energy. ................................................................................................................ 96 7.3 Rate of energy release and fire intensity. ................................................................................. 97 7.4 Vertical distribution of heat energy........................................................................................... 98 7.5 Fuel and its characteristics ..................................................................................................... 100 7.6 Air Temperature and relative humidity ................................................................................... 111 7.7 Topography ............................................................................................................................... 112 7.8 Fire barriers ............................................................................................................................... 113 7.9 Conclusion ................................................................................................................................ 114 8. Fire climate and fire weather ........................................................................... 115 8.1 Introduction ............................................................................................................................... 115 8.2 Fire and wind ............................................................................................................................ 115 8.3 South Africa .............................................................................................................................. 117 8.4 Australia .................................................................................................................................... 121 8.5 The United States of America .................................................................................................. 124 8.6 Mediterranean countries .......................................................................................................... 126 8.7 Conclusion ................................................................................................................................ 126 9. Fire danger rating systems .............................................................................. 127 9.1 Overview .................................................................................................................................... 127 9.2 US National fire danger rating system .................................................................................... 127 9.3 Australian fire danger systems ............................................................................................... 128 9.4 Southern African fire danger index systems ......................................................................... 131 9.5 Conclusion ................................................................................................................................ 133 10. Predicting fire induced flashovers ............................................................... 135 10.1 Introduction ............................................................................................................................... 135 10.2 Fire Induced Flashover Probability Index (FIFPI) .................................................................. 135 10.3 Conclusion ................................................................................................................................ 141 11. Fire tracking systems .................................................................................... 143 11.1 Introduction ............................................................................................................................... 143 11.2 Remote sensing: monitoring fires from Space ...................................................................... 143 11.3 Background to the Advanced Fire Information System (AFIS) ............................................ 144 11.4 Extending AFIS Functionality to other users ......................................................................... 147 11.5 New developments ................................................................................................................... 150 11.6 Conclusions .............................................................................................................................. 151 12. Mitigation of fires under power lines ............................................................ 153 12.1 Background ............................................................................................................................... 153 12.2 Routing of power lines ............................................................................................................. 153 12.3 Vegetation management .......................................................................................................... 153 12.4 Fire Free Servitudes in sugar cane fields ............................................................................... 153 12.5 The legal approach ................................................................................................................... 153 12.6 Design and technical considerations ..................................................................................... 157 12.7 The approach followed in Mexico ........................................................................................... 157 12.8 The approach in Brazil ............................................................................................................. 157 12.9 The approach followed in Australia ........................................................................................ 158 12.10 The approach of South Africa ................................................................................................. 159 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 8 12.11 Fire risk mapping ...................................................................................................................... 159 12.12 Noise detection ......................................................................................................................... 160 12.13 Conclusion ................................................................................................................................ 160 13. Conclusion ..................................................................................................... 161 14. References ..................................................................................................... 163 15. Abbreviations and definitions ....................................................................... 171 16. Annex .............................................................................................................. 175 16.1 Comments received about the effect of fires on conductors ............................................... 175 16.2 The Breakdown of the insulating properties of a gas ........................................................... 177 16.3 Electrical conductivity in wildfires .......................................................................................... 195 16.4 Calculation example: The safe distance to a discharge of fault current into the soil ........ 196 16.5 Example of a cost analysis by Eskom to acquire cane free servitudes. .............................. 200 TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 9 1. Introduction and Background 1.1 Background Vegetation fires have been occurring on Earth for millennia and will continue to do so. Steven Pyne (1997) in his book “World Fire” says that Earth is a uniquely fire planet. Other planetary bodies in the universe have elements of fire: Jupiter has an ignition source in lightning, Mars has traces of free oxygen; Titan (the largest moon of Saturn) has methane based fuel. Only Earth has all three and only Earth has the means to combine them - Earth has life. Marine life has pumped the atmosphere with oxygen and terrestrial life has stocked the continents with carbon fuels. (Pyne, 1997) Earth is also the only planet with a species that can both start and stop fires. Payne states that by interfering with the natural cycle of fires, fuel loads are not contained but rather expand and when fires then occur, they are devastating and in many cases impossible to control (Pyne ,1997). Except for the arctic continents, all the other continents experience fires and these fires are such a part of the landscape that plant species subjected to them have over time adapted and are in many cases dependent on fire for reproduction and vigorous growth. The presence of fires was recorded by many early European explorers. The southern tip of the South American continent was named “Tierra del Fuego” (Spanish) by the Portuguese explorer Ferdinand Magellan in 1520, who witnessed fires and smoke visible from the sea. Magellan undertook this voyage on behalf of king Charles 1 of Spain. (Bergreen, L., 2003). At the southern tip of Africa a similar case exists. The first recording of a veld fire was probably done by Bartolommeo Dias when he became the first European to round the Cape in 1488. He called the current Cape St. Francis “Ponta das Queimadas” (Axelson, 1973) (Queimada = forest fire Anon (no date). Ponta de Queimada, São Jorge Island in the Azores, is another example. Where power lines traverse areas where fires occur, a number of effects are experienced by these lines and, major system disruptions have resulted from wild fires under lines. Although huge wild fires occur in Australia, South Africa, the United States, Russia, and Mediterranean Europe, the effects of sugar cane fires and smaller grass fires also have a considerable impact on both power lines and the rest of the electrical supply system. Because of the problems associated with fire under power lines in many countries around the world which have considerable impact on the total electrical system as well as on the supply of electricity to customers a work group (WG B2-45) under Gigré Study Committee B2 was established to study this problem and produce a Technical Brochure on the subject. This paper contains some extracts from the work done to date. This document will discuss certain aspects of fire such as the costs resulting from fire, the weather and its influence on the onset and behaviour of fires. Attention will also be devoted to ways to predict the inception of dangerous fire weather as well as a novel approach which refines the conventional systems for power lines. Finally the steps that can be taken by the designer of overhead lines (OHL) as well as the vegetation manager in order to manage and minimize fire effects on OHL within their managed easements. Where OHL traverse areas where fires occur, a number of effects are experienced by these lines. Major system disruptions have resulted from wild fires under lines. Although huge wild fires in Australia, California, Colorado in the Rockies, Russia, Greece, the effects of cane fires and smaller grass fires also have considerable impact on OHL. Because the problems associated with fire are experienced in many countries around the world and have considerable impact on the total electrical system, as well as on the supply of electricity to customers, it has become necessary to study this subject and produce a Technical Brocure based on contributions from around the World. This document will be the first to consider in one document, factors such as fire and the various parameters that describe its behaviour, the various fuels and its effect on fires and the weather and it influence on the onset and behaviour of fires, in addition to those subjects that have been studied to date by engineers and physicists. TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 10 Attention will also be devoted to the various impacts of fires under power lines on the whole electrical system and its customers. Finally, the document is of interest for the utilities, namely OHL designers, operation and maintenance (O&M) personnel dealing with vegetation management, the system operators, but also for firefighting authorities to be aware of the risk when working near an OHL. TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 11 2. Costs associated with fires under power lines 2.1. Societal costs of fires The costs associated with fires, specifically wildfires can be enormous notably the destruction of property and even human lives. These costs are often difficult, if not impossible to quantify. Fires affect wide areas and can encompass all areas and components of the electrical system within a fire zone, inflicting heavy costs on the utility and communities. Fires are present all over the world. Uncontrolled slash and burning in Amazonia, Indonesia and India all threaten biodiversity. Smoke plumes cover the Amazon Basin and pastoral burns pull the Sahara southwards. Multimillion-acre wildfires regularly burn throughout the boreal forest and endless annual savanna fires burn throughout the tropics. The Great Black Dragon fire incinerated the Hinggan forest in China, the East Kalimantan fires that turned 9 million acres of Borneo rain forest into a smoking hole. In Australia the Ash Wednesday almost brought an industrialized nation to its knees. The conflagration of 1988 through the Yellowstone National Park swallowed $130 million for the provision of fire fighters, without any effect on the fire. (Pyne, 1997) ). The debates and discussions within communities after such fires have wide ranging effects. An early example is the proclamation of forest reserves in 1891 as well as huge fires in the USA since had a significant effect on the debate and policies concerning fire, its prevention and suppression. America developed the most expensive firefighting operation in the world, but with a capacity to mobilize against wildfire, being a marvel of the modern world. Only Canada could demonstrate anything like it. Within days the fire establishment could assemble and deploy around the country tens of thousands of fire fighter, hundreds of fire engines and tractors and the mobile kitchens, power tools and gasoline tanks, even pay fire fighters and support staff. The suppression apparatus could flood the sky with scores of air tankers, helicopters and observation planes, but at a cost of $1 million per day. The rehabilitation of the landscape after the Foothills fire near Boise cost $24 million. Over the 1994 season $925 million was spent by the fire establishment (Pyne, 1997). Fires in Greece, Italy, Portugal and Spain make the news headlines from time to time. Driven by hot, dry winds, they leave destroyed property, vegetation and livestock in their wake. People that are not able to flee, perish. On 12 August 2014 the press service of the Russian forestry administration reported heavy forest fires which broke out in the Russian Far East since the forest fire season began. These fires have already destroyed huge forest territories. According to the administration the fires have already destroyed more than 1.6 million hectares of forests, this year alone (2014). This is double the area of forest territory destroyed in the same period the previous year. TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 12 Figure 1 - Example of a fire whirl in a commercial forest fire in South Africa (photo Mauritz Bam) Given this huge society cost of fire effects, fire research experienced an intensive era since the early 1970’s, with the focus changing to predicting fire behaviour, and not only establishing fire danger as given by the national fire danger rating system (NFDRS). A model for this purpose was developed by Richard Rothermel, of the Northern Fire Lab in 1972, with the prospect of mathematical forecast of fire spread. Due to the popularity of the model, fire officers embraced it but extrapolated it far beyond its originating conditions, necessitating redefining the model and making it more universal. The computer based fire behaviour system that emerged was the now well-known BEHAVE system (Pyne, 1997). 2.2 Costs associated with loss of supply The problem with fire-induced flashover is experienced worldwide in countries such as the U.S.A., Australia, South Africa, Brazil, Mexico, and others. The experience of each of these countries is discussed in the annexure. Fires burning under overhead lines normally cause short-circuits resulting in the operation of the protection system. In some extreme cases, however, the line may suffer damage to either the conductor or the structure, leading to a permanent and sometimes prolonged outage. In cases where the outage occurs and the protection system operates, successfully re-closing the breakers, this is referred to as a momentary outage. Depending on the voltage of the line and the fault current, this may last between 0.1 s to 2 s and could affect customers with continuous process plant. The case with line faults caused by fires is regarded as more serious than other transient faults, as it has been observed that after a successful auto-reclose the breakdown conditions still exists under the line and a second breaker operation then results in a lock-out. (Sadurski, 1977). A study conducted in 2001 in the USA commissioned by the Electric Power Research Institute (EPRI), the Consortium for Electric Infrastructure for a Digital Society (CEIDS) indicated that poor power quality cost the American economy between US$15,000 million to US$25,000 million per year (Primen, 2001). TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 13 In South Africa, a single fire-induced flashover causes an average of three voltage dips on the transmission system, which can cause damage to a customer’s production ranging between R5000 and R150 000 per dip (Taylor 1999, Vajeth et al. 2003). Outages caused by two fires 295 km distant on two 765 kV lines in South Africa, lead to the separation of the Cape load network on the 29th of July 2002. After the trip and lockout of the second line, the remaining 400kV lines were not able to transmit sufficient power, causing the Cape network to island. The deficit of active power in this islanded network resulted in a frequency drop to 47.72Hz. This in turn caused the activation of the under-frequency load shedding scheme and a collapse of the Cape network. The outage lasted for 10 minutes and resulted in a load loss of 1,593 MW. Bushfires in the Sydney region, during the 4th to 6th December 2002, caused an unprecedented number of faults on the main NSW 330 kV and 500 kV networks. The main 330 kV / 500 kV network was subjected to approximately 70 faults due to fires between 2.30pm and 12.00am on the 4th December 2002. On the following two days more than 45 faults were recorded. These faults caused considerable number of network outages, resulting in major reductions in supply security and the service quality. It was estimated that on the 4th December alone, 4,000 MWh of energy were not consumed by customers due to loss of load caused by voltage dips. The economic impact on electricity users, on that afternoon, was in the order of A$40 million using the value of lost load (cost of unserved energy) as A$10,000/MWh. (Ref. Investigation on the impact of Bushfires on TransGrid Extract supplied by Francis Lirios,AU) In other cases in Australia with a long interconnected transmission grid extending over 4,500 km there is a danger during high fire danger season, for sustained trips to cause power swings on the network that can cause voltage instability and major and extensive regional outages. For example, in a transmission line with 1,000 MW of continuous load over a 500 km interregional transfer, a suddend loss of the line would cause voltage instability and major black outs; but if for a short time the electrical load is reduced to 400MW, to allow a fire front to pass under the line, it significantly reduces the potential risk. 2.3 The effect of fires on overhead conductors Overhead lines are subject to different environmental conditions such as fires, lightning and even faults caused by birds. The resulting outages not only affect the line but also impact the equipment in substations. Although equipment is normally designed to withstand the effects of short circuits caused by these events, repeated faulting stresses the equipment, sometimes outside their design parameters and cause premature ageing. A variety of responses has been obtained from utilities about damage caused to conductors of OHL during fires. As the duration and intensity of the fire plays an important role in the effect on conductors, it is important to distinguish between low intensity grass fires and high intensity forest fires. TB 767 - Vegetation fire characteristics and the potential impacts on overhead line performance 14 Figure 2 - Examples showing a phase to...

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