BS-7733-1994 IEC-60826-1991.pdf

BRITISH STANDARD BS 7733:1994 IEC 826:1991 (Incorporating Corrigendum September 1991) Guide to Loading and strength of overhead transmission lines Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 This British Standard, having been prepared under the direction of the Power Electrical Engineering Standards Policy Committee, was published under the authority of the Standards Board and comes into effect on 15 April 1994 © BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference PEL/17 Draft for comment 87/29345 DC ISBN 0 580 22818 5 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Power Electrical Engineering Standards Policy Committee (PEL/-) to Technical Committee PEL/17, upon which the following bodies were represented: Aluminium Federation Association of Consulting Engineers BECCAMA (BEAMA Electrical Cable Conductor Accessory Manufacturers Association) British Non-Ferrous Metals Federation British Railways Board Department of Trade and Industry (Electricity Division) Electrical and Electronic Insulation Association (BEAMA Ltd.) Electricity Association Overhead Transmission Line Contractors (BEAMA Ltd.) Amendments issued since publication Amd. No.DateComments Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 © BSI 10-1999i Contents Page Committees responsibleInside front cover National forewordiv Introduction1 Section 1. General 1.1Scope2 1.2Definitions2 1.3Symbols and abbreviations3 1.4Design basis6 1.5System design6 1.6Assessment of line reliability7 1.7Security requirements12 1.8Safety requirements12 Section 2. Design criteria 2.1General12 2.2Design criteria15 2.3Design criteria for towers18 2.4Design criteria for foundations18 2.5Design criteria for conductors and earth wires19 2.6Design criteria for insulators and hardware19 Section 3. Loadings 3.1General19 3.2Climatic loads, wind19 3.3Climatic loads, ice without wind31 3.4Climatic loads, combined wind and ice loadings38 3.5Loads for construction and maintenance44 3.6Loads for failure containment46 Section 4. Strength of components and limit states 4.1General47 4.2Components of transmission line systems48 4.3Limit states of line components48 4.4Strength data of line components48 Section 5. Technical justifications 5.1Scope53 5.2Relation between load and strength53 5.3Effect of span dispersion on reliability57 5.4Strength coordination of line components57 5.5Number of components subjected to maximum load intensity59 Annex A (informative) The effect of span dispersion on load-strength relationship: calculation of span use factor61 Annex B (informative) The Beta distribution function65 Annex C (informative) Statistical distribution of maximum yearly wind velocity and ice load66 Annex D (informative) Characteristic strength of components68 Annex E (informative) Determination of the meteorological reference wind velocity70 Annex F (informative) Gradient wind speeds71 Annex G (informative) Temperature measurements and their interpretation71 Annex H (informative) Types of icing72 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 ii © BSI 10-1999 Page Annex J (informative) Atmospheric icing process and terrain influences73 Annex K (informative) Guidelines for the implementation of an ice observation programme74 Figure 1 Relations between load and strength8 Figure 2 Methodology for the design of transmission lines14 Figure 3 Ground roughness A25 Figure 4 Ground roughness B25 Figure 5 Ground roughness C26 Figure 6 Ground roughness D26 Figure 7 Determination of the apparent length of 2 Lm of the two adjacent spans on the considered support27 Figure 8 Gust factor of insulators and towers: Gi, Gt28 Figure 9 Definition of the solidity ratio29 Figure 10 Overall normal drag coefficients for rectangular towers composed of flat-sided members CxT29 Figure 11 Overall normal drag coefficients for rectangular towers composed of circular section members CxT30 Figure 12 Value of CxTc drag coefficient of cylindrical elements having a large diameter, as a function of Reynolds number Re31 Figure 13 Factor related to the influence of the number of years of observation of ice33 Figure 14 Factor related to the influence of conductor diameter34 Figure 15 Variation of Kh with the height of conductors above the ground34 Figure 1636 Figure 1737 Figure 18 Cases of combined ice-wind loadings given as an indication39 Figure 19 Factor related to the influence of the number of years with observation of wind combined with ice41 Figure 20 Definition of cylindrical ice shape42 Figure 21 Application of longitudinal loads45 Figure 2246 Figure 2347 Figure 24 Diagram of transmission line system48 Figure 25 Diagram of a transmission line diagram49 Figure 26 Values of Pf = (1 Ps) for various distributions of Q and R for T = 50 years55 Figure 27 Values of Pf = (1 Ps) for various distributions of Q and R for T = 150 years56 Figure 28 Values of Pf = (1 Ps) for various distributions of Q and R for T = 500 years56 Figure B.1 Different forms of the Beta distribution66 Figure E.1 Relationship between meteorological wind velocities at a height of 10 m71 Figure K.175 Table 1 Typical strength coordination7 Table 2 Reliability corresponding to various assumptions of load and strength11 Table 3 Relationship between reliability levels and return periods of design loads11 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 © BSI 10-1999iii Page Table 4 Reliability levels of transmission lines13 Table 5 Design conditions15 Table 6 Range of spatial coverage of maximum load intensity (given in number of towers)17 Table 7 Number of towers subjected to maximum load intensity17 Table 8 Strength factor ÌN related to the number N of components subjected to the critical load intensity17 Table 9 Values of ÌS18 Table 10 Definition of ground roughness20 Table 11 Values of high wind velocity21 Table 12 Values of KR for different ground roughness21 Table 13 Statistical parameters of ice loads32 Table 14 Values of ice load coefficient KÖg33 Table 15 Return period of combined events38 Table 16 Combined wind and ice loading conditions39 Table 17 Values of KÖL and KÖH40 Table 18 Values of factors KiH and KiL40 Table 19 Drag coefficients of ice-covered conductors42 Table 20 Damage and failure limits of supports50 Table 21 Damage and failure limits of foundations51 Table 22 Damage and failure limits of conductors and ground wires52 Table 23 Damage and failure limits of interface components52 Table 24 Strength parameters of supports52 Table 25 Strength parameters of foundations (in uplift)53 Table 26 Strength parameters of interface components53 Table 27 Strength parameters of conductors and ground wires53 Table 28 Values of ratio of average strengths (RAS) and ÌS required to insure that component R2 will fail after component R1 with a 90 % probability59 Table 29 Strength coefficient ÌN related to N components in series subjected to the critical load60 Table A.1 Statistical parameters of wind span variation62 Table A.2 Statistical parameters of weight span variation62 Table A.3 Values of ¾u as a function of U and N for vR= 10 %63 Table A.4 Values of ¾u for different strength dispersions64 Table C.1 Values of the constants C1 and C266 Table C.2 Probability of exceeding an event having a return period T67 Table C.3 Ratios of for an extreme type 168 Table D.1 Values of Kc69 Table D.2 Value of quality factor ÌQ for lattice towers69 Table E.1 Values of factor Kj70 Table H.1 Physical properties of ice72 Table H.2 Meteorological parameters controlling ice accretion72 List of referencesInside back cover x x Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 iv © BSI 10-1999 National foreword This British Standard has been prepared under the direction of the Power Electrical Engineering Standards Policy Committee. It is identical with IEC 826:1991 Loading and strength of overhead transmission lines, incorporating Corrigendum September 1991, published by the International Electrotechnical Commission (IEC). This British Standard was approved for publication in November 1993 as a guide to what should be considered in determining the loading and strength of overhead transmission lines. The Technical Committee has reviewed the provisions of IEC 383, to which informative reference is made in the text, and has decided that they are acceptable for use in conjunction with this standard. A related standard to IEC 383 is BS 137-1:1982. The graphic signs, symbols and letters used in this standard may be found in: A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-references IEC 652:1979BS 7732:1994 Guide to the loading tests on overhead line towers (Identical) BS 3939Graphic symbols for electrical power, telecommunications and electronics diagrams Part 1:1986General information, general index Part 2:1985Symbol elements, qualifying symbols and other symbols having general application Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 76, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 © BSI 10-19991 Introduction Probabilistic methods are recommended for the design of transmission lines as opposed to deterministic methods because they openly acknowledge that in practice there is always some risk that design loads can be exceeded and as a result complete reliability cannot be achieved. The proposed methods also provide for designing according to different levels of reliability depending on either the importance of lines in the system, or on varying requirements for public safety. The techniques described enable the designer to assess the reliabilities of existing lines or to design new lines for target reliabilities provided that the data required for such analysis are available. However, it is recognized that for many locations and situations much of the data may not be available to the extent necessary for confidence in the calculation of absolute reliability. In such cases the recommended methods will be effective for estimating the relative reliabilities of different designs. It will be noted that the alternative to designing to target reliabilities ends up as one of designing for varying return periods of climatic events, specifically 50, 150 and 500 years. It is considered that these represent reasonable differences between reliability levels, although different return periods may be selected if desired. The actual reliability of lines is sensitive to the accuracy of many design parameters. Some of the typical parameters which may affect reliability are discussed hereafter. Although the basic formula for calculating wind loads from measured velocities is well known, it requires the use of a number of coefficients that are not precise. For instance drag coefficients for conductors and bundles depend on conductor stranding and bundle configuration. It is not possible to give an absolute recommendation covering all conductors in all situations in a report such as this, and the best that can be achieved represents some sort of compromise. Similarly, the effects of varying terrain are not exact and inaccuracies in selecting the appropriate coefficient may lead to differences as large as that separating reliability levels. The estimation of wind speeds between widely-spaced measuring situations is likely to be inexact and can lead to unpredictable errors in load calculations. Data on ice loads and wind-on-ice are sparse and the recommendations given in this report are based on the judgement of knowledgeable and experienced engineers and scientists. But for many situations they will be only broad approximations. Similar comments may be made with respect to the strength of line components, although in general they are more precisely known than climatic loads with the exception of foundations. The use factors of these components (the percentage of their rated strength that is used to carry loads) may not be known and can affect reliability. Although the report does propose a method for estimating use factor, there is room for error which in general should be on the side of safety. The above discussion does not represent a complete catalogue of all grounds for uncertainties but does indicate the type of analysis that the designer shall go through in order to design for target reliabilities with confidence. Having done this and if the designer is satisfied with the completeness and accuracy of the data for the particular situation, the report may be used as originally intended, i.e., providing for a reliability based design of transmission lines. Notwithstanding the uncertainties of the existing probabilistic methods, it shall be pointed out that deterministic methods have many of the same pitfalls that are generally not acknowledged. The approach recommended in this report provides a consistent and logical way of relating loads and strengths, and will result in economic and safe transmission lines whenever the required data is available. Finally, it is important to compare the results obtained by the proposed methods with existing ones which have proved to be satisfactory. This comparison should allow further adjustment of some of the proposed factors according to local experience. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 12:30:58 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7733:1994 2 © BSI 10-1999 Section 1. General 1.1 Scope This report applies to overhead lines of nominal voltage above 45 kV. It may also be applied to lines with a lower nominal voltage. The purpose of this report is to propose relationships between loads imposed on transmission lines and the strengths of transmission line components in order to obtain safe and economical designs. This report also provides a framework for the preparation of national st