179

GUIDELINES FOR FIELD MEASUREMENTOF ICE LOADINGS

ON OVERHEAD POWER LINE CONDUCTORS

Task Force22.06.01

February 2001

GUIDELINES FOR FIELD MEASUREMENT OF ICE LOADINGS ON OVERHEAD POWER LINE

CONDUCTORS

PREPARED BY

TASK FORCE 22.06.01 (ATMOSPHERIC ICING)

Members of Task Force 22.06.01 S.M. FIKKE (convenor - Norway), F. DOWNES (Ireland), J-F. DRAPEAU (Canada), A.J. ELIASSON (Iceland), M. ERVIK (Norway), M. FARZANEH (Canada), A.P. GOEL (Canada), E.J. GOODWIN (United States), J. HRABANEK (Czech Rep.), J. JAKSE (Slovenia), S. KRISHNASAMY (Canada), L. MAKKONEN (Finland), F. POPOLANSKY (Czech Rep.), J. RUHNAU (Germany), C.C. RYERSON (United States), Y. SAKAMOTO (Japan), V. SHKAPTSOV (Russia), J.B. WAREING (United Kingdom).

GUIDELINES FOR FIELD MEASUREMENT OF ICE LOADINGS ON OVERHEAD POWER LINE CONDUCTORS

TABLE OF CONTENTS

Summary 1. INTRODUCTION 2 2. FIELD DATA 3 2.1. Need for field data 3 2.2. Direct measurements related to other sources of ice load information 3 2.3. Icing processes 4 2.4. Aspects regarding collection of representative ice samples 4 2.5. Instrumentation of transmission lines 4 2.6. Ice load observations on (not instrumentaed) lines 6 2.7. Effects of climatic variations 7 3. PREPARATIONS BY UTILITIES AND FIELD STAFF 8 3.1. Information and training of field staff 8 3.2. Equipment for the field staff 8 3.3. Headquarter routine 9 4. PROCEDURES FOR MEASUREMENTS 9 4.1. Introduction 9 4.2. Ice measurements 9 4.2.1. Physical dimensions 9 4.2.2. Weight of ice on conductors 10 4.2.3. Density 10 4.3. Classification of ice types 10 4.4. Moulds 11 5. OTHER OBSERVATIONS 12 5.1. General 12 5.2. Pollution (Observation sheets B5 and C3) 12 6. OBSERVATION SHEETS 13 6.1. Basic information 13 6.2. Observation sheet A : Weather data 13 6.3. Observation sheet B : General icing data 13 6.4. Observation sheet C : Weight, cross section and density measurement 13 6.5. Simplified observation sheets D and E 14 6.6. Additional information 14 7. ICING DATABASE 14 8. REFERENCES 15 APPENDIX A : Icing processes 16 APPENDIX B : Examples of ice observation tools 18 APPENDIX C : Observation sheets 21

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GGUUIIDDEELLIINNEESS FFOORR FFIIEELLDD MMEEAASSUURREEMMEENNTT OOFF IICCEE LLOOAADDIINNGGSS

OONN OOVVEERRHHEEAADD PPOOWWEERR LLIINNEE CCOONNDDUUCCTTOORRSS

Summary When criteria for ice loadings on overhead transmission lines shall be established, experiences from existing lines and line networks are of great economical value. This document describes the importance of such data and procedures for collecting them. Guidelines for company routines as well as recommendations for tools, training and observation procedures are given. Two sets of observation forms are presented: 1) A comprehensive set of forms recommended for collecting fundamental and accurate

information on ice loadings and shedding on various conductor types and configurations, and

2) A simplified set for collecting the most basic information regarding ice loadings. Utilities are strongly encouraged to establish appropriate routines, considering their own needs, prior to each coming season in order to be prepared to collect the data needed within the limited time window they may have for this task.

1. Introduction In many parts of the world ice loading is the most important parameter influencing the investments and performance of electric overhead lines. Ice loading data is also crucial where upgrading old lines is considered. In particular, information on ice loading is important when the reliability of electrical networks is to be assessed. IEC Technical Report 826 (1991), “Loading and Strength of Overhead Lines” [1], (hereafter referred to as IEC 826) provides the basis for National Standards for project specifications on overhead line design using probabilistic methods. To establish the necessary data on ice loading for the use of probabilistic methods, IEC presented the Technical Report 61774 (1997): “Meteorological Data for Assessing Climatic Loads” [2] (hereafter referred to as IEC 61774). The targets of IEC 61774 were:

- Reporting on the availability and use of climatic data. - Recommending standardised measurement techniques for ice loading. - Reviewing models for computing ice loads.

IEC 61774 does not cover gathering ice load data from existing transmission lines, mainly because such guidelines ideally need a relatively close follow-up of practices and data collection, which was outside the scope of IEC TC11. Cigré SC22 therefore supported further work on gathering and evaluating of ice data in 1996. The objective of this report is to provide utilities wishing to collect icing data from their overhead lines and test configurations with guidelines and recommendations on how such

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measurements should be conducted and evaluated. Some background information, which is considered relevant, is included. For more information on atmospheric icing, the reader will find references to more detailed literature. The need for improved design loads is backed both by the economic consequences for new lines as well as for upgrading of older lines. In the latter case, the loads may decide whether upgrading would be possible or not. An alternative approach to collect ice data on transmission lines would be to instrument lines for real time monitoring of ice accretion. Due to the costs involved, only a few line sections could be equipped and ice events on other lines would be missed. Such instrumentation is recommended for research purposes and is not discussed further in this report. As a cheaper alternative, real time monitoring (possibly with on-line alarm functions) in the vicinity of lines is recommended. The term “conductor” used in this report includes also earth wires, optical ground wires, covered conductors, and similar.

2. Field data 2.1 Need for field data In many countries ice loadings influence the life cycle costs of power lines in many ways, such as investment, maintenance cost, repairs after failures or loss of delivered power during outage periods. A more recent consequence is the potential loss of telecommunication facilities, due to the increased use of fibre optic wires on transmission lines. It is outside the scope of this report to discuss such economic consequences in detail. However as an example for steel towers, when ice loadings increase, the weight of individual towers increases and the average span length decreases. This means that the investment increases rapidly with ice loadings. An overestimate in the design ice loadings means that the investment costs of new 420 kV lines may be 5-10% higher than necessary. An underestimate in the design load can, on the other side, imply catastrophic costs regarding maintenance, tower restoration and compensation for non-delivered energy. For wood pole lines the similar sensitivity is valid for ice loads of 3-4 kg/m or higher.

2.2 Direct measurements related to other sources of ice load information Ice load measurements on overhead line conductors, combined with other information such as meteorological conditions and load estimates from ice models, are key factors to improve line design criteria. In particular the complementaary information of all these sources is important. IEC 61774 recommends a strategy for implementing the different data sources to obtain the best possible information basis to form the design load. This strategy is demonstrated in Figure 2.2.1. The upper right hand box of this figure represents field data for ice. Such data may come from especially designed measuring racks or test spans. However, the numerous kilometres of transmission lines that pass through icing exposed areas, represent probably by far the most comprehensive source of information to cover this point. Therefore, ice loads obtained from existing transmission lines represent a major key to improved line design in the future. In

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particular, such data are in many cases the most relevant when evaluating the possibilities for upgrading of older lines, e.g. installing larger conductors or optical cables, or increasing the number of sub-conductors (bundles). Such issues have been raised already in many regions where the need for increased transmission capacity cannot be met by building new lines. Investments will be greatly affected depending on whether it is possible to utilise existing towers or new towers have to be built. Decisions can in most cases only be made in terms of probabilities. This means that owners have to take into consideration the probability of failure they are willing to accept or, in other words, how much they are willing to pay for a greater reliability. It is important to compile primarily good records of major icing events, for instance with significant damage to the lines. But for statistical purposes, it is also important to record all icing events. The statement of NO ice during an observation period is for the same reason of great value.

2.3 Icing processes It is assumed that the quality of information is improved when the observer has some knowledge about the icing processes. The various icing processes are described in [2]. An extract is given in Appendix A. The observers should know which type(s) of ice is (are) most relevant for their regions.

2.4 Aspects regarding collection of representative ice samples The collection of ice samples from transmission lines is far from being a simple task. Samples from conductors 20-30 m above ground are mostly impossible to obtain (even from un-energised lines) and samples fallen from the conductors to the ground are in general only fragments of the complete ice accrual. Partial fragmentation during the ice accretion as well as sublimation and fragmentation after the icing has stopped must also be considered when ice samples are collected. Furthermore, the ice layers may vary from the span ends to mid-span and also from spacers in bundle conductors over the sub-span. The experiences this report is based upon are mainly from lines of 22 kV - 132 kV. This report is therefore more direct applicable for voltages below 145 kV rather than for higher voltages. It is however important to have the described methods and guidelines in mind also for the higher system voltages and apply them whenever possible, especially by describing ice accruals by scalable photographs (e.g. by including the bare conductor in the picture).

2.5 Instrumentation of transmission lines Instrumentation of transmission lines for direct measurements of ice loadings on live lines would deliver optimum results on ice data. Such measurements would also give data for various unbalanced loads like transversal, longitudinal and torsional loads. Although this is done by several utilities, mostly for research purposes, this document does not include recommendations on this topic. Instrumentation is most relevant to extremely exposed lines, and the results may be difficult to transfer to other lines. The use of external expertise should be considered.

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General meteorological data

Icing models

Icing data

Use transfer

functions to convert to local

meteorological data

Use transfer functions to

convert to local icing data

Evaluate

liquid water content and droplet size

Calculate local icing data

Compare calculated and measured icing data.

If difference is not acceptable this is used to adjust icing model Calculate final icing data taking conductor and span data into account

Calculate wind force

on iced conductor

Statistical processing of the effect of wind and temperature,

wind on iced conductor and ice load

Design data

Figure 2.2.1. Strategy flow chart for obtaining design loads [2].

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2.6 Ice load observations on (not instrumented) lines It is urgent for utilities in regions exposed to icing to implement a program for ice load recordings on existing lines. In the case of failures or serious disturbances, the number one priority is to re-establish the system without delay. Nevertheless, someone should, in parallel with the restoration work, be appointed to secure the evidences of ice accretion before they are removed or melted. In fact, the more extreme or unusual the event seems to be, the greater is the value and importance of detailed and comprehensive data and information from the event. Therefore, motivation and proper training of the people involved are necessary. The grid owners are encouraged to predefine a group of people dedicated to this task. The weight (ice load) and the diameters (shape) of the icing are parameters that are dependent on each other. If both have been measured, the density can be calculated. It is mostly easier to measure the diameter than the weight, and hence the diameter measurements have the number one priority. If the weight cannot be measured directly, it may be estimated from the diameter and proper assumption of density (based on ice type or measurements elsewhere in the area). The accuracy of the weight depends more on the accuracy of the ice diameter measurements than on the density, since it is proportional to the square of the diameter. This is illustrated in Figure 2.6.1. However, the importance of density measurements must not be underestimated since they are important for model calculations. Some standards (countries) use the load (mass per unit length), other use the ice thickness (radial ice) as input. In both cases the density is needed, in case a) for combined wind load calculations and in case b) to calculate the vertical loads or tension loads on towers. The representativity of ice data is very important, but also difficult to ensure. It is not possible to describe general rules other than to keep this in mind when recording such data, and take notes on variations that are not clear from the measurements. Photographs are important and helpful, in particular scalable close-ups (with zoom lens) showing both the ice accretion and the conductor. The time window for observing ice accretions on overhead lines may often be less than ideal. In such cases the priority of observations should stand as follows:

- Photo or sketch - External diameter and other dimensions accessible for observation - Weight and length of a representative sample - Ice type

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Figure 2.6.1. Ice load as a function of diameter and density.

2.7 Effects of climatic variations Atmospheric icing will have a great variability when measured on one location. Figure 2.7.1 shows annual maxima (and running 5-year mean values) from the longest homogeneous time series of ice measurements in the world, on the top of Studnice Mountain (800 m above sea level) in the Czech Republic [4]. This test site represents nearly 60 years of continuous measurements and is fortunately still in operation. This figure clearly shows that the design ice loads based on data from the 1950 - 60’s would be quite different from those selected from the 70’s and 80’s. Furthermore, it is interesting to notice the development through the last decade of this century. It is relevant to ask whether this dramatic increase of ice accretion is related to the parallel increase of atmospheric temperature in the same decade. This underlines the importance of careful awareness regarding ice accretions in the future, especially in the light of possible global warming which will lead to higher atmospheric humidity.

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The most important conclusion from this figure is that it is more important than ever to monitor ice accretions and ice loads for power utilities in all countries where damage cost due to ice loads is high.

Figure 2.7.1 Ice load measurements from Studnice 1940 – 1999.

3 Preparations by utilities and field staff 3.1 Information and training of field staff The importance of ice data collection on overhead lines should be explained to the field staff, who should be motivated to be as accurate and thorough as possible. The various icing processes, measurement methods, tools and safety procedures should be clearly explained. A regular training program for field staff, every year just before the icing season, is strongly recommended. An overview of the lessons learned from the previous year’s data or experiences from other utilities should be considered.

3.2 Equipment for the field staff Appendix C includes a set of observation sheets that should be carefully studied before each icing season. The quality of data recorded on these sheets will be substantially improved by providing the staff with appropriate tools, as well as good headquarter routines during the icing season. These topics are dealt with in the next two subclauses. Since the conditions and demands vary significantly between utilities, these sections indicate what is recommended as mandatory (M) and what may be optional (O), according to local needs. Table 3.2 Equipment for field measurements. “M” is considered mandatory and “O” optional. M Observation sheets, pencils, clipboard and a hard cover binder. M Instructions for data collection. M Measuring tape and a compass. M Sampling devices such as a knife, saw and plastic hammer. M Plastic bags to store samples and tags to identify (bags must be watertight and strong).

0

5

10

15

20

25

40/41 50/51 60/61 70/71 80/81 90/91t (years)

Q [kg.m-1] Studnice 800 m , n=59

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M Digital camera with facility to record time and date, or camera with high speed film (Video cameras are generally more sensitive to low light than still cameras).

O Weighing device. O Flexible tape (wire) to record and store the shape of the accretion. O Proper equipment for measuring wind speed and temperature. O Bag containing all equipment. O Dictaphone (if writing at the site is difficult). The minmum requirement should be the items marked “M” in Table 3.2. Appendix B shows some examples of well-designed observation tools and field staff training.

3.3 Headquarter routine It is necessary to have management acceptance and a staff member assigned to oversee and manage the ice data collection, training of field staff, maintain and update the equipment kit, and analyse and report of the collected data. This will include: • Developing and organising yearly training programs for field staff according to utility

needs. • Maintaining and updating the equipment and procedures for data collection. • Making sure that data is collected during or after each significant icing event and

according to the instructions. • Gathering data from other sources, e.g. local people and other utilities (telecom). • Presentation of data (check for accuracy, preparation of reports and permanent storage of

data, electronic or otherwise). • Liason with the overhead line designers and other users.

4. Procedures for measurements 4.1 Introduction In order to advice the observer to make optimum measurements or observations, this clause gives some guidelines and recommendations regarding both methods and instruments. This clause also provides some additional information or recommendations related to the observation sheets in Appendix C. The procedures may be adjusted to local circumstances and requirements by each utility. Procedures may vary depending on whether the line is energised or not. Section 4.2 gives some alternatives to be applied on energised lines where it is not possible to take ice samples directly from the conductors. Safety warning:

Before any measurements are made near energised lines, all appropriate electrical safety requirements must be met and all necessary regulatory procedures followed. When observing or making actual measurements, the probability of ice shedding from other conductors should be considered, especially under galloping.

4.2 Ice measurements

4.2.1 Physical dimensions Number one priority is to measure the representative diameter (shape) of the icing.

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Physical dimensions of the ice accrual are important for three purposes: 1) wind exposed area (drag coefficient), 2) weight of ice pr. meter (can be estimated from the diameter and the ice type) and 3) if the weight has been measured, the density of ice may be calculated.

If the physical dimensions of a cross section cannot be taken by means of a centimetre scale or a ruler, it may be useful for instance to cut paper strips or small branches from a tree, equal to the smaller and greater diameter of (an elliptic) ice accretion, for later measurements. It is useful to apply any indirect method that gives as accurate representation as possible, for instance: - Make sketches – use manual scaling methods by eye. - Take photographs – use zoom lens. - Use binoculars (or better: scaled monocular for estimating geometry and dimensions). - A distance meter (range finder), or a scale fitted to a hot stick, may also be useful to find

exact dimensions.

4.2.2 Weight of ice on conductors For load calculations it is important to know the average weight of ice per meter of conductor. Choose a representative piece of ice, cut plane ends and measure the length. Then the sample can be crushed or melted and kept in a watertight container or plastic bag for weighing on site or later. If this is not possible indirect methods may be used, for instance by measuring the sag of conductors or earth wires together with ambient air temperature. The additional loads may then be calculated using a conductor design program directly if the line is out of service, or possibly corrected for current for an operating line.

4.2.3 Density When the ice load (mass/length) is given, the cross section is needed for the calculation of wind pressure. On the other hand, if the radial ice concept is used, the load must be calculated. In both cases it is necessary to know the density of the ice. When both the physical dimensions of a (representative) cross section, as described in 4.2.1, and the weight of the same sample, as described in 4.2.2, are properly taken, the density of the ice accrual can be calculated (=mass/volume), see 6.4. Density measurements should preferably be taken from the conductors or earth wires, but samples from guy wires or other nearby structures may also be valuable. Density measurements are generally not necessary for pure glaze ice since it is close to 900 kg/m3. However, if air bubbles are visible then density is lower and should be measured. Data on density are especially important for model calibrations.

4.3 Classification of ice types The structure and composition of both surface and internal of the accretion may generally identify the actual type of ice. The ice types and densities may be identified by the descriptions given in Table 4.1. The main ice types are listed on the observation sheets and the observer can tick the appropriate box. Further information regarding granular inclusions, air bubbles, adhesion or internal strength, etc. can be noted in the comments.

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Table 4.1. Classification of ice types with typical density ranges. Ice and snow type

Density (kg/m3)

Description

Glaze ice 700-900 Pure solid ice, sometimes icicles underneath the wires. The density may vary with the content of air bubbles. Very strong adhesion and difficult to knock off.

Hard rime 300-700 Homogenous structure with inclusions of air bubbles. Pennant shaped against the wind on stiff objects, more or less circular on flexible cables. Strong adhesion and more or less difficult to knock off, even with a hammer.

Soft rime 150-300 Granular structure, “feather-like” or “cauliflower-like”. Pennant shaped also on flexible wires. Can be removed by hand.

Wet snow 100-850 Various shapes and structures are possible, mainly dependent on wind speed and torsional stiffness of conductor. When the temperature is close to zero it may have a high content of liquid water, slide to bottom side of the object and slip off easily. If the temperature drops after the accretion, the adhesion strength may be very strong.

Dry snow 50-100 Very light pack of regular snow. Various shapes and structures are possible, very easy to remove by shaking of wires.

Hoar frost <100 Crystal structure (needle like). Low adhesion, can be blown off. It is important to notice that an ice accretion may be a mixture of two or more ice types, for instance soft rime, hard rime and wet snow, due to variations in the meteorological parameters during the icing event. Various shapes, densities, adhesion strengths etc. result accordingly. The density measurement (see chapter 6.4) is however the mean value of the sample.

4.4 Moulds Moulds are useful and informative for a number of reasons, in particular:

- to get exact volume - to study drag factors for combined wind and ice loadings - to study surface structure for modelling purposes, especially relating to wind impacts

on wet surfaces, including icicle formation on glaze ice. Moulds have to be taken in two steps according to recommendations in [5]: 1. Make a negative mould by putting the original ice sample in a low-temperature-cure RTV

rubber. This compound cures at -8°C in a few hours and the original can be easily removed by slitting the rubber mould.

2. The empty mould is used to cast a plastic replica by pouring in polyurethane foam. This material expands rapidly and completely fills the interstitial cavities.

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5. Other observations 5.1 General In addition to the observations summarised in the observation sheets given in chapter 6 (Appendix C), there are other types of information that are worthwhile to consider. The most important is related to topography, line configurations, extent of the icing zone, secondary effect such as galloping, etc. It is therefore important to take additional notes on: - Icing on ground wires compared to conductors. - Number of spans affected. - Variations along the affected section, especially related to changes in topography and

height above sea level. - Variations with height above ground level. - Ice accretions on bundle conductors versus single conductors. - Difference in ice accretion between conductors in the same bundle. - Icing versus line direction. - Icing on parallel or nearby lines. - Icing on other components such as structures, insulators, spacers, etc. - Icing on vertical and horisontal surfaces. - Icing on other structures or vegetation. - Galloping. - Whenever possible keep a video record of icing on conductors and galloping. All the above points should be checked before ice shedding when possible. Whenever possible pictures should be taken, because pictures can often explain much better than any written message. A few well-taken pictures can easily reveal important details of the ice. For instance, if the only ice samples are on the wires and not obtainable, a picture with a zoom lens showing the bare conductor together with the ice, will make it possible to assess the dimensions of the ice when the diameter of the conductor is known. Pictures taken at night with flash can be even clearer because of sharper contrasts. If conductors or earth wires are galloping, this should be reported separately.

5.2 Pollution (Observation sheets B5 and C3) Polluted ice on insulators reduces their electrical withstand capacity. There are many evidences of flashovers on both lines and in substations. They may even lead to outages lasting many hours or days [6]. The pollution may stem from sea salt and/or long range transported anthropogenic (“man-made”) pollution from the combustion of fossile fuels. Both phenomena are considered to be local phenomena (near the coast or industrialised areas), but it may as well occur far inland and even in mountains. If reduced insulation withstand is expected due to this, it is recommended that ice samples are taken for chemical analyses, see e.g [6, 7].

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6. Observation sheets 6.1 Basic information There are five types of observation sheets: A: Registration of weather data for the whole icing area, or part of it. B: Registration of the diameter of icing. Make one report for each transmission line. C: Measurement of weight per metre conductor, or measurement of density. D: Simplified weather observation sheet. E: Simplified ice observation sheet. Each sheet should be filled in as carefully and accurately as possible, remembering the recommendations in previous chapters. In order to complete the measurements as accurately as possible, the tools listed in Table 3.2 are necessary or recommended.

6.2 Observation sheet A: Weather data. - General information about the weather may often be obtained from the Meteorological

Service. - Use sheet A to record local observations made in the icing area. - Mark on maps or describe in words the extent of the icing affected zone. If the

affected area is great, or there are distinctive variations within the area, it is recommended to make more than one report.

- Mark on maps the iced sections of the overhead lines. Add any comments. - Further guidelines are on sheet A.

6.3 Observation sheet B: General icing data. - Use sheet B to record the diameter of icing, and other information about the accretion.

The sheet is in two parts and should be printed double sided. - Measure the diameter(s) of a representative sample for each span. - If the ice varies significantly along the span take note of the variations. - If the icing episode is extensive, it is recommended to make several reports according

to local variations or variations between different lines. - It may not be possible to complete all of this form at the site. Some information, such

as that described in parts B2 and B3 can be obtained later from the office. - The last table in part B4 is a choice for special studies. - Further guidelines are given on sheet B.

6.4 Observation sheet C: Weight, cross section and density measurement. - Cut clean and plane ends of an ice sleeve with a shape as regular as possible, e.g. 50

cm long. - Make an accurate drawing of one of the end planes (cross section) on this sheet or on

a separate sheet (put the paper over the cross section of the ice). If a drawing is not possible (e.g. due to weather conditions) indicate approximate shape by ticking the right box in part C3 on the sheet.

- Measure as accurately as possible: - the length of the sample. - the diameter of the conductor or wire.

- Measure for sections 1 and 2 (end 1 and end 2):

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- the greatest (max) diameter of the cross section (it is often elliptic). - the smallest (min) diameter of the cross section.

- Put the sample in a watertight plastic bag or box (now it may be crushed or melted). - Measure the mass (weight) on a scale or the water volume with a gauge. - Further guidelines are given on sheet C.

If the icing on the conductor is very irregular so that measurement of diameter is hard to define, it is important to select a sample which is as representative as possible for the span, to get the weight per meter of conductor.

6.5 Simplified observation sheets D and E The simplified observation sheets concentrate on the most important data if a complete measurement procedure cannot be accomplished. There are two types: D “Weather Observation Sheet” and E “Ice Observation Sheet”. They may preferably be printed double sided in a pocket book format.

6.6 Additional information In addition to completing the observation sheets, please mark the iced sections of the lines on maps or tower lists. Also remember to take photographs or videos which describe the icing event (and damage) both on the overhead lines and surrounding objects, including trees and vegetation. Systematic observations of trees have proved to be of great value for both evaluating the magnitude of loads as well as spatial distributions.

7. Icing database Although this report mainly focuses on the registration of icing data, the next step is to collect the reports and make them accessible as an information source of icing. There are many ways in which this can be done:

- File the prime sources (or copies of them). - Publish the main information in reports. - File the icing data in a computer database

See [8] for further information.

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7. REFERENCES [1] Loading and Strength of Overhead Lines International Electrotechnical

Commission. (IEC) TR 826. Second edition 1991-04.

[2] Overhead Lines – Meteorological Data for Assessing Climatic Loads. (IEC) TR 61774. First edition 1997-08.

[3] Fikke, S.M., K. Schjetne, and B.D. Evensen: “Ice Load Measurements and Design Practice”, First International Workshop on Atmospheric Icing of Structures, CRREL, New Hampshire, 1982.

[4] Popolanský F. et.al.: Ice Monitoring at Stand Studnice. Tuned Vibration Control of Overhead Line Conductors. Cigré Paper 22-105 Session 1998.

[5] Nigol, O. and P. Buchan: “Conductor Galloping Part 1 – den Hartog Mechanism”, IEEE Paper F79714-7.

[6] Cigré TF 33.04.09: “Influence of ice and snow on the flashover performance of outdoor insulators”, Electra No 187 1999, pp 91 – 111.

[7] Fikke, S.M., J.E. Hanssen and L. Rolfseng: “Long range transported pollution and conductivity of atmospheric ice on insulators”, IEEE Trans. on Power Delivery, Vol 8, No. 3, 1993, pp.1311-1321.

[8] Isaksson, S.P., A.J. Eliasson and E. Thorsteins: “Icing database. Acquisition and registration of data.” IWAIS 1998, Reykjavik 1998.

Appendices: A: Icing Processes (taken from [2]) B: Examples of observation tools C: Observation sheets

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APPENDIX A Icing processes

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The following text is taken from IEC TR 61774 [[[[2]]]] clause 2.2.1:

Atmospheric icing is a result of two main processes in the atmosphere which are named accordingly:

1. in-cloud icing and 2. precipitation icing.

The latter one occurs in several forms among which the most important are:

2.1. freezing rain 2.2. wet snow accretion 2.3. dry snow accretion

There is a third process resulting in the formation of so-called ‘hoar frost’ but this does not lead to significant ice loads on overhead lines and will not be considered further. In - cloud icing is a process where suspended, supercooled droplets in a cloud (or fog) freeze immediately upon impact on an object exposed to the airflow, for instance, a high level power line above the cloud base. The ice growth is said to be dry when the available heat transfer rate away from the object is greater than the release of the latent heat of fusion. The density of the accretion is a function of the flux of water to the surface and the temperature of the layer. The resulting accreted ice is called soft or hard rime according to the density. A typical density for soft rime is 300 kg/m3

and 700 kg/m3 for hard rime. The ice growth is said to be wet when the heat transfer rate is less than the rate of latent heat release. The growth then takes place at the melting point, resulting in a water film on the surface. The accreted ice is called glaze with a density of 900 kg/m3. Precipitation icing can occur in several forms, including freezing rain, wet and dry snow. Freezing rain comprises supercooled droplets which freeze immediately upon impact on objects. The resulting accretion is also glaze. The ambient temperature is below the freezing point. When snowflakes fall through a layer of air with temperatures slightly above the freezing point, the flakes may partly melt, become sticky and thus accrete on objects. This is called wet snow accretion. The density and the adhesion may vary widely. If the ambient temperature drops significantly below freezing after a wet layer of snow has accreted, the adhesive and mechanical strength of the layer may become very high. In exceptional cases, wet snow accretions are known to have occurred with ambient temperatures slightly below freezing. Dry snowflakes may accrete at temperatures significantly below freezing and can - under very low wind speed conditions - accumulate on objects to form a dry snow accretion.

It should be noted that the accretion on a conductor may be the result of more than one process occurring during an icing event.

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APPENDIX B Examples of ice observation tools

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Figure B.1.

Basic observation tools: Saw, plastic hammer, measuring tape, steelyard for weight measurements and clipboard with instructions and forms. See also plastic bag in fig. B.6.

Figure B.2. Bag for measuring kit, shown in figures B.1 and B.6.

Figure B.3. Bag is easy-to-carry.

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Figure B.4. Greatest diameter is 19.7 cm.

Figure B.5. A calliper may be a useful supplementary tool.

Figure B.6. After measuring diameters and length, the ice sample is collected in a special plastic bag for weighing.

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APPENDIX C Observation sheets

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COMPANY NAME OBSERVATION SHEET – A ICE ACCRETION REPORT (WEATHER DATA) (Recommended appendices: Maps and photographs).

1. ICING PERIOD (EPISODE): Ice accretion started: Ice accretion stopped:

Date: ____/___/_______ Time: ____:____ Date: ____/___/_______ Time: ____:____

Note: _______________________________________________________________________________

_______________________________________________________________________________

2. DESCRIPTION OF ICING AREA: Describe in words and/or mark on maps (or tower lists) the iced sections of the overhead lines. Record the line identification, and the tower numbers of the iced sections. The diameter of icing should be recorded on observation sheet, B, or on the maps.

3. DESCRIPTION OF THE ICING WEATHER: From the Meteorological Service it is possible to get general information about the weather. Here you can record local information, which can deviate considerably from the general. Direction of ice-keels on exposed tower parts shows the wind direction at the time of ice accretion. 3.1 Wind direction and wind speed: Direction: (Degrees or main directions)

Observ. date Time Location Wind speed Magnetic Geographic

3.2 Direction of ice-keels:

Location Line no. Structure no. Magnetic Geographic

3.3 Gustiness: ! - Normal ! - Strong ! - Extreme

3.4 Temperature: °C: _____ Observation date and time: ____/___/_______ ____:____

Location: ____________________________________________________________

3.5 Type of precipitation, mist or cloud: ___________________________________________________

Please send this report to: Company / Person __________________________________________ Address Tel: xxxxxxxx / Fax: xxxxxxxx / E-mail: xxxxxxxx Date Signature Appendices: _______________________

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COMPANY NAME OBSERVATION SHEET – B

ICE ACCRETION REPORT – GENERAL ICING DATA (Recommended appendices: Maps, tower lists and photographs).

1. ICING PERIOD (EVENT): From, date: ____/___/_______ To, date: ____/___/_______

2. TRANSMISSION LINE: kV Name: Number:

Type of conductor:

Conductor bundle (single, twin, etc.):

Devices on line (pendulum, air-flow spoilers, etc.):

Supporting structure (towers, poles, etc.):

3. LOCAL CONDITIONS:

Height above sea level (m):

Describe local conditions which may have affected the ice accretion:

4. DIAMETER OF ICE ACCRETION: If the icing event is extensive, it is recommended to make several reports, one for each line or area. In case of cylindrical shapes, it may be easier to measure the circumference of the icing (special column). Maximum and Minimum diameter refer to the same section, e.g. when the icing is elliptical or wing-shaped. If you cannot reach the conductor, or if it is energised, you can estimate the diameter (last column). Measure the diameter of a representative sample for each span.

(In the same section)

Span between Circumf. Measured diameter (cm) Estimated Date. structure no. and no. (cm) Max. Min. diam. (cm)

(If you wish, you can mark the sites of measurement on maps. The results can also be recorded there).

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In the case of wet snow icing it often slips on the conductor, so that the diameter is largest in the middle of each span. The table below may be used to describe the distribution of the icing, and find the average value.

Line identification. Structures no. 0-span cm ¼ span cm ½ span cm ¾ span cm 1-span cm

5. DESCRIPTION OF ICING: (Mark x in the appropriate box).

Type of icing: ! - Hard rime ! - Soft rime ! - Glaze ice

! - Icicles ! - Wet snow ! - Wet snow (frozen)

! - Dry snow ! - Snowy (with layers of ice)

! - Mixed ! - Hoar frost ! - Unknown

Pollution of icing: ! - Unknown ! - Salt ! - Industrial ! - Sample _________

Surface: ! - Smooth ! - Rough ! - Very rough

Shape of icing: ! - Circular ! - Elliptic ! - Wing-shaped, draw >>

! - Other, draw yourself >>

6. GALLOPING: ! - No ! - Yes (Please describe it. A video is recommended). 7. LINE DAMAGE: ! - No ! - Yes 8. COMMENTS:

Photographs of icing samples, and overall views of icing and overhead line damages are very important. Please use some kind of scale, when photographing icing samples. FURTHER DATA:

Maps: ! - Yes > _____________________________________________________________

Tower lists: ! - Yes > _____________________________________________________________

Photographs: ! - Yes > _____________________________________________________________

Videos: ! - Yes > _____________________________________________________________

Samples: ! - Yes > _____________________________________________________________ Please send this report to: Company / Person __________________________________________ Address Tel: xxxxxxxx / Fax: xxxxxxxx / E-mail: xxxxxxxx Date Signature

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COMPANY NAME OBSERVATION SHEET – C

ICE ACCRETION REPORT – WEIGHT AND DENSITY MEASUREMENTS 1. ICING PERIOD (EVENT): From, date: ____/___/_______ To, date: ____/___/_______ 2. MEASUREMENT OF DENSITY: If possible, sample should be taken from the conductor. If not, sample can be taken from some part of the line that is more accessible, e.g. a stay wire.

Mark out an ice sample, e.g. 50 cm, with as regular form as possible. Cut plane ends. Measure the sample's diameter (max/min) and/or circumference at sections 1 and 2 (at both ends), the length of the sample and the diameter of the wire. Then measure the weight of the sample. Take care to collect every bit of the sample into the weight bag. Make an accurate drawing of the cross section below or on the back of this sheet. Circumf. Diameter (cm) Length Weight D-wire

Sect. (cm) Max. Min. (cm) (g) (mm) Date and time: 1

Structure no. no. 2

Date and time: 1

Structure no. no. 2

Did rain or thaw affect the sample? ! - Yes ! - No, it was intact. 3. DESCRIPTION OF ICING IN THE SAMPLE: (Mark x in the appropriate box).

Type of icing: ! - Hard rime ! - Soft rime ! - Glaze ice

! - Icicles ! - Wet snow ! - Wet snow (frozen)

! - Dry snow ! - Snowy (with layers of ice)

! - Mixed ! - Hoar frost ! - Unknown

Pollution of icing: ! - Unknown ! - Salt ! - Industrial ! - Sample _________

Surface: ! - Smooth ! - Rough ! - Very rough

Shape of icing: ! - Circular ! - Elliptic ! - Wing-shaped, draw >>

! - Other, draw yourself >> 4. COMMENTS:

Please send this report to: Company / Person __________________________________________ Address Tel: xxxxxxxx / Fax: xxxxxxxx / E-mail: xxxxxxxx Date Signature

Cross-sections: end 1 & end 2

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WEATHER OBSERVATION SHEET - DSheet No :

Meteorological Conditions during Ice Accretion

Area : Location : Observation : Time : Date :

Starting of Event End of EventTime : Time :Date : Date :

Slow ( 0 - 5 m/s )

Moderate ( 5 - 12 m/s ) Unknown

Strong ( 12 - 20 m/s )

Very Strong ( > 20 m/s )

Unknown Clear

Foggy

Cloudy

N In-Cloud

NE Stormy

E Rain

SE Freezing Rain

S Drizzle

SW Freezing Drizzle

W Snow Wet Dry

NW Drifting snow

Unknown Unknown

Number of ice observation sheets (E) attached :

Observator name and signature

Wind Velocity Temperature

Weather

Wind Direction

°C

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ICE OBSERVATION SHEET - ESheet No :

Report of Ice on Overhead Power Lines

Line Identification : Span between Structures Structures No : No :

Type of Icing Diameter of Icing Shape of icing at same point Draw the shape around the conductor shown

Rime Conductor

Glaze MAX

Icicle MIN

Snow Wet Dry Ground Wire

Mix (to be precised) MAX

Unknown MIN

Samples / Pictures ....... Samples Yes No

Pictures Yes No

Comments (Damage, vibration, ice shedding, galloping, ...)

Incident date : Incident time :

Reference sheet (D) no :

Observator name and signature

Le CIGRÉ a apporté le plus grand soin à la réalisation de cette brochure thématique numérique afin de vous fournir une information complète et fiable. Cependant, le CIGRÉ ne pourra en aucun cas être tenu responsable des préjudices ou dommages de quelque nature que ce soit pouvant résulter d’une mauvaise utilisation des informations contenues dans cette brochure. Publié par le CIGRÉ 21, rue d’Artois FR-75 008 PARIS Tél. : +33 1 53 89 12 90 Fax : +33 1 53 89 12 99 Copyright © 2000 Tous droits de diffusion, de traduction et de reproduction réservés pour tous pays. Toute reproduction, même partielle, par quelque procédé que ce soit, est interdite sans autorisation préalable. Cette interdiction ne peut s’appliquer à l’utilisateur personne physique ayant acheté ce document pour l’impression dudit document à des fins strictement personnelles. Pour toute utilisation collective, prière de nous contacter à sales-meetings@cigre.org

The greatest care has been taken by CIGRE to produce this digital technical brochure so as to provide you with full and reliable information. However, CIGRE could in any case be held responsible for any damage resulting from any misuse of the information contained therein. Published by CIGRE 21, rue d’Artois FR-75 008 PARIS Tel : +33 1 53 89 12 90 Fax : +33 1 53 89 12 99 Copyright © 2000 All rights of circulation, translation and reproduction reserved for all countries. No part of this publication may be produced or transmitted, in any form or by any means, without prior permission of the publisher. This measure will not apply in the case of printing off of this document by any individual having purchased it for personal purposes. For any collective use, please contact us at sales-meetings@cigre.org