CIGRE 179

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

Task Force 22.06.01

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).

G UIDELINES FOR FIELD MEASU RE M E NT OF ICE LOA DIN GS ON OVERHEAD PO WER LINE CONDUCTORS

TABLE OF CONTENTS

Sum mary 1. INTRODUCTION

2

2. FIELD DATA 2.1. Need for field data 2.2. Direct measurements related to other sources of ice load information 2.3. Icing processes 2.4. Aspects regarding collection of representative ice samples 2.5. Instrumentation of transmission lines 2.6. Ice load observations on (not instrumentaed) lines 2.7. Effects of climatic variations

3 3

3. PREPARATIONS BY UTILITIES AND FIELD STAFF 3.1. Information and training of field staff 3.2. Equipment for the field staff 3.3. Headquarter routine

8 8 8 9

3 4 4 4 6 7

4. PROCEDURES FOR MEASUREMENTS 4.1. Introduction 4.2. Ice measurements 4.2.1. Physical dimensions 4.2.2. Weight of ice on conductors 4.2.3. Density 4.3. Classification of ice types 4.4. Moulds

9 9 9 9 10 10 10 11

5. OTHER OBSERVATIONS 5.1. General 5.2. Pollution (Observation sheets B5 and C3)

12 12 12

6. OBSERVATION SHEETS 6.1. Basic information 6.2. Observation sheet A : W eather data 6.3. Observation sheet B : Generalicing data 6.4. Observation sheet C : W eight, cross section and density measurement 6.5. Simplified observation sheets D and E 6.6. Additionalinformation

13 13 13 13 13 14 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

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

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 unenergised 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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Icing data

Icing models

General meteorological 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.

-1 Q [kg.m ]

25

Studnice 800 m ,n=59 20

15

10

5

0 40/41

50/51

60/61

70/71 80/81 t(years)

90/91

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 M M M M

Observation sheets, pencils, clipboard and a hard cover binder. Instructions for data collection. Measuring tape and a compass. Sampling devices such as a knife, saw and plastic hammer. Plastic bags to store samples and tags to identify (bags must be watertight and strong).

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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). Weighing device. Flexible tape (wire) to record and store the shape of the accretion. Proper equipment for measuring wind speed and temperature. Bag containing all equipment. 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

>

Type of icing:

1-span cm

! - Sample _________

! - Other, draw yourself >> ! - No

6.

GALLOPING:

7.

LINE DAMAGE: ! - No

8.

COMMENTS:

! - Yes (Please describe it. A video is recommended). ! - Yes

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.

Cross-sections: end 1 & end 2

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.

Sect. 1

Date and time: Structure no.

no.

Date and time: Structure no.

Diameter (cm) Max. Min.

Length (cm)

Weight (g)

2

no.

2 ! - Yes

DESCRIPTION OF ICING IN THE SAMPLE:

! - No, it was intact. (Mark x in the appropriate box).

! - 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

Surface:

! - Smooth

! - Rough

! - Very rough

Shape of icing:

! - Circular

! - Elliptic

! - Wing-shaped, draw >>

Type of icing:

D-wire (mm)

1

Did rain or thaw affect the sample? 3.

Circumf. (cm)

! - Sample _________

! - Other, draw yourself >> 4.

COMMENTS:

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

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WEATHER OBSERVATION SHEET -

D

Sheet No :

Meteorological Conditions during Ice Accretion Area :

Location :

Observation : Time : Date :

Starting of Event

End of Event

Time : Date :

Time : Date :

Wind Velocity

Temperature

Slow

( 0 - 5 m/s )

Moderate

( 5 - 12 m/s )

Strong

( 12 - 20 m/s )

Very Strong

(

°C Unknown

Weather

> 20 m/s )

Unknown

Clear Foggy

Wind Direction

Cloudy

N

In-Cloud

NE

Stormy

E

Rain

SE

Freezing Rain

S

Drizzle

SW

Freezing Drizzle

W

Snow

NW

Drifting snow

Unknown

Unknown

Number of ice observation sheets (E) attached : Observator name and signature

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Wet

Dry

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ICE OBSERVATION SHEET -

E

Sheet No :

Report of Ice on Overhead Power Lines Line Identification :

Span between Structures Structures No :

Type of Icing

Diameter of Icing at same point

Rime

No :

Shape of icing Draw the shape around the conductor shown

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

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