SIST IEC/TR 61774:1999

SLOVENSKI STANDARD
SIST IEC/TR 61774:1999
01-november-1999
Nadzemni vodi - Meteorološki podatki za ocenjevanje klimatskih obtežb
Overhead lines - Meteorological data for assessing climatic loads
Lignes aériennes - Données météorologiques pour calculer les charges climatiques
Ta slovenski standard je istoveten z: IEC/TS 61774
ICS:
19.040 Preskušanje v zvezi z Environmental testing
okoljem
29.240.20 Daljnovodi Power transmission and
distribution lines
SIST IEC/TR 61774:1999 en,fr
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST IEC/TR 61774:1999

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SIST IEC/TR 61774:1999
RAPPORT
CEI
TECHNIQUE – TYPE 2
IEC
61774
TECHNICAL
Première édition
REPORT – TYPE 2
First edition
1997-08
Lignes aériennes –
Données météorologiques pour calculer
les charges climatiques
Overhead lines –
Meteorological data for assessing
climatic loads
 IEC 1997 Droits de reproduction réservés  Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in
utilisée sous quelque forme que ce soit et par aucun any form or by any means, electronic or mechanical,
procédé, électronique ou mécanique, y compris la photo- including photocopying and microfilm, without permission in
copie et les microfilms, sans l'accord écrit de l'éditeur. writing from the publisher.
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Telefax: +41 22 919 0300 e-mail: inmail@iec.ch IEC web site http: //www.iec.ch
CODE PRIX
Commission Electrotechnique Internationale
X
PRICE CODE
International Electrotechnical Commission
Pour prix, voir catalogue en vigueur
For price, see current catalogue

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 3 –
CONTENTS
Page
FOREWORD . 7
Clause
1 Scope. 11
2 Normative reference. 11
3 General. 11
3.1 Meteorological data. 11
3.2 Ice loads. 11
3.2.1 Icing processes. 11
3.2.2 Icing measurements. 15
3.2.3 Icing models. 17
3.3 Galloping (informative). 17
3.4 Strategy for employing data and models . 17
4 General meteorological data . 23
4.1 Introduction. 23
4.2 Weather parameters as required by IEC 60826 . 23
4.2.1 General. 23
4.2.2 Wind and thermal loads. 23
4.2.3 Weather elements required by icing models. 23
4.3 Availability of meteorological data for overhead line design. 25
4.4 Recommended procedures. 25
5 Ice load measurements. 27
5.1 Introduction. 27
5.2 Standard methods and recommended options for ice load measurements
associated with overhead line conductors. 29
5.2.1 General. 29
5.2.2 Consideration for ice load measurements . 29
5.2.3 Minimum recommendations. 31
5.2.4 Rods for additional investigations . 33
5.2.5 Characteristics of sites. 35
5.2.6 Standard procedures for measuring ice load on simple rods . 35
5.2.7 Training of observers . 37
5.3 Test spans. 37
5.4 Recommended procedures. 39
6 Icing models. 39
6.1 Introduction. 39
6.2 Types of icing models . 39
6.2.1 Empirical and deterministic icing models . 39
6.2.2 Climatological data used in icing models . 41
6.2.3 Application of icing models . 41

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 5 –
Page
Figure 1 – Strategy flow chart . 21
Table 1 – Ice measurement parameters. 29
Annexes (informatives)
A Overview of weather parameters required by IEC 60826. 45
B Overview of meteorological terms, data handling programs and forecasting models . 49
B.1  Weather elements and weather parameters . 49
B.2  General observation procedures . 53
B.3  Meteorological forecasting models . 57
C Examples of construction of ice load measuring rods and applicability to various
types of icing . 59
C.1  Glaze caused by freezing rain. 59
C.2  In-cloud icing including hard rime and soft rime . 59
C.3  Wet snow accretion. 59
C.4  Dry snow accretion. 59
D Examples of test spans. 67
E Examples of icing models . 69
E.1  Glaze ice. 69
E.2  Rime ice. 69
E.3  Wet snow . 71
E.4  Glaze ice, rime ice and wet snow . 71
E.5  Rime ice and wet snow . 73
F Evaluation of icing models . 75
F.1  Introduction . 75
F.2  Glaze ice . 75
F.3  Rime ice . 77
F.4  Wet snow . 77
F.5  Relevance of local weather data . 77
F.6  Availability of information on icing models . 79
G Basic icing model concepts. 85
H Bibliography. 89
J Bibliography for further reading. 91

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
_________
OVERHEAD LINES –
METEOROLOGICAL DATA FOR ASSESSING
CLIMATIC LOADS
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards.
In exceptional circumstances, a technical committee may propose the publication of a technical
report of one of the following types:
type 1, when the required support cannot be obtained for the publication of an
International Standard, despite repeated efforts;
type 2, when the subject is still under technical development or where for any other reason
there is the future but no immediate possibility of an agreement on an International
Standard;
type 3, when a technical committee has collected data of a different kind from that which is
normally published as an International Standard, for example "state of art".
Technical reports of types 1 and 2 are subject to review within three years of publication to
decide whether they can be transformed into International Standards.
Technical reports of type 3 do not necessarily have to be reviewed until the data they provide
are considered to be no longer valid or useful.

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 9 –
IEC 61774, which is a technical report of type 2, has been prepared by IEC technical
committee 11: Overhead lines.
The text of this technical report is based on the following documents:
Committee draft Report on voting
11/115/CDV 11/125/RVC
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.
This document is being issued in the Technical Report (type 2) series of publication (according
to subclause G.3.2.2 of the ISO/IEC Directives) as a "prospective standard for provisional
application" in the field of climatic load databases because there is an urgent need for
guidance on how standards in this field should be used to meet an identified need.
This document is not to be regarded as an "International Standard". It is proposed for
provisional application so that information and experience of its use in practice may be
gathered. Comments on the content of this document should be sent to the IEC Central Office.
A review of this Technical Report (type 2) will be carried out not later than three years after its
publication with the options of: extension for another three years, conversion into an
International Standard, or withdrawal.
Annexes A to J are for information only.

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 11 –
OVERHEAD LINES –
METEOROLOGICAL DATA FOR ASSESSING
CLIMATIC LOADS
1 Scope
This Technical Report (type 2) aims at providing advice on methods for developing climatic
load databases.This is necessary for the implementation of IEC 60826 which provides the
framework for National Standards on overhead line design. However, for its practical use, it is
required that design engineers acquire and utilise climatic data, since sufficient information is
often not available in existing building codes and standards. In particular there is a lack of
information on ice loads.
The objective of this report is met by:
a) reporting on the availability and proper use of climatic data
b) recommending simple standardized measurement techniques
c) reviewing icing models for computing ice loads.
The details of each of the foregoing aspects of overhead line design loads are presented in the
following clauses. Clause 3 describes the framework – or strategy – linking these separate
aspects together.
2 Normative reference
The following normative document contains provisions which, through reference in this text,
constitute provisions of this Technical Report. At the time of publication, the edition indicated
was valid. All normative documents are subject to revision, and and parties to agreements
based on this Technical Report are encouraged to investigate the possibility of applying the
most recent edition of the normative document indicated below. Members of IEC and ISO
maintain registers of currently valid International Standards.
IEC 60826: 1991, Loading and strength of overhead transmission lines
3 General
3.1 Meteorological data
The purpose of clause 4 is to provide an introduction to the meteorological data referred to
in IEC 60826, the weather elements and parameters from which they are derived, and an
explanation of some of the terminologies used by meteorologists. Information about the
availability of meteorological data for overhead line design obtained from questionnaires sent
to several utilities and national meteorological institutions is presented in 4.3. In 4.4
recommendations concerning procedures for obtaining the data most appropriate to IEC 60826
are given.
3.2 Ice loads
3.2.1 Icing processes
Atmospheric icing is a complex phenomenon which can take a number of forms. It is essential
both for the specification of measuring instruments and icing models that the distinguishing
features of these different forms are recognized.

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SIST IEC/TR 61774:1999
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Atmospheric icing is a result of two main processes in the atmosphere which are named
accordingly:
a) in-cloud icing,
b) precipitation icing.
The latter one occurs in several forms among which the most important are:
1) freezing rain,
2) wet snow accretion,
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
3
is called soft or hard rime according to the density. A typical density for soft rime is 300 kg/m
3
and 700 kg/m 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
3
surface. The accreted ice is called glaze with a density of 900 kg/m .
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 snow flakes 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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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 15 –
3.2.2 Icing measurements
Clause 5 deals with ice load measurements. This is a complex problem requiring careful
interpretation of the results if they are to be applied reliably to overhead line design. Such
interpretations require an understanding of the different types of icing and their associated
meteorological conditions, the physical processes associated with ice accretion and their
interaction with the mechanical characteristics of the conductor system. There is a range of
conductor and overhead line parameters which should ideally be employed (conductor
diameter, stranding, torsional stiffness, height above ground, etc) and a range of icing and
meteorological parameters (ice weight, density, shape, precipitation, droplet size, wind speed,
air temperature, etc) that should ideally be measured at a test site. The cost of a test site is
often high and decisions have to be made between the merits of using basic measurement
systems or more expensive test spans.
3.2.2.1 Basic icing measurements
To obtain icing data covering a range of locations – comparable to meteorological stations –
basic, relatively inexpensive techniques that can be replicated have to be defined. Such
techniques should allow measurements to be made so that they can be related to the ice loads
which would be experienced by a real line at the measurement site. These techniques may
also provide a means for assessing the comparative exposure to icing at the site, relative to
other sites.
By defining certain minimum common requirements for the basic standard rigs, comparisons
can be made between measurements recorded at different sites in different areas or countries.
This will assist in validating the measurements, increase the confidence in the data and allow a
better understanding of conductor icing in relation to location, climate and meteorology. In
order to achieve this, minimum standard instrumentation and measurement requirements are
also specified. Both manual and automatic measurement systems are described because, in
some locations, manual data collection is either not practicable or may be more expensive.
In addition to the minimum requirements for the basic rigs, optional additional facilities and
measurements are described. These can improve the quality of the data and make it more
readily related to the specific lines under consideration.
3.2.2.2 Test spans
Test spans are an advantageous way of obtaining wind and ice load data as they more closely
represent real overhead lines. Because test spans are expensive to build and maintain they are
generally restricted to a few sites. Sometimes they are formed by building a few spans of
unenergised line on wood poles. They may also be provided by instrumenting or making
detailed observations of spans of in-service lines.
Organizations in a number of countries have operated test lines of varying complexity,
reflecting different objectives and subject to constraints imposed by cost, manpower and
location. It is not proposed to define a standard test span; a list of both "purpose-built" test
spans and "real-line" test spans is provided in annex D, giving some information on their
construction, location, instrumentation, etc.
However, it is strongly recommended that at each new or continuing test site, an ice rack
meeting at least the minimum requirements, as introduced in 5.2.3, should be erected. By
means of comparisons between the ice rack data and the test line data, transfer functions can
be constructed to make allowance for the differences in, for example, torsional stiffness or
diameter of the conductor and its height above ground.

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SIST IEC/TR 61774:1999
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3.2.3 Icing models
The use of icing models to provide ice load data is obviously attractive. Some icing models use
relatively simple approaches and are restricted to a particular type of icing, while others start
from more fundamental equations and can address a range of icing types. A review of some
icing models is given in clause 6. Ice accretion involves complex processes in fluid mechanics
and thermodynamics. Empirical data are usually required for certain aspects of the theory, for
example the relationship between accretion density and windspeed, or the heat transfer
characteristics of a surface.
Icing models may be used both to estimate ice loads for defined conditions and to compute
icing statistics from a historical database of meteorological conditions. In most cases, the
precise information required as input to icing models is not available from such database. For
example, droplet size or snow intensity must be estimated from the available data.
Icing models enable transfer functions to be determined from ice measurements on rigs or test
spans. These transfer functions enable ice loads to be estimated for lines employing
conductors of different diameters, different mechanical characteristics and at sites with
different intensities of icing conditions. In this way, the models and measurements interact with
each other to improve the quality of such extrapolations and predictions.
3.3 Galloping (informative)
In addition to ice loads on conductors, ice accretion may also lead to a wind-induced oscillation
of the conductors known as galloping. This phenomenon, which is quite distinct from aeolian
vibration or sub-conductor oscillation, is a low frequency, mainly vertical oscillation of high
amplitude. Frequencies range up to about 1 Hz and amplitudes usually range from a few
metres to the sag of the span. The air temperatures are usually only a few degrees above or
below zero and the wind speed may range between 5 m/s and 25 m/s. The ice accretion
produces a conductor profile which generates both aerodynamic lift and moment. These
characteristics may be such that the conductor becomes aerodynamically unstable and gallops.
The start of the instability may often be observed in the middle of the span, and the intensity is
mainly influenced by the wind speed, the wind direction, the torsional stiffness of the conductor
and whether it is a single conductor or a bundle; in the case of bundles, other aspects such as
the tower and hardware characteristics may also be important.
This icing-related phenomenon is mentioned for completeness, but no further consideration is
given to it in this report.
3.4 Strategy for employing data and models
Figure 1 shows the general strategy for linking together the three aspects described in 3.2, 3.3
and 3.4 to provide the input data to IEC 60826. In the figure they are represented by the top
three boxes and are the resources ideally available to the overhead line engineer.
A simple case exists if the overhead line is not subject to icing and extensive data on extreme
wind speeds are available from the general meteorological database. Then the requirements of
IEC 60826 are met without modification, except for the transfer of data from their sites of origin
to the site of the overhead line.

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 19 –
The situation is far more complicated as soon as icing occurs. However, a second level of
simplification would exist if an icing model or empirical equation of sufficient reliability for use
with the general meteorological database was available: ice rack data would not then be
necessary. On the other hand, if ice rack data were available to a sufficient extent, historically
and geographically, there would be no need for icing models. In general, however, data
acquired through the procedures indicated in figure 1 will be required.
The historical meteorological and icing data (top left and right boxes) need to be transferred
from sites of origin to the site of the overhead line (local data). Theoretical icing data are then
generated from the local meteorological data, using the icing model (top middle box). Since
droplet size and liquid water content are not directly available from the historical meteorological
data, they have to be evaluated from the other historical data before the calculation of
icing data.
A comparison between the theoretical and measured icing data is needed to ensure that the
experimental results can be correctly interpreted and that the theoretical models are realistic,
thereby improving the quality of the transfer functions derived from each. This is indicated by a
feedback from the "comparison box" to the icing model.
If the comparison turns out to be acceptable, the resulting icing data have to be further
modified by taking conductor and span data into account. The final icing data can then be
statistically processed to give the design data on ice load. In addition, together with the wind
data, the wind force on iced conductors is obtained and can be statistically processed.

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 21 –
Measured
General Icing
icing data
meteorological data model
Use transfer functions
Use transfer
to convert to local
functions to convert
meteorological to local icing data
data
Evaluate
liquid water
content and
Calculate local
droplet size
icing data
Compare calculated and measured icing data.
If the 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 1 – Strategy flow chart

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SIST IEC/TR 61774:1999
61774 © IEC:1997 – 23 –
4 General meteorological data
4.1 Introduction
In view of the need for general meteorological data in overhead power line design, it is the
purpose of this clause to clarify the present situation and to outline methods for a more
efficient way of accessing the data needed. Meteorology is a science strongly dependent on
evolution in computer capacity, and the users should play an active role in taking advantage
of this trend.
In 4.2 a review is given on the specifications of IEC 60826. Annex A gives a more detailed
survey.
Based on questionnaires to overhea
...