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Evolut ion of IEC 60826 Loading and Strength of Overhead Lines

Elias Ghannoum1

Abstract

This paper provides an overview of IEC Publication No. 60826 (old number IEC 826)

entitled Loading and Strength of Overhead lines . This publication has been a

milestone in the introduction of improved structural design criteria of overhead

transmission lines based on reliability methods. Originally published by IEC in 1991

as a technical report type II (i.e. a pre-standard phase), this document was then

extensively reviewed by CIGRI~ and IECfrC11/WG08, and a revised version of this

document is currently being circulated to National Committees of IEC/TCll for

adoption as an IEC Standard.

This IEC publication specifies loadings and strengths requirements of overhead lines

derived from reliability-based design principles. It is based on the concept whereby a

transmission line is designed as a system made of components such as supports,

foundations, conductors and insulator strings. This approach enables to coordinate the

strengths of components within the system taking into account the fact that in such a

series system, the failure of any component could lead to the loss of power

transmitting capability. It is expected that this approach should lead to an overall

economical design without undesirable mismatch between strengths of line

components.

Many improvements were introduced in the revised version of IEC 60926 (IEC 2002)

such as: dividing the document into a normative section containing all requirements

and another section consisting of a commentary to the document and technical

annexes, providing default load and strength factors when statistical data are scarce,

1 Eng., M.Sc.; Fellow IEEE, Chairman IEC/TC11/WG08, Consultant,Chairholder, Hydro-Quebec

Chair on Overhead TransmissionLines, Departmentof Civil Engineering,Universit6of Sherbrooke,

Qu6bee J1K 2R1;phone 1-514-3444127;[email protected]

59

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60 ELECTRICAL TRANSMISSION N A NEW AGE

i m pr ov e m e n t s / s im p l i f ic a t i ons t o s om e l oa d i ng r e qu i re m e n t s t ha t e x i s t e d in t he

p r e v i ous v e r s ion , bu t m a y n o t c on t r o l t he d e s i gn , et c.

T he I E C 60826 ha s a l r e a dy be e n u s e d i n m a ny na t i ona l s t a nda r ds (e x . CS A C22 . 3 , e x .

I S 802 , 2002 , CE N E L E C) a nd u t i l i ty p r a c t ic e s , a nd ha s p r ove n t o be a n e s s e n t ia l t oo l

f o r t hos e m i g r a t ing t ow a r d t he m or e e f f ic i e n t a nd e c o nom i c a l Re l i a b i l i t y Ba s e d

D e s i gn ( R BD ) c onc e p ts .

I n t r o d u c t i o n

D ur i ng t he l a s t de c a de s , t he I E C ( I n t e m a t i ona l E l e c t r o t e c hn i c a l Com m i s s i on )

C o m m i t t e e T C l l a n d C I G R E S t u d y C o m m i t t e e S C 2 2 p i o n e e r e d i m p r o v e m e n t o f

ove r he a d t r a ns m i s s i on l i ne s de s i gn c r i t e r i a a s w e l l a s t he i n t r oduc t i on o f

r e l i a b i l i t y / p r oba b i l i t ybased de s ign concept s (G hanno um , 1986) , (CIGRI~ 1990).

A m i l e s t one o f th i s e vo l u t i on oc c u r r e d in 1991 w he n t he I E C 826 doc u m e n t e n t it l e d

L oa d i ng a nd S t r eng t h o f O v e r he a d L i ne s w a s pub l i s he d (I E C, 1991). T h i s

doc u m e n t i n t r oduc e d r e l ia b i l i t y a nd p r oba b i l is t i c c onc e p t s f o r c a l c u l a t ion o f l oa d i ng

a nd s t r e ng t h r e qu i r e m e n t s f o r ove r he a d l i ne s c om pone n t s . T h i s doc um e n t w a s

publ i shed as a t echnica l repor t type 2 , i . e . a p re -s t andard document tha t wi l l be

rev ie we d in a few years for the purpose o f conver t ing i t t o an IE C s tandard .

W h e n d o c u m e n t I E C 8 2 6 w a s p u b l i s h e d , I E C a s k e d C I G R E t o p r e p a r e a p p l i c a t i o n

e xa m pl e s o f t h i s doc um e n t a nd t o s i m p l i f y i t fo r a n e ve n t ua l c onv e r s i on t o t he s t a tu s

o f a n I E C i n t e r na t i ona l s t a nda r d . T he CI G RE t a s k w a s c om pl e t e d i n 1998 a nd a

r e v i s e d / s i m p l i f i e d ve r s i on o f IE C 826 ( now I E C 60826) w a s s e n t by CI G RE S C22 t o

IE C T C l l .

T he ne w ve r s i on o f I E C 60826 ( I E C, 2002 ) i s c u rr e n t l y be i ng c i r c u l a t e d f o r vo t e (a s

o f M a y 2 0 0 2 ) t o N a t io n a l C o m m i t t e es m e m b e r s o f I E C / T C 1 1 .

T h i s pa pe r p r ov i de s a n ove r v i e w o f I E C a nd CI GRt~ w or k a nd s um m a r i z e s t he m a i n

fea tures of IE C 60826.

T h e I E C I n t e r n a t io n a l E l e c t r o t e c h n i c a l C o m m i s s io n )

F ou nde d i n 1906 , t he I n t e rna t i ona l E l e c t r o te c hn i c a l Co m m i s s i on ( I E C) i s the g l oba l

organiza t ion tha t p repares and publ i shes in te rna t iona l s t andards for a l l e l ec t r i ca l ,

e l e c t r on i c a nd r e l a t e d t e c hno l og i e s . T he m e m be r s h i p c ons i s t s o f m or e t ha n 60

pa r t i c i pa t i ng c oun t r ie s , inc l ud i ng a l l t he w or l d ' s m a j o r t r a d i ng na t i ons a nd a g r ow i ng

num be r o f i ndus t r i a li z i ng c oun t r ie s .

T he m i s s i on o f I E C i s t o p r om ot e , t h r ough i ts m e m be r s , i n t e r na ti ona l c oope r a t i on on

a l l ques t ions of e l ec t ro technica l s t andard iza t ion and re la t ed mat te r s , such as the

a s s e s s m e n t o f c on f o r m i t y to s t a nda r ds i n t he f i e ld s o f e l e c t r ic i t y , e l e c tr on i c s a nd

re la ted t echnologies .




ELECTRICAL TRANSMISSION N A NEW AGE 61

T he de f i n i ti on g ive n i n a l l I E C s t a nda rds r ea ds : " A no r m a t i ve doc um e n t , de ve l op e d

a c c o r d i ng t o c ons e ns us p r oc e du r e s , w h i c h ha s be e n a pp r ove d by t he I E C N a t i ona l

Co m m i t te e m e m b e r s o f t he r e spons i b l e c om m i t t e e i n a c c o r da nc e w i t h P a r t 1 o f t he

I S O / I E C D i r e c t i ve s a s a c om m i t t e e d r a f t f o r vo t e a nd a s a f i na l d r a f t I n t e r na t i ona l

S t a nda r d a nd w h i c h h a s be e n pub l i s he d by t he I E C Ce n t r a l O f f ic e . "

A dop t i on o f I E C s t a nda r ds by a ny c oun t r y , w he t he r i t i s a m e m be r o f t he

Co m m i s s i on o r no t , i s e n t i r e l y vo l un t a ry .

W h a t is C I G R E

CI G RE ( I n t e r na t i ona l Counc i l on L a r ge E l e c t r i c S ys t e m s ) i s a pe r m a ne n t non -

gove r nm e n t a l a nd non p r o f i t - m a k i ng I n t e r na t i ona l A s s oc i a t i on ba s e d i n F r a nc e . I t

w a s f oun de d in 1921 a nd a i m s to :

9 F a c i l i ta t e a nd de ve l op t he e xc ha nge o f e ng i ne e r i ng know l e dg e a nd

i n f o r m a t ion , be t w e e n e ng i ne e r i ng pe r s onne l a nd t e c hn i c a l s pe c i a l is t s i n a l l

c oun t r i e s a s r e ga r ds ge n e r a t ion a nd h i gh vo l t a ge t r a ns m i s s i on o f e l e c t r i c it y .

9 A dd va l ue t o t he kno w l e d ge a nd i n f o r m a t i on e xc ha n ge d by s yn t he s i z i ng s t a te -

o f - t he - a r t a nd w o r l d p r a c t i c e s .

9 M a ke m a n a ge r s , de c i s i on - m a ke r s a nd r e gu l a t o r s a w a r e o f the s yn t he s is o f

CI G RE ' s w or k , i n t he a r e a o f e l e c tr i c pow e r .

M or e s pe c i f i c a ll y , i s s ue s re l a t e d t o t he p l a nn i ng a nd ope r a t i on o f pow e r s y s t e m s , a s

w e l l a s t he de s i gn , c ons t r uc t i on , m a i n t e na nc e a nd d i s pos a l o f H V e qu i pm e n t a nd

p l a n t s a re a t the c o r e o f CI G RE ' s M i s s ion .

R e l a ti on s h ip b e t w e e n C I G R E a n d I E C

M a ny I E C pub l i c a t i ons a nd s t a nda r ds o r i g i na t e d f r om CI G RE t e c hn i c a l w or k . I t i s

ge ne r a l l y t he p r a c t i c e t ha t CI G RE de ve l ops ne w t e c hn i c a l w o r k un t il s uc h w or k

r e a c he s t e c hn i c a l c ons e ns us a nd m a t u r i t y . O nc e t h i s s t a ge i s r e a c he d , t he r e s u l t i ng

w o r k m a y b e u s e d b y I E C a s a b a s i s f or p r ep a r a t io n o f a n e w s t an d a r d o r r e v is i n g an

e x i s t i ng one .

T h e c o o p e r a ti o n b e t w e e n I E C a n d C I G R E i s o b v io u s i n t h e d e v e l o p m e n t a n d r e v i si o n

of IEC 60826 as expla in ed in th i s paper (CIGRt~ 1990) .

O b j e c ti v e s o f I E C 6 8 2 6

W he n de ve l ope d , I E C 60826 w a s o r i e n t e d t o c o r r e c t s om e i de n t i f ie d s ho r t c om i ngs o f

sa fe ty fac tor m e thod s such as (Ghann oum , 1984):

9 P os s i b l e i nc ons i s t e nc ie s a nd unba l a nc e i n st r eng t hs o f c om pone n t s

9 U nk now n r e l i a b i l i t y l e ve l ( e xc e p t by ge ne r a l i n f e r e nc e f r om e xpe r i e nc e )

9 D i f f i c u l t y t o a d j u s t ove r a l l l i ne r e l i a b i l i ty

9 D i f f i c u l t y t o de s i gn c om pos i t e s tr uc tu r e s ( s te e l a nd w oo d f o r e xa m pl e )

9 D i f f i c u l t y t o e vo l v e w i t h ne w t e c hno l og i e s

9 D i f f i c u l t y t o a d j u s t de s i gn t o loc a l c ond i t i ons




62 ELECTRICALTRANSMISSION N A NEW AGE

D i f f i c u l t y t o c o n t r o l t h e s e q u e n c e i n w h i c h l i n e c o m p o n e n t s f a i l i n c a s e a

f a il u r e e v e n t i s t ri g g e r e d ( f o r e x a m p l e : U l t i m a t e c a p a c i ty o f f o u n d a t i o n s c o u l d

b e l e s s th a t t h e s u p p o r t e d t o w e r , o r a c r i ti c a l a n g l e t o w e r m a y f a i l b e f o r e a l e s s

c r i ti c a l t a n g e n t t o w e r ) .

M a i n fe a t u r es o f I E C 6 8 2 6

Scope

I E C 6 0 8 2 6 s p e c i f i e s t h e l o a d i n g s a n d s t r e n g t h s r e q u i r e m e n t s o f o v e r h e a d l i n e s

d e r i v e d f r o m R e l i a b i l i t y B a s e d D e s i g n ( R B D ) p r i n c ip l e s . I t a ls o p r o v i d e s a

f r a m e w o r k f o r t h e p r e p a r a t i o n o f N a t i o n a l o v e r h e a d t r a n s m i s s i o n l i n e s S t a n d a r d s

u s i n g r e l i a b i l i ty c o n c e p t s a n d e m p l o y i n g p r o b a b i l i s ti c o r s e m i - p r o b a b i l i st i c m e t h o d s .

H o w e v e r , N a t i o n a l S t a n d a r d s s t il l n e e d t o e s t a b l i s h th e l o c a l c l i m a t i c d a t a fo r t h e u s e

a n d a p p l i c a t i o n o f th i s s t a n d a r d .

T h e d e s i g n c ri t e ri a i n I E C 6 0 8 2 6 , a l th o u g h i n t e n d e d t o a p p l y t o n e w l in e s , t h e y c a n

a l so b e u s e d t o a d d r e ss t h e r e l i a b i li t y r e q u i r e m e n t s f o r r e f u r b i s h m e n t a n d u p r a t i n g o f

e x i s t i n g l i n e s .

I t is n o t e d t h a t I E C 6 0 8 2 6 d o e s n o t c o v e r th e d e t a i l ed d e s i g n o f l in e c o m p o n e n t s s u c h

a s t o w e r s , f o u n d a t i o n s , c o n d u c t o r s o r i n s u l a t o r s . N e v e r t h e l e s s i t p r o v i d e s l o a d i n g a n d

s t re n g t h r e q u i r e m e n t s t h a t a l l o w a c o o r d i n a t e d a n d c o n s i s t e n t d e si g n b e t w e e n t h e se

c o m p o n e n t s .

Basis o f load-strength relation in IEC 60826

A m a j o r b r e ak t h r o u g h i n p r o b a b i l i t y m e t h o d s o c c u r r e d w h e n a r e la t i o n t h a t le a d s t o

a n a l m o s t c o n s t a n t p r o b a b i l i ty o f f a il u re , w a s u n c o v e r e d b e t w e e n l o a d a n d s t re n g t h

( G h a n n o u m 1983 1986) . Th i s r e l a t ion i s :

Q T = ( 1 0 ) R , o r

L o a d w i t h a r e t u r n p e r i o d T = S t r e n g t h m e t w i t h 9 0 p r o b a b i l i t y ( o r h a v i n g a 1 0

e x c l u s i o n l i m i t ) .

T h e a b o v e r e l a t i o n w a s f o u n d t o g i v e c o n s i s te n t r e li a b i l it y (o r p r o b a b i l i ty o f s u r v i v a l

Ps ) a lm os t equ a l to (1 - 1 /2T ) , w i th a typ ica l r ange o f (1 - 1 /T ) to ( 1 - 1 /2T ) . Th ese

r e s u lt s r e m a i n v a l i d f o r v a r i o u s d i s tr i b u t io n s o f l o a d c u r v e s Q s u c h a s e x t r e m e t y p e

( G u m b e l ) , l o g - n o r m a l a n d F r e c h e t, a s w e l l a s f o r N o r m a l a n d lo g - N o r m a l s t r e n g th R

d i s t r i b u t i o n s . R e f e r t o F i g u r e 1 f o r t h e c a s e w h e r e T = 5 0 y e a r s a n d t h e c o e f f i c i e n t o f

v a r i a t i o n ( C O V ) Q w a s v a r i e d f r o m 1 2 t o 5 0 .




ELECTRICALTRANSMISSION N A NEW AGE 63

Figure 1- Fai lure probabil i ty Pf = 1 - Ps) for various values of Q and R, for T =

50 years, Q is extrem e type I and R is Norm al.

System design

IEC 60826 design methodology is based on the concept whereby a transmission line

is designed as a system made of components such as supports, foundations,

conductors and insulator strings. This approach enables to coordinate the strengths of

components within the system taking into account the fact that in such a series

system, the failure of any component could lead to the loss of power transmitting

capability. It is expected that this approach should lead to an overall economical

design without undesirable mismatch between strengths of l ine components.

Design methodology o IEC 60826

The design methodology as per IEC60826 can be summarized in the Figure 2 note

that the activities a) and h) listed in this figure are not parts of the scope of IEC

60826):




64 ELECTRICAL TRANSMISSION1N A NEW AGE

Figure 2 Transmission line design methodology according to IEC 60826

Source o f design requirements

The design according to IEC60826 (see boxes b l, b2 and b3 in Figure 2) originates

from the follo wing requirements:

9 Relia bility: These requireme nts consist of climatic loads (wind, ice,

temperature and their combinations) and aim to provide l ines with satisfactory

service performance. Statistical tools are used to q uantify these loads.

9 Security: These requirements aim to prevent or reduce r isk of uncontrollable

or casc ading failures.

9 Safety: These requirements aim to prevent human injury.

eliability levels

Three R elia bilit y levels (I, II, III) are prov ided in IEC 60826. These levels correspon d

to return periods o f design loads of 50, 150 and 500 years. In general,

9 Level I is considered minim um for all permanent l ines

9 Level II applies to lines with voltages equal or exceeding 230 kV

9 Lev el III applies to important lines in excess of 230 kV that are a unique

source of supply.

Security requirements

Security requirements relate to behav ior of l ines once failure is init iated. They aim to

prevent un controlled propagation of failures (cascading). In such case, components





are allowed to be reach stresses very close to their ultimate limit state. It is noted that

in IEC 60826, security is a deterministic concept, while reliability is probabilistic.

Security and reliability requirements are interrelated because both tend to increase

strength of components. Security measures, if more critical than climatic loads

(reliability requirements), can also increase reliability.

Safety requirements

These are required to protect people from injury. They consist of construction and

maintenance loads. It is aimed that the probability of failure under such loads should

be very low.

Design equation general format

Load effect < Strength or,

QT < Rc or,

Load corresponding to a return period T < Characteristic strength Rc

The above equation has been expanded in the document to the form below:

YQT = ~R Rc, where,

y factor for span dispersion, default value equal to 1.0

QT load corresponding to a return period T

global strength factor equal to the product of ~S * ~N * ~Q * ~C

~s factor related to coordination of strength (sequence of failure)

factor related to number N of components

~ Q

factor related to the difference between tested and installed component

~c factor related to the statistical parameters of the characteristic strength

It is important to note that the load QT shall be the maximum along the space covered

by the line. Furthermore, not only the maximum load intensity is important, but also

its spatial coverage, as both affect design requirements and line reliability. Directional

tendencies of wind or ice loads can be taken into account if confirmed; otherwise, it

should be assumed that load direction always occurs in the most critical direction.

Loading conditions an d limit states

Limit states of strength of line components are defined for each component: a damage

limit state (serviceability) and a failure (ultimate) limit state. Each group of loading

requirements is associated with one of the limits states given below:

CONDITION LOADS STRENGTH LIMIT

STATE

Reliability

Security

Safety

climatic, ice, wind, wind + ice, with a damage limit

return period T

failure limit (torsional and longitudinal) Failure limit

construction and maintenance loads damage limit





Differences between theoretical and actual reliabilities

IEC 60826 recognizes that the actual reliability may differ from the theoretical

reliability when factors such the ones listed below are not accounted for:

9 Actual use factors of components as opposed to assuming them equal to 1.0

9 The degree of correlation between loads and strengths

9 Direction of wind speed in relation to that of the line

9 Exclusion limit of strength different from the assumed 10%

9 Number of components subjected to maximum load intensity

9 Quality control during fabrication and construction

Methods to take into account the above factors are covered in the subject document.

Use factor o f components

Use factor in IEC 60826 is defined as the ratio of the actual load (as built) to the limit

design load of a component. For tangent towers, it is virtually equal to the ratio of

actual to maximum design spans (wind or weight) and for angle towers, it also

includes the ratio of the sines of the half angles of deviation (actual to design angles).

Use factor cannot exceed 1.0 and its influence on line reliability has been covered in

the Document. The Use factor variation in overhead lines is inevitable because of the

following reasons:

9 Line components are mass fabricated

9 Components are not specifically designed for each tower location or use

9 Their design parameters reflect maximum usage along the line

9 Effective loads on line components are location dependent (span and tower

height at each location)

Globally, the use factor variation should increase reliability. However, a large

dispersion of U may be an indication of a poor optimization (e.g. not enough tangent

tower types or their parameters incorrectly selected). It is important to recognize that

the preferred sequence of failure could also be altered if the use factor is not taken

into account.

The hara cteristic streng th Rc_

IEC 60826 makes reference to the characteristic strength which is defined as the

strength value guaranteed in relevant Standards. Sometimes, it is also called the

guaranteed strength, the minimum strength, or the minimum failing load, and usually

corresponds to an exclusion limit, from 2 to 5%, with 10% being an upper practical

(and conservative) limit.

The strength distribution function is usually Normal (Gaussian). With stringent

quality control, it tends to become a Log-normal function.

The characteristic strength can thus be calculated from the following equation,

assuming it corresponds to a 10% exclusion limit:




ELECTRICALTRANSMISSION1N A NEW AGE 67

Rc = (10%) R = R ( 1 - k VR), where

k = 1,28 for Normal distribution

k = 1,08 to 1,26 for Log-normal distribution

In case the maximum intensity of load is widespread and covers a large number (N)

of structures, the strength distribution becomes that of chain or a series of N

components whose strength is controlled by the weakest. Although the original

distribution of strength can be Normal, that of the series of N structures will tend to

be an Extreme (minima) type. Correction factors are provided in order to take into

account the effect of the spatial coverage of the maximum load event on reliability

(~N factor).

Strength coe f f ic ien t ~ re la ted to sequence o f fa i lure

In IEC 60826, line components can be designed to fail (with a 90% probability) in a

preferred mode called "preferred sequence of failure". The best (or least damaging)

failure mode is the one where the consequences of the first failure on the line are

minimized. Strength factors allowing targeting a preferred sequence of failure are

provided in the document. It is generally accepted that angle towers, dead-end towers,

conductors or foundations should not fail first, thus leaving tangent towers as the one

to fail first. The following table specifies the strength factors applicable to the

strength of the component not to fail first.

COV of 0,05-0,10

R2 0,10-0,40

T a b l e 1 - V a l u e s o f ~)S

C o e f f i c i e n t o f V a r i a t i o n ( C O V ) o f R1

5% 7,5% 10% 20%

0,92 0,87 0,82 0,63

0,94 0,89

0,86

0,66

Note: in the abo ve Table R2 is the comp one nt desig ned mo re re liable than R1

W ind loads an d limitat ions o f wi nd calculat ions

Wind loads on conductors and tower structures are the source of important and

critical loading requirement for overhead transmission lines. Methods to calculate

wind forces starting with a reference wind speed are provided in the document for the

following conditions:

9 Spans between 200 m and 800 m

9 Height of supports less than 60 m

9 Altitude below 1300 m

Ground roughne ss

For the purpose of calculating wind pressure and forces, four (4) categories of ground

roughness (also called terrain types) are provided:

9 A - Flat coastal areas and deserts

9 B- Open country, cultivated fields

9 C- Numerous low height obstacles

9 D - Suburban areas





Reference w ind velocity

The reference wind velocity V considered in IEC 60826 consists of a 10 min.

average, at 10 m height, in a terrain type B. The document provides for conversion

from other wind data, having different averages or located in a different terrain

category, to the above reference value

Wind spee d design cases

High wind is combined with average minimum daily temperatures and a reduced

wind (60% of the reference value) is combined with the 50 year minimum

temperature.

Wind load model

The conversion from wind speed to forces follows the equation:

Load = k (V2 9V2), where k is the product of:

9 height factor

9 span factor

9 response factor

9 shape factor

In IEC 60826, the k factor in the above equation takes the form of:

Wind force = A Cx Gc ( V2 9 V2 ), with

Gc = the combined wind factor dependent on spans, height and terrain roughness

category

Cx = Drag (or force) coefficient

9 is the air mass per unit volume = 1,225 kg/m3 (this is a default value, but

adjustments of p to different temperatures and altitudes are provided.

A similar equation provides for calculation of wind forces applied to various types of

transmission structures such as those made of angle sections, round pipe sections or

steel poles. Drag coefficients are also provided for these tower types and take into

account the compactness (or solidity ratio) of the windward face to reflect the

shielding of wind on the leeward face.

Validation of lEC6 826 wind model

IEC wind load calculations (or wind model) have been satisfactorily validated. The

results of some experimental validations can be found in "Houle, Hardy, and

Ghannoum (1991)" and "EPRI, TR-104480".

Icing types

Ice accretion on conductors and structures are the source of important loads, and often

control the design in many northern countries. The document covers three types of ice

accretion: precipitation icing, wet snow, and in-cloud icing. Methods to calculate

design icing are provided and cover a range of cases with various availabilities of

statistical data.





I c e l oad i ng c as e s

Once design ice thickness or weight per unit length of conductors has been

statistically defined, this value is used in the following loading cases:

9 Uniform ice formation

9 Non uniform ice (longitudinal unbalanced icing, with all phases in a span

subjected to the same unbalanced conditions)

9 Torsional condition (unbalanced icing conditions occurring in opposite

longitudinal directions thus creating a torsional moment on the structure)

Com bi ne d i c e loads w i t h w i nd

The presence of wind during or after icing episodes requires special loading cases and

combinations of ice and wind loads in order to account for their combined effects.

The calculation of combined forces due wind on ice covered conductors are provided

in the document and take into account: the ice thickness or ice weight per unit length

of conductor, ice density, wind speed during icing, and drag coefficient of ice covered

conductor.

Three combinations of: ice, wind speed during icing, ice density/drag coefficient are

provided for in the document and consist of combining an extreme value of one

variable (such as the 50 year value) with average values of the other variables.

Con s t ruc tion and main tena nce loads sa fe ty requi remen ts )

The loading conditions provided in the IEC document supplement National

Regulations and safety codes. They are focused on reducing the risk of injuries to

personnel working during construction and maintenance of the lines. These

requirements should result in a very high reliability (risk of failure practically nil).

The approach to deal with such loads is deterministic and consists of applying

overload factors of 1.5 to 2.0 in order to insure such a high reliability. These loads are

not usually combined with climatic loads, because construction and maintenance

operations are not commonly undertaken during strong weather events.

For example, loads during erection of supports are simulated by designing each

support point for twice the static loads at sagging conditions. Under some conditions,

and under controlled construction operations, the factor of 2.0 could be reduced to

1.5.

Secur i t y r e la ted loads

As explained earlier, these loads are intended to prevent cascading or uncontrollable

failures. Minimum requirements are specified as follows:

9 A broken phase load (torsional load) is applied on any one phase or g/w

attachment point, and is equivalent to the Residual Static load (RSL)

calculated with bare conductors at average temperatures.

9 A longitudinal load is specified, equivalent to a simulated fictitious ice load

equal to the conductor weight applied on one side of the tower.




70 ELECTRICALTRA N SMI SSIO N N A N EW A G E

F o r l i n e s t h a t r e q u i r e a h i g h e r s e c u r i t y l e v e l , a d d i t i o n a l s e c u r i t y m e a s u r e s c a n b e

c o n s i d e r e d s u c h a s : I n c r e a s in g t h e n u m b e r o f p o in t s w h e r e t h e R S L i s a p p l i ed ,

C o n s i d e r i n g t h e R S L i n c o n j u n c t i o n w i t h s o m e c l i m a t i c l o a d , a n d / o r i n s e r t i n g a n t i -

c a s c a d i n g t o w e r s .

L i m i t s t a te s o f c onduc tor s and g r ound w i r e s

A n e x a m p l e o f l i m i t s ta te s o f s tr e n g th o f c o n d u c t o rs a n d g r o u n d w i r e s i s p r o v i d e d i n

T a b l e 2 .

T a b l e 2 - D a m a g e a n d f a i l u r e l i m i t s o f c o n d u c t o r s a n d g r o u n d w i r e s

T y p e s D a m a g e l i m i t F a i l u r e

l i m i t

l o w e s t o f :

- V i b r a t i o n l i m i t , o r

A l l t y p e s - t h e i n f r in g e m e n t o f c r it ic a l c l e a r a n c e s d e f i n e d b y U l t i m a t e t e n s il e

app r opr i a t e r egu l a t i on s , o r s t r e s s ( r up t u r e )

- 7 5 % o f t h e c h a r a c t e r i s ti c s t r e n g t h o r r a te d t e n s i l e

s t re n g t h ( t y p i c a l r a n g e i n 7 0 % t o 8 0 % )

L i m i t s t a t e s o f i n t er fac e c om p one n t s

A n e x a m p l e o f s tr e n g th l i m i t st at e s o f i n te r f a c e c o m p o n e n t s i s p r o v i d e d i n

E r r o r

N o t a v a l i d b o o k m a r k s e l f- r e fe r e n c e .

T a b l e 3 - D a m a g e a n d f a i lu r e l i m i t o f i n te r fa c e c o m p o n e n t s

T y p e o f in t e r f a c e

c o m p o n e n t s

C a b l e c o n n e c to r s : D e a d -

e n d a n d ju n c t i o n f i tt i n g s

a n d S u s p e n s i o n f i t t i n g s

I n s u l a t o r s ( p o r c e l a i n a n d

g l a s s )

H a r d w a r e

D a m a g e U m i t2 F a i l u r e

l i m i t

R u p t u r e

n a c c e p t a b l e p e r m a n e n t

d e f o r m a t i o n o r s l ip p a g e

7 0 % s t r e n g th r a t i n g o r b r o k e n

s h e d ( g l a s s o n l y )

C r i t ic a l 3 p e r m a n e n t d e f o r m a t i o n

R u p t u r e o f p i n , c a p ,

c e m e n t o r s h e d

R u p t u r e o f h a r d w a r e

o r s h e a r o f b o l t s

l im i la r it ie s be t we e n I E C 6 826 a nd A S CE 74

B o t h d o c u m e n t s r e p r e s e n t

9 S i m i l a r d e s i g n p h i l o s o p h y o f r e l i a b i li t y , s e c u r i t y a n d s a f e t y c r i te r i a .

9 B o t h t r e a t l o a d a n d s t r e n g th a s r a n d o m v a r i a b l e s

9 T h e d e s i g n e q u a t i o n ( lo a d - s t r e n g t h r e l a ti o n ) i s s i m i l a r i n f o r m a t

9 R e l i a b i l i t y l e v e l s a r e e x p r e s s e d i n r e t u r n p e r i o d s o f d e s i g n l o a d s

2 Norm ally, hardware is designed in a mann er to reduce or eliminate wear. Should wear be expected

because of point to point contact, i t should be considered in the design. In such case, the dam age limit

becomes: exceeding the excepted wear.

3 De fined as the state wh ere the hardw are canno t be easily taken apart.




ELECTRICALTRANSM ISSION N A NEW AGE 71

9 T h e 5 0 y e a r l o a d i s c o n s i d e r e d a r e f e r e n c e m i n i m u m l o a d r e q u i r e m e n t

9 W i n d m o d e l i n b o t h d o c u m e n t s a re s i m i l ar l y b a s e d o n t h e D a v e n p o r t m o d e l

9 B o t h a p p r o a c h e s c o o r d i n a t e t h e s t re n g t h o f c o m p o n e n t s

D i f fe r e n c es b e t w e e n I E C a n d A S C E

9 T h e A S C E i s c o m p o n e n t b a s e d, w h i l e I E C is s y s t e m (l in e ) b a s e d

9 R e l i a b i l i t i e s i n A S C E a r e n o w r e f e r r e d t o a s r e l a t i v e re l i a b i l it i e s , b u t i n b o t h

I E C a n d A S C E , r e t ur n p e r i o d s o f l o a d s a r e u s e d .

9 L o a d d a t a u s e d i n IE C r e l a t e t o t h e s p a c e o r t h e a r e a c o v e r e d b y t h e li n e w h i l e

A S C E i s b a s ed o n p o i n t d a ta

9 F a s t e s t m i l e s ta t is ti c s i n e a r l y v e r s io n o f A S C E ( n o w 3 s e c o n d g u s t ) a re u s e d

v e r s u s 1 0 m i n a v e r a g e w i n d s p e e d i n I E C .

9 C o m b i n a t i o n s o f w i n d a n d i c e l o a d s a r e s o m e h o w d i f f e r e n t

N e w v e r s i o n o f IE C 6 0 8 2 1i

B a s e d o n C I G R E r e c o m m e n d a t io n s a n d I E C / T C 1 1 / W G 0 8 r e v i e w , a r e v i se d v e rs i o n

o f th i s d o c u m e n t i s b e i n g c i r c u la t e d to N a t i o n a l C o m m i t t e e s f o r a d o p t i o n a s an I E C

s t an d a r d ( r e f e r t o I E C 6 0 8 2 6 , 2 0 0 2 ) . T h e m a i n c h a n g e s i n th i s d o c u m e n t c o m p a r e d t o

t h e e a r l ie r 1 9 9 1 v e r s i o n a r e s u m m a r i z e d h e r e a f t e r:

S t r uc tu r e o f r e v is e d I E C 826

T h e d o c u m e n t i n n o w s p l i t i n t w o p a r t s : T h e f i r s t p a r t c o n t a i n s d e s i g n r e q u i r e m e n t s

a n d i s N o r m a t i v e . E f f o r t s w e r e m a d e i n t h i s p a r t t o d e l e t e r e q u i r e m e n t s t h a t d o n o t

c o n t r o l t h e d e s i g n . T h e s e c o n d p a r t i s i n f o r m a t i v e a n d c o n t a i n s c o m m e n t a r y ,

e x p l a n a t i o n s a n d u s e f u l a n n e x e s r e l a t e d t o t h e f i r s t p a r t . T h i s r e v i s e d d o c u m e n t , t a k e s

i n t o a c c o u n t t h e c o m m e n t s r e c e i v e d o n t h e 1 9 91 v e r s i o n f r o m C I G R I ~ a n d N a t i o n a l

C o m m i t t e e s o f I E C .

M ain f ea tu res in t roduced in the rev i sed 1EC 6 826

9 I n t h e r e v i s e d v e rs i o n , u s e r s c a n e i th e r d e te r m i n e t h e l o a d c o r r e s p o n d i n g t o a

r e t u r n p e r i o d T f r o m s t a t i s t i c a l d a t a o r a p p l y l o a d f a c t o r s t o t h e 5 0 y e a r l o a d s

i f t h e l a t t e r a r e p r o v i d e d i n N a t i o n a l S t a n d a rd s .

9 T h e v a l u e s o f s tr e n g th f a c t o r d ~ a n d R c a r e n o w p r o v i d e d f o r t w o a s s u m p t i o n s

o f s t r e n g t h d i s t r i b u t i o n s : N o r m a l a n d l o g - N o r m a l d i s t r i b u t i o n s .

9 A i r d e n s i t y c o r r e c ti o n f a c to r s a r e n o w p r o v i d e d f o r t h e p u r p o s e o f c a l c u l a ti n g

w i n d f o r c e s o n t o w e r s a n d c o n d u c t o r s f o r v a r i o u s c o m b i n a t i o n s o f a l t i t u d e s

a n d t e m p e r a t u r e s .

9 T h e g u s t re s p o n s e f a ct o r s G c h a v e b e e n s i m p l i f ie d a n d a ll t h e v a l u e s o f G c

c a n n o w b e o b t a i n e d g r a p h i c a l l y f ro m o n e F i g u re .

9 T h e s p a n e f f e c t o n w i n d l o a d w a s p r e v i o u s l y in c l u d e d i n th e g u s t r e s p o n s e

f a c t o r , b u t i t i s n o w p r o v i d e d s e p a r a t e l y a s s p a n f a c t o r G L

9 I n c a s e s t a ti s ti c a l d a t a f o r s t r e n g th o f c o m p o n e n t s a r e n o t a v a i l a b l e , d e fa u l t

v a l ue s o f C O V ' s a r e p r o v id e d .




72 ELECTRICAL TRANSMISSION N A NEW AGE

onc lus ion

T hi s pa pe r s um m a r i z e s t he ba c kg r ound , ke y f e a t u r e s o f a nd t he e vo l u t i on o f

doc um e n t I E C 60826 t ha t p r ov i de s fo r : a ) a de si gn m e t ho do l o gy ba s e d on r e l i a b i l it y

c onc e p t s , b ) m o r e c ons i s t e n t de s i gn t ha t t ar ge ts a m i n i m u m r e l i a b i li t y , a nd c ) a vo i ds

m i s m a t c h be t w e e n l i ne c om p one n t s .

L oc a l w e a t he r c ond i t ions a r e t a ke n in t o a c c oun t du r i ng t he de s i gn p r oc e s s , a nd t oo l s

a r e p r ov i de d i n o r d e r t o i nc r e a s e r e l i a b i l i t y a nd s e c u r i t y i f w a r r a n t e d e i t he r by t he

i m po r t a nc e o f t he l i ne o r by l oc a l c ond i ti ons .

T h i s doc um e n t s hou l d p r ov i de f o r m or e e c onom i c a l de s i gn f o r a g i ve n t a r ge t

r e l i a b i l i t y c om pa r e d t o s a f e t y f a c to r m e t hods o r , i nve r s e l y , shou l d p r ov i de f o r a

h i ghe r r e l i a b i l i ty f o r g i ve n l i m i t l oa ds .

T he I E C 60826 ha s be e n i n t e g r a t e d t o m a ny i n t e r na ti ona l s t a nda r ds (e x . IS 802 , C S A

C22 . 3 , 2002 , C E N E L E C a nd u t i l i t y p r a c t ic e s a nd r e p r e s e n ts a m a j o r c on t ri bu t ion t o

t he i n t e r na t iona l t re nd i n m i g r a t i ng t ow a r d r e l i a b i l i t y ba s e d de s i gn c onc e p t s i n

ove r he a d l i ne de s i gn .

References

I E C/ 60826 - E d . 2 . 0 ( 1991 ) : L oa d i ng a nd s t r e ng t h o f ove r he a d t r a ns m i s s i on l i ne s ,

June 1991.

H ou l e , S . , H a r dy , C . , a nd G ha nn oum , E . ( 1991) , S t a ti c a nd D yna m i c T e s t i ng o f

T r a ns m i s s i on L i ne s S ub j e c t e d t o Re a l W i nd Cond i t i ons , C I G RI ~ S ym pos i um on

Co m p a c t O ve r he a d L i ne s , L e n i ng r a d , J une 1991.

E l e c t r i c P ow e r Re s e a r c h I n s t it u t e Re po r t , Cond uc t o r W i nd L o a d i ng - Re s u l t s o f

E P RI F i e l d V a l i da t i on s t ud ie s E P R I , T R- 104480 .

G H A N N O U M , E . , O r a w s k i , G . (1986 ), Re l i a b i l it y Ba s e d D e s i gn o f T r a ns m i s s ion

L i ne s A c c o r d i ng t o Re c e n t A dva nc e s by I E C a nd CI G R]~ , I n t e r na t iona l S ym p os i um

of P r ob a b i l i s ti c D e s i gn o f T r a ns m i s s ion L i ne s , T o r on t o , J une 1986 .

G H A N N O U M , E . (1983 ) , P r oba b i l i st i c D e s i gn o f T r a ns m i s s i on L i ne s - P a r t I :

P r ob a b i l i t y Ca l c u l a t ions a nd S t r uc tu r a l Re l i a b i l it y , I E E E / P E S 1983 W i n t e r M e e t i ng ,

N e w - Y o r k .

G H A N N O U M , E . ( 1983 ), P r oba b i l is t ic D e s i gn o f T r a ns m i s s i on L i ne s - P a r t I I

D e s i gn Cr i t e r ia Cor r e s pond i ng t o a T a r ge t Re l i a b i l i ty , I E E E / P E S 1983 W i n t e r

M e e t i n g , N e w - Y o r k .

G H A N N O U M , E ., (1 9 8 4 ) I m p r o v i n g T r a n s m i s si o n L i n e D e s i g n b y U s i n g

Re l i a b i l i t y T e c hn i que s , I E E E / P E S 1984 W i n t e r M e e t ing , D a l l a s .




ELECTRICALTRANSMISSION1N A NEW AGE 73

A S CE M a nu a l 74 ( 1991 ), G u i de l i ne s f o r E l e c t ri c a l T r a ns m i s s ion L i ne S t r uc t u ra l

L oa d i ng , 1991

I E C 60826 E d . 3 , ( 2002 ), D e s i gn c r i te r i a o f o ve r he a d t r a ns m i s s ion l i ne s ,

11 / 165A / CD V , M a r c h 2002 .

CI GRI ~ W G 0 6 , ( 1990 ), L o a d i ng a nd s tr e ng th o f ov e r he a d t r a ns m i s s i on l ine s ,

E lec t ra No. 129, Marc h 1990 .

[IEEE] Evolution of IEC 60826 Loading and Strength of Overhead Lines []

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