38874892 fatigue endurance present knowledge

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David G. Havard, Ph.D., P.Eng.Havard Engineering Inc.

FATIGUE OF CONDUCTORS A SUMMARY OF PRESENT KNOWLEDGE

January 12, 2010

Conductors and Accessories WG Meeting

Disney Contemporary Resort

Orlando, FL


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OUTLINE OF PRESENTATION

z Examples of conductor fatigue

z Conductor types

z Clamp types

z Fretting behaviour in stranded conductors

z

Design toolsz Aeolian vibration

z Assessment of vibration severity on actual lines

z Determination of fatigue endurance capabilityz Examples of conductor fatigue data

z Evaluation of conductor residual life

z Conductor and clamp types lacking fatigue data


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EXAMPLE OF CONDUCTOR FATIGUE

Fatigue failure of a conductor

next to a metal clamp

Conductor fatigue occurswhen wind inducedvibration is not controlled

Fatigue damage occurs

most often next to thesuspension clamp

Fatigue usually takes many

years to become apparentSteel core can fail byoverheating after aluminum

layers are separated


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EXAMPLE OF FATIGUE DAMAGE

Conductor fatigue damage

visible at a clamp due toaeolian vibration after fiveyears service

Showing the conductor after

removing the clamp Damage locations are atboth ends of the keeper

Includes damage in thesecond layer


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TYPICAL CONDUCTORCONFIGURATIONS

z Conductors comprise layers of strands wound in alternatedirections around a central "king" wire

z

The conductor size is chosen to suit electrical and mechanicalrequirements

z The conductor cost is up to about 40% of total capital investment.

z The most common conductor type is ACSR (Aluminum ConductorSteel Reinforced)

z The ratio of steel to aluminum areas vary widely


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SOME SPECIAL CONDUCTORS

z Trapezoidal

z

Z-shaped compactz Self-damping

z Expanded

z

Optical Ground Wirez TP conductor


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COMMON CONDUCTOR MATERIALS

z Core:Mainly galvanized steel (sometimes greased)

Some aluminized steelAluminum alloy 6201-T6Composite

z

Outer layers:Electrical grade aluminum (high conductivity, lowstrength)

Aluminum alloy (higher strength, minor loss of

conductivity)Annealed aluminum (ACSS) (low tensile and fatigue

strengths)


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SOME CHARACTERISTICS

METAL SUSPENSION CLAMPSz The ideal profile of the

clamp body follows thenatural curvature of theconductor

z The ends of the clamp

body and the keeper mustbe rounded to avoidindenting the conductor

z

The clamp incorporates apivot either below, aboveor at the conductor axis toallow rotation in the plane

of the conductor


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OTHER SUSPENSION CLAMPS

z Armor grip suspension (AGS)

Elastomeric bushing withcage of preformed rods

z Metal clamp with elastomericinsert

z

Special river crossing clamp Long saddle to reduce

contact stress


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CONTACT AREAS BETWEEN

ROUND STRANDSz Fatigue of conductors is due to

microslip movements of wiresinducing fretting fatigue

z The phenomenon is complex andits exact modelling has yet to be

completedz Fatigue of conductors is due to microslip movements of wires

z Contact areas between round strands are elliptical

z Fretting and microslip occur in these contact areasz Fatigue cracks develop out of these contact areas

z Fatigue cracks can occur on top and on bottom of the strand

in the second layer


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CONTACT AREAS BETWEENTRAPEZOIDAL STRANDS

z The knowledge on fatigue performance of conductors mostlyrelies on results of laboratory tests made on conductors in fixed

short metallic clampsz It is not possible at the moment to determine the fatigue

endurance of a conductor alone

z There is a wide diversity of design and geometry of conductors

and supports

z Contact areas between trapezoidalstrands are diamond shaped

z Stress levels are lower betweentrapezoidal strands

z Poorly formed trap wire can have

small contact areas and higherstresses


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DESIGN TOOLS: AEOLIAN VIBRATIONS

AND CONDUCTOR FATIGUEz There is no analytical solution that will

predict fatigue of conductor-clamp systems

due to the complex fatigue process and thevariety of conductors and clamps

z Approximate engineering solutions havebeen developed and serve as reliable designtools

z When applied correctly, they lead to anacceptable level of control of the vibration to

avoid fatiguez The CIGR report includes a review of those

design tools and gives to the transmissionline engineer a clear indication of the limits to

their application


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PREDICTION OF AEOLIAN

VIBRATION AMPLITUDESz Many utilities have their own design rules (for number of

dampers) based on past experience

z Vibration severity can also be measured on existing linesz A useful analytical approach is the "Energy Balance Principle

(EBP)

z

The EBP leads to an estimate of conductor vibration amplitudebased on equating the energy input from the wind with theenergy absorption (damping) of the conductor and dampers

z The EBP can also be used for the direct design of the damping

system for a new linez The estimate of the expected vibratory motion from EBP is

considered an upper bound and is therefor a safe value


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LIMITATIONS ON THE USE OF EBP

"The strains predicted by the different researchers exhibitconsiderable variability. Nevertheless analytical methodsbased on the EBP and shaker-based technology can provide auseful tool for use in design of damping systems for theprotection of single conductors against aeolian vibrations. It

should be used with circumspection and be supplemented byreferences to field experience. Greater accuracy can beobtained by evaluating damper dissipation on laboratory spanrather than on the shaker"

Ref: "Modelling of aeolian vibrations of a single conductorplus damper: assessment of technology "

CIGR TF B2.11.01, Electra, No 223, December 2005, pp.28-36


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CONDUCTOR PROFILE DURING

AEOLIAN VIBRATION

Parameters describing conductor vibration include:

Bending amplitude Yb, Free loop amplitude ymaxBending angle , Wave length and Loop length

This representation applies to metal clamps, not toelastomer lined clamps


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The bending amplitude Yb is the most practical field measurement :

z The peak to peak displacement of the conductor at 89 mm (3.5

inch) from the last point of contact with the clampz Recommended by IEEE in 1966 (also in the 2007 revision IEEE

P1368)

z

Recommended in CIGR SC22 WG04 1979 and SC22 WG11 TF021995

MEASUREMENT OF

CONDUCTOR MOTIONS


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MEASUREMENT OFCONDUCTOR MOTIONS

Pavica

Ontario Hydro Recorder Vibrec 400ALCOA Scolar III

Vibration recorders sample conductor vibration fora few seconds every 15 minutes

Each record is summarized as the maximum peakto peak amplitude and the average frequency

The records are stored for subsequent analysis


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An idealized bending stress in the top-most outer-layer strand in the

plane of the last point of contact) is calculated from the bendingamplitude (Poffenberger-Swart formula)

Ea: modulus of elasticity of outer wire material (N/mm2)

d: diameter of outer layer wire (mm)H: conductor tension at average temperature during test period (N)

EI: sum of flexural rigidities of individual wires in the cable (N mm2)

x: distance from the point of measurement to the last point ofcontact between the clamp and the conductor.

ANALYTICAL REPRESENTATION

OF THE FATIGUE PHENOMENON

( ) bpxaa Ypxe pdE += 142

EI

H

p=


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The same idealized bending stress can be derived from the free loopamplitude, ymax, which is the vibration parameter often measuredin indoor test spans

Ea: Youngs modulus for the outer-layer strand material (N/mm2)

d: diameter of outer layer wire (mm)f: frequency of the motion (Hz)

m: conductor mass per unit length (kg/m)

EI: sum of flexural rigidities of individual wires in the cable (N.mm2

)

maxaa fyEI

m

Ed=

ANALYTICAL REPRESENTATION OF

THE FATIGUE PHENOMENON


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LABORATORY FATIGUE TESTSRESONANT TYPE TEST BENCHES

Pneumatic tensioning system

Slider

DynamometerAmplitude measuring system

Rubber dampers

Wire break detection

Vibrator

Active length : 7 m2 m 2 m

Suspension clamp

End clamp

Turnbuckle

5.5(

Constant amplitude excitation Measurement of the bending

amplitude Yb and/or the freeloop amplitude ymax Most tests with conductorssupported in short metallic

clamps Clamps usually held in a fixedposition on the test bench


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z The results of fatigue tests ultimatelylead to the presentation of a fatigue

(S-N) curvez Note scatter in the data

z The endurance limit is determined at

500 megacyclesz Idealized bending stress at conductor

surface vs megacycles to failure

z Endurance limits

22.5 MPa for single-layer ACSR 8.5 MPa for multi-layer ACSR

FATIGUE ENDURANCE DATA


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FATIGUE OF TWO LAYER ACSR

CONDUCTORS


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FATIGUE OF THREE LAYER ACSR

CONDUCTORS


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FATIGUE OF ALDREY AND 6201

ALUMINUM ALLOY CONDUCTORS


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CONDUCTOR ENDURANCE LIMITS

(IN METAL CLAMPS)CONDUCTOR TYPE ENDURANCE LIMIT

a ksi fymax in/sec

ALL ALUMINUM 3.19 5.04ALL 5005 ALLOY 3.19 5.04

ALL ALDREY or 6201 2.18 3.43

ACSR (Except 7/1) 3.19 4.65

ACSR (7/1) 3.19 5.87

COPPER (Cu) 5.08 3.39

COPPERWELD (Cw) 5.08 4.61

6 Cu/1 Cw 5.08 3.66

2 Cu/1 Cw 5.08 3.82

EHS Steel (Galv) 27.85 15.16

EHs Steel (Aluminized) 19.58 10.71

ALUMOWELD 19.58 10.87


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z Based on Cumulativedamage theory (Miners

rule)z Total damage D at several

stress levels icumulateslinearly:

D = ni/Niz Failure is predicted when

D =

ni/Ni=1z The accuracy of the

resulting estimate oflifetime is between 50%

and 200%

EVALUATION OF CONDUCTOR

RESIDUAL LIFE (CIGR)


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RULE OF THUMB APPROACH TO

INTERPRETING FATIGUE DATA (IEEE)

z Widely used set of empirical criteria (Guide forAeolian Vibration Field Measurements ofOverhead Conductors, IEEE P1368, 2007)

z The bending amplitude may exceed the endurance

limit during no more than 5% of total cyclesz No more than 1% of total cycles may exceed 1.5

time the endurance limit

z No cycle may exceed 2 times the endurance limit


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CONDUCTOR AND CLAMP TYPES

LACKING FATIGUE DATAz The extrapolation of fatigue data available to other types

of conductors or to different types of support is notrecommended

z Bending amplitude method is valid only for armored orunarmored conductors fitted with solid metal-to-metal

clampsz Not valid for cushioned clamps (armored or unarmored)

z Little test data for conductors except ACSR andaluminum alloys

z Some data for ACSR conductors with armor rods

z There is a need for more published data on conductorfatigue


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CONDUCTOR FATIGUE - SOURCES

Engineering Guidelines Relating to Fatigue Endurance

Capability of Conductor/Clamp Systems, CIGRTechnical Brochure No. 332, 2007EPRI Transmission Line Reference Book: WindInduced

Conductor Motion, (The Orange Book), Second Edition,Chapter 3 Fatigue of Conductors, 2006

Guide for Aeolian Vibration Field Measurements ofOverhead Conductors, IEEE P1368, 2007 (a revision of

IEEE 1966 Report)


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SPEAKERS CONTACT INFORMATION

z President: Havard Engineering Inc.

z

Tel: 1-905-273-3076z Fax: 1-905-273-5402

z E-Mail: [email protected]

z Web Page: www.havardengineering .comz Address: 3142 Lindenlea Drive

Mississauga, Ontario

Canada, L5C 2C2
mailto:[email protected]://www.havardengineering/http://www.havardengineering/mailto:[email protected]

38874892 fatigue endurance present knowledge

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