Study Committee B2 Technical Advisory Group B2-AG-06
CIGRÉ B2-AG-06 Seminar Bangkok
Tutorial Conductor Fatigue Louis Cloutier Cloutier, Convenor André Leblond, Secretary CIGRÉ WG B2.30 "Engineering Guidelines Relating to Fatigue Endurance Capability of Conductor/Clamp Systems" February F b 28 28, 2011 © CIGRÉ 2011
Outline of Presentation
Introduction Examples p of Conductor Fatigue g Typical Conductor Configurations Some Characteristics of Suspension Clamps CIGRE Technical Brochures Prediction of Aeolian Vibration Amplitudes
Field Measurements of Conductor Vibrations Analytical Representation of the Fatigue Phenomenon
Conductor Profile During Aeolian Vibrations
Laboratory Fatigue Tests – Resonant Type Test Benches Fatigue Endurance Data
Vibration Measurement Analysis Case Study Conductor and Clamp Types Lacking Fatigue Data Conclusion
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Introduction (I)
Aeolian vibrations :
Small vibration amplitudes exceeding rarely the conductor diameter; frequency range : 3 to 150 Hz; winds : 1 to 7 m/s
May lead to fatigue failure of conductor strands at suspension clamps
Such failures are caused by dynamic stresses resulting from reverse bending
Other wind-induced conductor motions such as wakeinduced oscillations and galloping may also be responsible for fatigue conductor strand failures
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Introduction (II)
Natural vibrations are not sinusoidal but show beats Examples :
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Examples of Conductor Fatigue (I)
Conductor fatigue occurs when wind-induced vibration is not controlled
Fretting fatigue has long been recognized as being the cause of strand failures in outer as well as inner layers of the conductors
Steel core can fail by overheating after aluminum layers are separated
Interstrand microslip amplitude increases, small cracks are generated t d and d some propagate t up to t complete l t strand t d failures f il
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Examples of Conductor Fatigue (II)
Strand failures occur mainly at suspension p clamps p where such singular conditions are created To a lesser extent, a similar phenomenon can occur at damper damper, marker or spacer clamps Early detection of conductor failure or risk i k off ffailure il ((attentiveness tt ti to t early warnings) Wear and failure of conductor strands due to spacer clamp loosening Fatigue usually takes many years to become apparent
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Typical Conductor Configurations (I)
An important component of an overhead power line
The conductor cost is up to about 40% of total capital investment
The conductor size is chosen to suit electrical and mechanical requirements The most common conductor type is ACSR (Aluminum Conductor Steel Reinforced) The ratio of steel to aluminum areas vary widely
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Typical Conductor Configurations (II) Some Special Conductors
Trapezoidal
Z-shaped compact
Self-damping
Expanded
Ri River crossing i conductor d t
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Some Characteristics of Suspension Clamps (I)
Most of the conductor fatigue test results refer to those obtained bt i d when h th the conductor d t is supported in a short metallic clamp The ideal profile of the clamp body follows the natural curvature of the conductor The ends of the clamp body and the keeper must be rounded to avoid indenting the conductor The clamp should be able to rotate in a longitudinal vertical plane to accommodate asymmetrical t i l lloads d 2011-02-28
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Some Characteristics of Suspension Clamps (II) Some Other Suspension Clamps
Armor grip A i suspension i (AGS)
Elastomeric bushing with cage of preformed rods
Metal clamp with elastomeric insert Special river crossing clamp
Long saddle to reduce contact stress
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CIGRE Technical Brochures (I) This TB covers a state of the art review on the following aspects of the problem :
Fretting behaviour in stranded conductor
Determination of fatigue endurance capability
Inner conductor mechanics
Assessment of vibration severity on actual lines
Evaluation of conductor residual life
This TB covers the determination of possible damage and ways to predict remaining life of conductors as well as new methods to t t conductor/clamp test d t / l systems t 2011-02-28
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CIGRE Technical Brochures (II)
This TB is a complement to TB 332, which was a state of the art review : Meant to be a reference for the practicing line engineer in the application of the latest technology Reviews the available design tools to achieve engineering solutions Identifies the inherent gaps in their application li ti Gives the engineer a better comprehension of the two related phenomena Fatigue of conductors
Aeolian vibrations
This TB includes a review of those design g tools and g gives the transmission line engineer the limits to their application 2011-02-28
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Prediction of Aeolian Vibration Amplitudes
Many utilities have their own design rules (for number of dampers) based on past experience Vibration severity can also be measured on existing lines A useful analytical approach is the "Energy Balance Principle" (EBP) Principle The EBP leads to an estimate of conductor vibration amplitude based on equating the energy input from the wind with the energy absorption (damping) of the conductor and dampers The EBP can also be used for the direct design of the damping system for a new line The estimate of the expected vibratory motion from EBP is considered an upper bound and is therefore a safe value
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Conditions can also be assessed through measurements on existing lines Study Committee B2 - Technical Advisory Group B2-AG-06
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Conductor Profile During Aeolian Vibrations
Parameters describing conductor vibration include: Bending amplitude Yb, Free loop amplitude ymax Bending angle β, Wave length λ and Loop length ℓ This representation applies to metal clamps, not to elastomer lined clamps 2011-02-28
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Field Measurements of Conductor Vibrations (I)
Several methods to measure the vibration intensity of a conductor have been employed The bending amplitude Yb method finallyy comes out as the most practical It measures the differential displacement of the conductor at 89 point of contact with the clamp p mm from the last p The reverse bending amplitude was presented as an alternative to permit the installation of the vibration recorder directly onto the conductor The bending amplitude method must be properly interpreted when cushioned clamps are used Recommended by IEEE in 1966 (also in the 2007 revision) and CIGRE SC22 WG04 1979 and SC22 WG11 TF02 1995 2011-02-28
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Field Measurements of Conductor Vibrations (II)
Ontario Hydro Recorder
Vibrec 400
HILDA
Ribe LVR
TVM 90
Pavica
Scolar III 2011-02-28
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Analytical a yt ca Representation ep ese tat o o of tthe e Fatigue at gue Phenomenon e o e o ((I))
σa =
(
Ea d p 2
4 e − px − 1 + px
)
Yb
H p= EI
An idealized bending stress in the top-most outer-layer strand (in the plane of the last point of contact) is calculated from the bending amplitude (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 of contact between the clamp and the conductor 2011-02-28
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Analytical a yt ca Representation ep ese tat o o of tthe e Fatigue at gue Phenomenon e o e o ((II))
σ a = π d Ea
m fymax EI
The idealized bending stress can be derived from the free loop amplitude, ymax, which is the vibration parameter often measured in indoor test spans Ea: Young’s 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) g of individual wires in the cable ((N·mm2) EI: sum of flexural rigidities 2011-02-28
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Analytical a yt ca Representation ep ese tat o o of tthe e Fatigue at gue Phenomenon e o e o ((III))
Fatigue of conductors is due to microslip movements of wires inducing fretting fatigue
Contact areas between round strands are elliptical Fretting and microslip occur in these contact areas Fatigue cracks develop out of these contact areas The knowledge on fatigue performance of conductors mostly relies on results of laboratory tests made on conductors in fixed short metallic clamps
The phenomenon is complex and its exact modelling has yet to be completed
It is not possible at the moment to determine the fatigue endurance of a conductor alone
There is a wide diversityy of design g and g geometry y of conductors and supports 2011-02-28
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Laboratory Fatigue Tests – Resonant Type Test Benches Pneumatic tensioning system Suspension clamp
Dynamometer Amplitude measuring system
End clamp
Rubber dampers
Turnbuckle
Wire break detection Slider 2m
5.5 deg.
Vibrator Active length : 7 m
2m
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Constant amplitude excitation Measurement of the bending g amplitude Yb and/or the free loop amplitude ymax Most tests done with conductors supported in short metallic clamps Clamps usually held in a fixed position on the test bench
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Fatigue Endurance Data Data* (I)
The results of fatigue tests ultimately lead to the presentation of a fatigue (S (S-N) N) curve Note scatter in the data The endurance limit is determined at 500 megacycles Idealized bending stress relative to Yb vs megacycles to failure Endurance limits 22.5 MPa for single-layer ACSR
8.5 MPa for multi-layer ACSR
*R f EPRI Orange *Ref.: O Book B k
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Fatigue Endurance Data (II) Estimated bending amplitude endurance limits
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Vibration Measurement Analysis (I) Rule of Thumb Approach to Interpreting Fatigue Data (IEEE)
Widely used set of empirical criteria (“Guide for Aeolian Vibration Field Measurements of Overhead Conductors”, IEEE P1368, 1368 200 2007)) The bending amplitude may exceed the endurance limit during no more than 5% of total cycles No more than 1% of total cycles may exceed 1.5 time the endurance limit No cycle may exceed 2 times the fatigue endurance limit
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Vibration Measurement Analysis (II) Multi-Layer ACSR Fatigue Endurance Data
Statistical analysis S-N curves without wire f il failure Average
95% p probability y of survival
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Vibration Measurement Analysis (III)
Based on Cumulative d damage th theory (Mi (Miner’s ’ rule) Total damage D at several stress levels σi cumulates linearly: D = Σ ni/Ni
Failure is predicted when D = Σ ni/Ni =1 1
The accuracy of the resulting estimate of lifetime is between 50% and d 200% 2011-02-28
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Case Study (I) Report evaluated on 02/05/2002 Transmission line Voltage (kV) Conductor type Location of measurement Terrain Span length (m) Recorder (Type and No.) Remarks
Circuit 3002 315 ACSR Pheasant Tower 313 facing Tower 312 Flat 348 4 348.4 PAVICA n° 5P02 Installation date : November 21, 2001 @ 0°C
First measurement at Last measurement at Duration of one measurement (s) Measurement cycle time (s)
21/11/2001 18:00 24/02/2002 18:00 10 900
Total duration of measurement (s) Factor for extrapolation to one year Total number of measurements taken
91200 345.789 9120
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Case Study (II) Fatigue Endurance Limit Approach 12
PAVICA Fatigue endurance limit
PS stress rellative to Y b, (MP Pa)
10
8
6
4
2
0 0
10
20
30
40
50
Frequency, (Hz)
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Case Study (III) Cumulative Damage Approach 12
5% S-N curve 50% S-N curve 95% S-N curve Accumulated stress curve per year
PS stress rela ative to Y b, (MP Pa)
10
8
6
4
2
0 0.01
0.1
1
10
100
1000
10000
100000 1000000
N = Accumulated megacycles per year
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Case Study (IV)
Evaluation of remaining lif ti lifetime, D D=1 1 There is a 86% probability that the remaining lifetime exceeds 20 years
1200
Remaining lifetime e, (years)
1400
1000 800 600 400 200 0 0
10
20
30
40
50
60
70
80
90
Probability of survival, (%)
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100
Case Study (V) 65
Assessment of the mostt damaging d i frequencies Helpful for choosing the right damping system
55 50 45
Frequency, (Hz))
60
40 35 30 25 20 15 10 5 0 0
4
8
12
16
20
24
28
Damage share, (%)
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Conductor and Clamp Types Lacking Fatigue Data
The extrapolation of fatigue data available to other types of conductors or to different types of support is not recommended
Bending amplitude method is valid only for armored or unarmored conductors fitted with solid metal-to-metal clamps
Not valid for cushioned clamps (armored or unarmored)
Little test data for conductors except ACSR and aluminum alloys
Some data for ACSR conductors with armor rods
There is a need for more published data on conductor fatigue
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Some Important Recent Contributions
Guide for Aeolian Vibration Field Measurements of Overhead Conductors,, IEEE P1368, 3 , 2007 (a ( revision of IEEE 1966 Report) Transmission Line Reference Book, Wind Induced p 3,, Conductor Motion,, Second Ed. EPRI 2007 ((Chapter Fatigue of Overhead Conductors), a revision of the 1979 “Orange Book” Fatigue g Endurance Capability p y of Conductor/Clamp p Systems – Update of Present Knowledge, CIGRE TF B2.11.07, TB No. 332, October 2007 Engineering g g Guidelines Relating g to Fatigue g Endurance Capability of Conductor/Clamp Systems, CIGRE WG B2.30, TB No. 429, October 2010
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Conclusion
Fatigue endurance capability of conductors is a very useful parameter at the design stage as well as for a maintenance program
Aeolian vibrations and conductor fatigue are both highly complex phenomena
So far, design tools proposed are a good example of the engineering g g approach pp to solve a complex p p problem Adequate determination of the fatigue characteristics of a conductor/clamp system is very important in the design of a line Acceptable level of conductor vibrations
Determination of safe design tensions
Future work is needed to better understand the importance of many other parameters
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CIGRE WG B2.30 B2 30
Members : L. L Cloutier (Convenor) (Convenor), A A. Leblond (Secretary) (Secretary), U U. Cosmai, P. Dulhunty, M. Ervik, D.G. Havard, D. Hearnshaw, H.J. Krispin, M. Landeira, P. Mouchard, K. Papailiou, D. Sunkle, B. Wareing
Corresponding p g Members : J.A. Araújo, j , H. Argasinska, g , J.M. Asselin, O. Cournil, G. Diana, K. Halsan, C.B. Rawlins, R. Stephen, P. Timbrell
Associated Experts : T. Alderton, J. Duxbury, A. Goel, C. Hardy, A. Laneville, A. Manenti Diana, S. Pichot, T. Seppä, P. Van Dyke
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Speaker's Speaker s Contact Information
André Leblond, Ph.D., Eng. Tel: 1 1-514-879-4100 514 879 4100 ext. 5734 Fax: 1-514-879-4855 E-Mail:
[email protected] Address : 85, rue Ste-Catherine Ouest, 2nd floor Montreal Quebec Montreal, H2X 3P4 CANADA
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Thank you !
QUESTIONS
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