IEC TS 61400-9:2025
(Main)Wind energy generation systems - Part 9: Probabilistic design measures for wind turbines
Wind energy generation systems - Part 9: Probabilistic design measures for wind turbines
IEC TS 61400-9:2025 sets out minimum requirements to the use of probabilistic design measures in order to ensure the structural and mechanical integrity of wind turbines. The document is based on the general approach in ISO 2394, which also forms the basis for IEC 61400-1. In 61400-1, the design verification approach is based on deterministic design using safety factors. However, edition 4 of IEC 61400-1:2019 opens for introduction of probabilistic design in an informative annex specifying requirements to the calibration of structural material safety factors and structural design assisted by testing. IEC 61400-1 is the governing standard. This document provides appropriate methodologies and requirements for full probabilistic design by taking into account specific uncertainties on not only material properties but also on environmental conditions, design models and the degree of validation. This document also provides provisions for semi-probabilistic design, including reliability-based calibration of partial safety factors and assessment of existing wind turbines. The probabilistic methods in this document are formulated generically and can be applied to structural and mechanical failure modes where a limit state equation can be formulated.
General Information
Standards Content (Sample)
IEC TS 61400-9 ®
Edition 1.0 2025-07
TECHNICAL
SPECIFICATION
Wind energy generation systems -
Part 9: Probabilistic design measures for wind turbines
ICS 27.180 ISBN 978-2-8327-0494-3
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CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviated terms . 7
3.1 Terms and definitions. 7
3.2 Symbols and abbreviated terms . 10
3.2.1 Symbols . 10
3.2.2 Abbreviated terms . 12
4 Principal elements . 13
4.1 General . 13
4.2 Minimum reliability level and component classes . 14
4.3 Limit states . 14
4.4 Data validity . 15
5 Uncertainty representation and modelling . 16
5.1 General . 16
5.1.1 Uncertainties . 16
5.1.2 Types of uncertainty . 16
5.1.3 Interpretation of probability and treatment of uncertainty . 16
5.1.4 Probabilistic model . 17
5.1.5 Uncertainties for wind turbines. 18
5.2 External condition uncertainty modelling . 19
5.2.1 General . 19
5.2.2 Wind conditions . 19
5.2.3 Normal wind conditions . 20
5.2.4 Other conditions . 21
5.2.5 Electrical network conditions . 22
5.3 Load uncertainty modelling . 23
5.3.1 General . 23
5.3.2 Aeroelastic model . 23
5.3.3 Extreme loads . 24
5.3.4 Fatigue loads . 25
5.4 Structural resistance uncertainty modelling . 25
5.4.1 General . 25
5.4.2 Geometrical properties . 25
5.4.3 Material properties . 25
5.4.4 Resistance models . 26
5.4.5 Fatigue strength and damage accumulation . 26
6 Performance modelling . 26
6.1 General . 26
6.2 Structural performance of primary structures. 26
6.2.1 General . 26
6.2.2 Load performance calibration for ultimate limit states . 27
6.2.3 Evaluation of serviceability limit states. 30
6.3 Performance of primary mechanical and electrical components . 30
6.3.1 General . 30
6.3.2 Requirements for mechanical components . 31
6.3.3 Serviceability limit states for mechanical components . 31
6.3.4 Requirements for electrical components and control and protection
systems . 31
6.4 Robustness . 32
7 Assessment of reliability . 32
7.1 Overview . 32
7.1.1 General . 32
7.1.2 Reliability measures . 32
7.1.4 Accuracy requirements . 34
7.1.5 Sensitivity analysis . 34
7.2 Reliability-based method . 35
7.2.1 General . 35
7.2.2 Probability of failure for extreme design situations . 35
7.2.3 Probability of failure for fatigue design situations . 35
7.2.4 Updating probability of failure using test or inspection data . 36
7.3 Semi-probabilistic method . 36
7.3.1 General . 36
7.3.2 Representative and characteristic values . 36
7.3.3 Partial factor method for extreme and fatigue design situations . 37
7.3.4 Reliability-based calibration of partial safety factors . 37
8 Site suitability analysis . 37
8.1 General approach and scope . 37
8.2 Reliability models for site suitability analysis . 38
8.2.1 General . 38
8.2.2 Load models for site suitability assessment . 39
8.2.3 Resistance model for site suitability assessment . 40
8.2.4 Structural performance on site specific conditions . 41
8.3 Site specific uncertainty modelling . 41
8.3.1 General . 41
8.3.2 Quantification of site-specific uncertainties . 45
8.4 Reliability assessment . 46
Annex A (informative) Uncertainty quantification . 47
A.1 General . 47
A.2 Bayesian methods . 47
A.2.1 General . 47
A.2.2 Closed form solutions for parameter estimation . 48
A.2.3 Exact inference for continuous parameters . 51
A.2.4 Sampling-based inference . 51
A.2.5 Exact inference for discretized parameters . 51
A.3 Maximum likelihood . 51
A.4 Model uncertainties . 52
A.4.1 General . 52
A.4.2 Example: Model uncertainty quantification . 54
Annex B (informative) Inverse FORM . 63
Annex C (informative) Example calculations for reliability assessment . 67
C.1 General . 67
C.2 Ultimate limit state . 67
C.2.1 Design equation. 67
C.2.2 Limit state equation . 67
C.2.3 Reliability assessment . 68
C.2.4 Direct reliability-based design . 69
C.2.5 Reliability-based calibration of partial safety factors . 69
C.2.6 Assess the accuracy of the computation and perform sensitivity studies . 70
C.3 Fatigue limit state . 71
C.3.1 General . 71
C.3.2 Limit state equation . 72
C.3.3
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