Advances in the Treatment of the Influence of Meteorological Variables on Wind Turbine Power Curves in Warranties

Introduction

Wind turbine power curves are influenced by meteorological conditions, e.g. by the turbulence intensity and the wind shear. This must be considered for power curve warranties, which are normally part of wind turbine supply or maintenance agreements. Wind turbine suppliers cannot overtake the risk that a warranted power curve is not met due to site specific meteorological conditions significantly different from the reference conditions of the warranted power curve. On the other hand, for wind farm developers and operators the real world power curve examined at their wind farm side is the relevant measure for economic considerations and should ideally be warranted. Advanced procedures how to deal with this conflict have been developed over the past years.

Approach

The classical procedure to cover the influence of meteorological conditions on wind turbine power curves in warranties by requiring additional data filtering has been found to have shortcomings in practice. Different new approaches did evolve over the past years in the frame of designing and negotiating power curve warranties, in discussions of the Power Curve Working Group and in the frame of the maintenance of the power curve testing standard IEC 61400-12-1. The pros and cons of the different methods have been systematically worked out. In addition, practical experience with the different methods from negotiations of power curve warranties and executing power curve verifications will be presented.

Main body of abstract

Wind turbine power curves can be warranted by wind turbine suppliers in a meaningful way only for certain reference meteorological conditions or ranges of such conditions. 6 different approaches are considered:
1. Additional data filtering of the verified power curve around the reference conditions of the warranted power curve. Despite this is the conventional procedure, nearly all standard warranties offered by the wind turbine suppliers include a severe gap in the description how this filtering should be executed.
2. Normalisation of power curve verification data to reference conditions of the warranted power curve according to the draft revision of IEC 61400-12-1 and extensions of these normalisation procedures
3. Consideration of deviations of meteorological conditions at the power curve verification from the reference conditions of the warranted power curve in the uncertainty of the verified power curve. The uncertainty is taken to the advantage of the warranty provider when comparing the verified power curve with the warranted power curve.
4. Warranting of a site specific power curve
5. Application of the Inner/Outer Range approach as introduced by the Power Curve Working Group
6. Provision of a model by the wind turbine supplier that simulates the (warranted) power output for the conditions present at the power curve verification test within each 10-minute period
Also combinations of the above methods are investigated. Guidance on avoiding shortcomings of the different solutions has been developed.


Conclusion

All of the considered procedures have their advantages and disadvantages. Combining different approaches often helps compromising pros and cons and to reach a fair treatment of the problem for all involved parties. A trend is observed that wind turbine suppliers get more open for new approaches.


Learning objectives
The audience will learn:
- Risks linked to power curve warranties due to the treatment of the impact of meteorological variables
- How the impact on meteorological variables can be addressed in power curve warranties
- Advantages and disadvantages of the different methods
- How to best avoid certain shortcomings of the methods when designing or negotiating power curve warranties