The publication of IEC 61400-12-1:2017 earlier this year marks a key milestone in the wind power industry's continuous improvement of its energy assessment methods. Wood Group strongly recommends its adoption for power performance testing of wind turbines. This second edition replaces the previous standard published in 2005.

Wood Group can support all stages of power performance testing, making the process of adopting the new standard painless and hassle free for our clients. We are members of MeasNet, recognised by IECRE, and accredited to IEC 17025 in relation to PPT. Our procedures have been fully revised to streamline compliance with the new standard.

Wood Group personnel were involved in writing the standard and have been using it from its earliest drafts, and so have unsurpassed familiarity with its content and requirements, enabling us to ease the adoption of the standard and unlock its benefits.  

The second edition includes a number of new provisions that allow us to sharpen the focus with which wind turbine performance is scrutinised, making best use of the most up-to-date tools and techniques. While this means the new edition is the appropriate standard to use, it introduces some discontinuity with the previous edition. Wood Group is clear that this change represents an improvement over previous methods. Our understanding of the origin of this discontinuity enables us to guide you step-by-step through any challenges that arise as a consequence.  

Power performance testing is used to characterise the efficiency with which a wind turbine converts the incident wind resource to electrical power. This is represented by a power curve, which relates a wind speed to the power output. The new standard supports a more detailed evaluation of the incident wind resource, reflecting real-world complexity in the test. This provides opportunities to reduce uncertainty and obtain a more precise assessment of efficiency which can be applied under a wider set of circumstances.

Challenges arise in relation to uncertainty contributions which were neglected by the previous standard because of the way it over-simplified the evaluation of the incident wind resource. To illustrate the benefit of the new standard consider an analogy that uses the ideas of focus and pixelation.

Consider the precision of the underlying measurement of the power curve to be represented by how blurred or sharply focussed an image of it is. Now imagine the methods by which the performance is assessed on the basis of these measurements is represented by the resolution of this image. Cruder, more coarse assessment methods provide highly pixelated images. Limited measurements that do not fully characterise the incident wind resource provide a picture that is out of focus.

This coarse, pixelated definition is adequate while the underlying image itself is blurred. However, when more refined measurements are made, the underlying image can be pulled into sharper focus. Uncertainty can be reduced because the influence of variations in wind conditions that were previously neglected due to the limitations of the measurements, which were manifested only as statistical scatter in the power curve as a consequence, can now be accommodated in the analysis.

The previous standard neglected the contribution to uncertainty of how sharply focussed the characterisation of the incident wind resource was. The new standard includes an assessment of these uncertainties. Therefore, if the assessment method proceeds with the same degree of pixelation, the uncertainty apparently increases. The uncertainty is only reduced if the resolution is refined by the adoption of improved methods to reveal the sharper underlying image made possible by the measurements.

For example, the previous standard neglected wind shear (the variation in wind speed with height). The new standard provides guidance for measuring shear and assessing its influence on power performance. If the previously neglected uncertainty associated with this is not mitigated by suitable measurement and analysis, a higher uncertainty is evaluated using the new standard compared to the previous standard for equivalent measurement campaigns. However, if you take full advantage of the guidance provided by the new standard to assess the influence of shear on performance, the power performance is revealed with sharper focus and correspondingly reduced uncertainty.  

Wood Group welcomes the new standard as it incentivises the industry to adopt improved methods that provide a clearer picture of reality. Our priority is always to arrive as closely as possible at the right answer, rather than a temporary expediency, as the basis of long term confidence in wind energy projects.

The standard should be viewed in the wider context in which wind energy assessment procedures are continually improved by the use of better tools and techniques as these become available. Inevitably this process reveals unforeseen and potentially inconvenient insights which nevertheless should be embraced rather than ignored as they provide a clearer image of reality. The requirements of bankability, as described in IEA Wind Recommended Practices 15 Section 1.7, include evaluation of uncertainty that is robust, unbiased and complete. The continuous improvement of our instruments and methods can be seen as the way our uncertainty assessments become more complete.