The maturity of onshore wind energy specifically means that there is now the impetus to reflect on past industry performance and learn from operational experiences over the last 25 years.
As further accuracy and enhancements are made in wind prediction and measurement, and regulatory regimes showing potential signs of stability (particularly in the US following the renewal of the Production Tax Credit) so the attention now turns from the top line to the bottom line as project stakeholders seek to increase yields and reduce cost.
While these two variables would seem to be in conflict, a carefully constructed O&M programme can achieve precisely that by proactive measures that will increase yield through lower downtime, achieved at a potentially lower cost.
In order to construct and craft an effective O&M programme, an in-depth knowledge of the plant is required - including service intervals, spares inventory, access to labour, crane hire agreement, access restrictions etc. Typically, there is a reliance on the engineering expertise that resides at owner or project level, leveraging on the experience of the engineers who have managed and worked on wind turbines for many years, in some cases since the dawn of the industry itself. While the input of project engineers is essential, the application of data analytics to O&M can assist in pinpointing loss drivers from a time and cost perspective. These outputs can then be used and applied by experienced engineers to reconfigure and optimise existing O&M programmes.
The rapid upscaling of wind turbines has created a situation whereby the majority of equipment in the field is considered ´prototypical´ and today´s dominant technology will not have reached end of life before a new technology becomes dominant. This makes it incredibly difficult to take lessons learned and apply that to the next generation of wind turbines. Only now are projects from the mid 1990s starting to reach the end of their theoretical design life. Examining performance by technology is therefore a useful exercise in terms of both reflection, and providing relevant analysis to operators of each specific technology.
The analysis shows that the Direct Drive and hybrid PMG machines are some of the most reliable on the market - in part down to the superior reliability of the PMGs (0.11 failures per year) in comparison to the other generator technologies shown (0.12-0.14 failures per year), but also due to the comparatively low number of electrical faults and failures (0.59 failures per year vs. 0.69 for other technologies).
The analysis also shows that the removal of the gearbox has not necessarily led to an elevated generator failure rate in PMGs, whereas Electro Magnet machines do exhibit this tendency. It is hypothesised that this is a function of the application of Full Converters to connect to the farm transmission infrastructure and results in better ability to ride through grid faults. Once more operational experience is gained with these technologies, and as they reach similar maturity to Danish Concept, Stall Regulated and DFIMs, then more accurate analysis is possible.
Of the geared machines, Stall Regulated wind turbines exhibit more frequent failure of gearboxes (0.16 failures per year vs. ~0.09 failures per year) than any other geared technology; it is hypothesised that this is a direct function of the different stresses placed upon a twin-speed gearbox than a fixed or variable speed unit.
Impact of O&M strategy on performance
In order to consider the impact of changes to maintenance programmes, a number of different project scenarios were constructed and simulated:
The analysis conducted within the project simulation software WindRAT shows that bringing in defined SLA´s assist in reducing project downtime by eliminating portions of the lead time between failure identification and repair. In particular access to spares pooling and tracking with other operators is a potentially effective way to reduce downtime - by 18.1 per cent according to the simulation. This equates to a potential extra 1-1.5 per cent of project availability alone and will also have a knock on effect on the cost of risk transfer - by lowering insurance premiums and removing elements of capital risk from the project balance sheet. The use of an SLA with a crane hire provider may incur additional upfront cost - but some of that cost can be justified by the 14.6 per cent upturn in project downtime, bringing in additional production revenue and also having a similar effect on risk transfer as per the spares pooling. With the application of novel up-tower repair methods and inbuilt service lifts the number of failures which require a crane are less than historically, but a complete gearbox, generator or blade replacement will still require a crane in the majority of cases and as the analysis shows, even getting the response time down to 3 days still demonstrates marked improvement.
Finally, the analysis of an access restricted scenario shows that, for onshore applications, being impeded by access restrictions results in only a marginal (3.8 per cent) worsening of project downtime. This is because the initial repair lead time is more dependent on access to cranes and spares rather than labour access to the turbine itself.
This article has been authored by James Ingham, Analyst of Renewables, Sciemus Ltd.
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