Condition Monitoring for Increasing the Availability of Wind Turbines

The trend towards damaged mechanical components in wind turbines in the period from 2000 – 2002 caused some larger insurance companies to change their insurance terms by introducing a so-called revision clause. This clause required the roller bearings of the main components of the power train to be replaced regularly, irrespective of their condition. The financial implications of meeting this requirement were so significant to plant operators that it was necessary to seek out alternatives [1, 2]. One such alternative was to measure and analyse the structure-borne vibration of these main components, since this technology has already been used very successfully for early damage detection in other branches of industry. In recent years, two routes have been taken in this respect:

Meanwhile, experience over several years has shown that online measurement offers significantly higher diagnostic security than offline measurement. This result becomes easy to understand when one considers the specific characteristics of the wind turbine.

In addition to these very specific constraints, the following article will consider the different aspects that are required for successful early damage detection. In this context, however, it is necessary first of all to define the terms Condition Monitoring or Condition Monitoring System more closely than occurs at present, since up to now this term has mostly included only the vibration test and/or monitoring of the main components of the wind turbine’s power train. Condition Monitoring in the form of continuous recording of the overall condition of the wind turbine and the responsible management of this knowledge will shape the operation of this plant in the future. The main emphasis of this article is therefore concerned with permanent monitoring, whereby many of the aspects presented should be understood to serve by way of illustration.

2. Developments in Technical Plant Management

Many operators (owners) of wind turbines in Germany appoint service companies to take on the commercial and technical management of the plant on their behalf.

Plant management companies are given a mandate by the owner to operate the wind turbine plant in the most economic manner possible. The emphasis here is on “efficient operation“ – this means that the plant management company, whilst producing the highest possible amount of energy from the plant, must also weigh up whether or not the operation of the plant is, under certain circumstances, creating other difficulties. The following is an example in order to clarify this statement: Imperfect formation of lubricating film in the plant’s lower performance range can cause increased wear on the components, to the extent that significantly higher maintenance costs can be expected which later nullify the revenues from energy production. The professional plant manager is aware of these and other important interrelationships and can therefore decide to adjust the parameters of the start-up conditions.

In the past, suitable tools were not available to plant managers to enable them to efficiently operate wind turbine plant from a variety of manufacturers. In the past two years, a number of technical plant management companies have put in place intensive processes in order to select a producer-independent software solution which will satisfy the special requirements of plant management in the wind energy sector. In certain cases, these processes have even led to the placement of contracts or proprietary developments. The goal of the plant management companies in relation to these software tools is to obtain a powerful resource with which both the parameters and measurements of specific types of plant controller can be monitored and adjusted whilst at the same time enabling detailed analysis of fault messages from the plants. Automated tools for maintenance and repair are generally an integral component of these software solutions which are often constructed in modular form. User friendly interfaces for these software solutions, which are always based on a database, simplify individual adaptation to the requirements of technical plant management. Active use by plant managers of the tools described will make an increasing contribution towards locating existing weak points in the technology of the plant and even taking active control countermeasures, especially in older plant. The manufacturers of wind turbines can also benefit significantly from these processes if communication between all participants can be placed on a sustainable, goal-oriented footing.

3. Plant Management and Successful Condition Monitoring

It can quickly be realized that, given the described development towards professional plant management with the aid of suitable tools, at the same time important foundations are being laid for successful condition monitoring – in the broadest sense of knowledge about the overall condition of the plant. The variety of information which is already available in the controller can be presented in suitable combinations (especially graphically) and can significantly increase understanding of causal interrelationships as well as drawing attention to differences in the plant. Additional modules and sensors for plant monitoring and early damage detection to rotor blades, power train, transmission lubricant, hydraulic oil and structural components, for example, can be successively integrated. The prerequisites for this are standard interfaces, compatibility of the data structures¹ to be exchanged and fast internal and external communications. Fast, stable communications solutions play a key role in the high data transfer rates required for modern plant management. Many shortcomings suffered in the past by a large number of wind farms in Germany can be eliminated by applying this very basic principle, indeed tried and tested solutions are already being offered in the market for this purpose. In other countries, suitable wind farm communications (internal and external) and the associated network connection always present a special challenge, which should as far as possible be resolved at the project implementation stage and should therefore necessarily be taken into consideration during the planning phase.

Experience to date clearly demonstrates that technical plant management occupies a key position in relation to profitable plant operation. In the wind energy sector, it is taking on the function of an integrated platform for the various suppliers and service providers and at the same time managing the interfaces to the manufacturers and operators. The intensive moderation of such communications - against the interests of reluctant manufacturers and operators for intelligent plant management - is highly significant and will determine the success of “modern” condition monitoring. Therefore, old structures from an already outdated understanding of “classic”, purely administrative plant management are very likely to disappear at an increasing rate in the near future.

¹IEC 61400-25 “Communications for monitoring and control of wind power plants“

The Institut für Solare Energieversorgungstechnik (Institute for Solar Energy Supply Engineering) (ISET) in Kassel is campaigning for the integration of Standards for early damage detection in wind turbines in the quoted IEC-Standard.

4. Potential for Increased Condition Monitoring

After the initial euphoria in the wind energy sector concerning the anticipated introduction of tested and certified Condition Monitoring Systems, certain disillusionment has arisen in relation to the speed of proliferation of such systems [4]. As ever, there are naturally many reasons for this: In addition to the important financial aspect whereby retrospective installation often provides a challenge to the short-term liquidity of some operating companies, the concrete monetary value of these systems is not, for the most part, self-evident (http://www.my-sen.de/presse.htm). Although CMS suppliers have carried out a great deal of explanatory work on the technical side, when considering some constraints, certain rigid opinions and viewpoints have arisen on the part of operators and often also plant managers in relation to early damage detection and replacement services under insurance, which are to some extent fostered by a variety of misconceptions on the part of the persons concerned. In addition, there is an associated lack of awareness on the part of owners who rightly expected that, when concluding business with the manufacturer of the wind turbine plant, the reliability of the chosen plant technology had reached an adequate level. The lack of awareness referred to is related above all to the methods available for preventive maintenance. These methods cannot at a stroke eliminate the existing design defects of plant which has already been installed; however they can significantly increase the actual availability of energy (electricity production from available wind energy) and thereby easily justify expenditure on additional technology and personnel.

In principle it can be established that the information provided by the plant controller until now has not, on its own, been sufficient to guarantee efficient early damage detection or the avoidance of faults, since these controllers hitherto reported almost exclusively on faults/plant stoppages which had already occurred. Due to the stated shortage of finance within the operating companies (caused by years of low wind availability or over optimistic wind forecasts leading to overpriced projects with high interest payments) these complementary methods cannot in the main be implemented simultaneously, for example CMS and oil monitoring using particulate counters, vibration measurement and endoscopy, monitoring of rotor blades and power train etc.. Naturally, the suppliers of the respective methods and associated diagnostic systems are also aware of the position in which the operators find themselves and often deliberately add confusion to this situation by playing one method off against the other and not addressing the content of the operator’s priority requirements. For this reason, the role of enlightened technical plant management is particularly important, especially with a view to representing the interests of the operators in relation to the various manufacturers and insurers.

However, in addition to technical plant management, operators seeking assistance can also turn to independent consultants (for example IBH, http://www.conditioncontrol.de), since every Euro well spent on good advice often prevents significant costs at a later date, e.g. in the case of necessary repair to the plant.

5. Conclusion

Professional plant management and the independent expert make decision-making principles available to the operator in order to better exploit the available potential of their wind turbine plant with the aid of recognized procedures, diagnostic systems and tried and tested resources.

This is of vital importance, especially for existing wind parks which are facing special challenges due to the low availability of wind in Germany in recent years combined with frequent, expensive damage to the main power train components. The installation of tested CMS and other systems and methods demands detailed consideration of the applicable constraints (e.g. number of plants and their performance, available communications) and the financial means of the operating companies.

The experience of recent years shows that unexpectedly high financial returns can be realized by means of CMS, by reducing repair and consequential costs which arose previously, to the extent that, in general, the time period for the amortization of tried and tested CMS now lies between two and five years. However, the integration of systems into a coherent overall concept (plant management, maintenance, repair and insurance) is an essential prerequisite for success.

Condition Monitoring for Increasing the Availability of Wind Turbines

1. Introduction

2. Developments in Technical Plant Management

3. Plant Management and Successful Condition Monitoring

4. Potential for Increased Condition Monitoring

5. Conclusion

6. References