concepts 4.1 General
5.1 Methods to improve reliability
5.1.1 Reliability results and future turbines:
The results presented above have all been obtained on existing WTs of historic design of size ranging from 200 kW to 2 MW. To what extent can these data be used to predict the reliability performance of new designs of WT of much larger size, say 3 – 5 MW?Reliability analysis is of necessity backward looking and rarely produces data which is younger than 5 years, however, its great advantage is that data is numerical and comparable.
It is proposed that the WT failure rates shown in Fig. 4a can be used as a datum against which future designs should be measured. For example while an average failure rate of 1 failure per turbine per year could be acceptable onshore,
it is unlikely to be acceptable offshore where access may be limited to one visit a year.
The WT subassembly failure rates can also be used as a datum for comparison between different concepts and different designs, however, they must also be considered against the MTTR, as the gearbox data has shown.
Reliability improvement analysis will be useful for WT and subassembly manufacturers to define where design and testing effort can be deployed to improve future reliability.
5.1.2 Design:
One simple approach to improve reliability, taken by Enercon, has been to remove the gearbox and use a direct drive configuration. Enercon also adopted an all- electric approach, avoiding the use of hydraulics for pitch or yaw control.Taking up the comparison between direct and geared drive WTs, raised by Polinder et al. [5], this paper and an earlier reference [15] have shown that:
1. From Fig. 5 direct drive WTs do not necessarily have better reliability than geared drive WTs. In Fig. 5 the Figure 8 Variation between the failure intensities of gearbox subassemblies, using the PLP model, in the LWK population of German WTs
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The Institution of Engineering and Technology 2009 doi: 10.1049/iet-rpg:20080060direct drive E40 has a higher failure rate than its geared drive partners of the same size, whereas the direct drive E66 has a lower failure rate than its partners, although the E66 data is rather limited in number of WTs.
2. From Fig. 6 and reference [15] the aggregate failure rates of generators and converters in direct drive WTs are generally greater than the aggregate failure rate of gearboxes, generators and converters in geared WTs. Therefore the price paid by direct drive WTs for the reduction of failure rate by the elimination of the gearbox is a substantial increase in failure rate of electrical-related subassemblies.
3. On the other hand from Fig. 4c it can be seen that the MTTR of electronic subassemblies is lower than the MTTR of gearboxes suggesting that an all-electric direct drive WT may ultimately have an intrinsically higher availability than a geared drive WT.
4. From Figs. 6b and 6c the failure intensities of larger direct drive generators are up to double that of the geared drive generators of similar size. The following explanation is
offered. The direct drive machines in this paper are wound- rotor, synchronous generators with high pole pair number, incorporating a large number of rotor and stator coils. Whereas the geared drive machines are four or six switchable pole, high-slip, induction generators or double- fed, induction generators, with far fewer coils. It is suggested that the disparity in failure intensities is because of: † The much larger number of coils in the direct drive machine. The failure rate could be improved by replacing field coils by permanent magnets, but this would introduce other, reactive control issues.
† The larger diameter of the direct drive generator making it difficult to seal the more numerous coils from the environment, exposing coil insulation to damage because of the air contaminants and environmental humidity.
† Insufficient standardization in the manufacture of the large direct drive machines, as a consequence of smaller production runs, compared to the more common doubly fed induction generator.
Figure 9 Variation between the failure intensities of converter subassembly, using the PLP model, in the LWK population of German WTs
The upper two are for fully rated converters applied to low-speed direct drive generators, whereas the lower one is a partially rated converter applied to a high-speed geared drive generator
From a general consideration of direct drive or geared concept WTs the following arises from this work associated with the design:
† The reliability of these WT generators, from Fig. 7, is worse during early operational life than that achieved by generators in other industries.
† From Fig. 8 the reliability of these WT gearboxes are seen to be that of a mature technology, constant or slightly deteriorating with time. The reliabilities are comparable with those obtained by gearboxes in other industries. Therefore substantial improvements in the designed reliability of these gearboxes are unlikely in the future, although design improvements in gearboxes for newer, larger designs of WTs are being actively pursued and it appears that maybe a greater onus is being placed on wind turbine gearbox reliability by the stochastically varying torque to which it is subjected
† From Fig. 9 the reliability of these WT converters is
considerably worse throughout their operation than
achieved by converters in other industries.
† From the observations above improvements in
generator and converter reliability design will be crucial to improving the reliability of both direct drive and geared concept WTs.
5.1.3 Subassembly
testing:
Testing of these subassemblies, particularly converters and generators, can also achieve higher WT reliability at the start of operational life by eliminating early failures.A suggestion is that for offshore WTs nacelles could be tested complete, at full or varying load, at elevated temperature, to accelerate the occurrence of early failures.
This is a standard practice in the electrical machine and gearbox industry where prolonged heat runs at elevated temperatures are done as type tests on new products. These type tests are then repeated on individual machines from batch sizes specified for example by IEC Standards 60034 and 61852.
It is also a standard practice, in the volume production of low-rating power converters, ,100 kW, to routinely age test key converter subassemblies and then carry out extended load tests on assembled converters from batch sizes specified for example by IEC Standard 60700 to identify generic weaknesses before despatch.
5.1.4 Operation and maintenance and condition
monitoring:
The improving reliability of generators and converters in Figs. 7 and 9 indicates that operations and maintenance (O&M) activities are already having a reliability effect.Condition monitoring measures machine performance indicating the need for remedial action when performance deteriorates. The wind industry has been very successful in applying Supervisory Control and Data Acquisition (SCADA) systems to WTs and most wind farms now have a SCADA system providing data to remote control rooms. However, agreement has not yet been reached on condition monitoring the large quantities of data generated to indicate incipient failures. O&M methods need to use this information to predict failure and thereby schedule maintenance, although work is currently going on in this area [16, 17]. If the design and testing suggestions above are heeded and the condition monitoring technique is resolved the O & M approach will require:
† Maintenance based on the measured condition of the WT so that failures of vital subassemblies like the generator, gearbox and converter can be pre-empted.
† The provision of adequate spares to reduce downtime when maintenance on the basis of condition takes place.
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Conclusions and
recommendations
The paper has investigated the reliability of more than 6000 WTs in Denmark and Germany over 11 years and particularly the changes in reliability of generators, gearboxes and converters in a subset of 650 of these WTs
in Schleswig Holstein, Germany. The paper has
demonstrated the following:
1. The subassemblies with the highest failure rates are, in descending order of significance:
† Electrical system
† Rotor (i.e. blades and hub)
† Converter (i.e. electrical control, electronics, inverter) † Generator
† Hydraulics † Gearbox
2. Larger WTs have a lower reliability than smaller WTs. 3. Results are supported by other surveys in Sweden and Germany.
4. Technological advances in WT variable- speed and pitch- control have conferred reliability improvements with time. 5. Direct drive WTs are not necessarily more reliable than geared WTs. Aggregate failure intensities of generators and
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The Institution of Engineering and Technology 2009 doi: 10.1049/iet-rpg:20080060converters in direct drive WTs are greater than the aggregate failure rate of gearboxes, generators and converters in geared WTs. Therefore the elimination of a gearbox has resulted in a substantial increase in the failure rate of electrical-related subassemblies. However, it has been shown that the downtime of electrical-related subassemblies is lower than the downtime of gearboxes suggesting that an all-electric, direct drive WT may ultimately have an intrinsically higher availability than a geared drive WT.
6. WT gearboxes are a mature technology with a constant or
slightly deteriorating reliability with time. Therefore
substantial improvements in the designed reliability of existing gearboxes are unlikely in the future. The reliabilities of WT gearboxes are comparable with those obtained by similar size gearboxes in other industries and do not have the highest failure rate compared to other WT subassemblies. However, the results do show that the gearbox exhibits the highest downtime. This rather than the failure rate is the real reason for the industry’s focus on the gearbox.
7. WT direct drive and geared drive generators in these surveys exhibit higher failure rates during the initial phases of operation than generators in other industries.
8. The failure intensities of larger direct drive generators in these surveys are up to double that of the geared drive generators. This disparity may be because of the much larger number of coils used in the direct drive machine, the larger diameter of the machine and the fact that it is not a standard machine produced in large numbers. The failure rate of the direct drive generator could be improved by replacing the field coils by permanent magnets.
9. The power electronic converters of direct and geared drive WTs exhibit higher failure intensities throughout their operation than converters in other industries.
10. For offshore WTs it is recommend that their subassemblies should be tested more thoroughly than at present, particularly converters and generators, to eliminate early failures. A suggestion is that WT nacelles could be tested complete, at full or varying load, at elevated temperature, to accelerate early failures during test so that they enter service with an improved reliability.
7
Acknowledgments
The authors are grateful to Bill Grainger formerly of AMEC Wind Energy and now of Clipper Wind who suggested some of the ideas which initiated this work. They have also received helpful comments and encouragement from the New & Renewable Energy Centre at Blyth, Northumberland, the
Centre of Renewable Energy Systems Technology,
Loughborough University and Dr Polinder at TU Delft. The work on reliability was funded by an EPSRC CASE
Award, ‘Towards a Zero Maintenance Wind Turbine’, GR/P03636/01. The work also contributes towards the EPSRC-funded Supergen Wind Energy Technologies Consortium, EP/D034566/1, in which Durham and Loughborough Universities and AMEC Wind Energy are partners.
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