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Content uploaded by Kurt Rohrig
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All content in this area was uploaded by Kurt Rohrig
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Reliability of Wind Turbines
Experiences of 15 years with 1,500 WTs
Berthold Hahn, Michael Durstewitz, Kurt Rohrig
Institut für Solare Energieversorgungstechnik (ISET)
Verein an der Universität Kassel e.V., 34119 Kassel, Germany
INTRODUCTION
With the rapid expansion of wind energy use in Germany over the past fif-
teen years, extensive developments in wind turbine technology have taken
place. The new technology has achieved such a level of quality, that wind
turbines obtain a technical availability of 98 percent.
This means that an average wind turbine (WT) will be inactive for
around one week per year for repairs and maintenance. Considering that
the WTs operate over years without operating personnel, this average
downtime seems short. The paper gives some figures about reliability of
wind turbines, failures and downtimes for wind turbines and components.
DATA BASIS
In the framework of the „250 MW Wind“ Programme, ISET is monitoring
over 1,500 WTs in operation. Over a period of 15 years now, WTs with a
variety of different technical conceptions and installed in different regions
in Germany have been included in the programme. So, from these turbines,
the experiences of up to 15 operating years are readily available. On aver-
age, the participating turbines have completed ten years of operation.
Operators of the supported WTs regularly report to ISET concerning en-
ergy yields, maintenance and repairs, and operating costs. In form sheets
for maintenance and repair, the operators report on the downtimes caused
by malfunctions, the damaged components and - as far as possible - the
causes and obvious effects on turbines and operation.
Most of these supported WTs have a rated power below 1 mega-watt.
Thus, in the recent years operators of mega-watt WTs where asked quite
2 Berthold Hahn, Michael Durstewitz, Kurt Rohrig
successfully to contribute to the programme on a voluntary basis. So, ex-
periences of the recent models can be included into the evaluations as well.
Up to now, over 60,000 reports on maintenance and repair have been
submitted to ISET. Standardized evaluations are published in the ‘Wind
Energy Report’ [1], which is updated annually.
Break down of Wind Turbines
Usually, WTs are designed to operate for a period of 20 years. But, no fi-
nal statement can be made yet concerning the actual life expectancy of
modern WTs as, until now, no operational experience of such period is
available. Changes in reliability with increasing operational age can, how-
ever, provide indications of the expected lifetime and the amount of up-
keep required. Reliability can be expressed by the number of failures per
unit of time, i. e. ‘Failure Rate’. In the following, the failure rates of WTs
depending on their operational age will be depicted (Fig. 1).
It is clear that the failure rates of the WTs now installed, have almost
continually declined in the first operational years. This is true for the older
turbines under 500 kW and for the 500/600 kW class. However, the group
of mega-watt WTs show a significantly higher failure rate, which also de-
clines by increasing age. But, including now more and more mega-watt
WT models of the newest generation, the failure rate in the first year of
operation is being reduced.
Fig. 1. Frequency of ‘failure rate’ with increasing operational age
The principal development of failure rates is well known in other technical
areas. ‘Early failures’ often mark the beginning of operation. This phase is
0
1
2
3
4
5
6
7
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Operational Year
Annual Failure Rate
Rate Power < 500 kW
Rated Power 500 - 999 kW
Rated Power >= 1000 kW
Reliability of Wind Turbines
Experiences of 15 years with 1,500 WTs 3
generally followed by a longer period of ‘random failures’, before the fail-
ure rate through wear and damage accumulation (‘wear-out failures’) in-
creases with operational age.
The total life period and the individual phases are naturally distinct for
different technical systems. For WTs, hardly any experience is available in
this respect. Based on the above evaluations, however, for the WTs under
500kW it can be expected that the failure rate due to ‘wear-out failures’
does not increase before the 15
th
year of operation.
MAL-FUNCTIONS OF COMPONENTS
The reported downtimes are caused by both regular maintenance and un-
foreseen malfunctions. The following evaluations refer only to the latter,
which concerned half mechanical and half electrical components. (Fig. 2).
Fig. 2. Share of main components of total number of failures
Besides failure rates, the downtimes of the machines after a failure are an
important value to describe the reliability of a machine.
The duration of downtimes, caused by malfunctions, are dependent on
necessary repair work, on the availability of replacement parts and on the
personnel capacity of service teams. In the past, repairs to generator [2],
drive train, hub, gearbox and blades have often caused standstill periods of
several weeks.
Electrical
System
23%
Generator
4%
Rotor Blades
7%
Rotor Hub
5%
Structural
Parts / Housing
4%
Yaw System
8%
Sensors
10%
Plant Control
System
18%
Gearbox
4%
Mechanical
Brake
6%
Drive Train
2%
Hydraulik
System
9%
Reports in Total: 34582
4 Berthold Hahn, Michael Durstewitz, Kurt Rohrig
Taking into account all the reported repair measures now available, the
average failure rate and the average downtime per component can be given
(Fig. 3). It gets clear, that the downtimes declined in the past five to ten
years. So, the high number of failures of some components is now bal-
anced out to a certain extent by short standstill periods. But still, damages
of generators, gear boxes, and drive trains are of high relevance due to
long downtimes of about one week as an average.
Drive Train
Supporting Structure /Housing
Generator
Gearbox
Rotor Hub
Mechanical Brake
Rotor Blades
Yaw System
Hydraulic System
Sensors
Electronic Control
Electrical System
Down time per failure [days]
A
nnual failure frequency [-]
1 0
,
75 0
,
5 0
,
25 0 2 4 6 8
Fig. 3. Failure Frequency and downtimes of components
Conclusion
Wind turbines achieve an excellent technical availability of about 98% on
average, although they have to face a high number of malfunctions. It can
be assumed that these good availability figures can only be achieved by a
high number of service teams who respond to turbine failures within short
time. In order to further improve the reliability of WTs, the designers have
to better the electric and electronic components. This is particularly true
and absolutely necessary in the case of new and large turbines.
References
1. C Ensslin, M Durstewitz, B Hahn, B Lange, K Rohrig (2005) German
Wind Energy Report 2005. ISET, Kassel
2. M Durstewitz, R Wengler (1998) Analyses of Generator Failure of Wind
Turbines in Germanys ‘250 MW Wind’ Programme, Study. ISET, Kassel
