RCM For Wind Turbines
RCM For Wind Turbines
RCM For Wind Turbines
1, MARCH 2012
Abstract—The concept of reliability-centered maintenance in onshore installations [3] and up to 30% of the-considerably
(RCM) is applied to the two wind-turbine models Vestas V44- higher-LCC in offshore installations [4]. In order to make wind
600 kW and V90-2MW. The executing RCM workgroup includes power cost competitive with conventional generation technol-
an owner and operator of the analyzed wind turbines, a main-
tenance service provider, a provider of condition-monitoring ser- ogy even in offshore and remote onshore locations, key factors
vices, and wind-turbine component supplier as well as researchers are improvements in wind-turbine design for enhanced inherent
at academia. Combining the results of failure statistics and as- reliability, but also systematic solutions for maintenance man-
sessment of expert judgement, the analysis is focused on the most agement. Research has shown that the present maintenance of
critical subsystems with respect to failure frequencies and con- both on- and offshore installations is not optimized. It has re-
sequences: the gearbox, the generator, the electrical system, and
the hydraulic system. This study provides the most relevant func- vealed large potential savings by optimizing maintenance deci-
tional failures, reveals their causes and underlying mechanisms, sions over the lifetime to reduce the total cost 1) for maintenance
and identifies remedial measures to prevent either the failure itself activities and component failure, and 2) costs due to production
or critical secondary damage. This study forms the basis for the losses, especially for large offshore wind parks [5]–[10].
development of quantitative models for maintenance strategy se- In this paper, the proven concept of reliability-centered main-
lection and optimization, but may also provide a feedback of field
experience for further improvement of wind-turbine design. tenance (RCM) is applied to wind turbines. This paper is struc-
tured in the following way. In Section II, the relevance of RCM
Index Terms—Availability, failure, failure mode and effect anal- in the context of data-based maintenance management and the
ysis (FMEA), maintenance, reliability, reliability-centered mainte-
nance (RCM), wind energy, wind turbines. implemented analysis procedure are explained and placed in
the context of related scientific work. Section III provides a
description of the two analyzed wind-turbine models. For the
I. INTRODUCTION subsystems selected for in-depth analysis in Section IV, Sec-
tion V presents the results of the RCM study, giving particular
IND power technology has undergone an immense
W growth during the past decades, both with respect to
turbine size and to worldwide installed capacity. As one of the
emphasis on the discussion of the technical failure causes and
suitable preventive measures. The central findings are summa-
rized together with the identified key challenges for achieving
key technologies for sustainable power generation, ambitious
improved reliability, availability, and profitability of wind tur-
goals have been set for its continued development. A 2009 EU
bines in the conclusions in Section VI.
Directive has set a target share from renewable energy of 20%
for the EU by 2020. From a rated capacity of 84 GW installed
in the EU end of 2010, the baseline scenario of the European
Wind Energy Association targets a capacity of 230 GW by the II. METHODOLOGY AND RELATED WORK
end of 2020, including 40 GW of offshore wind power [1], [2].
The cost for operations and maintenance (O&M) of wind tur- The study presented here is part of a combined approach that
bines, required to ensure their technical availability, presently aims at achieving cost-effective maintenance for wind power
constitutes a considerable portion of the life-cycle cost (LCC) plants by means of data-based methods: Reliability-centered
and thus of the cost of wind energy: approximately 20–30% asset maintenance (RCAM) merges the proven systematic ap-
proach of RCM, explained, e.g., in [11] and [12], with quan-
titative maintenance optimization techniques (described, e.g.,
by [13] and [14]). While sole RCM as a qualitative method is
Manuscript received June 23, 2011; revised September 23, 2011; accepted
November 7, 2011. Date of publication December 13, 2011; date of current ver- limited in assessing the cost effectiveness of different mainte-
sion February 17, 2012. This work was supported by the Research Foundation nance strategies, mathematical maintenance optimization tech-
of Göteborg Energi, Gothenburg, Sweden. Paper no. TEC-00312-2011. niques alone do not ensure that the maintenance efforts address
The authors are with the Division of Electric Power Engineering, De-
partment of Energy and Environment, Chalmers University of Technol- the most relevant components and failures. By combining these
ogy, SE-412 96 Gothenburg, Sweden (e-mail: katharina.fischer@chalmers.se; two approaches, the RCAM method, which was originally de-
francois.besnard@chalmers.se; lina.bertling@chalmers.se). veloped for the application to electric power distribution sys-
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org. tems [15], provides a promising framework also for the mainte-
Digital Object Identifier 10.1109/TEC.2011.2176129 nance strategy selection and optimization of wind turbines.
0885-8969/$26.00 © 2011 IEEE
FISCHER et al.: RELIABILITY-CENTERED MAINTENANCE FOR WIND TURBINES BASED ON STATISTICAL ANALYSIS 185
Fig. 3. Structure of the V90-2MW system with DFIG and partially rated
Fig. 2. Structure of the V44-600kW system with wound-rotor asynchronous converter.
generator and “OptiSlip” technology for RCC.
tion to the grid and smoothly reduces the current to zero during
available experience with O&M of these turbines in the RCM
disconnection from the grid. The reactive power required by
workgroup.
the generator is partially provided by a capacitor bundle at the
It should be noted that there is usually some variance in
bottom of the tower, the power factor correction or phase com-
design even between wind turbines of the same model. It is
pensation unit.
common practice that wind-turbine manufacturers have several
The main function of the V44 system, and the only one being
subsuppliers for major components like, e.g., the gearbox. This
of relevance in the scope of this study, is the conversion of
applies also to the systems analyzed here. In addition, small
kinetic wind energy to electric energy, which is provided to the
changes in design can have been implemented over the years of
electric power grid. More specifically, the system function is
production, as it is explicitly stated for the V90-2MW in [28].
to provide up to 600-kW electric power at 690 V and 50 Hz to
the electric power grid, at wind speeds of 4 to 20 m/s and in an
A. V44-600kW Wind Turbine
operating temperature range of −20–+40 ◦ C.
The Vestas V44-600kW, launched in 1996, is an upwind tur- Failures on the system level that are relevant in this study
bine with three blades and an electrically driven yaw system. Its are both a complete and a partial loss of energy conversion
rotor has a diameter of 44 m, a weight of 8.4 t, and a rated rota- capability of the turbine.
tional speed of 28 r/min. A hydraulically actuated pitch system
is used for speed control, optimization of power production,
start-up, and aerodynamic braking of the turbine. Additional B. V90-2MW Wind Turbine
breaking functionality is provided by a disk brake located on Fig. 3 shows the structure of the Vestas V90-2MW system.
the high-speed side of the gearbox. The first turbines of this type were installed in 2004. Like the
The structure of the V44-600kW system is shown in Fig. 2. V44, the V90-2MW is an upwind turbine with three blades and
During operation, the main shaft transmits the mechanical power electrically driven yaw. Its rotor has a diameter of 90 m, a weight
from the rotor to the gearbox, which has either a combined of 38 t, and a nominal rotational speed of 14.9 r/min. The pitch
planetary-parallel design or, as in the case of the early V44 tur- control system with individual pitching capability for each blade
bines analyzed in this study, a parallel-shaft design. The gearbox continuously adapts the blade angle to the wind conditions in
and the generator are connected with a Cardan shaft. The gen- order to provide optimum power output and noise levels. In
erator is an asynchronous four-pole generator with integrated addition, it serves for speed control, turbine start-up and stop
electronically controllable resistance of the wound rotor (so- by aerodynamic braking. Similarly to the V44, a disk brake is
called OptiSlip technology, see Fig. 2), which requires neither located on the high-speed shaft (HSS).
brushes nor slip rings. The variability of the rotor resistance is In contrast to the V44 turbine, all V90-2MW systems ap-
provided by the rotor current control (RCC) unit which is bolted ply hybrid gearboxes with one planetary and two parallel-shaft
to the nondrive end of the generator rotor and, thus, permanently stages, from which the torque is transmitted to the generator
rotates during wind-turbine operation. It consists of a micropro- through a composite coupling. A major difference from the V44
cessor unit to which the control signal is optically transmitted, system is the generator concept: the V90-2MW contains a four-
of a power electronics unit and a resistor bundle. As shown in pole doubly fed asynchronous generator (DFIG) with wound
Fig. 2, the rotor resistance is varied in the way that the resistor rotor. A partially rated converter controls the current in the rotor
bundle is short-circuited at varying frequency by means of an circuit of the generator, which allows control of the reactive
insulated gate bipolar transistor (IGBT) in the power electronics power and serves for smooth connection to the electric power
unit. This OptiSpeed technology allows the rotational speed of grid.
the generator to vary between 1500 (idling) and 1650 r/min. In particular, the applied DFIG concept (so-called OptiSpeed
The generator stator is connected to the electric power grid technology) allows the rotor speed to vary by 30% above and
through a thyristor unit, also called “soft starter.” This limits below synchronous speed. The electrical connection between
the cut-in current of the asynchronous generator during connec- the power converter and generator rotor requires slip rings and
FISCHER et al.: RELIABILITY-CENTERED MAINTENANCE FOR WIND TURBINES BASED ON STATISTICAL ANALYSIS 187
the slip rings and carbon brushes providing the electrical contact The different failure causes explained require different mea-
to the V90-2MW generator rotor. Carbon dust originating from sures for prevention: vibration-related failure can be influenced
the wear of the brushes favors electric spark-over, which dam- through the level of vibration, e.g., by applying continuous vi-
ages the slip ring. This issue has been solved successfully by bration monitoring and preventive bearing replacement before
means of a suction system for continuous removal of the carbon strong vibrations occur. Age-based preventive replacement is a
dust during operation. In addition, the quality of the brushes theoretically possible, but not an economic measure.
has improved significantly, which significantly extended slip- 2) Converter (V90-2MW): In a variable-speed wind turbine
ring life to approximately two years today. According to [22], with a DFIG like the V90-2MW, the function of the converter
carbon brush life is usually affected by the environmental con- is to feed the generator rotor circuit with an electric current
ditions temperature and humidity. Slip rings and carbon brushes of desired amplitude and frequency in order to ensure the de-
are inspected every six months during service and are replaced sired output of active and reactive power from the stator to
depending on their condition. Continuous condition monitoring the grid. Depending on the operating speed, the converter ei-
of the brushes, having the benefit of preventing damage from ther transmits electric power from the generator rotor circuit to
over-worn brushes, can be implemented using electrical con- the grid (supersynchronous operation) or vice versa (subsyn-
tacts in the brushes or visual monitoring of the brush thickness chronous operation) (see e.g., [42]). As shown in Fig. 3, the
by means of a camera. converter system consists of a rotor-side and a grid-side con-
verter in back-to-back configuration. The rotor-side converter
controls the current to the rotor whereas the grid-side converter
C. Electrical System controls the dc-link voltage [43]. In addition to these two power-
1) RCC (V44-600kW): The most frequently failing item in electronic units with IGBT switches, the converter subsystem
the electrical system of the V44-600kW is the RCC unit ex- includes corresponding microprocessor (or control) units. The
plained in Section III-A. This is clearly indicated by the turbine- converter is water-cooled by means of a pump-driven cooling
specific failure statistics and in agreement with both the experi- circuit. High failure rates of the converter systems found in
ence of the RCM workgroup and [41]. In average, the RCC unit V80 and early V90-2MW turbines have decreased. However,
has to be replaced at least once during the wind-turbine lifetime. the converter system is still a source of failure in later turbines
In spite of its attachment to the generator, the RCC is discussed of the V90-2MW series. Salty environments, grid disturbances,
as a part of the electrical subsystem here in accordance with the insufficient cooling, and condensation problems negatively im-
categorization of failure data of the V44 (see Fig. 4). Please note pact the failure frequency of the converter system according to
that the discussion in the following refers to the RCC of genera- the RCM workgroup. The consequences of converter failure are
tors manufactured by Weier, out of two generator models being mainly limited to production loss. In case of failure, the turbine
the one found predominantly in V42 and early V44 turbines. is stopped smoothly by means of the aerodynamic brake so that
The function of the RCC unit is a fast control of the generated excessive loading of the drive train components and potential
electric power by regulating the rotor current, as explained in secondary damage of this are avoided.
Section III-A. Failure of the RCC unit results in a loss of this The failure modes of the converter identified in this study are
current control capability. However, the impact on the system 1) open-circuit failure, i.e., an interruption of the electrical con-
function, i.e., if a full or a partial loss of the energy conversion nection; and 2) a malfunction of the rotor current and frequency
capability is caused, depends on the failed part of the RCC. control. A common cause is the failure of the microprocessor
Due to the fact that failure of the power electronics unit clearly units, a problem that can be addressed, e.g., with software up-
dominates over failures of the RCC microprocessor unit and dates or by using dust- and moisture-protected circuit boards,
the resistor unit, only failure modes and causes of the first are but usually not with preventive maintenance. Another cause
discussed in the following. Failure of this unit usually permits is the failure of the power-electronic components with poten-
continued operation of the wind turbines at reduced power of tial causes ranging from high ambient temperature, vibration,
maximum 300 kW. In that case, the generator operates with a moisture or condensation problems, and component aging to
fixed rotor resistance (see also Fig. 2). In some cases, failure of disturbances from the grid. Based on that, possible preventive
the power-electronic unit of the RCC is caused by loose con- measures are those reducing or monitoring the given failure
tacts or cable twist and, thus, by mechanical impact, e.g., due to causes, such as reduction of vibration in the power-electronics
vibrations. However, more frequent is the occurrence of IGBT unit by design modifications, or monitoring of vibration and
failure. According to the RCM workgroup, this has been ob- bearing temperatures. Potential design modifications of con-
served predominantly in the mornings and evenings, i.e., during verters for wind turbines aiming at increased fault tolerance are
times with large changes in power demand and generation and, discussed in [44].
therefore, frequent coupling activities in the power grid. This 3) Phase Compensation Unit (V44-600kW only): Besides
observation suggests that the failure is related to grid distur- the RCC unit discussed previously, a second frequently fail-
bances like voltage dips or frequency variations. The frequency ing component of the V44 electrical system is the capacitor
of failure of the RCC power-electronics unit tends to increase bundle at the bottom of the tower providing reactive power to
with component age. Overheating-related failure of this unit oc- the generator. Failure of capacitors is a common cause of fire
curs also as a secondary failure due to high temperature in the in wind turbines. A lack in manufacturing quality is consid-
nacelle, i.e., after a defect in the cooling system of the gearbox. ered to be the main cause of failure. According to the current
FISCHER et al.: RELIABILITY-CENTERED MAINTENANCE FOR WIND TURBINES BASED ON STATISTICAL ANALYSIS 193
maintenance practices, the capacitors are visually inspected dur- hydraulic system can also be a result of oil leakage, e.g., due to
ing the regular service. a rupture of an oil hose.
The other failure mode of interest, the unwanted activation
of the mechanical brake, is disadvantageous as it imposes large
D. Hydraulic System loads on the wind-turbine drive train. It is mostly caused by an
The function of the hydraulic system in the considered wind error in the control system and can, thus, hardly be influenced by
turbines is 1) to operate the rotor-blade pitch mechanisms for maintenance. However, in some case, it is caused by oil leakage
controlling the angle of attack of the wind, which includes aero- through an upstream valve which causes a pressure buildup at
dynamic braking capability, and 2) to actuate the mechanical the disk brake, a problem that could be overcome by means of
disk brake. The hydraulic system comprises two different pres- a drainage system [45].
sure levels, but only a single hydraulic pump: the pitch system In addition to the scheduled exchange of hydraulic oil ex-
is operated at a level of 200 bar, while the mechanical brake, plained previously, the present maintenance practices for the
connected with a pressure reducing valve, is actuated at a level hydraulic system cover a visual check of its components to de-
of 30 bar. tect oil leakage, the exchange of filters as well as functional tests
The theoretical worst case scenario of a full loss of braking of the control valves, and the hydraulic pump during the regular
capability resulting in overspeeding of the turbine, which is service every six months.
afflicted with considerable risk for persons and installations
near the wind turbine, is considered highly improbable as this is
prevented by two different safety systems for the pitch system VI. CONCLUSION
and the mechanical brake. In case of emergency, a safety valve An RCM analysis of the two wind-turbine models V44-
releases the pressure from the hydraulic pitch actuators and 600kW and V90-2MW has been carried out. This study has
in this way allows the blades to be pitched out of the wind been performed in a workgroup involving a wind-turbine owner
solely by the aerodynamic forces; emergency operation of the and operator, a maintenance service provider, a provider of
mechanical brake is ensured by a hydraulic accumulator with condition-monitoring services, and wind-turbine component
stored pressure. supplier as well as researchers at academia. The analysis forms
The two failure modes selected by the RCM workgroup for the basis for the development of quantitative models for main-
analysis are 1) a loss of pitching functionality or a too slow tenance strategy selection and optimization. Its scope is limited
pitch as well as 2) an unintentional activation of the mechanical to those subsystems and failure modes being the main drivers of
brake. The consequences of these include a partial or full loss wind-turbine unavailability, namely, the gearbox, the generator,
of power generation capability, but no risk for personnel or the electrical system, and the hydraulic system. For these sub-
the environment. Secondary damage to the drive train can result systems selected for in-depth analysis based on failure statistics
from pitch errors of single blades, which lead to unbalances and, and expert opinion, it has identified the most relevant functional
thus, impose high cyclic load on the drive train components. failures, their causes, and suitable preventive measures.
The most common cause of too slow pitching or loss of pitch- Regarding the failure modes, causes, and underlying mech-
ing functionality is a failure of a control valve due to wear out. anisms, wide-ranging parallels between the considered wind-
The failure characteristic is, thus, a failure rate increasing with turbine models are recognized. Compared to the V44-600kW
the age of this component. At present, hydraulic valves are design, comprehensive additional failure-preventive measures
maintained with a run-to-failure strategy. A possible preventive have been found implemented in the V90-2MW series. In nu-
measure is a scheduled replacement of the valves at the age of merous cases, the identified issues were found to be valid also
around ten years. However, a central cause for the wear out of for a wide variety of other wind turbines, as a comparison with
the hydraulic control valves is moisture and particle contamina- the cited references reveals.
tion of the hydraulic oil. A more effective damage-preventing Among the failure causes identified in this study, vibration has
measure in this context is, thus, an additional offline filtration been found to play a central role as a cause for mechanical fail-
of the hydraulic oil or a measurement of the moisture content ure of a variety of wind-turbine components. Measures aiming
for early detection. According to the present common practice, at prevention or early detection of bearing damage are, there-
the hydraulic oil of the V44-600kW is exchanged every four fore, concluded to be particularly effective. A second conclusion
to five years, while an oil change in the V90-2MW hydraulic from this concerning maintenance modeling is that quantitative
system is usually required after three years. It is expected that maintenance-optimization models that are based on the com-
the implementation of offline filtration significantly extends the mon assumption of independent failure of components run the
oil-change intervals for the hydraulic oil. risk of systematically underestimating the benefit of vibration
Another common cause of pitch failure is too low pressure in monitoring.
the hydraulic system. This can result from failure of a hydraulic Both the frequency and the quality of service maintenance
pump, failure of a control valve or the 24-V transformer feeding have been found to significantly impact the technical condi-
it, or failure of an accumulator. Accumulators store energy by tion and failure rate of wind turbines. This underlines the im-
means of pressurized gas contained in a rubber bladder. As the portant role of maintenance besides the inherent, design- and
rubber ages, the probability of cracks or rupture leading to a quality-dependent reliability of a wind turbine for its operat-
pressure loss in the accumulator increases. Low pressure in the ing availability. A perceived challenge in this context, the lack
194 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 1, MARCH 2012
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[42] J. F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Energy Explained: Lina Bertling (S’98–M’02–SM’08) received the
Theory, Design and Application, 2nd ed. Chichester, U.K.: Wiley, 2009. M.Sc. degree from the Royal Institute of Technol-
[43] K. Nilsson, “Torque estimation of double fed induction generator using a ogy, School of Electrical Engineering, Stockholm,
dynamic model and measured data,” M.S. thesis, Dept. Ind. Electr. Eng. Sweden, in 1997, where she received the Ph.D. de-
Automat., Lund University, Lund, Sweden, 2010, 2012. gree on reliability-centered maintenance for electric
[44] H. Polinder, H. Lendenmann, R. Chin, and W. M. Arshad, “Fault tolerant power distribution systems in 2002.
generator systems for increasing availability of wind turbines,” presented She holds the Chair of Professor in Sustainable
at the Eur. Wind Energy Conf. Exhib., Marseille, France, 2009. Electric Power Systems and has been the Head of
[45] C. Roed, O. M. Jeppesen, K. L. Jensen, and C. Ahler, “Hydraulic sys- the Division of Electric Power Engineering, Depart-
tem and method for operating a brake of a wind turbine,” US Patent ment of Energy and Environment, Chalmers Univer-
20110014048A1, Jan. 1, 2011. sity of Technology, Gothenburg, Sweden, since Jan-
uary 2009. During 2007–2009, she was with Svenska Kraftnät, i.e., the Swedish
Transmission Systems Operator. During 2007–2009, she was with the Royal
Institute of Technology, School of Electrical Engineering, Stockholm, where
she became an Associate Professor in 2008 During the Spring semester 2000,
she was a Visiting Researcher at the University of Saskatchewan, a Postdoctoral
Fellow at the University of Toronto, and was associated with Kinectrics, Inc.,
Katharina Fischer (M’10) was born in Germany in during 2002–2003. Her research interests include the evolution of the electric
1979. She received the Diploma degree in electri- power systems into smart grid and the application of probabilistic methods for
cal engineering from Leibniz Universitaet Hannover, reliability analysis and maintenance management.
Hannover, Germany, in 2002, where she received the Dr. Bertling is the Chair of the Swedish PE/PEL Chapter, and the Chair of the
Ph.D. degree in the field of thermomechanical fail- IEEE Power and Energy Society (PES) Subcommittee on Risk, Reliability and
ure of high-temperature fuel cells from the Faculty of Probability Applications. She is a member of the Editorial board of the IEEE
Mechanical Engineering in 2008. TRANSACTIONS ON SMART GRID. She was a General Chair of the first IEEE PES
Since 2009, she has been a Postdoctoral Research Innovative Smart Grid Technologies Europe Conference, Gothenburg, 2010.
Fellow in the Division of Electric Power Engineering, She is a member of the Swedish National Committee of Congrès International
Department of Energy and Environment, Chalmers des Réseaux Electriques de Distribution and the World Energy Council, and a
University of Technology, Gothenburg, Sweden, and member of the International Council on Large Electric Systems. She is a mem-
part of the Wind Power Asset Management Group. Her current research inter- ber of the Scientific Board of the Swedish Civil Contingencies Agency and a
ests include reliability analysis and maintenance optimization for wind power Board member of the Swedish Wind Power Technology Center hosted by the
plants. Chalmers University of Technology. She is a member of the Advisory Council
of the Energy Markets Inspectorate.