Condition Monitoring of Wind Turbine Gearbox

Here's a summary of the provided content in one paragraph:

Investigation of Transient
Load in Gearbox

TABLE OF CONTENTS
CHAPTER - LITERATURE REVIEW...........................................................................................4
REFERENCES..............................................................................................................................12
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Illustration Index
Illustration 1: Gearbox structure......................................................................................................4
Illustration 2: Wind turbine gearbox................................................................................................5
Illustration 3: Emergency stop rotor speed......................................................................................6
Illustration 4: Emergency stop gear input........................................................................................7
Illustration 5: Emergency stop damage accumulation.....................................................................8
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CHAPTER - LITERATURE REVIEW
Mechanism of transient loads in gear box
Power is a form of energy which is useful in driving the entire load over a machine or
machinery components. Antoniadou and et. al., (2015) stated that power trains in wind turbines
have an expected service life of about 20 years. In this huge life span the diversity of dynamic
loads belongs to a wide spectrum. In order to understand the factors which lead to failure of
gearbox in power transmission systems, it is important to get quick insight about the mechanism
of transient loads (Walker and Zhang, 2013). The failure of gearboxes is a common phenomenon
which has been a major concern for the industries that have been working in the wind turbine
manufacturing and operation. According to Zhao and et. al., (2014), E-stops or emergency stops
are considered as the most frequent process which affects the functioning of gearbox in
transmission systems.
Transmission systems are of different types. These include automatic, semi-automatic
and continuously variable transmissions. Despite of their complexity in structural design, the
usage principles are very easy and individuals can grasp the entire working procedure quite
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Illustration 1: Gearbox structure

quickly (Hansen, 2015). Often the transient-torsional events were considered as the primary
cause of failure of the gearbox which is installed in wind turbine or the power transmission
systems. The entire power transmission system is considered as a machine that is developed for
controlling the application and usage of power in a system. Gears and gear trains are considered
as the basic components of the gearbox (Li and et. al., 2013). This particular element provides
torque and speed to rotate the or convert the power from source to any other device. On the
contrary, the drivetrain includes clutch, gearbox, final drive shafts, rear wheel drives and
differential. According to Fu, Li and Yi (2015), the use of gearbox in wind turbines is
considered as stationary application.\
The basic mechanism of gearbox is of conversion. The slow, high-torque rotation is
converted into very fast rotation of the other device. The application of this mechanism is
witnessed in wind turbines and electrical generators (Elasha, Mba and Teixeira, 2015). The entire
gearbox model is kept separate from the structure because of their tendency to fail due to
transient loads. According to Viadero and et. al., (2014), the moving load which comes and
moves out very quickly and frequently is considered as transient load. In case of wind turbines,
this type of load is introduced on a continuous basis and very often. Drivetrain has to experience
the harshness of this load because of not so steady rotation and certain difficulties in managing
the changing torque which finally leads to damage of the device (Jiang, Karimirad and Moan,
2014).
Factors leading to failure of the gearbox
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Illustration 2: Wind turbine gearbox

The reliability of a device or machine reduces when there is continuous threat of failure.
This kind of possibility exists in the current case of wind turbines. The gearbox which consists of
aforementioned components fails in to perform its current operation because of the transient
loads that come and go frequently in a continuous motion (Carroll, McDonald and McMillan,
2015). The different types of transient loads which are addressed by gearbox include input side
torsional load, output end torsional load, radial and axial transient loads and loads during braking
or pausing events. According to Bruce (2016), the requirement from machinery is quite high as
compared to the structural design estimations. It is not possible practically to generate high
power with maximum weight. The lack of understanding and application of aerodynamics and
wind shear forces has also caused deficiencies in the structural design of gearbox (Sinha and et.
al., 2014).
Antoniadou and et. al., (2015) stated that maintenance of huge device requires lot of
labour, money and time. Unlike automated systems, the one that are manually controlled cannot
be maintained unless there is keen supervision provided. Hence, lack of proper maintenance on a
regular basis is also one of the chief reasons behind failure of gearboxes. As per the views of
Girsang and et. al., (2014), the structural designs of wind turbines and other machines with
power transmission systems has transformed over the past few years. This transition has occurred
due to advances in technology and changing demands of individuals. When area of application
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Illustration 3: Emergency stop rotor speed

differs, then consequent changes have to be introduced in the structure also. As a result, there are
complications or complexities introduced in the design and manufacturing organisations have to
bear large cost (Hansen, 2015). The power capacity of gear box has increased from 250kW
(1985) to about 4,400kW in 2009. This demand of high power transmission requires high
capacity gearboxes. Furthermore, there is an overload experienced over the machine and no
provision for commitment of error.
According to Bruce, Long and Dwyer-Joyce, (2015), alignment problems also cause the
transient loads to fail the gearbox. Some of the operational misalignments include mechanical
looseness, aerodynamic unbalance, wind shear, blade or tower shadow, etc. These are considered
as the essential elements of the deflective survey which causes failure of the gearbox (Bruce,
Long and Dwyer-Joyce, 2015). The parts or features of gearbox may be torn or undergo
manufacturing defect sometimes. This automatically creates a negative impact over functioning
and the gearbox fails.
Importance of transient loads in gearbox
Considering a system in which there is no gearbox. Then the entire process of
transmitting energy or rotational power to another device has to be shifted to some other device
or machine (Antoniadou and et. al., 2015). Furthermore, the requirements of this prospective
device is will be the same i.e. torque driving capacity and ability to transform the energy from
one form to another. The layout of the structure must be in accordance with cost requirements
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Illustration 4: Emergency stop gear input

and efficiency standards. The gearbox fulfils all these criteria and brings in great mechanical
advantage to the entire system. According to Girsang and et. al., (2014), there must be some role
of the transient load in complete functioning of this mechanical system so that efficiency of wind
turbine is not affected.
The functioning or working of wind turbines is always related to transient loads. They are
also considered as torque reversal loads which occur so frequently that a significant impact is
experienced over gearbox (Walker and Zhang, 2013). The end results happens to be failure of
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Illustration 5: Emergency stop damage accumulation

the gearbox. As per the view of Bruce, Long and Dwyer-Joyce, (2015), the transient loads or
torque reversal loads are considered as consecutive changes in the magnitude and direction of
torque load. The lack of proper research and existence of support material results in
complications and misunderstandings relative to the subject. This further brings in defects in the
manufacturing and leads to failure (Hansen, 2015). The transient load is important because this
rotation or changes in the magnitude and direction causes the turbine shaft to move and produce
energy which is further transmitted in the form of power.
According to Bruce, Long and Dwyer-Joyce, (2015), the life cycle of gearbox can be
maintained or targeted to up to 25 years if proper measures are taken during the settlement and
designing of the entire structure. It has been witnessed that despite of making efforts in
improving the gearbox design, the systems fail to comply with transient loads. This is because of
the existence of dynamic loads in the form of transient response and changing situations of winds
in the form of shear loads. The impact of such situational consequences is experienced in the
form of field recordings. Carroll, McDonald and McMillan, (2015) stated that emergency stops
are considered as the major causes of transient loads and these occur in the situation when worst
torsional vibrations and torque reversals occur. The severity of transient loads is much more in
wind turbines gearbox as compared to any other equipment or device (Sinha and et. al., 2014).
Working of transient load in different conditions of gear box
As stated by Viadero and et. al., (2014), there are certain implications experienced when
considering the development of transient loads and functioning of gearbox. Emergency stops or
E-stops is the primary condition that occurs due to transient loads. The process of start ups and
shut downs are affected by these transient loads which further affects the efficiency of gear box
(Bruce, 2016). Wind turbines will not function properly and may even fail the minimum working
criteria due to these conditions of the operational environment. According to Antoniadou and et.
al., (2015), micropitting and fretting are some of the issues which are the outcomes of transient
loads. There are various cracks and pitfalls which occur due to torning of the device. The surface
cracks are considered as fatigue. Primary reason behind development of fatigue structures on the
surface of gearbox is due to uneven thickness of the film which means manufacturing defect (Fu,
Li, and Yi, 2015).
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The different working conditions of the gearbox include coupling with the generators.
Since, the power from the rotational motion of the turbine is transmitted to the electric device so
that further transmission can take place; there is a coupling which takes place between the
gearbox and the generator (Jiang, Karimirad and Moan, 2014). The transient load can affect this
entire coupling process and bring in more damage to the device. According to Girsang and et. al.,
(2014), the agricultural fields that use turbines for generating power to pump water and transfer it
in fields, has a bit different functional model. However, performance of these turbines is quite
varying as compared to the huge wind energy plants. The aim of the entire power plant is to
handle costs and bring in cheap energy for consumption in both domestic and industrial aspects
(Hansen, 2015).
The only aspect which needs to be considered for the functioning of gearbox with
transient loads is the complete manufacturing and maintenance cost. The alignment defects or
operational requirements manufacturing and design modelling have to be considered by the
companies so that negative impact of transient loads is reduced. According to Viadero and et. al.,
(2014), the requirement of keen supervision by the maintenance staff and regular conditioning
can help in tearing of gearbox. As a result, the life span of the machine is not affected and
company will gain greater benefits. Different operating conditions have different take on the
transient loads. The drivetrain which is designed in this system has blades (Li and et. al., 2013).
The pitch of blades provides deceleration to the rotor which makes the generator to decelerate
and which further reduces the efficiency of entire wind turbine.
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According to Bruce, Long and Dwyer-Joyce, (2015), the bearings which have been
implied in the design also experience a transient load. Oscillation of torque loads is experienced
when the braking system is considered. Sudden application of brakes in the wind turbine
machine on a regular basis causes the system to reduce its efficiency and causes upliftment of
error. The aero-only breaking takes place in the negative side and transforms into positive when
there is application of mechanical brakes (Sinha and et. al., 2014). As a result, the torque load
starts oscillating and brings severe damage to the bearings. Furthermore, the bearings start
moving in the forward and reverse motion which causes the worse case scenario.
The entire functioning of wind turbine is totally dependent on the design and modelling
of concerned system. According to Antoniadou and et. al., (2015), the design of gearbox is
totally based on requirements and resources available. In order to minimise the manufacturing
costs, the functioning and efficiency is compromised. Torque reversal system is initiated and
causes defects in the components of turbine. Spike of the gear system and measurements have to
be standardised so that certain precautionary measures are prevalent to reduce the impact of
transient loads (Walker and Zhang, 2013). The major competitor in terms of destruction of the
machine is considered to be fatigue load. Transient load is sometimes ignored or considered to be
less dominant when fatigue load exists. Hence, it is important to reduce the probability of
occurrence of certain events that can cause torsional reversals. The amount of damage which can
be caused to the gearbox includes damage to elements and increase in maintenance costs (Elasha,
Mba and Teixeira, 2015). Furthermore, the major defects that lead to such damage include grid
faults, crowbar events, resonant vibrations, wind gusts, control malfunctions, etc.
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REFERENCES
Books and Journals
Antoniadou, I. and et. al., 2015. A time–frequency analysis approach for condition monitoring of
a wind turbine gearbox under varying load conditions. Mechanical Systems and Signal
Processing. 64. pp.188-216.
Walker, P. D. and Zhang, N., 2013. Modelling of dual clutch transmission equipped powertrains
for shift transient simulations. Mechanism and Machine Theory. 60. pp.47-59.
Zhao, C. and et. al., 2014. Design and stability of load-side primary frequency control in power
systems. IEEE Transactions on Automatic Control. 59(5). pp.1177-1189.
Fu, X., Li, H. N. and Yi, T. H., 2015. Research on motion of wind-driven rain and rain load
acting on transmission tower. Journal of Wind Engineering and Industrial Aerodynamics.
139. pp.27-36.
Jiang, Z., Karimirad, M. and Moan, T., 2014. Dynamic response analysis of wind turbines under
blade pitch system fault, grid loss, and shutdown events. Wind Energy. 17(9). pp.1385-
1409.
Carroll, J., McDonald, A. and McMillan, D., 2015. Failure rate, repair time and unscheduled
O&M cost analysis of offshore wind turbines. Wind Energy.
Bruce, T., 2016. Analysis of the premature failure of wind turbine gearbox bearings (Doctoral
dissertation, University of Sheffield).
Sinha, Y. and et. al., 2014. Significance of Effective Lubrication in Mitigating System Failures—
A Wind Turbine Gearbox Case Study. Wind Engineering. 38(4). pp.441-449.
Bruce, T., Long, H. and Dwyer-Joyce, R. S., 2015. Dynamic modelling of wind turbine gearbox
bearing loading during transient events. IET Renewable Power Generation. 9(7). pp.821-
830.
Girsang, I. P. and et. al., 2014. Gearbox and drivetrain models to study dynamic effects of
modern wind turbines. IEEE Transactions on Industry Applications. 50(6). pp.3777-3786.
Antoniadou, I. and et. al., 2015. A time–frequency analysis approach for condition monitoring of
a wind turbine gearbox under varying load conditions. Mechanical Systems and Signal
Processing. 64. pp.188-216.
Viadero, F. and et. al., 2014. Non-stationary dynamic analysis of a wind turbine power
drivetrain: Offshore considerations. Applied Acoustics. 77. pp.204-211.
Elasha, F., Mba, D. and Teixeira, J. A., 2015. Gearboxes prognostics with application to tidal
turbines. In Proceedings of twelfth international conference on condition monitoring and
machinery failure prevention technologies. Oxford, UK.
Li, J. and et. al., 2013. A new noise-controlled second-order enhanced stochastic resonance
method with its application in wind turbine drivetrain fault diagnosis. Renewable Energy.
60. pp.7-19.
Hansen, M. O., 2015. Aerodynamics of wind turbines. Routledge.
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