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 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.
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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).
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 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.
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 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 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).
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).
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.
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.
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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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