The coupling service factor is very important and allows for various modes of off-design operation that can result from factors such as a change in power demand, unequal load sharing between generator units, fouling, torsional oscillations, driver output at maximum conditions or possible future up-rating.
A good example of future uprating is augmentation of the power output of a gas turbine by water injection. For metallic flexible-element couplings, the minimum service factor is recommended as 1.
Coupling capable of withstanding the cyclic torque associated with start-up or transient conditions is required particularly for drivers that introduce transient load variations such as reciprocating engines. The worst transient case for power generation trains is usually generator short circuit.
Limited life fatigue stress analysis with respect to transient cases is carried out to verify coupling selection suitability. For initial coupling selection, a large cyclic torque requirement is typically assumed until all conditions are known so that the torsional response analysis can be completed.
Sufficient spacer length known as shaft separation is required for removal of coupling. It allows for the maintenance of adjacent bearings and seals without the removal of the shaft or disturbance of the train alignment. The distance between shaft-ends of about mm or more is preferred for trouble-free installation and maintenance.
Metallic, deformed-thread, self-locking fasteners are only acceptable for special purpose couplings. Keyless connection methods coupling hub type are always preferred. Attention is also needed for coupling guard details. Coupling guards need a rigid, liquid-tight and spark-resistance design. Coupling guards should withstand a kg static load with a deflection of less than 0. Proper radial clearance, of perhaps a minimum of 25 mm, is required. Otherwise personnel protection from contact with the coupling guard may be considered.
Oil spray to control temperature is not recommended. A useful hint for operation inspection is to ask the coupling manufacturer to study and indicate which component s should be inspected or replaced following the occurrence of torque greater than the peak torque rating. Some metallic-flexible couplings, such as single element convoluted diaphragm styles, can exhibit an un-damped response to forced axial vibration.
Multi-disc, multi-diaphragm and non-convoluted single element flexible couplings typically do not exhibit such axial vibration behaviour. The axial movements of the generator rotor have to be limited by locating the proper axial thrust bearing in the power generation train, whether in the driver unit or in the gear unit via the coupling.
Suitable coupling should be used and proper limits set. The generator has to be aligned magnetically. If the generator-rotor is pulled out of the magnetic centre during operation, considerable axial forces occur, which can cause problems.
Considering available-in-market coupling and generator technologies, train axial movement may exceed generator allowable axial deviation from the magnetic centre. Extensive studies and reviews are required to ensure coupling selection meets the specified generator and train requirements. There are usually two main concerns: the train rotor can make excessive force on the axial end-stop s of the thrust bearing s ; axial movement is usually limited by the axial bearing via coupling.
An axial vibration study may be required. Complex power generation trains contain several machine casings and gear units.
A good example is trains using low-pressure and high-power steam turbines and gear units to drive a large generator. Axial alignment is critical in complex power generation trains. It ensures that during operation, when operating temperatures have been reached and all final thermal expansions have taken place, the magnetic centre of the generator rotor is in proper position. This is done by proper coupling selection and giving couplings defined pre-stretches. Train design and installation using coupling pre-stretches and correct alignment procedure should ensure that the generator-rotor stays in magnetic centre position during cold operation as well as hot.
During cold assembly, with the generator stopped, the generator-rotor will stay in the magnetic centre position, regardless of temperature changes, because of the large axial friction forces in the bearings during standstill compared with the coupling spring forces.
Both foil and magnetic bearings offer longer lifetimes, with magnetic bearings outperforming foil bearings when used in large rotating machinery under high loads and a relatively low speed Clark, Large, heavily loaded, and relatively slow rotating provides a nearly perfect description of a modern utility scale wind turbine generator.
A common criticism of magnetic bearings is the high power requirement to generate ample current to generate a magnetic field great enough to yield an ample magnetic force to handle the large loads. This criticism is simply outdated, as recent advances in permanent magnets allow similarly strong magnetic fields to be generated by said magnets instead of via a current. It is these same permanent magnet advances that have allowed the construction of the aforementioned direct-drive generators.
Magnetic bearings appear well-poised to mitigate some of the current gearbox problems, but their application to wind turbines lies well behind the current state of development of direct-drive and torque splitting solutions.
This solution has the potential to aid in the solution of gearbox problems on the lower end of utility scale wind turbines, as it may be adaptable to existing gearbox designs with minimal design changes required.
As the technology matures, magnetic bearings have the potential to allow conventional gearbox designs to approach turbine rated powers of as much as 4 MW, if specific design constraints call for the use of a conventional gearbox. This gearing design has only recently reached mass production in passenger vehicles, although it has been in use for a long time on farm machinery, drill presses, snowmobiles, and garden tractors.
Transmissions of the CVT type are capable of varying continuously through an infinite number of gearing ratios in contrast to the discrete varying between a set number of specific gear ratios of a standard gearbox.
It is this gearing flexibility that allows the output shaft, connected to the generator in wind turbine applications, to maintain a constant rate of rotation for varying input angular velocities. The variability of wind speed and the corresponding variation in the rotor rpm combined with the fixed phase and frequency requirements for electricity to be transmitted to the electrical grid make it seem that CVTs in concert with a proportional Position, Integral, Derivative PID controller have the potential to significantly increase the efficiency and cost-effectiveness of wind turbines.
One disadvantage of CVTs is that their ability to handle torques is limited by the strength of the transmission medium and the friction between said medium and the source pulley. Through the use of state of the art lubricants, the chain-drive type of CVT has been able to adequately serve any amount of torque experienced on buses, heavy trucks, and earth-moving equipment.
In fact, the Gear Chain Industrial B. Company of Japan appears to have initiated work on a wind application for chain-driven CVTs. In addition to being able to handle minor shaft misalignments without being damaged, CVTs offer two additional potential benefits to wind turbines. As reported by Mangliardi and Mantriota , , a CVT-equipped wind turbine is able to operate at a more ideal tip speed ratio in a variable speed wind environment by following the large fluctuations in the wind speed.
The dynamic results were even more promising, as a CVT-equipped turbine subjected to a turbulent wind condition demonstrated increased efficiencies of on average 10 percent relative to the steady wind stream CVT example. Mangliardi and Mantriota go on to determine the extraction efficiency of a CVT-equipped and a CVT-less wind turbine as a function of wind speed, and this is presented below in Fig. As observable in Fig. The wind speed range where the CVT-equipped turbine is at a disadvantage is centered on the design point of the conventional wind turbine, where both turbines exhibit similar aerodynamic efficiencies, but the CVT-equipped turbine is hampered by energy losses in its gearing system.
It should be noted that this is a rather narrow range, and the value by which the CVT wind turbine trails the conventional wind turbine is much smaller when compared to its benefits over the rest of the range of wind speeds. As one moves from an ideal constant and uniform wind field to a turbulent wind field, the potential benefits of a CVT-equipped wind turbine increase. Potential challenges to turbines equipped with a CVT center mainly on the lack of knowledge about the scalability of such designs.
Questions such as what is the upper limit to the amount of torque that may be transmitted through a belt drive have yet to be answered. The potential benefits exist, but it appears that more research and turbine test platforms are needed before the range of applicability of CVTs on wind turbines is known Department of Energy, and their commercial benefits quantified. Hydrostatic drives are one type of CVT that has been studied for wind turbine applications, but it appears, at least initially, that this may replace one problem, gearbox oil filtration, with another, increased maintenance and hydraulic fluid cleanliness requirements.
According to Fig. However, they are costly compared with the other failures when they occur. Percentage of needed repairs and maintenance on utility scale wind turbines. Data: AWEA. While wind turbines are designed for a lifetime of around 20 years, existing gearboxes have exhibited failures after about 5 years of operation.
The costs associated with securing a crane large enough to replace the gearbox and the long downtimes associated with such a repair affect the operational profitability of wind turbines.
A simple gearbox replacement on a 1. Additionally, fires may be started by the oil in an overheated gearbox. The gusty nature of the wind is what degrades the gearbox, and this is unavoidable. Figure 11 summarizes the estimates of the economic rated power ranges of applicability for each of the considered wind turbine gearbox solutions.
The direct-drive approach to the current wind turbine gearbox reliability problem seems to be taking a strong hold in the 3 MW and larger market segment, although torque splitting is also being used in this range. For the 1. These options may gain traction over direct-drive solutions due to the approximately 30 percent cost premium of a direct-drive system, and the larger sizes and capital costs associated with such a system.
If the magnetic bearing route is to be used, the answer may lie with gas turbine manufacturers, as their design criteria already call for bearings that are highly reliable, damage tolerant, and capable of handling large loads. CVTs appear to also offer aerodynamic efficiency benefits.
Identified rated power applicability ranges of existing and possible wind turbine gearbox options. For this reason, magnetic bearings appear to provide a potential solution to a slightly wider range of turbine rated powers than CVTs would.
Licensee IntechOpen. Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by Rupp Carriveau. We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Introduction The reliability issues associated with transmission or gearbox-equipped wind turbines and the existing solutions of using direct-drive gearless and torque splitting transmissions in wind turbines designs, are discussed.
X series Norway 3. Table 1. Direct-drive wind turbine rated power MW and rotor blade diameters. More Print chapter. How to cite and reference Link to this chapter Copy to clipboard. Cite this chapter Copy to clipboard Adam M. Ragheb and Magdi Ragheb July 5th Available from:. Over 21, IntechOpen readers like this topic Help us write another book on this subject and reach those readers Suggest a book topic Books open for submissions.
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Access personal reporting. Load cases that result in torque reversals may be particularly damaging to bearings, as rollers may be skidding during the sudden relocation of the loaded zone.
Seals and lubrication systems must work reliably over a wide temperature variation to prevent the ingress of dirt and moisture, and perform effectively at all rotational speeds in the gearbox. Gear and bearing fatigue standards by AGMA and ISO are used for design, these only capture a subset of the potential failure modes of the components.
For instance, the ISO gear standard provides an established method for calculating resistance to subsurface contact failure and for tooth root breakage. The standards are doing their job, but these are not the most common failure modes observed in windturbine gearboxes. More common causes of failure are manufacturing errors such as grind temper or material inclusions, surface related problems, such as scuffing or micropitting, and fretting problems from small vibratory motions, such as may occur when a machine is parked.
Scuffing is adhesive wear and subsequent detachment and transfer of particles from one or both of the meshing teeth ref ISO It can happen quickly and is generally considered to be associated with an absence or breakdown of the lubricant film under high loads ISO Micropitting is a surface fatigue resulting from generation or numerous surface cracks, and is associated with insufficient film thickness ISO Film thickness is affected by sliding speed, load, temperature, surface roughness, and chemical composition of the lubricant.
Many wind-turbine gearboxes have also suffered from fundamental design issues such as ineffective interference fits that result in unintended motion and wear, ineffectiveness of internal lubrication paths and problems with sealing.
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