Study of Performance Improvement Technique for Wind Turbine Components

IJIRST 窶的nternational Journal for Innovative Research in Science & Technology| Volume 1 | Issue 6 | November 2014 ISSN (online): 2349-6010

Study of Performance Improvement Technique for Wind Turbine Components Radheshyam Verma UG Student Mechanical Engineering Kirodimal Institute of Technology, Raigarh, Chhattisgarh, India 496001

Sheelesh Kumar Patel UG Student Mechanical Engineering Kirodimal Institute of Technology, Raigarh, Chhattisgarh, India 496001

Alok Kumar Agrawal UG Student Mechanical Engineering Kirodimal Institute of Technology, Raigarh, Chhattisgarh, India 496001

Shailendra Kumar Bohidar Faculty Department of Mechanical Engineering Kirodimal Institute of Technology, Raigarh, Chhattisgarh, India 496001

Prakash Kumar Sen Faculty Department of Mechanical Engineering Kirodimal Institute of Technology, Raigarh, Chhattisgarh, India 496001

Abstract In the wind power industry, maintenance and technological evolution/improvement are critical factors to ensure low operation and maintenance (O&M) cost and keeps the wind turbine available to generate greater power. Because of enlarging wind power generation, growing nonlinear load and competitive electricity markets operation mechanism of power systems are facing problems like damping of power oscillation, voltage regulation, power loss via generators etc. So aiming at the solution of these troubles, this study is carried out with advanced control systems like shunt FACTS device in which STATCOM and SVC have been identified as a good device and perfect compensators, active flow control technique using Air Jet Vortex Generators (AJVG), high power generators with gearbox fatigue ranking. The use of such a device on full scale wind turbines may lead to greater net gains in power output, as well as reducing the magnitude of aerodynamic loads associated with dynamic stall.

Keywords: Wind Power, Wind turbine, Voltage Regulation, Operation and maintenance O&M), Gearbox, Air Jet Vortex Generators (AJVG), Flexible Alternate Current Transmission Systems (FACTS), Static Synchronous condenser (STATCOM). _______________________________________________________________________________________________________

I. INTRODUCTION The wind is natural phenomenon related to the movement of air passes caused primary by differential solar heating of the earth surface. Wind energy has a great potential to overcome excessive dependents on fossil fuel to meet energy demand. A modern wind energy conversion system is shown in Fig.3 with their major components. Electricity is the key factor for industrialization, urbanization, economic growth and improvement of quality of life in society. India is the world's fifth prevalent in the electricity sector. The technology evolution of wind turbine is recognised in Fig. 1, according to their power generating capacity from 1975 to 2020.Also shown the corresponding Rotor Diameter, Rotational speed, Pitch, generator and need of power converter.

During the past decades, great efforts have been undertaken to make wind power a competitive source for electrical energy.

By the end of 2012, global installed wind capacity had risen to 264GW, almost a tenfold increase of the capacity in 2002.

Nevertheless, the wind energy sector is still far too expensive to be profitable, especially the strong growing offshore branch.

However, a significant part (about 25%) of the cost is related to operation and maintenance (O&M), in particular the failures of the main components (i.e. gearbox and drive train) resulting in long downtimes(shown in figure 4) and hence high O&M costs.

Various studies today discuss if condition monitoring systems, which allow the forecasting of failures at a very early stage, might be the cure to the problems related to the reliability of the gearbox. Rather than formulating yet another methodology to forecast upcoming failures, the aim of this paper is to identify the underlying cause of the reliability issues related to the gearbox.[3]

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Fig 1 The Evolution of Wind Turbine Technology shown as a function of Generating Capacity (kW) for the years 1975 to 2020[1]

In India wind power generation is spread over states, that shown in fig. 2 wind energy system in the Indian scenario along with enough future scope for these renewable sources through “Grid Parity�. The aim of this paper is to present in a coherent and integrated way the major constraints hampering the improvement and development of renewable energy in India.

Fig 2 Wind power density map at 80m (W/m2) [2]

Fig 3 Typical layout of drive train components in a modern wind turbine, with a hub carrying three blades, a main low speed shaft, a step-up gearbox and a high speed connection shaft to the generator[3]

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Fig 4 Experienced downtime and experienced availability of the OWEZ Offshore wind farm in 2008.[3]

II. DAMPING SYSTEM FOR OFFSHORE WIND TURBINE The potential of installing dampers inside the tower of an offshore wind turbine is investigated through simulations. Dampers are installed at the bottom to act on the curvature of the tower, and it is shown that dampers installed in suitable braces have the potential to increase the critical damping ratio of the two lowest tower modes by 1 percent. By using a toggle-brace system, damper stroke is increased, while the damper force is reduced. Finally, by installing the dampers in a symmetric configuration, tuning for maximum damping is approximately independent of the orientation of the rotor, thereby making this installation of dampers feasible.

Design of monopile support structures for large offshore wind turbines, as the wind turbine illustrated in figure 5, is usually driven by fatigue. Due to the relatively low inherent damping of cross-wind tower vibrations, the fatigue lifespan is significantly influenced by cross-wind vibrations caused by wave loading misaligned with the wind [13]. In the future larger offshore wind turbines will be operating at larger water depths, whereby the critical tower frequency will be lowered and potentially approach the excitation frequency of the waves. This may cause fatigue damage due to wave loading to increase significantly, whereby the monopile support structure may reach its limit of structural feasibility. A way to extend the feasibility of the monopile support structure is by means of structural control, in which external devices are installed in order to reduce the dynamic response. For structural control of fixed offshore wind turbines, resonant damping is the concept most widely used by the industry [13], and it is also the damping concept that has received the most attention in the literature. Installation of a resonant damper like a Tuned Liquid Damper (TLD) or a Tuned Mass Damper (TMD) have been shown to lead to a reduction of the fatigue damage accumulated in the monopile of an offshore wind turbine [14,15]. In order to be effective a resonant damper like a TMD should be installed where the absolute motion of the targeted vibration mode is large, which is at the top of the tower or inside the nacelle. However, with a TMD efficient damping is associated with large damper motion and large damper mass, which is highly undesirable and maybe even unfeasible at the top of a slender wind turbine tower. In order to avoid these installation issues, a concept with semi-active dampers installed in a stroke amplifying toggle-brace-damper system inside the tower has been proposed by Fischer et al. [16]. The brace is installed in order to amplify the horizontal relative motion of the tower walls to a larger displacement of the damper.

A toggle-brace system is a well known concept for application in shear frames [17], where it is used to amplify the horizontal drift of the frame to a larger displacement over the damper. This has been proven to enhance the energy dissipation of a damper installed in a shear frame, when compared to the same damper installed using a traditional diagonal brace [18]. Likewise, the present paper considers installation of passive dampers to act on the relative motion of the tower, though using a more realistic model for the tower wall deflection, that takes into account both horizontal and vertical displacements of the tower walls.

A passive damper is by definition collocated, which means that the two concepts of controllability and observability fuse together. In this paper the two concepts are represented in the term ’modal connectivity’. Critical for the effective implementation of passive dampers inside the tower is sufficient damper stroke and sufficient attainable damping, both related to modal connectivity. The damper stroke is represented by the displacement of the damper with respect to the targeted vibration form, while attainable damping is associated with the ability of the damper to change the natural frequency of the structure when the damper is locked [19]. Vibrations of the tower are dominated by a combination of the two lowest tower modes, fore-aft mode and side-to-side mode, as seen in figure 6 and figure 7, respectively. This means that both modes need to be addressed. In the two modes the tower is primarily deformed in bending, while rotor, nacelle and blades act as lumped inertia at the free end of the tower. Modal connectivity is maximized, by positioning the dampers at the bottom of the tower, where the curvature associated with bending is largest. Although the modal connectivity is maximized, the displacement of the damper is likely to be small, thereby making installation of passive dampers inside the tower unfeasible. In order to increase the displacement of the damper and thereby the feasibility of installing passive dampers inside the tower, a toggle-brace-damper concept is introduced. The toggle brace concept amplifies the relative displacement of the tower to a larger displacement of the damper, whereby at the same time the damper force is decreased.

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Another important aspect of installing passive dampers inside the tower is the relative circumferential positioning of the dampers. Since the wind turbine rotor can turn relative to the tower, dampers should be positioned in a layout so that damping is independent of the position of the rotor. In this paper it will be demonstrated that equally tuned dampers can be positioned in a circumferential symmetric layout, so that the damping ratio of the two lowest tower modes remain approximately constant with respect to the orientation of the rotor.

Fig 5 Oshore wind turbine [4]

Fig 6 Fore-aft mode [4]

Fig 7 Side-to-side mode [4]

It is clearly seen that optimum tuning to get maximum damping is given as a ratio between the changes in frequency associated with locking the dampers and the sum of the displacement across the dampers. Numerical simulations The objective of this numerical example is to demonstrate the performance of the curvature-brace and curvature toggle-brace in damping of tower vibrations of an offshore wind turbine. Time simulation results may be included to study from HAWC without dampers and with dampers .[4]

III. ACTIVE FLOW CONTROL TECHNIQUE In order to maximize the extraction of mechanical power from the wind resource, modern Horizontal Axis Wind Turbines (HAWT) are often equipped with large blade spans resulting in huge swept areas. The ability of a wind turbine to react to rapid fluctuations in wind velocity is blunted by the massive rotational inertia of the rotor assembly as a whole, as well as the mass of individual blades bearing upon pitch change mechanisms. Thus, wind turbines often operate with a less than optimal relationship to the instantaneous wind conditions. A wind turbine interacting with slow fluctuations in wind velocity may suffer a loss in potential energy extraction due to stalling of the blades. Interaction with rapid fluctuations in wind velocity can subject a wind turbine to the phenomenon of dynamic stall .

A. Dynamic / Non dynamic stall Dynamic stall is an aerodynamic phenomenon related to cyclical changes in Angle of A these cyclical changes are due to rapid variations in wind velocity combined with the relatively steady angular velocity of the turbine rotor. Dynamic stall is often characterized by the formation of vortex degree of an airfoil undergoing rapid changes in AOA[20]. The shedding and advection of the DSV along the chord of the airfoil produces large overshoots in lift coefficient and rapid, significant changes in the moment coefficient. Such rapid variations in lift and moment coefficients pose serious structural issues particularly for the large .

Figure 8 contrasts the lift and moment coefficient behaviour for an airfoil under conditions of dynamic and non-dynamic stall.[5]

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Fig 8 Lift and Moment coefficient behaviour for dynamic and non-dynamic stall [5]

Non-dynamic stall is the type of behaviour generally associated with aircraft wings. Non-dynamic stall can also result in marked changes in aerodynamic force coefficients; however, the absence of a DSV tends to significantly reduce the magnitude of any changes. Dynamic and non-dynamic stall may both be heavily influenced by flow separation. Techniques for controlling flow separation may hold the key to reducing the detrimental effects.[20] B. Air Jet Vortex Generators One technique for reducing the magnitude of flow separation involves the generation of a series of vortices over the surface of an airfoil. These vortices act in a manner such that high momentum fluid in the free-stream is bought down to the near wall region.

This high energy fluid endows sluggish boundary layers with additional momentum, which allows them to penetrate further against adverse pressure gradients before separating. A passive method for realising such behaviour involves the use of fixed Vane Vortex Generators (VVG) (Fig. 9a) simplicity; however, such devices may not be able to provide the rapid, active control required to alleviate symptoms of dynamic stall. Air Jet Vortex Generators (AJVG) replaces the VVG with a series of small air jets (Fig. 9b). Although more complicated than the VVG, the AJVG possess the advantage of being more amicable as a rapid response separation control device. The air jets themselves are relatively small fluid dynamic structures, however, their effect on a flow field can be marked.

Fig. 9 (a) Vane Vortex Generator [6];

(b) Air Jet Vortex Generator [7]

The first practical application of the air jet technique is usually attributed to Wallis. Since that study, much research has been carried out in the laboratory on two dimensional wings subjected to cyclical and non-cyclical net increases in power output when used on a full size HAWT , maximized by configuring the devices with certain physical characteristics. Prior studies have highlighted the advantages of carefully selecting the pitch and skew angles of the jet axis, as well as the orientation and preference for certain orifice shapes and configurations . Further increases in AJVG efficiency may require exploration of additional factors to those listed prior. Controlling the manner in which the jet fluid is delivered can produce significant changes in the resulting vortex behaviour. Experiments with jets issuing into quiescent bodies of fluid demonstrated enhanced penetration of jet fluid that was started impulsively, or issued in a non-steady manner with respect to time. Studies conducted with fluid jets issuing into cross flows are particularly relevant to separation control applications about airfoils. Adding a non steady characteristic to the jet injection scheme appears to result in a fluid jet penetrating much further into a cross flow compared with a fluid jet issuing in a steady manner. The exponential injection scheme of Eroglu and Breidenthal may hold much promise as a practical separation control device for airfoils, as the jet injection scheme varies with space, not time .[20]

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IV.

GEAR BOX FATIGUE DAMAGE RANKING

An inspection and maintenance planning map based on the fatigue damage of gears and bearings is presented in Figure 10. The procedure for calculating the short-term fatigue damage for gears and bearings is described and exemplified for the NREL GRC 750 kW gearbox. The gearbox components are then sorted based on their fatigue damage. A ”vulnerability map” is constructed indicating the components with highest to lowest fatigue damage. This maintenance map can be used for maintenance planning and inspection of components during routine preventive maintenance inspections. This approach can give the advantage of detecting the source of fault in shorter time. By using this plan, the maintenance inspector looks for defects from those with higher probability of failure, rather than examining all gears and bearings.

Fig 10 Failure modes of wind turbine gears (adapted after Niemann [8]

It is emphasized that the vulnerability map (shown in Figure 11) demonstrated here in is wind turbine specific and should not be generalized for other gearboxes. Moreover, ideally one should use the long-term fatigue damage estimation − for instance by the method described by Nejad et al. [9] − for gear and bearing design and maintenance planning. It is therefore proposed to devote future work to develop vulnerability maps based on long-term fatigue and for different gearboxes in different sites and sizes.

Fig 11 “vulnerability map” of 750 kW case study gearbox based on component fatigue damage ranking.[9]

V.

HIGH POWER GENERATORS SOLUTION

In this section, the industry and academic solutions for high power generators are investigated and discussed. These solutions can be classified into three groups: upscale the current design, modify the current design, and develop new solutions. Less system components, less generator mass and higher efficiency are the concerns of these solutions .

A. Direct-driven DFIG It is attractive to investigate the direct-driven DFIG shown in Figure 12. Because of low rating converter, no gearbox and no PM used, the system cost is expected to be low, and the system reliability and efficiency are expected to be high. However, in order to produce enough torque, direct-driven generators are normally heavy and large, and big air gap is required for the largediameter structure. This is the challenge to the efficiency of the DFIG. It propose an elastic structure and the deformations in both the rotor and the stator are expected to be equalized. In order to achieve the low voltage drop and thus high efficiency, the stator voltage is kept high whereas the rotor voltage is kept low. Furthermore, fractional winding is used in stator to allow the

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low leakage inductance. In rotor, the slot per pole per phase is kept as high as possible to decrease the no-load current. However, detail analysis of torque ripples is necessary to make this design convincing.[21]

Fig 12: DFIG entirely interfaced with network integrated in WECS [10]

B. Conventional radial-flux PM generators It is now common to find iron-cored radial-flux PM generators in wind turbines. Therefore, it looks straightforward to upscale this mature technology. Also shows an air-cooling 10MW PM machine based on conventional iron-cored technology. In this machine, most of the mass goes to the construction which accounts for 80% of the total mass, though this machine is not optimized. One approach to minimize the machine dimension is to use more efficient cooling methods. Direct water-cooling PM generator is considered. Tooth coil is used with the slots per pole per phase at 0.4 for a six phase machine. The stator has 12slots10poles segments. With the electric load of 150 kA/m, this 8MW machine is only 34% volume of the 10MW.[21] Ironless PMSG Ironless PMSG has negligible normal force between stator and rotor. Therefore, the requirement to the strength of the supporting structure is relatively low, and the total weight and cost can be reduced dramatically. In ironless machine, the iron loss in rotor can be neglected, and has no cogging torque. The synchronous inductance is also low, which allows the cheap full-scale converter. However, in ironless machine, normally more magnet material is required in order to produce the necessary field, and because of directly facing the rotor field, considerable eddy current loss is expected in the stator winding.[21] Furthermore, the design of the large-diameter support structure is a challenge. Different configurations of ironless PMSG generators are investigated, and shows when it comes to large diameter, the axial-flux machine shows the same performance as radial-flux machine, and the machine with 2 rotors is better than the machine with 1 rotor.

C.

Super conducting generator Using superconductive material in electrical machines can reduce the synchronous reactance and the excitation losses, increase the magnetic flux density in the air gap, and eliminate ferromagnetic cores, therefore, high efficiency and compact design can be achieved. When compared to PM, up to 50% of the generator mass can be saved with High Temperature Superconducting (HTS) generator, which also means that the cost of construction and installation can be significantly minimized. Furthermore because of the low driving voltage (around 100mV), the rotor of the HTS generator is subject to low thermal aging of the insulation, thus without risk of insulation breakdown. In addition, HTS generators also have high overload capability without thermal excursion.

Because of the high loss at AC condition (in stator) and the challenge to remove it, the super conducting winding is currently limited to DC condition, i.e., in rotor for producing excitation field, and the stator winding still employs copper winding. The main challenge of the super conducting generator is the cryo cooler used to cool down the rotor, which adds extra cost and the reliability of the cooling technology has not been proven yet for offshore operations. Nonetheless, HTS generator is a promising candidate for future large offshore wind turbines.[21] D.

HVDC generator High-voltage variable-capacitance DC generator is proposed and investigated. Contrary to conventional magnetic machine where the power flows through magnetic field, this machine performs the energy conversion by varying capacitance in electric field as the rotor rotates. The power is proportional to the square of the terminal voltage which relies on the air gap thickness. A 7.3MW generator is reported to have 200kV output with 4mm air gap. Because of the high terminal voltage and vacuum gap employed, high efficiency can be expected. Furthermore, there will be no need for transformer and AC-DC converter station as that in conventional HVDC offshore wind farm. However, this concept is still far from practical application. One key factor is the low power density, which makes this machine bigger than conventional machines. If the reliable high voltage insulation across the gap can be realized, the generator dimension and mass can be reduced. Nonetheless, the early stage estimation shows that the total system mass is comparable to conventional system.[21] E.

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VI. STATCOM DEVICE FOR WIND FARM The STATCOM (Static Compensator) whose block diagram is inserted into the 6MW wind farm as shown in Figure 13, uses the same principle of compensation of reactive power. It consists of a capacitor with an inverter-side current (DC) and an inductor coupling the AC side (AC), to be connected to the mains. The STATCOM regulates the voltage at the CPC by controlling the amount of reactive power absorbed or injected into the network. When the voltage is low (V <V2) the STATCOM generates reactive power (capacitive STATCOM) and when the voltage is high (V> V2), it absorbs reactive power (inductive STATCOM).

This is justified by simulation results (Figure 14 in MATLAB.[11]

Fig 13-Block diagram of Wind farm 6 MW with STATCOM.[11]

Fig 14(a, b, c): Simulation Results of wind Farm with STATCOM and capacitor banks at Bus Connection B25.[11]

VII. TOPFARM TECHNOLOGY

A new approach has been developed, which allows for wind farm topology optimization in the sense that the optimal economical performance, as seen over the lifetime of the wind farm, is obtained[12]. This is fundamentally different from conventional power output optimization and is achieved by determining the optimal balance between capital costs, operation and maintenance costs, cost of component fatigue degradation and power output income on a rational background. From a modelling perspective, the main difference between conventional wind farm power output optimization and the present approach can be summarized as:

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(1) Contrary to conventional wind farm power output optimization, the economical approach requires a detailed instationary modelling of the wind farm flow field in order to enable realistic wind turbine load simulations. The DWM model offers simulation of in-stationary wind farm flow fields with an acceptable computational effort, which is essential in an optimization context; (2) Contrary to conventional wind farm power output optimization, the economical approach requires main component loads/degradation to be determined from detailed aeroelastic simulations. This in turn enables wind turbine control to be accounted for, thus at the same time paving the way for tuning the control system to wind farm operation, which finally may impact the individual wind turbine power output that result from the aeroelastic simulations rather than from a simplistic power curve approach; (3) Contrary to conventional wind farm power output optimization, the economical approach requires cost models for the synthesis of different types of essential quantities (i.e. investment costs; wind turbine degradation costs; operation and maintenance costs; and power production) into the objective function defining the optimization problem. The resulting comprehensive optimization problem is solved in an iterative manner, taking advantage of a multi-fidelity optimization approach. Constraints on the design space may be imposed either as a direct pre-defined reduction of the design space or, indirectly, in terms of restrictions on integral values resulting from calculations in addition to the cost functions.

Examples of the latter category could be power quality and maximum allowable turbine loads. Computational speed of all basic elements of the TOPFARM platform is of utmost importance, and this challenge has thus been met on all levels ranging from the wind farm flow field simulation to the aeroelastic simulation and the optimization strategy itself. In a future perspective, inclusion of wind farm control features and refinements of the embedded (cheapest possible) grid layout sub-optimization problem are planned along with general improvements of the code and the optimization strategy in order to increase the computational speed.

VIII. CONCLUSION It presents a thorough investigation of the global operational offshore wind farms from the perspective of generators, AJVG, STATCOM, TOPFARM, Damping technique and Gearbox fatigue map. It is found that the dominant solution for offshore energy conversion system is the multi-stage geared drive train with the induction generators. It confirms the dominance of supporting structure in the total mass and cost of the high-power generators. It is therefore not economic to simply upscale the conventional technology of iron-cored PM generator if larger parts of the mass and cost go to the supporting structure.

Furthermore, developing lightweight technology or other cost-effective solutions becomes necessary. It reviews the generator solutions for high-power offshore wind turbines. Also new toggle-brace-damper concept for installing dampers at the bottom of fixed offshore wind turbines has been presented. Compared to a brace design where the damper is connected directly to the tower wall, the damper stroke is significantly increased, thereby increasing the feasibility of installing dampers inside the tower. In the numerical example presented a single circumferential row of toggle brace dampers is seen to increase the damping ratio of the two lowest tower modes. In a practical implementation 2-3 rows of dampers at different heights could be installed together in order to further increase attainable damping. As offshore wind turbines continue to increase in size and are moved to larger water depths, the toggle-brace-damper concept could become a feasible alternative to the tuned mass damper concept for damping of offshore wind turbine tower vibrations and STATCOM with its reactive energy intake stabilizes the voltage at a relatively constant value even in the presence of the fault in the network. The major significance of this study is the reduced maintenance (costs and time) and improving the unit operation efficiency and reliability and providing guidance for designers.

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