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THERES breezy activity in wind power, pan-India, or so it would seem with the installed base adding up to 10,000 mw at a fast clip - it's almost 10% of overall generation capacity. Yet the plant load factor (PLF), read capacity utilization, in our wind power sector is pathetic, often in the single digits. The fact is that panoply of investment subsidies, generous tax holidays and sheer 'irrational exuberance' at the bourses has revved up significant wind power potential without commensurate generation. This amounts to misallocation of resources. We certainly need to generate environment-friendly power that raises our energy efficiency levels and is sustainable as well. But the way ahead is to move away from investment-based incentives and to policy induce norms that are generation-linked instead. And in the entire renewable energy sector, wind power seems particularly apt for a change of policy track to incentivize actual generation. Now the capital cost of wind power remains about a third higher than conventional thermal power. Also, the potential of wind power here is huge, estimated at about 50,000 mw. But then the objective ought to surely be to boost generation with reasonable PLF. The current policy seems more conducive for fast-paced capacity addition. It's wholly suboptimal indeed.Â
Â Â Â Â The existing policy of 80% accelerated depreciation in the very first year does make wind power projects particularly attractive as an investment vehicle. Concurrently, there are regular income-tax benefits available for wind power. Additionally, the compulsory 'renewable portfolio standard' - the requirement that utilities source 'green' power - all do shore up wind power capacity on the ground. Further, the stock market seems to greatly value wind power. The result is that cash-rich entities like hotel companies and spinning mills, and not power generators, seem more likely to foray into wind power. This needs to change. The policy emphasis needs be on capacity utilization in wind power so as to bring about better allocation of resources under the Clean Development Mechanism.
The aim of this project is to formulate certain solutions to the problems faced by generation and use of Wind energy; particularly in India.
The precursor to wind turbine electric power generation was the horizontal axis windmill for mechanical power generation, used since about 1000 AD in Persia, Tibet and China. Diffusion of mechanical windmill technology from the Middle East to Europe took place between 1100 and 1300, followed by further development of the technology in Europe. During the 19th century many tens of thousands of modem mechanical windmills with rotors of 25 meters in diameter were operated in France, Germany and the Netherlands, where at one time 9000 of the mechanical power used in industry was based on wind energy. Further diffusion of mechanical windmill technology to the United States took place during the 19th Century, with the invention and installation of self-regulating windmills for water pumping reaching a maximum of about 600,000 installed units in about 1925.
The advent of DC electric power plants in 1882 in New York and 1884 in Germany, followed by introduction of 3-phase AC power production in the early 1890s, provided a technological basis for constructing wind turbines that generated electricity rather than mechanical power. The Danish scientist and engineer Poul La Cour, the most widely recognized entrepreneur pioneer of electricity generation using wind power, in 1891 in Askov, Denmark introduced a four shuttle sail rotor design generating approximately 10kW of DC electric power. He also applied the DC current for water electrolysis, and utilized the hydrogen gas thus produced for gas lamps to illuminate the local school grounds. La Cour's efforts sparked research, development and commercialization of wind electricity in Denmark and Germany in the 20th Century that provided Europe with its initial leadership role in wind energy electricity generation. Though less internationally recognized than La Cour, Charles F. Brush in 1888 introduced in Cleveland Ohio the first automatically operating wind turbine generator, a 12kW, 17-meter-diameter machine, operated for 20 years.
The earliest recorded (traditional) windmill dates from the year 1191 at the Abbey of Bury St Edmunds in Suffolk. Replacing animal power for grinding grain, the popularity of windmills grew steadily until by 1400 there were some 10,000 in England, mostly in the East of England, Kent and Sussex. With the introduction of iron components in the 19th century, the traditional windmill reached its high point and was a common sight in towns and villages across the country. The first use of a windmill to generate electricity was by a Charles F Brush in Cleveland, Ohio in 1888 and by 1908 there were over seventy in operation, with capacities ranging from 5-25 kW. By the 1930s, windmills with capacities as high as 100 kW were in widespread use as a source of electricity, particularly in areas where centralized distribution systems had not been installed. In the modern era the rate of technological development has increased dramatically, driven mainly by rising fossil fuel prices and concerns over climate change, and turbine capacity has roughly doubled every five years:
Typical Blade Length
Fixed rotational speed and fixed blade pitch angle
Dual rotational speed and fixed blade pitch angle
Variable rotational speed and variable blade pitch angle
Within 5 years
Today wind turbines are generally considered to be the most mature form of renewable energy technology, with industrial giants such as Siemens and GE amongst the leading manufacturers. In 2006, some 11,000 turbines were produced with a combined capacity of 16,000 MW and the global market was worth an estimated £13,000 million. Installed capacity is expected to pass 100 GW in 2008, the equivalent of 50-100 nuclear power stations.
The energy available in the wind varies as the cube of the wind speed, so an understanding of the characteristics of the wind resource is critical to all aspects of wind energy exploitation, from the identification of suitable sites and predictions of the economic viability of wind farm projects through to the design of wind turbines themselves, and understanding their effect on electricity distribution networks and consumers. From the point of view of wind energy, the most striking characteristic of the wind resource is its variability. The wind is highly variable, both geographically and temporally. Furthermore this variability persists over a very wide range of scales, both in space and time. The importance of this is amplified by the cubic relationship to available energy. On a large scale, spatial variability describes the fact that there are many different climatic regions in the world, some much windier than others. These regions are largely dictated by the latitude, which affects the amount of insolation. Within any one climatic region, there is a great deal of variation on a smaller scale, largely dictated by physical geography - the proportion of land and sea, the size of land masses, and the presence of mountains or plains for example. The type of vegetation may also have a significant influence through its effects on the absorption or reflection of solar radiation, affecting surface temperatures, and on humidity. More locally, the topography has a major effect on the wind climate. More wind is experienced on the tops of hills and mountains than in the lee of high ground or in sheltered valleys, for instance. More locally still, wind velocities are significantly reduced by obstacles such as trees or buildings. At a given location, temporal variability on a large scale means that the amount of wind may vary from one year to the next, with even larger scale variations over periods of decades or more. These long-term variations are not well understood, and may make it difficult to make accurate predictions of the economic viability of particular wind-farm projects, for instance. On time-scales shorter than a year, seasonal variations are much more predictable, although there are large variations on shorter time-scales still, which although reasonably well understood, are often not very predictable more than a few days ahead. These variations are associated with the passage of weather systems. Depending on location, there may also be considerable variations with the
time of day (diurnal variations), which again are usually fairly predictable. On these time-scales, the predictability of the wind is important for integrating large amounts of wind power into the electricity network, to allow the other generating plant supplying the network to be organized appropriately. On still shorter time-scales of minutes down to seconds or less, wind-speed variations known as turbulence can have a very significant effect on the design and performance of the individual wind turbines, as well as on the quality of power delivered to the network and its effect on consumers. Van der Hoven (1957) constructed a wind-speed spectrum from long- and short-term records at Brookhaven, New York, showing clear peaks corresponding to the synoptic, diurnal and turbulent effects referred to above (Figure 2.1). Of particular interest is the so-called 'spectral gap' occurring between the diurnal and turbulent peaks, showing that the synoptic and diurnal variations can be treated as quite distinct from the higher-frequency fluctuations of turbulence. There is very little energy in the spectrum in the region between 2 h and 10 min.
Major factors that have accelerated the wind-power technology development are as follows:
1. high-strength fiber composites for constructing large low-cost blades.
2. falling prices of the power electronics.
3. variable-speed operation of electrical generators to capture maximum energy.
4. improved plant operation, pushing the availability up to 95 percent.
5. economy of scale, as the turbines and plants are getting larger in size.
6. accumulated field experience (the learning curve effect) improving the capacity factor.
India has 9 million square kilometers land area with a population over 1 billion, of which 75
percent live in agrarian rural areas. The total power generating capacity has grown from 1,300
MW in 1950 to about 144 gW in 2008, at an annual growth rate of about nine percent.
Power in a wind stream:
A wind stream has total power given by Pt = m. (K.E.w)
where m = mass flow rate of air, kg/s
Vi = incoming wind velocity, m/s
Air mass flow rate is given by
m = á¿¤A Vi
where á¿¤= Density of incoming wind, kg/m2 = 1.226 kg/m2 at 1 atm., 15 0C
A = Cross-sectional area of wind stream, m2
Substituting the above and accounting for the constants, we arrive at the following:
Pw = extracted power from the wind,
Ï= air density, (approximately 1.2 kg/m3 at 20Â¤ C at sea level)
R = blade radius (in m), (it varies between 40-60 m)
Vw = wind velocity (m/s) (velocity can be controlled between 3 to 30 m/s)
Cp = the power coefficient which is a function of both tip speed ratio (Î»), and blade pitch angle, (Î²) (deg.)
Power coefficient (Cp) is defined as the ratio of the output power produced to the power available in the wind.
No wind turbine could convert more than 59.3% of the kinetic energy of the wind into mechanical energy turning a rotor. This is known as the Betz Limit, and is the theoretical maximum coefficient of power for any wind turbine. The maximum value of CP according to Betz limit is 59.3%. For good turbines it is in the range of 35-45%. The tip speed ratio (Î») for wind turbines is the ratio between the rotational speed of the tip of a blade and the actual velocity of the wind. High efficiency 3-blade-turbines have tip speed ratios of 6-7.
Capacity factor is one element in measuring the productivity of a wind turbine or any other power production facility. It compares the plant's actual production over a given period of time with the amount of power the plant would have produced if it had run at full capacity for the same amount of time.
Capacity Factor = Actual amount of power produced over time / Power that would have been produced if turbine operated at maximum output 100% of the time
A conventional utility power plant uses fuel, so it will normally run much of the time unless it is idled by equipment problems or for maintenance. A capacity factor of 40% to 80% is typical for conventional plants.
A wind plant is "fueled" by the wind, which blows steadily at times and not at all at other times. Although modern utility-scale wind turbines typically operate 65% to 90% of the time, they often run at less than full capacity. Therefore, a capacity factor of 25% to 40% is common, although they may achieve higher capacity factors during windy weeks or months.
It is important to note that while capacity factor is almost entirely a matter of reliability for a fueled power plant, it is not for a wind plant-for a wind plant, it is a matter of economical turbine design. With a very large rotor and a very small generator, a wind turbine would run at full capacity whenever the wind blew and would have a 60-80% capacity factor-but it would produce very little electricity. The most electricity per dollar of investment is gained by using a larger generator and accepting the fact that the capacity factor will be lower as a result. Wind turbines are fundamentally different from fueled power plants in this respect.
India's Market Overview of Wind Energy:
India has a vast supply of renewable energy resources. India has one of the world's largest Programs for deployment of renewable energy products and systems 3,700 MW from renewable energy sources installed.
Table: States with strong potential: (potential MW /installed MW)
A wind turbine is a rotating machine which converts the kinetic energy in wind into mechanical energy. If the mechanical energy is then converted to electricity, the machine is called a wind
generator, wind turbine, wind power unit (WPU), wind energy converter (WEC), or aero generator.
Wind turbines can be separated into two types based by the axis in which the turbine rotates.
Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.
Wind Turbine Generator units:
Turbine subsystems include:
a rotor, or blades, which convert the wind`s energy into rotational shaft energy;
a nacelle (enclosure) containing a drive train, usually including a gearbox* and a generator;
a tower, to support the rotor and drive train; and
Electronic equipment such as controls, electrical cables, ground support equipment, and interconnection equipment.
HORIZONTAL AXIS WIND TURBINES (HAWTs):
Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator. Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount. Downwind machines have been built, despite the problem of turbulence, because they don't need an additional mechanism for keeping them in line with the wind, and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclic (that is repetitive) turbulence may lead to fatigue failures most HAWTs are upwind machines.
â€¢ Variable blade pitch, which gives the turbine blades the optimum angle of attack.
Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
â€¢ The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.
â€¢ High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.
â€¢ The tall towers and blades up to 90 meters long are difficult to transport. Transportation can now cost 20% of equipment costs.
â€¢ Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
â€¢ Massive tower construction is required to support the heavy blades, gearbox, and generator.
â€¢ Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
â€¢ Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
â€¢ HAWTs require an additional yaw control mechanism to turn the blades toward the wind.
Types of HAWTs:
Mono-Blade Horizontal Axis Wind Turbine (HAWT):
1. They have lighter rotor and are cheaper.
2. Blade are 15-25 m long and are made up of metal, glass reinforced plastics, laminated wood, composite carbon fiber/ fiberglass etc.
3. Power generation is within the range 15 kW to 50 kW and service life of plant is 30 years.
1. Simple and lighter construction.
2. Favorable price
3. Easy to install and maintain.
1. Tethering control necessary for higher loads.
2. Not suitable for higher power ratings.
1. Field irrigation
2. Sea-Water desalination Plants
3. Electric power supply for farms and remote loads.
1. They have large sizes and power output in range of 1 MW, 2 MW and 3MW.
2. These high power units feed directly to the distribution network.
1. 3 blade propeller type wind turbines have been installed in India as well as abroad.
2. The rotor has three blades assembled on a hub. The blade tips have a pitch control of 0 - 30
for controlling shaft speed.
3. The shaft is mounted on bearings.
4. The gear chain changes the speed from turbine shaft to generator shaft.
Vertical axis Wind Turbines:
Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. Key
advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable. VAWTs can utilize winds from varying directions. With a vertical axis, the generator and gearbox can be placed near the ground, so the tower doesn't need to support it, and it is more accessible for maintenance. Drawbacks are that some designs produce pulsating torque. Drag may be created when the blade rotates into the wind.
â€¢ A massive tower structure is less frequently used, as VAWTs are more frequently mounted with the lower bearing mounted near the ground.
â€¢ Designs without yaw mechanisms are possible with fixed pitch rotor designs.
â€¢ A VAWT can be located nearer the ground, making it easier to maintain the moving parts.
â€¢ VAWTs have lower wind startup speeds than HAWTs. Typically, they start creating electricity at 6 M.P.H. (10 km/h).
â€¢ VAWTs may have a lower noise signature.
â€¢ Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as their blades rotate into the wind.
â€¢ While VAWTs' parts are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts nearly impossible without dismantling the structure if not designed properly.
â€¢ Having rotors located close to the ground where wind speeds are lower due to wind shear, VAWTs may not produce as much energy at a given site as a HAWT with the same footprint or height.
â€¢ Because VAWTs are not commonly deployed due mainly to the serious disadvantages mentioned above, they appear novel to those not familiar with the wind industry. This has often made them the subject of wild claims and investment scams over the last 50 years.
Types of VAWTs:
1. The Persian windmill was the earliest windmill installed. (7th Century A.D. - 13th Century
A.D. in Persia, Afghanistan and China)
2. It is a vertical axis windmill.
3. This windmill was used to grind grains and make flour.
Savonius Rotor VAWT:
1. Patented by S.J. Savonius in 1929.
2. It is used to measure wind current.
3. Efficiency is 31%.
4. It is Omni-directional and is therefore useful for places where wind changes direction frequently.
Darrieus Rotor VAWT:
1. It consists of 2 or 3 convex blades with airfoil cross-section.
2. The blades are mounted symmetrically on a vertical shaft.
3. To control speed of rotation mechanical brakes are incorporated. Those brakes consist of steel discs and spring applied air released calipers for each disc.
VAWT's Used in Practice:
Windterra Eco 1200 1Kw VAWT:
The Windterra ECO1200 Wind Turbine is a revolutionary Vertical Axis Wind Turbine (VAWT). Thanks to its advanced technical design, the ECO1200 is ideally suited to both rural and urban installations, generating green energy from a freely available source -- the wind! Wind generation provides a viable solution for addressing such issues as the increasing demand and cost of power, and directly addresses world environmental issues.
The advantages of the Windterra ECO1200 Vertical Axis Wind Turbine system are in its revolutionary design:
Omni-directional: The ECO1200 can instantaneously accept wind from any direction as opposed to HAWTs (Horizontal Axis Wind Turbines, which require an on-board motor to rotate the unit relative to wind direction.
Turbulent-wind friendly: The ECO1200 is easily roof mountable and is less affected by turbulent air, making ECO1200 suited for areas where houses and trees may disturb airflow.
Low rotation speed: The ECO1200 rotates up to 200rpm during normal operation and has a maximum rpm of 270.
Industry-leading annual output: The ECO1200 blade design is optimized for performance at typical lower wind speeds. The result is a higher annual output which makes the ECO1200 a cost effective choice for green energy.
All-in-one system: The ECO 1200is a complete power-generation package, including turbine, controller/inverter, and mounting system. This system can typically be installed and ready to use in four to five hours.
Roof-top mounting: The ECO1200 is designed for roof top use, eliminating the need for a pole or tower installation that significantly increase cost and complicate routine maintenance.
Internal Components of a Wind Turbine:
Anemometer: This device is used for measurement of speed. The wind speed is also fed to the controller as it is one of the variables for controlling pitch angle and yaw
Blades: These are aerodynamically designed structures such that when wind flows over them they are lifted as in airplane wings. The blades are also slightly turned for greater aerodynamic efficiency.
Brake: This is either a mechanical, electrical or hydraulic brake used for stopping the turbine in high wind conditions.
Controller: This is the most important part of the turbine as it controls everything from power output to pitch angle. The controller senses wind speed, wind direction, shaft speed and torque at one or more points. Also the temp of generator and power output produced is sensed
Gear box: This steps-up or steps down the speed of turbine and with suitable coupling transmits rotating mechanical energy at a suitable speed to the generator. Typically a gear box system steps up rotation speed from 50 to 60 rpm to 1200 to 1500 rpm
Generator: This can be a synchronous or asynchronous Ac machine producing power at 50Hz
High-speed shaft: Its function is to drive the generator.
Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
Nacelle: The nacelle is the housing structure for high speed shaft, low speed shaft, gear box, generator, converter equipment etc. It is located atop the tower structure mostly in the shadow of the blades.
Pitch: This is basically the angle the blades make with the wind. Changing the pitch angle changes weather the blades turn in or turn out of the wind stream.
Rotor: The hub and the blades together compose the rotor.
Tower: Towers are basically made up of tubular steel or steel lattice. Taller the towers greater is the amount of power generated as the wind speed generally goes on increasing with height.
Wind direction: Generally erratic in nature, hence the rotor is made to face into the wind by means of control systems.
Wind vane: Basically the job of a wind sensor, measuring the wind speed and communicating the same to the yaw drive, so as to turn the turbine into the wind flow direction.
Yaw drive: This drive controls the orientation of the blades towards the wind. In case the turbine is out of the wind, then the yaw drive rotates the turbine in the wind direction
Yaw motor: Powers the yaw drive.
The following is a graph between Power Coefficient (CP) vs. Tip Speed Ratio (Î»):
Degree of freedom:
Optimization of the power generated in a wind generator is done my means of two basic degrees of freedom:
1. Yaw orientation: It basically refers to the freedom that we have to change the orientation of the entire nacelle unit so that the rotor is pointed directly into the ever-changing direction of wind flow. This is explained in detail under yaw control, and is done basically by help of motors. The motors are run on the information generated from the wind vanes, which act as the sensors for this system.
2. Pitch of blades: By changing the pitch of the blades we can keep a near-constant rotation rate under the ever varying wind speeds. Generally the control is done in a manner, such that the power-generation efficiency of the turbine is optimized.
Both the pitch of the blades and the Yaw control mechanism can act as brakes for the system in case its hit by strong gusts of wind.
The wind turbine control system consists of a of sensors, actuators, and a system consisting of hardware and software which processes the input signals from the sensors and generates output signals for the actuators.
The sensors might include, for example:
â€¢ An anemometer,
â€¢ A wind vane,
â€¢ At least one rotor speed sensor,
â€¢ An electrical power sensor,
â€¢ A pitch position sensor,
â€¢ Various limit switches,
â€¢ Vibration sensors,
â€¢ Temperature and oil level indicators,
â€¢ Hydraulic pressure sensors,
â€¢ Operator switches, push buttons, etc.
The actuators might include a hydraulic or electric pitch actuator, sometimes a generator torque controller, generator contactors, switches for activating shaft brakes, yaw motors, etc.
The system that processes the inputs to generate outputs usually consists of a computer or microprocessor-based controller which carries out the normal control functions needed to operate the turbine, supplemented by a highly reliable hardwired safety system. The safety system must be capable of overriding the normal controller in order to bring the turbine to a safe state if a serious problem occurs.
Functions Of a Controller in the Wind Turbine Construct:
Supervisory control :
Supervisory control can be considered as the means whereby the turbine is brought from one operational state to another. The operational states might, for example, include:
â€¢ Stand-by, when the turbine is available to run if external conditions permit,
â€¢ Power production,
â€¢ Shutdown, and
â€¢ stopped with fault.
The closed-loop controller is usually a software-based system, which obviates the need for manual control. This is a form of automation in general; it adjusts the operational state of the turbine in such a manner so as to keep it operational in the required range. Basically the operating range is found out on the basis of a operating curve or characteristic. Some examples of such control loops are:
â€¢ Control of blade pitch in order to regulate the power output of the turbine to the rated level in above-rated wind speeds;
â€¢ Control of blade pitch in order to follow a predetermined speed ramp during start-up or shut-down of the turbine;
â€¢ Control of generator torque in order to regulate the rotational speed of a variable speed turbine;
â€¢ Control of yaw motors in order to minimize the yaw tracking error.
The safety system:
The safety system is quite distinct from the main control system of the turbine. Its eponymous function is to protect the turbine and bring it to a safe condition in case of potentially hazardous situation. This usually means bringing the turbine to rest with the brakes applied.
The safety system might, for example, be tripped by any one of the following:
â€¢ Rotor overspeed, i.e., reaching the hardware overspeed limit - this is set higher than the software overspeed limit which would cause the normal supervisory controller to initiate a shut-down (see Figure for typical arrangement of rotor speed sensing equipment on low-speed shaft);
â€¢ Vibration sensor trip, which might indicate that a major structural failure has occurred;
â€¢ Controller watchdog timer expired: the controller should have a watchdog timer which it resets every controller time step - if it is not reset within this time; this indicates that the controller is faulty and the safety system should shut down the turbine;
â€¢ Emergency stop button pressed by an operator;
â€¢ Other faults indicating that the main controller might not be able to control the
Types Of Control Systems:
Pitch control is the most common means of controlling the aerodynamic power generated by the turbine rotor. It also has a major effect on all the aerodynamic loads generated by the rotor. In this control the system changes the pitch angle of the plates according to the speed of the wind. below rated wind speed, the turbine should simply be trying to produce as much power as possible, so there is no need to vary the pitch angle. Here, the pitch setting should be at its optimum value to give maximum power. Above rated wind speed, pitch control provides a very effective means of regulating the aerodynamic power and loads produced by the rotor so that design limits are not exceeded. A decrease in pitch, i.e., turning the leading edge downwind, reduces the torque by increasing the angle of attack towards stall, where the lift starts to decrease and the drag increases. This is known as pitching towards stall. Most pitch controlled turbines use full-span pitch control, in which the pitch bearing is close to the hub. It is also possible, though not common, to achieve aerodynamic control by pitching only the blade tips, or by using ailerons, flaps, airjets or other devices to modify the aerodynamic properties.
These strategies will result in most of the blade being stalled in high winds. If only the blade tips are pitched, it may be difficult to fit a suitable actuator into the outboard portion of the blade; accessibility for maintenance is also difficult.
In the process of controlling the pitch in cases of speeds above the wind speed, the rotor output power decreases, generally the input variable to the pitch controller is the error signal arising from the difference between the output electrical power and the reference power. Generally the operation below the rated speed has the controller changing the pitch in a manner so as to use the available wind stream most efficiently. The generator output hence has to be properly monitored, this would necessitate incorporation of better sensors, hence complete pitch control is generally not considered for smaller machines.
Many turbines are stall-regulated, which means that the blades are designed to stall in high winds without any pitch action being required. This means that pitch actuators are not required. Some means of aerodynamic braking is likely to be required, if only for emergencies. In order to achieve stall-regulation at reasonable wind speeds, the turbine must operate closer to stall than its pitch-regulated counterpart, resulting in lower aerodynamic efficiency below rated. This disadvantage may be mitigated in a variable-speed turbine, when the rotor speed can be varied below rated in order to maintain peak power coefficient. In order for the turbine to stall rather than accelerate in high winds, the rotor speed must be restrained. In a fixed speed turbine the rotor speed is restrained by the generator, which is governed by the network frequency, as long as the torque remains below the pull-out torque. In a variable speed turbine, the speed is maintained by ensuring that the generator torque is varied to match the aerodynamic torque. A variable-speed turbine offers the possibility to slow the rotor down in high winds in order to bring it into stall. This means that the turbine can operate further from the stall point in low winds, resulting in higher aerodynamic efficiency. However, this strategy means that when a gust hits the turbine, the load torque not only has to rise to match the wind torque but also has to increase further in order to slow the rotor down into stall. This removes one of the main advantages of variable-speed operation, namely that it allows very smooth control of torque and power above rated.
Generator torque control:
The torque developed by a fixed-speed (i.e., directly-connected) induction generator is determined purely by the slip speed. As the aerodynamic torque varies, the rotor speed varies by a very small amount such that the generator torque changes to match the aerodynamic torque. The generator torque cannot therefore be actively controlled. If a frequency converter is interposed between the generator and the network, the generator speed will be able to vary. The frequency converter can be actively controlled to maintain constant generator torque or power output above rated wind speed. Below rated, the torque can be controlled to any desired value, for example with the aim of varying the rotor speed to maintain maximum aerodynamic efficiency. There are several means of achieving variable-speed operation. One is to connect the generator stator to the network through a frequency converter, which must then be rated for the full power output of the turbine. Alternative arrangements include a wound rotor induction generator with its stator connected directly to the network, and with its rotor connected to the network through slip rings and a frequency converter. This means that the frequency converter need only be rated to handle a fraction of the total power, although the larger this fraction, the larger the achievable speed range will be. A special case is the variable slip induction generator, where active control of the resistance in series with the rotor windings allows the torque/speed relationship to be modified. By means of closed-loop control based on measured currents, it is possible to maintain constant torque above rated, effectively allowing variable speed operation in this region. Below rated it behaves just like a normal induction generator
Turbines whether upwind or downwind, are generally stable in yaw in the sense that if the nacelle is free to yaw, the turbine will naturally remain pointing into the wind. However, it may not point exactly into wind, in which case some active control of the nacelle angle may be needed to maximize the energy capture. Since a yaw drive is usually required anyway, e.g. for start-up and for unwinding the pendant cable, it may as well be used for active yaw tracking. Free yaw has the advantage that it does not generate any yaw moments at the yaw bearing. However, it is usually necessary to have at least some yaw damping, in which case there will be a yaw moment at the bearing. In practice, most turbines do use active yaw control. A yaw error signal from the nacelle-mounted wind vane is then used to calculate a demand signal for the yaw actuator. Frequently the demand signal will simply be a command to yaw at a slow fixed rate in one or the other direction. The yaw vane signal must be heavily averaged, especially for upwind turbines where the vane is behind the rotor. Because of the slow response of the yaw control system, a simple dead-band controller is often sufficient. The yaw motor is switched on when the averaged yaw error exceeds a certain value, and switched off again after a certain time or when the nacelle has moved through a certain angle. More complex control algorithms are sometimes used, but the control is always slow-acting, and does not demand any special design considerations. One exception is the case of active yaw control to regulate aerodynamic power in high winds, as used on the variable speed Gamma 60 turbine. This clearly requires very rapid yaw rates, and results in large yaw loads and gyroscopic and asymmetric aerodynamic loads on the rotor. This method of power regulation would be too slow for a fixed-speed turbine, and even on the Gamma 60 the speed excursions during above-rated operation were quite large.
A generator is an electrical machine which helps in generating electricity by using the mechanical energy of a prime mover. Wind or Aero-generators are basically wind turbine-generator sets, i.e. a propeller or rotor attached to a turbine which in turn is coupled with an electric generator. The generator is further connected to appropriate electronic devices that help in its connection and synchronization to the electrical grid.
Generators are basically of two different types:
a) Synchronous Generators
b) Asynchronous Generators
The basis of this categorization is the speed of operation of generators. Synchronous generators are run at synchronous speed (1500 rpm for a 4 pole machine at 50Hz frequency) while asynchronous generators run at a speed more than the synchronous speed.
Synchronous generators are doubly fed machines which generate electricity by the principle of electromagnetic induction. The rotor is rotated by a prime mover. The result is a current, which flows in the stationary set of rotor conductors. Now this produces a magnetic field which in turn induces a current in the stator conductors. This is the current which we use finally as the output.
This rotating magnetic field induces an Alternating voltage, by the principle of electromagnetic induction, in the stator windings. Generally there are three sets of conductors distributed in phase sequence, so that the current produced is a three phase current. The rotor magnetic field is generally produced by means of induction, where we use either permanent magnets (in very small machines) or electromagnets in larger machines. Also the rotor winding is sometimes energized with direct current through slip rings and brushes. Sometimes even a stationary field winding, with moving poles in the rotor may be the source of the rotor magnetic field. Now this very setup is been used in automotive alternators, where by varying the current in the field winding we can change and control the alternator voltage generated. This process is known as excitation control.
Basically the problem which plagues the electromagnets is the magnetization losses in the core, this is absent in the permanent magnet machines. This acts as an added advantage, but there is a size restriction owing to the cost of the material of the core.
Asynchronous generators or Induction generators are singly excited a.c. machine. Its stator winding is directly connected to the ac source whereas its rotor winding receives its energy from stator by means of induction. Balanced currents produce constant amplitude rotating mmf wave. The stator produced mmf and rotor produced mmf wave, both rotate in the air gap in the same direction at synchronous speed. These two mmf s combine to give the resultant air-gap flux density wave of constant amplitude and rotating at synchronous speed. This flux induces currents in the rotor and an electromagnetic torque is produced which rotates the rotor. Asynchronous generators are mostly used as wind turbines as they can be operated at variable speed unlike synchronous generator. Two kinds of asynchronous generators are used namely
a) Squirrel cage induction generator (SCIG)
b) Doubly fed induction generator (DFIG)
SQUIRREL CAGE INDUCTION GENERATOR:
A squirrel cage rotor is so named due to the shape which represents a cage like structure; it basically is the rotating part of the generator. Being cylindrical in nature, it's mounted on the shaft. The internal construction relates to the cage structure and contains longitudinal conductive bars (made of aluminum or copper) set into channel like constructs and connected together at both ends by shorting rings forming a proper cage-like shape. The core of the rotor is built of a stack of iron laminations, so as to decrease the eddy current losses.
Squirrel Cage rotor
The current flowing in the field windings in the stator results in the setting up of a rotating magnetic field around the rotor. This magnetic field cuts across the shorted rotor conductors resulting in electromagnetic induction which induces a voltage and in turn a current in the rotor windings. The magnitude of both the induced entities depends directly on the relative speed of the rotor with respect to the stator; this quality is basically called the slip of the motor. Slip basically signifies the difference between the speeds of the rotor and synchronous stator field speed. The rotor is carried around with the magnetic field but at a slightly slower rate of rotation.
DOUBLY FED INDUCTION GENERATOR:
DFIG is Double Fed Induction Generator, a generating principle widely used in wind turbines. It is based on an induction generator with a multiphase wound rotor and a multiphase slip ring assembly with different brushes for access to the varied rotor windings.
For wind power applications, this type of machine has distinct advantage over the conventional type of machines.
The rotor circuit is basically controlled by a power electronics converter. Now this makes it possible for the induction generator to act both as a source and sink for reactive power. This allows for power system stability and allows the machine to support the grid during severe voltage disturbances (low voltage ride through, LVRT) also it allows for reactive power compensation of the system.
The control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies. This allows for the proper usage of the wind stream, since a variable speed drive can derive greater power from the wind stream, as compared to a fixed sped drive
Another factor which reduces the cost of the converter, apart from the initial investment is that only fraction of the Mechanical power, typically 25-30 %, is fed to the grid through the converter, the rest is fed to grid directly from the stator. This in turn enhances the efficiency of the DFIG.