Wind Power System
The wind power system is fully covered in this and the following two chapters. This chapter covers the overall system level performance, design considerations and trades. The electrical generator is covered in the next chapter and the speed control in Chapter 7.
5.1 System Components
The wind power system is comprised of one or more units, operating elec- trically in parallel, having the following components:
• The tower.
• The wind turbine with two or three blades.
• The yaw mechanism such as the tail vane.
• The mechanical gear.
• The electrical generator.
• The speed sensors and control.
The modern system often has the following additional components:
• The power electronics.
• The control electronics, usually incorporating a computer.
• The battery for improving the load availability in stand-alone mode.
• The transmission link connecting to the area grid.
Because of the large moment of inertia of the rotor, the design challenges include the starting, the speed control during the power producing opera- tion, and stopping the turbine when required. The eddy current or other type of brake is used to halt the turbine when needed for emergency or for routine maintenance. In the multiple tower wind farm, each turbine must have its own control system for operational and safety functions from a remote location (Figure 5-1).
FIGURE 5-1
Control center at Baix Ebre wind farm in Catalonia, Spain. (Source: Institut Catalia d’Energia,
Barcelona, Spain. With permission.)
5.1.1 Tower
The wind tower supports the turbine and the nacelle containing the mechan- ical gear, the electrical generator, the yaw mechanism, and the stall control. The nacelle component details and the layout are shown in Figure 5-2. Figure 5-3 shows a large nacelle during installation. The height of tower in the past has been in the 20 to 50-meter range. For medium and large size turbines, the tower is slightly taller than the rotor diameter, as seen in the dimension drawing of a 600 kW wind turbine (Figure 5-4). Small turbines are generally mounted on the tower a few rotor diameters high. Otherwise, they would suffer due to the poor wind speed found near the ground surface(Figure 5-5). Both steel and concrete towers are available and are being used. The construction can be tubular or lattice.
The main issue in the tower design is the structural dynamics. The tower vibration and the resulting fatigue cycles under wind speed fluctuation are avoided by design. This requires careful avoidance of all resonance frequencies of the tower, the rotor and the nacelle from the wind fluctuation frequencies.
FIGURE 5-2
Nacelle details of a 150 kW/25 meter diameter turbine. (Source: Nordtank Energy Group/NEG Micon, Denmark. With permission.)
Sufficient margin must be maintained between the two sets of frequencies in all vibrating modes. The resonance frequencies of the structure are determined by complete modal analyses, leading to the eigenvectors and eigenvalues of complex matrix equations representing the motion of the structural elements. The wind fluctuation frequencies that are found form the measurements at the site under consideration. Experience on a similar nearby site can bridge a gap on the required information. Big cranes are generally required to install wind towers. Gradually increasing tower height, however, is bringing a new dimension in the installation
(Figure 5-6). Large rotors add to the transportation problem as well. Tillable
towers to nacelle and rotors moving upwards along with the tower are among some of the newer developments in the wind tower installation. The offshore installation comes with its own challenge that must be met.
5.1.2 Turbine Blades
The turbine blades are made of high-density wood or glass fiber and epoxy composites. Modern wind turbines have two or three blades. The steady
FIGURE 5-3
A large nacelle under installation. (Source: Nordtank Energy Group/NEG Micon, Denmark. With permission.)
mechanical stress due to centrifugal forces and fatigue under continuous vibrations make the blade design the weakest mechanical link in the system. Extensive design effort is needed to avoid premature fatigue failure of the blade. The mechanical stress in the blade under gusty wind is kept under the allowable limit. This is achieved by controlling the rotor speed below the set limit. This not only protects the blades, but also protects the electrical generator from overloading and overheating. One method that has been used from the early designs and continues to be used today is the stall control. At stall, the wind flow ceases to be smooth around the blade contour, but separates before reaching the trailing edge. This always happens at high pitch angle. The blades experience high drag, thus lowering the rotor power output. The high pitch angle also produces high lift. The resulting load on the blade can cause a high level of vibration and fatigue, possibly leading to the mechanical failure. Regardless of the fixed or variable speed, the design engineer must deal with the stall forces. Researchers are moving from
FIGURE 5-4
A 600 kW wind turbine and tower dimensions with specifications. (Source: Wind World Corporation, Denmark. With permission.)
the 2D to 3D stress analyses to better understand and design for such forces. As a result, the blade design is continually changing, particularly at the blade root where the loading is maximum due to the cantilever effect. The aerodynamic design of the blade is important, as it determines the energy capture potential. The large and small machine blades have significantly different design philosophies. The small machine sitting on the tower relatively taller than the blade diameter, and generally unattended, requires low maintenance design. On the other hand, the large machine tends to optimize the aerodynamic performance for the maximum possible energy capture. In either case, the blade cost is generally kept below 10 percent of the total installed cost.
5.1.3 Yaw Control
The yaw control continuously orients the rotor in the direction of the wind. It can be as simple as the tail vane, or more complex on modern towers. Theoretical considerations dictate free yaw as much as possible. However, rotating blades with large moments of inertia produce high gyroscopic torque during yaw, often resulting in loud noise. Too rapid yaw may generate noise exceeding the local ordinance limit. Hence, a controlled yaw is often required and is used.
5.1.4 Speed Control
The wind turbine technology has changed significantly in the last 25 years. 1 Large wind turbines being installed today tend to be of variable speed
FIGURE 5-6
WEG MS-2 wind turbine installation at Myers Hill. (Source: Wind Energy Group, a Taylor Woodrow subsidiary and ETSU/DTI, U.K.)
design, incorporating the pitch control and the power electronics. Small machines on the other hand must have simple, low cost power and speed control. The speed control methods fall into the following categories:
• No speed control whatsoever. In this method, the turbine, the electrical generator, and the entire system is designed to withstand the extreme speed under gusty wind.
• yaw and tilt control, in which the rotor axis is shifted out of the wind direction when the wind speed exceeds the design limit.
• pitch control, which changes the pitch of the blade with the changing wind speed to regulate the rotor speed.
• stall control. In this method of speed control, when the wind speed exceeds the safe limit on the system, the blades are shifted into a position such that they stall. The turbine has to be restarted after the gust has gone.
Figure 5-7 depicts the distribution of the control methods used in small wind turbine designs. Large machines generally use the power electronic speed control, which is covered in Chapters 7 and 11.
5.2 Turbine Rating
The wind turbines are manufactured in sizes ranging from a few kW for stand-alone remote applications to a couple of MW each for utility scale power generation. The grid-connected turbine as large as 2 MW capacity was installed in 1979 on Howard Knob Mountain in the United States, and 3 MW capacity was installed in 1988 at Berger Hill in the United Kingdom. The method of assessing the nominal rating of the wind turbine has no globally acceptable standard. The difficulty arises because the power output of the turbine depends on the square of the rotor diameter and the cube of the wind speed. The rotor of a given diameter will, therefore, generate different power at different wind speed. The turbine that can generate 300 kW at 7 m/s would produce 450 kW at 8 m/s wind. What rating should then be assigned to this turbine? Should we also specify the “rated speed”?
Early wind turbine designers created a rating system that specified the power output at some arbitrary wind speed. This method did not work well as everyone could not agree on one speed to specify the power rating. The “rated” wind speeds varied from 10 to 15 m/s under this practice. Manufacturers are lax on providing the higher side of the wind speed, claiming greater output from the same design.
To avoid such rating confusion, some European manufacturers refer to only the rotor diameter. But the confusion continues as to the maximum power the machine can generate under the highest wind speed the turbine can continuously operate. Many manufacturers have, therefore, adopted the combined rating designations — the wind turbine diameter following the generator peak electrical rating. For example, the 300/30 wind system means 300 kW electrical generator and 30-meter diameter turbine. The specific rated capacity (SRC) is often used as a comparative index of the wind turbine designs. It is defined as follows:
(5-1) SRC =
Generator electrical capacity
Rotor swept area
For the 300/30 wind turbine, the specific rated capacity is 300/π 152 =0.42 kW/m2. The specific rated capacity increases with the diameter, giving a favorable economy of scale to large machine. It ranges from approximately 0.2 kW/m2 for 10-meter diameter rotor to 0.5 kW/m2 for 40-meter diameter rotor. Some aggressively rated turbines have SRC of 0.7 kW/m2, and some reaching as high as 1.0 kW/m2. The operating stresses in rotor blades of the high SRC are high, generally resulting in shorter fatigue life. All stress concentration regions are carefully identified and eliminated in high SCR designs. Modern design tools, such as the finite element stress analysis and the modal vibration analysis can be of great help in the rotor design. The turbine rating is important as it indicates to the system designer how to size the induction generator, the plant’s transformer, connecting cables to the substation, and the transmission link interfacing the grid. The power system must be sized on the peak capacity of the generator, and the generator is rated in a different manner than the wind turbine. The turbine power depends on the cube of the wind speed. The system design engineer is, therefore, required to match the turbine and the generator performance characteristics. This means selecting the rated speed of the turbine to match with the generator. Since the gearbox and the generator are manufactured only in discrete sizes, selecting the turbine rated speed can be complex. The selection process goes through several iterations, trading the cost with benefit of the available speeds. Selecting a low rated speed would result in wasting much energy at high winds. On the other hand, if the rated speed is high, the rotor efficiency will suffer most of the times.
