Wind Power Systems

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.

System Components

The wind power system comprises one or more wind turbine units operating electrically in parallel. Each turbine is made of the following basic components:

  • • Tower structure
  • • Rotor with two or three blades attached to the hub
  • • Shaft with mechanical gear
  • • Electrical generator
  • • Yaw mechanism, such as the tail vane
  • • Sensors and control

Because of the large moment of inertia of the rotor, design challenges include starting, speed control during the power-producing operation, and stopping the turbine when required. The eddy current or another type of brake is used to halt the turbine when needed for emergency or for routine maintenance.

In a modern wind farm, each turbine must have its own control system to provide operational and safety functions from a remote location (Figure 4.1). It also must have one or more of the following additional components:

  • • Anemometers, which measure the wind speed and transmit the data to the controller.
  • • Numerous sensors to monitor and regulate various mechanical and electrical parameters. A 1 -MW turbine may have several hundred sensors.
  • • Stall controller, which starts the machine at set wind speeds of 8-15 mph and shuts off at 50-70 mph to protect the blades from overstressing and the generator from overheating.
  • • Power electronics to convert and condition power to the required standards.
  • • Control electronics, usually incorporating a computer.
  • • Battery for improving load availability in a stand-alone plant.
  • • Transmission link for connecting the plant to the area grid.

The following are commonly used terms and terminology in the wind power industry:

Low-speed shaft: The rotor turns the low-speed shaft at 30-60 rotations per minute (rpm).

Baix Ebre wind farm and control center. Catalonia. Spain. (From Wind Directions, Magazine of the European Wind Energy Association. London, October 1997. With permission.)

FIGURE 4.1 Baix Ebre wind farm and control center. Catalonia. Spain. (From Wind Directions, Magazine of the European Wind Energy Association. London, October 1997. With permission.)

High-speed shaft: It drives the generator via a speed step-up gear.

Brake: A disc brake, which stops the rotor in emergencies. It can be applied mechanically, electrically, or hydraulically.

Gearbox: Gears connect the low-speed shaft to the high-speed shaft and increase the turbine speed from 30-60 rpm to the 1200-1800 ipm required by most generators to produce electricity in an efficient manner. Because the gearbox is a costly and heavy part, design engineers are exploring slow-speed, direct- drive generators that need no gearbox.

Generator: It is usually an off-the-shelf induction generator that produces 50- or 60-Hz AC power.

Nacelle: The rotor attaches to the nacelle, which sits atop the tower and includes a gearbox, low- and high-speed shafts, generator, controller, and a brake. A cover protects the components inside the nacelle. Some nacelles are large enough for technicians to stand inside while working.

Pitch: Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds that have speeds too high or too low to produce electricity.

Upwind and downwind: The upwind turbine operates facing into the wind in front of the tower, whereas the downwind runs facing away from the wind after the tower.

Vane: It measures the wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

Yaw drive: It keeps the upwind turbine facing into the wind as the wind direction changes. A yaw motor powers the yaw drive. Downwind turbines do not require a yaw drive, as the wind blows the rotor downwind.

The design and operating features of various system components are described in the following subsections.

Tower

The wind tower supports the rotor and the nacelle containing the mechanical gear, the electrical generator, the yaw mechanism, and the stall control. Figure 4.2 depicts the component details and layout in a large nacelle, and Figure 4.3 shows the installation on the tower. The height of the tower in the past has been in the 20-50 m range, but in new'er installations it can be 100 m or higher. For medium and large-sized turbines, the tower height is approximately equal to the rotor diameter, as seen in the dimension drawing of a 600-kW wind turbine (Figure 4.4). Small turbines are generally mounted on the tower a few rotor diameters high. Otherwise, they would suffer fatigue due to the poor wind speed found near the ground surface. Figure 4.5 shows tower heights of various-sized wind turbines relative to some known structures.

Both steel and concrete towers are available and are being used. The construction can be tubular or lattice. Towers must be at least 25-30 m high to avoid the turbulence caused by trees and buildings. Utility-scale towers are typically twice or thrice as high to take advantage of the swifter winds at those heights.

Nacelle details of a 3.6-MW/l04-m-diameter wind turbine. (From GE Wind Energy. With permission.)

FIGURE 4.2 Nacelle details of a 3.6-MW/l04-m-diameter wind turbine. (From GE Wind Energy. With permission.)

A large nacelle under installation. (From Nordtank Energy group. Denmark. With permission.)

FIGURE 4.3 A large nacelle under installation. (From Nordtank Energy group. Denmark. With permission.)

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 the design. This requires careful avoidance of all resonance frequencies of the tower, the rotor, and the nacelle from the wind fluctuation frequencies. 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 are found from the measurements at the site under consideration. Experience on a similar nearby site can bridge the gap in 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 4.6). 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 wind tower installation. The offshore installation comes with its own challenge that must be met.

The top head mass (THM) of the nacelle and rotor combined has a significant bearing on the dynamics of the entire tower and the foundation. Low THM is generally a measure of design competency, as it results in reduced manufacturing and installation costs. The THMs of Vestas’ 3-MW/90-m turbine is 103 t, NEG Micon’s new 4.2-MW/110-m machine is 214 t, and Germany’s REpower’s 5-MW/125-m machine is about 3501, which includes extra 15-20% design margins.

A 600-kW wind turbine and tower dimensions with specifications. (From Wind World Corporation, Denmark. With permission.)

FIGURE 4.4 A 600-kW wind turbine and tower dimensions with specifications. (From Wind World Corporation, Denmark. With permission.)

 
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