Generator Drives

The turbine speed is generally much lower than the desired speed for the electrical generator. For this reason, the turbine speed in most wind systems is stepped up using a drive system. The system can be fixed-speed or variable-speed as described in this chapter.

The wind-power equation as derived in Chapter 3 is as follows:

where Cp = rotor power coefficient.

As seen earlier, the value of Cp varies with the ratio of the rotor tip-speed to the wind speed, termed as the tip-speed-ratio TSR. Figure 7-1 depicts a typical relationship between the power coefficient and the tip-to-speed ratio. As the wind speed changes, the TSR and the power coefficient will vary. The Cp characteristic has single maximum at a specific value of the TSR. Therefore, when operating the rotor at constant speed, the power coefficient will be maximum at only one wind speed.

For achieving the highest annual energy yield, the value of the rotor power coefficient must be maintained at the maximum level all the time, regardless of the wind speed. The theoretical maximum value of Cp is 0.59, but the practical limit is 0.5. Attaining Cp above 0.4 is considered good. Whatever value is attainable with a given wind turbine, it must be maintained constant at that value. Therefore, the rotor speed must change in response to the changing wind speed. To achieve this, the speed control must be incorporated in the system design to run the rotor at high speed in high wind and at low speed in low wind. This is illustrated in Figure 7-2. For given wind speeds V1, V2, or V3 , the rotor power curves versus the turbine speed are plotted in solid lines. In order to extract the maximum possible energy over the year, the turbine must be operated at the peak power point at all wind speeds. In the figure, this happens at points P1, P2, and P3 for the wind speed V1, V2, and V3, respectively. The common factor among the peak power production points P1, P2, and P3 is the constant high value of TSR, close to 0.5.

Operating the machine at the constant tip-speed ratio corresponding to the peak power point means high rotor speed in gusty winds. The centrifugal

Power Coefficient And Tsr

FIGURE 7-1

Rotor power coefficient versus tip-speed ratio has a single maximum.

FIGURE 7-1

Rotor power coefficient versus tip-speed ratio has a single maximum.

forces produced in the rotor blades under such speeds can mechanically destroy the rotor. Moreover, the generator producing power above its rated capacity may electrically destroy the generator. For these reasons, the turbine speed and the generator power output must be controlled.

7.1 Speed Control Regions

The speed and the power controls in the wind power systems have three distinct regions:

• the optimum constant Cp region.

• the speed-limited region.

• the power-limited region.

These regions are shown in Figure 7-3. Typically the turbine starts operating (cut in) when the wind speed exceeds 4-5 m/s, and is shut off at speeds exceeding 25 to 30 m/s. In between, it operates in one of the above regions. At a typical site, the wind-turbine may operate about 70 to 80 percent of the time. Other times, it is off due to wind speed too low or too high.

Wind Turbine Characteristics Figure

FIGURE 7-2

Turbine power versus rotor-speed characteristics at different wind speeds. The peak power point moves to the right at higher wind speed.

FIGURE 7-2

Turbine power versus rotor-speed characteristics at different wind speeds. The peak power point moves to the right at higher wind speed.

Rotor Speed Wind Speed

FIGURE 7-3

Three distinct rotor-speed control regions.

FIGURE 7-3

Three distinct rotor-speed control regions.

The maximum Cp region is the normal mode of operation, where the speed controller operates the system at the optimum constant Cp value stored in the system computer. Two alternative schemes of controlling the speed in this region were described in Section 5.6.

In the constant Cp region, the control system increases the rotor speed in response to the increasing wind speed only up to a certain limit (Figure 7-4). When this limit is reached, the control shifts into the speed-limiting region. The power coefficient Cp is no longer at the optimum value, and the rotor power efficiency suffers.

If the wind speed continues to rise, the systems will approach the power limitation of the electrical generator. When this occurs, the turbine speed is reduced, and the power coefficient Cp moves farther away from the optimum value. The generator output power remains constant at the design limit. When the speed limit and power limit cannot be maintained under extreme gust of wind, the machine is cut out of the power producing operation.

Two traditional methods of controlling the turbine speed and generator power output are as follows:

(1) The pitch control in which the turbine speed is controlled by controlling the blade pitch by mechanical and hydraulic means. The power fluctuates above and below the rated value as the blade pitch mechanism adjusts with changing wind speed. This takes some time because of the large inertia of the rotor. Figure 7-4 depicts variations in the wind speed, the pitch angle of the blades, the generator speed and the power output with respect to time in a fluctuating wind. The curves represent actual measurements on Vestas 1.65 MW wind turbine with OptiSlip® (registered tradename of Vestas Wind Systems, Denmark). The generator power output is held constant even with 10 percent fluctuation in the generator speed, thus, minimizing the undesired fluctuations on grid. The elasticity of the system also reduces the stress on the turbine and the foundation.

(2) The stall control in which the turbine uses the aerodynamic stall to regulate speed in high winds. The power generation peaks somewhat higher than the rated limit, then declines until the cut-out wind speed is reached. Beyond that point, the turbine stalls and the power production drops to zero (Figure 7-5).

In both methods of speed regulation, the power output of most machines in practice is not as smooth. The theoretical considerations give only approximations of the power produced at any given instant. For example, the turbine can produce different power at the same speed depending on whether the speed is increasing or decreasing.

7.2 Generator Drives

Selecting the operating speed of the generator and controlling it with changing wind speed must be determined early in the system design. This is

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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