WIND TURBINE OPERATING POWER CURVE BASIC INFORMATION AND TUTORIALS

What is the operating power curve of wind turbine?

The power curve of a turbine is built by connecting the maximum points of individual characteristic curves at various blade pitch angles, taking into account the maximum capacity of the turbine generator.

A curve specific to a wind turbine that determines the output power versus the wind speed. This curve is used by the wind turbine controller during operation to adjust the blade pitch.

Illustrates a typical characteristic curve of wind turbines based on which a turbine is to adjust its parameters and set points for the operation at various wind speed conditions. It is interesting to understand how this curve has been generated.

The performance (power coefficient and power capture) of a turbine changes with the tip speed ratio and that for a specific rpm of a turbine the tip speed ratio depends on the wind speed. On the other hand, we would like a turbine to always work, as much as possible, with maximum power coefficient, that is, around the peak of its characteristic curve at each wind speed.

If the maximum points of the characteristic curves for each wind speed are connected together, the resulting curve shows the desired points of operation of a turbine at various speeds. This curve, when blended with a cap for the maximum capacity of the turbine generator, defines a curve based on which a turbine is scheduled and controlled. That is the curve introduced in figure 10.3; it is referred to as a wind turbine power curve.

MECHANICAL POWER IN A TURBINE BASIC INFORMATION AND TUTORIALS

Mechanical power in a turbine is the amount of power that the turbine harnesses from wind.We can determine the available power based on the wind speed, air density, and the turbine size (blade diameter).

Out of this amount of power, the rotor of a turbine can harness up to a maximum theoretical value of 59%. In fact, a turbine rotor can harness between 0% and a maximum value, which cannot exceed 59%.

That maximum value depends on the quality of the rotor design. For instance, for a particular turbine it can be 52%. This is the energy on the shaft before the gearbox represents its efficiency.

Any device, such as a gearbox or a generator, has a definite efficiency. So, the final energy output from the turbine, aft er the gearbox and the generator, will be even less.

The efficiency determines how much of the input power is available on the device output. For example, in a generator, how many kilowatts of electrical energy can it deliver for each 100 kW of mechanical power input (certainly it is less than 100, since part of the power converts to heat during the process)?

Based on the preceding discussion, the energy grasped by the rotor of a turbine is further reduced in the succeeding components. In this section, nevertheless, we are not going to bring the effect of the efficiency of the components into picture.

The intention is to study the power grasped by a turbine rotor. In particular, we want to see if there is any difference between cases when a rotor rotates at different angular speeds.

When a turbine is stationary (it is not working), it grasps 0% of the wind energy. Th is is when a turbine is yawed out of wind and its blades are feathered. In this case, one wants a turbine not to grasp any power from wind.

The position when a turbine is yawed out of wind and its blades are feathered corresponds to the minimum power grasp. Any small amount of power that the rotor may grasp is canceled by the rotor brakes (in order to make sure that there is no rotor motion).

When a turbine is yawed into the wind, the blades capture the wind and a torque is created in the rotor shaft. In addition to the wind speed, the air density, the blade size, and the blade airfoil form, the magnitude of this torque depends on the pitch angle of the blades, if this angle can vary.

In fact, changing the pitch angle alters the design of the blade. So, we need to study
a. Th e effect of the angular speed change in a turbine, and
b. Th e effect of changing the blade pitch angle.