load is the input switch matrix of the power electronics. For a constant-speed wind turbine—one without power electronics—the generator load is the utility grid.

The nacelle yaw drive may be an electric or hydraulic motor acting through gearing to rotate the nacelle and rotor. The function of the yaw drive is to orient the wind turbine relative to the prevailing wind direction.

The blade pitch actuator effects the rotation of the rotor blades about their pitch axis. With the blade pitch angle set at the full-power angle, maximum power is extracted from the incident wind flow field. As the blade pitch angle is rotated toward the full feather position, the blades become less efficient at converting the power in the wind flow field to shaft power.

Also listed in the first column of Table 6-1 are two controls that are effected by the power electronics. The generator torque may be controlled by the power electronics acting on the electrical characteristics of the generator. The power electronics, acting as the load for the generator, can electronically vary the load and thus the drive train torque. The reactive power delivered to or received from the utility grid may also be controlled by the power electronics. This is important for maintenance of one of the desired electrical characteristics (power factor) of the power delivered to the utility system.

The second column of Table 6-1 lists the typical sensor complement associated with a wind turbine. Given also is the physical information measured by these sensors. The contactor status sensor indicates whether the contactor is open or closed, that is, whether the generator is connected to the load. The nacelle orientation sensor measures the angular position of the nacelle, either relative to a fixed reference or to the wind direction. The wind direction sensor measures the angular direction from which the wind blows. The wind speed sensor measures the wind speed. This sensor may be shared by a number of wind turbines or may be absent altogether. If absent, wind speed may be derived from knowledge of the blade airfoil characteristics, the power level, and the drive train rotational speed. The blade pitch angle sensor measures the pitch angle of the blades. The generator power sensor measures the real power flow into, or delivered by, the generator. Typically, the reactive power flow is also measured. The generator speed sensor measures the generator rotational speed and, through knowledge of the gearbox ratio, the speed of the low-speed shaft.

Of importance to control are two quantities that may be estimated from the sensor measurements. These are the wind speed and the torque in the low-speed shaft. Estimation of the wind speed value was discussed above. Estimation of the torque value proceeds from knowledge of the power, the rotational speed, and the gearbox ratio.

RECENT TRENDS IN CONTROL SYSTEM THEORY

The 1980s witnessed tremendous strides in the development of theory and algorithms applicable to the design of dynamic compensators for both single-input single-output (SISO) and multiple-input multiple-output (MIMO) systems under different sets of assumptions about the process to be controlled and with a variety of performance/robustness criteria satisfied.

In the area of linear time invariant (LTI) systems with parameters assumed to be known, perhaps the greatest breakthrough in the early phase was the unification of frequency and time domains so as to allow for the design of compensators that satisfy frequency domain specifications and robustness constraints using time domain synthesis tools (i.e., solutions to Riccati equations).

Clearly, the advantage of using time domain mathematics, which is finite dimensional and numerically robust, has greatly facilitated control system designs for arbitrary plant dimensions both for SISO and MIMO systems. In the H2 framework, a two-norm formulation was adopted, admitting the interpretation that root mean square (rms) tracking errors are to be minimized in a fixed spectrum, flat power spectral density (PSD), disturbance environment. Other measures of performance may be desirable, such as minimizing the worst possible error (in the frequency domain), captured by an infinity norm (H)



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