WIND ENERGY SYSTEMS - Cd3wd

Chapter 4—Wind Turbine Power 4–43
This is obviously not excessively large. In fact, since the cost of such a clutch would be a very
small fraction of the total turbine cost, it would normally be built larger than this minimum size to
allow a greater safety factor and to permit more frequent starts. A larger clutch plate will not get as
hot during start and will radiate this heat to the surroundings more rapidly because of a larger surface
area.
A clutch guarantees a smooth start on power lines of any capacity with minimum voltage
fluctuations and power flow transients. This allows considerable flexibility in the location of
the turbine as far as power line availability is concerned.
Each size of turbine needs careful study to determine the most economical and reliable
starting technique. In one study of several point designs10, Darrieus turbines of 10- and 30-
kW ratings were found to have sufficiently low inertias that it would be quite practical to start
these turbines without a clutch and with full voltage applied to the induction machine. Sizes
of 120 kW or more were found to require a clutch, and sizes of 200 kW or more were found
to require reduced voltage starting (discussed in the next chapter) for the induction machine
even with the clutch disengaged. Once the motor is running at rated speed and rated voltage,
the clutch is engaged and the turbine is started.
We see that starting a Darrieus turbine requires careful design of the starting system.
Smaller turbines can be started easily, but larger machines require a clutch and perhaps
reduced voltage starting for the motor.
9 TURBINE SHAFT POWER AND TORQUE
AT VARIABLE SPEEDS
Most wind turbines operate at fixed rotational speeds except when starting and stopping.
This simplifies system operation when using synchronous generators paralleled with the utility
grid. It also helps to prevent the turbine from being operated at a speed which will excite a
mechanical resonance that might destroy the turbine. However, fixed speed operation means
that the maximum coefficient of performance C pm is available only at one particular wind
speed. A lower coefficient of performance is observed for all other wind speeds, which reduces
the energy output below that which might be expected from variable speed operation. That
is, if the turbine speed could be adjusted in relation to the wind speed, a higher average
coefficient of performance and a higher average power output could be realized. Variable
pitch operation at a fixed speed also helps improve the average coefficient of performance, but
this is not feasible for turbines such as the Darrieus. Variable pitch operation also increases
complexity and cost, hence may not be the most economical solution for all applications. It is
therefore interesting to explore variable speed turbine operation. We shall now examine the
variation of shaft power and torque with turbine angular velocity, leaving the discussion of
specific methods of generating synchronous power from variable speed turbines to Chapter 6.
We shall proceed by examining the variation of P m as a function of ω m , with the wind speed
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001