Environmental Engineering Reference
In-Depth Information
high by 2.6 m wide. Rail transportation is even more dimensionally limited, although some
rail lines now have special cars designed for transporting large wind turbine components.
Crane requirements also limit the practical size of wind turbines, because of the combi-
nation of larger nacelle masses, higher lifts, and larger boom extensions. As these three fac-
tors increase, the number of available cranes decreases. Other limiting factors are that cranes
with very large lifting capacities are dificult to transport overland, require large crews, and
therefore have high mobilization, operation, and de-mobilization costs.
Offshore Turbine Size
It should be noted that the previous discussion of turbine size
vs.
cost is largely ap-
plicable only to land-based turbines, because the economic considerations driving the cost
tradeoffs for offshore turbines are much different. For offshore wind power plants, the cost
of the underwater support structure is a very signiicant portion of the total cost, as well as the
cost of working and installing a wind turbine at sea. Additionally, there is a much larger cost
advantage for high reliability in the operation and maintenance (O&M) of an offshore turbine
relative to an onshore turbine of the same size. This is due to the dificulty of servicing the
turbine during high wind periods, when access to offshore turbines may be prohibited for
periods of several days for safety reasons. As a result, minor faults causing turbine outages
must be eliminated by high-reliability design, even if the resulting costs are higher.
Variable Speed Controls
Today's commercial wind turbine controllers integrate the signals from dozens of sen-
sors to control rotor speed, blade pitch angle, generator torque, and power conversion voltage
and phase. The controller is also responsible for critical safety decisions, such as shutting
down the turbine when extreme conditions are realized. Today, most turbines operate at vari-
able speed for peak aerodynamic eficiency. The control system regulates the rotor speed to
keep the ratio of the blade tip speed to the wind speed approximately constant at its optimum
value as the wind speed varies. Continuously updating the rotor speed and generator loading
maximizes output power. The controller also regulates the electrical torque of the generator
to reduce drivetrain transient torque loads.
Operations at variable speed require the use of power converters, which also enable tur-
bines to deliver
fault ride-through protection
, voltage control, and dynamic reactive power
support to the grid. Because there are now large quantities of wind turbines supplying power
to the electrical system, utilities are requiring that wind power plants remain on line and
capable of providing power in spite of electrical faults that last as long as several electrical
cycles.
Evolution of Power Trains
Background
Wind generation of electricity places an unusual set of requirements on electrical drive
systems. Most applications for electrical drives are aimed at using electricity to produce
torque, rather than using torque to produce electricity. Moreover, other electricity generation
sources that produce electricity from torque usually operate at a constant power, or load.
Wind turbines, on the other hand, must generate electricity at all power levels and spend
a substantial amount of time at low power levels. Unlike most electrical machines, wind
generators must operate at the highest possible aerodynamic and electrical eficiencies in the
low-power/low-wind speed region to squeeze every kilowatt-hour out of the available wind
energy.
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