Civil Engineering Reference
In-Depth Information
TABLE 5-1
Advantages of Fixed and Variable Speed Systems
Fixed-Speed System
Variable-Speed System
Simple and inexpensive electrical system
Higher rotor efficiency, hence, higher energy capture
per year
Fewer parts, hence higher reliability
Low transient torque
Lower probability of excitation of
mechanical resonance of the structure
Fewer gear steps, hence inexpensive gear box
No frequency conversion, hence, no
current harmonics present in the
electrical system
Mechanical damping system not needed, the electrical
system could provide damping if required
Lower capital cost
No synchronization problems
Stiff electrical controls can reduce system voltage sags
or more. The high number of blades was used in old low, tip-speed ratio
rotors for water pumping, the application which needs high starting torque.
The modern high, tip-speeds ratio rotors for generating electrical power have
two or three blades, many of them with just two. The major factors involved
in deciding the number of blades are as follows:
•
the effect on power coefficient.
•
the design tip-speeds ratio.
•
the cost.
•
the nacelle weight.
•
the structural dynamics.
•
the means of limiting yaw rate to reduce gyroscopic fatigue.
Compared to the two-blade design, the three-blade machine has smoother
power output and balanced gyroscopic force. There is no need to teeter the
rotor, allowing the use of simple rigid hub. Three blades are more common
in Europe, where large machines up to 1 MW are being developed using the
three-blade configuration. The American practice, however, has been in the
two blade designs. Adding the third blade increases the power coefficient
only by about 5 percent, thus giving a diminished rate of return for the
50 percent more weight and cost. The two-blade rotor is also simpler to erect,
since it can be assembled on the ground and lifted to the shaft without
complicated maneuvers during the lift. The number of blades is often viewed
as the blade solidity. Higher solidity ratio gives higher starting torque and
operates at low speed. For electrical power generation, the turbine must run
at high speed since the electrical generator weighs less and operates more
efficiently at high speed. That is why all large-scale wind turbines have low
solidity ratio, with just two or three blades.
