Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 57

may include rotor control and safety devices (such as sensors and a rotor brake), rotary hy-
draulic couplings, sliprings (for power and data transfer), and attached electrical and mechani-
cal equipment with their necessary cables and piping.
The proper amount of torsional lexibility in the turbine shaft can attenuate rotor torque
oscillations before they reach the gearbox. However, the turbine shaft also needs to be strong
in bending, in order to support the rotor weight. In order to meet both of these requirements,
the assembly may be composed of two concentric shafts connected at the hub end and sepa-
rated at the gearbox end (Fig. 2-8(c)). The lexible inner shaft, sometimes called a quill shaft ,
transmits only torque to the gearbox.
A HAWT speed-increasing gearbox has a step-up ratio (equal to the generator shaft
speed divided by the turbine shaft speed) that is as high as 100 in a large-scale HAWT, de-
pending on blade tip speed, rotor diameter, and generator design. Parallel-shaft, epicyclic
(planetary), and hybrid designs are used. Parallel-shaft gearboxes are more readily available
than epicyclic units, cost less, and provide easier access to the rotor for cables and piping.
To minimize hub and gear case delections that can cause premature failure of bearings and
gears, nacelle structural supports must be designed to provide adequate stiffness [Rahlf et al.
1998]. Strength alone is not suficient. This is particularly true for parallel-shaft gearboxes.
The generator drive shaft is a conventional machine element with bolted langes on both
ends. If the HAWT has a rotor brake, its disk may be mounted on this shaft rather than on
the turbine shaft for multiplication of the braking power (equal to the square of the step-up
ratio). If there is a pitch control mechanism for stopping the rotor, the rotor brake is usually
used only for emergencies, parking, and maintenance. Other components that may be on the
generator drive shaft are a rotor-positioning device, a maintenance lock, and a torque-limiting
device (with friction or break-away action).
Generators
All types of electrical generators are used in HAWTs. Small-scale turbine rotors may
drive variable-speed alternators and DC generators , minimizing the amount of rotor speed
control required. Medium- and large-scale HAWTs use AC generators . The requirements of
size, weight, eficiency, and durability for HAWT applications can usually be met by modify-
ing commercially-available generators. The requirements that most often govern the design
of HAWT generating systems are those of cost, power quality (harmonic distortion), power
factor (reactive power), and torsional damping (to attenuate cyclic torques).
Induction AC generators are often selected because of the torsional damping provided
by their inherent slip (shaft speed in excess of synchronous speed by an amount proportional
to torque) and because their cost is relatively low. A high-slip induction generator can pro-
vide a modest amount of “softness” to the power train, although eficiency is reduced in the
process. Synchronous AC generators produce higher power quality and higher eficiency
than induction generators, but they require external voltage regulators and are unable to pro-
vide signiicant softness or damping to the power train.
A variable-speed constant-frequency (VSCF) generating system can provide more damp-
ing than a high-slip induction generator, electrical eficiency and power quality approaching
that of a synchronous machine, and the ability to control turbine speed (within limits) for
increased aerodynamic eficiency. VSCF systems require additional power electronic equip-
ment which is usually located on the ground. Two general types of VSCF systems are used
in HAWTs. The irst type employs a doubly-wound generator (windings in both rotor and
stator), in which most of the power is generated in the stator at line frequency. The fraction
of power generated in the rotor is equal to the ratio of the slip frequency to the line frequency.
Rotor power is generated electronically from the slip frequency to the line frequency using
a cycloconverter . In the second type of VSCF, the generator is a conventional synchronous