Electrical - Wind Energy: Renewable Energy and the Environment - page 135

available power is captured. Note : For this chapter, wind rotor will refer to the hub and blades, and
rotor will refer to the rotating part of a generator. The electrical conversion is with constant-wind
rotor rpm with squirrel cage induction generators or synchronous generators, or with variable-wind
rotor rpm with doubly fed (wound rotor) induction generators, permanent magnet alternators, or
direct-drive generators [9]. The variable-frequency output is then converted to constant frequency.
So there is a trade-off between wind rotor efficiency and the cost and efficiency of conversion for
variable frequency to constant frequency. The AC synchronous generator needs to be regulated to the
correct rpm and synchronized with the grid. Induction generators are essentially constant rpm with
a small variation, the slip. They are tied to the frequency of the grid as the grid supplies the reactive
power for the field coils of the generator.
Rural electric grids may only have one phase, so a wind turbine connected directly to these util-
ity lines would need a single-phase generator. If the wind turbine is connected through an inverter,
then the inverter can take care of the phase.
A generator is composed of the armature (coil of wire around metal core) and the field. Power is
taken from the armature, and the field, which controls the power, can be permanent magnets or an
electromagnet, an energized coil of wire. In the latter case, there are two coils of wire in the genera-
tor, with one being stationary (stator) and the other rotating (rotor). In a DC generator, the armature
rotates and power is taken off a commutator by brushes. Brushes need maintenance; therefore,
alternators are used. In an alternator, the field rotates and the variable AC output is converted to
DC by a rectifier circuit, which is then converted into constant voltage and constant frequency by
an inverter.
The advantage of the DC generator, permanent magnetic alternator, and doubly fed induction
generator is the variable-rpm (constant- C P ) operation, which is aerodynamically more efficient. For
small wind turbines (watts to 10 kW, 50 kW under development) the elimination of a speed increaser
is another advantage. Jacobs used a direct-drive, self-excited generator where the residual magneti-
zation gives the initial voltage output. Feedback from this is used to increase the field and give more
power output. The generator output can be single-phase or three-phase. The Danish Wind Industry
Association has a good explanation of types and operation of generators (see “ Links ” section).
For HAWTS, the power is transferred to ground level through slip rings, or the power cord has
enough slack to twist during yaw revolution. The second method has the desirable feature of elimi-
nating the slip rings, always a potential problem for control signals and even for power transfer.
However, strict observation schedules on length of the power drop cord or a trip relay for yaw have
to be maintained.
A number of wind turbines also use direct drive with a permanent magnet alternator. Output
is rectified to DC and then converted to AC by an inverter. Output is 120 or 240 V AC, single- or
three-phase, for small wind turbines.
Synchronous generators and self-commutated inverters require a means of disconnect for safety
during faults on the utility line because they are power sources. Induction generators at constant-
rpm operation drop offline during utility grid faults because the power for the field coils comes
from the grid. For small wind turbines, synchronous generators will probably not be acceptable for
interface with the grid, primarily due to complications of the control of the wind rotor rpm.
7. 2 .1 I NDUCTION G ENERATOR ,C ONSTANT -RPM O PERATION
Induction generators ( Figure 7.6 ) are used for wind turbines because induction motors are mass
produced, inexpensive, have reduced operation and maintenance costs, and controls are simple. The
induction motor/generator is brought up to synchronous speed and is then connected to the utility
line. All the features of synchronous generators for control of speed, excitation, and synchronizing
are eliminated as the utility line provides this aspect.
The rotor is in the center of a four-pole stator, where magnetic fields of the stator are supplied by
the three-phase utility grid. The rotor cage consists of a number of copper or aluminum bars that are