Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 226

a typical alternator carry only enough current to magnetize the field. There are no brushes
that need replacement. Like the DC generators before them, today's alternators are driven at
variable speeds. Variable-frequency AC output is then rectified to DC for charging batter-
ies. In other applications, the variable-frequency, variable-voltage AC is first rectified to DC
and then inverted back to constant-frequency AC, producing utility-grade electricity. This is
a so-called AC-DC-AC electrical system. Most wind turbine alternators use electromagnets
in the spinning field, but permanent magnets also play an important role because they do not
require slip rings. The Bergey Excel and the Kestrel e400 are examples of small-scale wind
turbines using permanent-magnet direct-drive AC alternators.

Inverters

As mentioned above, some form of inverter is needed to produce constant-frequency,
constant-voltage power from a variable-speed alternator. Inverters often use solid-state
switches to approximate the AC wave form of a true synchronous generator. Because invert-
ers can also convert DC electricity to AC, some power systems use a wind turbine to charge
batteries, and then use the battery-supplied DC to drive a motor-generator set (called a rotary
inverter ) to generate utility-grade AC. However, rotary inverters are only 60 percent effi-
cient, while solid-state inverters can be up to 90 percent efficient.
In interconnected applications, a synchronous or line-commutated inverter is used to
convert the alternator's output to line-quality AC. The synchronous inverter uses the utility-
line wave form as a signal to fire or switch a thyristor bridge . Thyristors act as gates that pass
current at the proper voltage as necessary to produce an AC wave form shaped like that of the
electric utility. Synchronous inverters are susceptible to line transients and lightning surges
and are costly to repair. Nevertheless, they are working reliably in hundreds of installations
across the United States.

Electrical Connections

Figure 4-5 is a one-line electrical diagram for a typical wind turbine connected to a
utility grid. Standard electrical protection devices are employed on commercial wind tur-
bines to detect the following fault characteristics:

- over- and under-voltage

- over- and under-frequency

- over-current (current detectors for each phase)

- line out-of-phase

- generator over-temperature

Lightning protection and capacitors for power factor correction are generally used with com-
mercial wind turbines. Electrical energy from the turbine generator is transferred from the mov-
able nacelle to the ground by use of slip rings or a droop cable . Droop cables are fixed at both
ends and can be wound up to the point where they are destroyed unless some provision is made
to periodically unwind them. Many wind turbines use an active yaw system to unwind the cable.
Other approaches employ a pop-out connector at the base of the tower to prevent damage to the
droop cable from excessive twisting. If there is no active yaw system on the wind turbine, the
cable must be untwisted periodically by shutting the turbine down, which has proven to be only a
minor inconvenience. Because droop cables lower initial costs and maintenance requirements by
eliminating the need for slip rings, they are now employed in most commercial wind turbines.
Most of the problems that have occurred with wind turbine electrical systems have been
traced to the use of components manufactured with marginal or substandard quality. The use