Page 391 - Wind Energy Handbook

TYPE OF GENERATOR 365

will be at blade passing frequency and it is an unfortunate coincidence that this
often matches quite closely the natural frequency of oscillation of a small synchro-
nous generator connected to an electrical network.
In practice, synchronous generators are often fitted with additional cage damper
windings but it is not practical to provide the degree of damping required for wind
turbine applications. Also at higher ratings (above, say, 1 MW) second-order effects
tend to reduce the damping available from induction generators (Saad-Saoud and
Jenkins, 1999). However, the basic principle remains that the damping provided by
induction generators is necessary for fixed-speed wind turbines.
In contrast, the generators of variable-speed wind turbines are not connected
directly to the network but are de-coupled through solid-state frequency converters.
Hence synchronous generators may be used.

6.10.1 Historical attempts to use synchronous generators

Induction generators are much less useful than synchronous generators for large-
scale power generation.

• The damping action results in higher energy losses in the rotor than with
synchronous generators. It is then, of course, necessary to arrange for the removal
of the heat dissipated in the rotor.

• All the reactive power necessary to energize the magnetic circuits must be
supplied from the network (or by local capacitors). If local capacitors are used
then there is the danger of self-excitation, when connection to the network is lost.

• There is no direct control over the terminal voltage or reactive power flow.

• Induction generators do not produce sustained fault current for three-phase faults
on the network.

• They suffer from problems of voltage instability. This was not an important issue
with limited wind generation but with very large wind farms is becoming of
concern.

Hence, in the early development of wind turbines considerable efforts were made
to use synchronous generators. These involved a number of innovative solutions to
the provision of damping. For example, both Westinghouse in the USA and
Howden in the UK used fluid couplings in the drive train to provide damping. The
Wind Energy Group in the UK mounted a 250 kW synchronous generator using a
spring-damper system and connected a 3 MW synchronous generator through a
sophisticated variable-speed mechanical gearbox (Law, Doubt and Cooper, 1984).
However, these and other similar approaches using synchronous generators on
large prototype wind turbines are now of historical interest only.