Civil Engineering Reference
In-Depth Information
Unlike the induction machine covered later in this chapter, the synchro-
nous machine, when used in the grid-connected system, has some advan-
tage. It does not require the reactive power from the grid. This results in a
better quality of power at the grid interface. This advantage is more pro-
nounced when the wind farm is connected to a small capacity grid using
long low voltage lines. For this reason, early California plants used synchro-
nous generators. Today's wind plants generally connect to larger grids using
shorter lines, and almost universally use the induction generator.
6.1.3
Induction Machine
Most of the electrical power in the industry is consumed by the induction
machine driving the mechanical load. For this reason, the induction machine
represents a well established technology. The primary advantage of the
induction machine is the rugged brushless construction and no need for
separate DC field power. The disadvantages of both the DC machine and
the synchronous machine are eliminated in the induction machine, resulting
in low capital cost, low maintenance, and better transient performance. For
these reasons, the induction generator is extensively used in small and large
wind farms and small hydroelectric power plants. The machine is available
in numerous power ratings up to several megawatts capacity, and even
larger.
The induction machine needs AC excitation current. The machine is either
self-excited or externally excited. Since the excitation current is mainly reac-
tive, a stand-alone system is self-excited by shunt capacitors. The induction
generator connected to the grid draws the excitation power from the net-
work. The synchronous generators connected to the network must be capable
of supplying this reactive power.
For economy and reliability, many wind power systems use induction
machines as the electrical generator. The remaining part of this chapter is,
therefore, devoted to the construction and the theory of operation of the
induction generator.
6.2
Induction Generator
6.2.1
Construction
In the electromagnetic structure of the induction generator, the stator is made
of numerous coils with three groups (phases), and is supplied with
three-phase current. The three coils are physically spread around the stator
periphery and carry currents which are out of time-phase. This combination
produces a rotating magnetic field, which is a key feature of the working of
