Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 49

1.4.3.1 Squirrel-Cage Induction Generators (SCIG)

A squirrel-cage induction machine is often operated as a motor but it can be operated as
a generator when driven by a prime mover to a speed exceeding the synchronous speed.
Induction machines are widely applied as generators in wind power applications due to the
reduced unit cost and size, ruggedness, lack of brushes, absence of a separate DC source, ease
of maintenance, self-protection against severe overloads and short circuits, etc. (Bansal 2005).
An induction generator produces real power but it needs reactive power to establish the
excitation (the magnetic field). This leads to a low power factor, which is often penalised by
utility companies. The reactive power needed for excitation can be provided by a capacitor
bank, the grid or a solid-state power electronic converter. The connection of an SCIG, in
particular a big one, to the grid often causes a large inrush current that is 7

8 times of
the rated current and a soft-starter is often needed. The pole pair number of SCIG used
in commercial fixed-speed wind turbines is often equal to 2 or 3, which corresponds to a
synchronous speed of 1500 rpm or 1000 rpm for a 50 Hz system. As a result, a three-stage
gearbox is often required in the drive train.
SCIGs are often applied in fixed-speed wind turbine systems directly connected to the
grid through a transformer, as shown in Figure 1.43(a). With this topology, the rotor blades
are directly fixed to the hub and adjusted only once when the turbine is erected. The power
limitation over the rated wind speed is achieved by stalling the rotor blades. Wind turbines with
this topology are completely passive and, hence, this topology is called passive stall control or
shortly stall control. In most cases, capacitors are connected in parallel to provide the reactive
power needed for excitation.
There are obvious advantages of using SCIGs. However, there are also disadvantages. The
speed of operation is not controllable and it can be varied only within a very narrow range
because the rotor circuit is not accessible, which makes it difficult to extract the maximum
available wind power. The need for a three-stage gearbox in the drive train considerably
increases the weight of the nacelle, and the investment and maintenance costs. Moreover, it is
necessary to obtain the excitation current from the grid, which makes impossible to support
the grid voltage.

∼

1.4.3.2 Doubly-fed Induction Generators (DFIG)

The fact that the rotor circuit of an SCIG is not accessible can be changed if the rotor circuit is
wound and made accessible via slip rings, which offers the possibility of controlling the rotor
circuit so that the operational speed range of the generator can be increased in a controlled
manner. The rotor circuit is often connected to back-to-back power electronic converters,
which consists of a rotor-side converter and a grid-side converter sharing the same DC bus, so
that the difference between the mechanical speed of the rotor and the electrical speed of the
grid can be compensated via injecting a current with a variable frequency into the rotor circuit.
Hence, the operation during both normal and faulty conditions can be regulated by controlling
the converters.
A DFIG can be excited via the rotor windings and does not have to be excited via the stator
windings. If needed, the reactive power needed for the excitation from the stator windings can
be generated by the grid-side converter. As a result, a wind power plant equipped with DFIGs
can easily take part in the regulation of grid voltage. The stator always feeds real power to the