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Design Considerations for Wind Turbine Systems 257

(1) PMSG, (2) self-excited induction generator (SEIG), (3) squirrel cage induction generator
(SCIG), and (4) DFIG. Figure 10.5 shows the most typical commercially available electrical gen-
erator systems for wind energy applications.
The wind energy systems can be classified in several ways, but they broadly fit into two catego-
ries: (1) the fixed and (2) the variable speed wind generator systems.

10.4.1 Fixed Speed Wind Turbines

The SCIG can be used as a fixed speed wind generator, directly connected to the power grid using
a step-up transformer. The rotor speed of an SCIG, which can be expressed in terms of the slip(s),
varies with the amount of power generated. However, the rotor speed variation is very small, in
the order of 2%–5% of the rated speed. This type of wind generator has many advantages, such
as very little maintenance; rugged construction; low-cost, short-circuit resilience; and operational
simplicity. On the other hand, as the rotor speed cannot be varied, fluctuations in the wind speed
are translated directly into drivetrain torque fluctuations, causing higher structural loads than with a
variable speed generator.
A very important factor for SCIGs directly connected to the grid is the need for reactive power
to establish the rotating magnetic field of the stator. In case of large turbines and weak grids, a
capacitor bank is connected at the terminal of the fixed speed wind generator. The capacitor bank
is designed for rated conditions to maintain unity power factor operation, so the voltage fluctua-
tion is natural in other operating conditions for fixed speed wind generator. Some turbines have
two IGs connected to the grid, a small one and a large one, with different number of poles, and
depending on the wind speed conditions, one of them will be selected to be connected to the grid.

10.4.2 Variable Speed Wind Turbines

The second type is the variable speed wind generator, which has again many subclassifications.
The commercially available variable speed wind generators can be IGs with front-end converters,
DFIGs, wound field synchronous generator (WFSG), or PMSG. The superior advantage of variable
speed operation over that of the fixed speed is that more wind energy can be extracted for a specific
wind speed regime. These variable speed wind generators, in general, are equipped with power elec-
tronic converters, rated partially or fully. Therefore, there are some losses in the power electronic
converters, but the aerodynamic efficiency increases due to variable speed operation increasing the
overall efficiency. The aerodynamic efficiency gain can exceed the electrical efficiency loss, result-
ing in higher overall efficiency. The mechanical stress is less with variable speed operation; the cost
of the variable speed wind generator is usually higher but can be paid by the extra energy produc-
tion. Further, it can provide an important ancillary service, that is, reactive power compensation for
weak grids along with fault ride-through capability. Nowadays, most of the wind turbines are using
the variable speed technology.
The modern variable speed wind turbine generator system topologies are shown in Figure 10.5.
For the DFIG, a frequency converter, which consists of two partial-rated (30%–35% of nominal
power of the generator) back-to-back voltage source converters, feeding the three-phase rotor wind-
ing through brushes and slip rings. Therefore, the mechanical and electrical rotor frequencies are
decoupled. This wind generator requires a gearbox as the DFIG is a high-speed machine and limited
to a few number of poles. On the other hand, with the direct-drive synchronous generator system
(PMSG or WFSG), the generator is completely decoupled from the grid by a fully rated frequency
converter, composed of back-to-back voltage source converters. The generator-side converter can
be either a voltage source converter or a diode rectifier for small power applications. The generator
is excited using either an excitation winding (in the case of WFSG) or permanent magnets (in the
case of PMSG).