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Design Considerations for Wind Turbine Systems 259

Other possible variations are either an SCIG or a conventional synchronous generator connected
to the wind turbine through a gearbox and to the grid by a power electronics converter of the full
rating of the generator. Recently, variable speed wind generator using switched reluctance generators
have also been developed as they can provide multiple poles [2].

10.5 WIND SPEED AND GENERATOR PERFORMANCE
The energy captured through the shaft of a wind turbine and converted into electrical energy can
be evaluated by data analysis of the historical wind power intensity (in W/m²) in order to access
the economic viability of a potential site; please refer to Chapter 7. It is appropriate to define
local wind power as proportional to the distribution of wind speed occurrence. A statistics-based
design should consider the different sites where the turbines will be installed. Thus, with the same
annual average speed, very distinct wind power characteristics may affect the optimal design of
the generator. Figure 7.4 displays a typical curve of wind speed distribution for a given site. If the
wind speed is lower than 3 m/s (denominated by “calm periods”), the power becomes very low
for the extraction of energy and the system is usually stopped. Calm periods will determine the
necessary time for energy storage. Power distribution varies according to the intensity of the wind
and with the power coefficient of the turbine. Then, a typical distribution curve of power may have
the form as shown in Figure 7.5. Sites with high average wind speeds do not have calm periods,
and there is no serious need for energy storage. However, high wind speed may cause structural
problems in the system or in the turbine. The vertical axis of Figure 7.4 is given in percentage of
hours/year per meter/second. For an optimal design on an electrical generator, the random nature
of wind distribution in a particular site is considered in order to design the best operating range,
defining electrical characteristics such as machine frequency and voltage ratings. The majority of
losses occur because a gearbox is used to match generator speed with turbine speed.
An electrical generator used for a wind turbine system has an efficiency that is imposed by three
main parts, (1) stator losses, (2) converter losses, and (3) gearbox losses, as depicted in Figure 7.2.
Stator losses are considered by a proper design of the machine for the right operating range; converter
losses are given by proper design of the power electronic circuits (on-state conduction losses of tran-
sistors and diodes, plus their frequency proportional switching losses, which may be neglected). One
of the main factors responsible for a noticeable power loss is the use of a gearbox, as discussed in
Chapter 7. The mechanical viscous losses due to a gearbox are proportional to the operating speed,
as indicated by the following equation:
η
P gear = P gear rated, (10.6)
η rated

where
is the loss in the gearbox at rated speed (in the order of 3% of rated power)
P gear rated,
η is the rotor speed (r/min)
η rated is the rated rotor speed (r/min)

Losses due to the use of a gearbox dominate the efficiency in most wind turbine systems, and
simple calculations show that there occurs significant power dissipation in the generator system
due to the gearbox. From the full energy available in the wind, just part of it can be extracted for
energy generation, quantified by the power coefficient, C p . The power coefficient is the relationship
of the possible power extraction and the total amount of power contained in the wind. The turbine
mechanical power P can be calculated by Equation 10.7.
t
C p ( ρ AV )
3
P t = (in kg ⋅m/s ) (10.7)
2