Powering Europe - European Wind Energy Association

operates below its peak efficiency in most of its operational
wind speed range. This has proven to be a costeffective
and robust concept and it has been scaled
up and optimised up to the 2 MW level.
The variable speed system uses power electronic converters
that enable the grid frequency to be decoupled
from real time rotational frequency as imposed
by the instantaneous wind speed and the wind turbine
control system. Variable speed operation enables
performance optimisation, reduces mechanical loading
and at the same time delivers various options for
active ‘power plant’ control. An essential feature of
variable speed wind turbines is an active blade pitch
control system, allowing full control of the aerodynamic
power of the turbine (almost comparable to the
fuel throttle of a combustion engine or gas turbine).
The decoupling of the electrical and rotor frequency
absorbs wind speed fluctuations, allowing the rotor
to act as a (accelerating and decelerating) flywheel,
and thus smoothing out spikes in power, voltage and
torque. It even enables the creation of “synthetic inertia
1 ” which is important in weak and poorly interconnected
power systems with high levels of wind power.
Until the turn of the century, the constant speed concept
dominated the market, and it still represents
a significant percentage of the operating wind turbine
population in pioneering countries such as Denmark,
Spain and regions of Germany. However, newly
chApTEr 2 Windgenerationandwindplants:theessentials
installed wind turbines are mostly variable speed
wind turbines.
Considering the wide range of technologies available,
it is useful to categorise electrical wind turbine concepts
by type of generator (including power electronics)
and by method of power control into four types A,
B, C and D, as described by Table 1 overleaf.
The significant move towards the two last concepts
(C + D represent almost 100% of sales in 2010 so
far) shows the efforts the industry has made to adapt
the design to the requirements of improved grid compatibility
with increasing wind power penetration. (The
term ‘wind power penetration’ indicates the fraction
of the gross (annual) electricity consumption 2 that is
covered by wind energy). Today’s share of the more
flexible wind turbine types accounts for approximately
75% 3 of the total installed and operating wind turbine
population worldwide. Because of historical factors
(periods of strong market growth), as well as commercial
(market position of manufacturers) and technical
ones (grid codes) there can be large regional differences
in the (cumulative) distribution of the wind turbine
types in specific regions or countries. Especially in the
first-mover countries (Germany, Denmark and Spain)
there still is a significant amount of type A technology
although this is rapidly changing, for example through
repowering. For example, in Spain 4 the distribution is:
Type A - 18%; Type B - 0%; Type C - 77%; Type D - 5%.
1 Inertia: for a definition plus brief explanation see glossary.
2 There are many ways to define ‘penetration level’. For example, wind power penetration can also be indicated as the total wind
power generating capacity (MW) in relation to peak load in the system area. If this meaning is used, it will be explicitly mentioned,
and referred to as ‘capacity penetration’. ‘Energy penetration’ is preferred in this report, because the majority of studies reviewed
measure wind power’s penetration in terms of its coverage of annual electricity consumption.
3 Own estimation based on market reviews by BTM consult (2009) and EEr (2009).
4 Based on data from AEE (Spanish wind turbine manufacturers association) 2010.
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