Page 191 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

178 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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discussed. The topics of power electronics, ranging from devices to circuit topologies, and similar
matters for electric generators, together with the results of optimal design studies, are included. It
is shown that the individual power rating of WTs has increased over the years and technologies
required in order to reach and even exceed a power rating of 10 MW are discussed. The role of
power electronics for improving the operation of WTs and ensuring compliance with power grid
codes is analyzed with a view to produce fully controllable generation units suitable for a tight inte-
gration into the power grid and large-scale deployment in the future smart power systems.

8.1 INTRODUCTION
The cumulative installations of wind turbines (WTs) have grown at a fast pace over the last two
decades. Even according to conservative estimations for continued developments, the installed wind
power generation, which is currently greater than 360 GW, is expected to exceed 760 GW by 2020,
making this form of renewable energy a significant component of modern power systems [1]. An
example is set by Denmark, which has a very high penetration of wind generation that covered
42% of the electric energy consumption in 2015. As an important milestone, on November 3, 2013,
Denmark set a record by having at national level wind power production in excess of power con-
sumption [2], and now it happens regularly.
Not only that the installations of wind farms have grown significantly, but also the size and the
power rating of WTs have increased dramatically. Just a few years ago, in 2011, the average rating
of WTs was 1.7 megawatts (MW) for onshore and 3.6 MW for offshore installations. By the end of
the decade, the number of high-power turbines, up to 10 MW rating, is expected to grow pushing
the average rating further up [3–10]. Currently, the world’s most powerful WT, the Vestas V-164,
is rated for 8 MW and employs a 164 m rotor [9]. Most manufacturers are developing WTs larger
than 4.5 MW, a trend that is aimed at significantly lowering the cost of wind energy delivered to the
power grid.
All these continuous WT developments would not have been possible without significant techno-
logical advancements, including those for power electronic drives, controls, and electric generators
that represent the scope of this chapter. Power electronics and related variable speed technologies
enabled, among other things, the reduction in mechanical stress and the increase in energy produc-
tion and made possible the operation of a wind turbine system (WTS) as a fully controllable genera-
tion unit suitable for a tight integration into the power grid.
This chapter first introduces the fundamentals, main components, and subsystems of WTS. The
main solutions, which are commercially available, will also be briefly reviewed in Section 8.2. The
specific issues of WTS, including complex mission profiles, power grid codes, and reliability, will
be discussed in Section 8.3. Section 8.4 covers power electronics, starting from the device to the
circuit level, including state-of-the-art technologies and future trends. The solutions and configura-
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tions of power electronics for wind farms will be introduced in Section 8.5. A MATLAB simula-
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tion example is presented in Section 8.6, ANSYS models are discussed in Section 8.7, and a final
summary is provided.

8.2 WIND TURBINE SYSTEMS

The main components employed for energy conversion from wind to electricity in a state-of-the-art
WTS include the rotor with turbine blades, possibly a gearbox (which is eliminated in direct drive
solutions), an electric generator, a power electronics converter, and a transformer (see Figure 8.1)
for interconnecting to the power grid.
Controlling a WTS involves both electrical and mechanical subsystems, as indicated in Figure 8.2,
where a general control structure for a WTS including turbine, generator, and converter is illustrated
at three different layers. The applied WT concept can either be the full-scale converter-based system