06-12-2012, 11:38 AM
Power Electronics Converters for Wind Turbine Systems
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Abstract
The steady growth of installed wind power together
with the upscaling of the single wind turbine power capability has
pushed the research and development of power converters toward
full-scale power conversion, lowered cost pr kW, increased power
density, and also the need for higher reliability. In this paper,
power converter technologies are reviewed with focus on existing
ones and on those that have potential for higher power but which
have not been yet adopted due to the important risk associated
with the high-power industry. The power converters are classified
into single- and multicell topologies, in the latter case with attention
to series connection and parallel connection either electrical
or magnetic ones (multiphase/windings machines/transformers).
It is concluded that as the power level increases in wind turbines,
medium-voltage power converters will be a dominant power
converter configuration, but continuously cost and reliability are
important issues to be addressed.
INTRODUCTION
WIND turbine system (WTS) technology is still the most
promising renewable energy technology. It started in the
1980s with a few tens of kW power production per unit. Today
multi-MW size wind turbines are being installed [1]–[4], and
they are very advanced power generators. There is a widespread
use of WTSs in the distribution networks as well as there are
more and more wind power stations which are connected to
the transmission networks. Denmark for example has a highpower
capacity penetration (> 30%) of wind energy in major
areas of the country, and today 25% of all the electrical energy
consumption is covered by wind energy. The aim is to achieve
a 100% nonfossil-based power generation system in 2050 [5].
Initially, wind power did not have any serious impact on the
power system control, but now due to its size.
WIND TURBINE SYSTEMS
Wind power has until now grown to a cumulative worldwide
installation level of 200 GW with close to 40-GW installed
alone in 2010, according to BTM Consult, indicating that
wind power is really an important player in some areas of the
world [9]. The worldwide penetration of wind power electricity
was 1.8%, and the prediction for 2019 is more than 8% or 1 TW
cumulative installations. China was the largest market in 2010,
and in general the EU, the USA, and China are sharing around
one third of the total market. The global accumulated installed
power capacity is shown in Fig. 1.
In 2010, the Danish company Vestas Wind Systems A/S was
still in the top position among the largest manufacturers of wind
turbines in the world, closely followed by the Chinese company
Sinovel as the second largest in the world. This company was
third in 2009, and this data gives some idea of the impressive
interest in Asia for wind energy. Fig. 2 shows the wind turbine
top suppliers in 2010, also published by BTM Consult.
Generators in Wind Turbine Systems
Synchronous generators, either externally excited or with
permanent magnets, are becoming the preferred technology
in the best seller power range [1]–[4]. Multipole permanent
magnet synchronous generator (PMSG) with a full power backto-
back converter looks to become the most adopted generator
in the near future due to the reduced losses and lower weight
if compared to the externally excited SG that is manufactured
successfully by, e.g., the German company Enercon. In the last
case, the generator is an anular generator, and rotor current
is used to regulate the dc link voltage. The transition seems
mainly to be valid for larger wind turbines (3–6 MW). However,
the increased prices of rare-earth magnets might change the
philosophy of wind turbine drive trains to avoid high risk in
expenses.
RELIABILITY ISSUES
The penetration of wind power into the power grid is fast
growing even expected to be 20% of the total electricity production
at 2020 in Europe [49]. Meanwhile, the power capacity of
a single wind turbine is increasing continuously in order to reduce
the price pr. produced kWh, and the location of wind farms
is moving from onshore to offshore because of land limits and
more wind energy production in the offshore. Consequently,
due to much more significant impacts to the power grid, as
well as higher cost to maintain and repair after failures than
ever before, the wind power generation system is required to
be more reliable and able to withstand some extreme grid or
environment disturbances. Reliability is one of the key issues
that concerns WTS manufacturers and investors in order to
ensure high-power security (availability). Market feedback has
shown that the control and power converters seem to be more
prone to failure even though the generator and gearbox have the
largest downtime, as shown in Fig. 19 [50]. The need for higher
power density in power converters, as already outlined, leads
to more compact design, reduced material use, and equipment
cost [51]. All may invoke new failure mechanisms in the power
and control electronics. Exposure to moisture, vibration, dust,
chemicals, high voltage, and temperature is predominant in
WTSs failure drivers.
CONCLUSION
The paper has given an overview of different power electronic
converters in WTSs with special attention paid to the
many possible topologies at low voltage and medium voltage.
An important trend is that the technology is moving toward
a higher power level, and it is inevitable that it goes for
higher voltage and as a consequence into multilevel singlecell
structures or to multicell modular structures that can even
use standard low voltage power converter modules. One current
concern beyond being able to upscale the power is being better
able to predict reliability of power electronic converters and
control, as it has been a major failure cause in WTS, and
better lifetime prediction and condition monitoring methods in
the future will be important to improve the technology.