The evolution of technology within the wind sector has experienced a very rapid development during the last years. Every year the most important manufacturers present new turbines. The price at auctions has gone hand in hand with this development, experiencing drastic reductions. Thus, manufacturers have upgraded the power rate of their newest turbines trying to reduce the Levelized Cost of Energy (LCoE). Ingeteam analyses the optimal wind turbine topology up to 15MW.

1. The evolution of technology within the wind sector has experienced a very
rapid development during the last years. Every year the most important
manufacturers present new turbines. The price at auctions has gone hand in
hand with this development, experiencing drastic reductions. Thus,
manufacturers have upgraded the power rate of their newest turbines trying
to reduce the Levelized Cost of Energy (LCoE).
Analysisofoptimal
windturbinetopology
upto15MW
Words: Jokin Aguirrezabal, Power Converter Platform Manager, Ingeteam Wind Energy
Itziar Kortazar, FC Product Manager, Ingeteam Wind Energy
Aritz Lorea, DFIG Product Manager, Ingeteam Wind Energy
Introduction
This rapid evolution, added to the impulse of
Full Converter (FC) topology and the
possibility of using different voltage levels
(low and medium voltage) unfolds a complex
debate about which topology will lead the
wind market in the future.
In parallel, the semiconductor market has
experienced a similar evolution providing
new chips with improved features that have
opened even more the possible solutions.
Thereby, the objective of this document is to
show a general view of the capabilities of
each topology in order to detect the
advantages for a new design. The main
topologies within the market will be
assessed: DFIG (DFIG), Full Converter Low
voltage (FC LV) and Full Converter Medium
voltage (FC MV).
In any event, it has to be said that the final
topology selection depends on lot of factors
of the whole system. Site emplacement
(wind class), price of the energy and the
power range are the main ones. The great
variability on these factors explains the fact
that several solutions coexist nowadays.
DFIG wind turbines
For the last two decades, Doubly Fed
Induction Generator (DFIG) converters have
been the most extended topology in the
power range of 2.XMW. The low power of the
converters, the efficiency and the overall low
cost of the drivetrain system have made this
topology almost a standard in low power and
low voltage wind turbines.
Taking advantage of the main benefits of the
DFIG topology, the wind power penetration
into the power generation system has
substantially increased during the last
twenty years. In this sense, and with the
increasingly importance of the power quality
and grid stability, the products have evolved
and are already capable to be installed in
designs of up to 6xMW in 1 power conversion
line configuration (PCL).
The main constraint of DFIG wind turbines
are mainly the FRT events. These are usually
caused by short circuits produced in the grid.
Generation systems must be able to keep
connected during these events in order to
support the grid stability.
In the past, most widely used solutions to
fulfil these requirements consisted in
actively controlled crowbar-base systems.
These systems, the so-called active crowbar
in DFIG turbines, a shunt circuit composed of
actively controlled switches and dissipation
elements which are connected in order to
shunt the high currents, avoiding damages to
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the converter, were identified to have
limitations. Besides, the system presented
the limitation that while the active crowbar
was connected, the power converter was not
in operation, the active and reactive power
set points couldn’t be tracked, and therefore
the wind turbine wasn’t being controlled.
Nonetheless,severalyearsago,Ingeteam
developedaDFIG‘Crowbarless’productrange
basedonamodularsolution,whichincludedan
FRTsystemthateliminatedtheneedtouse
activelycontrolledcrowbarsystems.
Ingeteam’ssolutiontofulfilthisdesigninputis
basedonaModularFRTsystemcomposedof
choppermodules.Inaddition,theModularFRT
systemisanoptimalsolution,whichmeans
thatdependingonthegridcodetobefulfilled,
andontheelectricalcharacteristicsofthe
system,especiallythestatorconfiguration,
thenumberofchoppermodulescouldbe
selectedallowingforacost-optimalpower
converterforeachwindfarmscenario.
FC wind turbines
The Full Converter topology, unlike DFIG
turbines, completely decouples the
generator from the grid. Thus, the generator
can work at any rotational speed. Besides,
grid transients do not affect the dynamics of
the generator. This kind of wind turbine also
provides complete control over active and
reactive power exchanged with the grid.
Apart from this, the smoother behaviour
during FRT where no surge currents appear
in the machine side converter and the extra
power capability, both active and reactive
power, make them possible to also work as
grid stabilizers. In this case, and depending
Fig 1. 3.X MW DFIG converter (Crowbarless)
Fig 2. 3.x MW FC converter (Low voltage solution)
on the wind farm location and turbine
number, the initial extra cost of a FC solution
can be justified.
Another interesting topic that arises when it
comes to FC topology is the voltage level.
Nowadays both low voltage (LV) and medium
voltage (MV) converters are a reality. The
decision of choosing one of them is not
related to the converter technology itself,
but also to the wind turbine design.
Converters for wind turbines of different
power rates
During the last years, the wind industry has
undergone a significant year-on-year growth
rate. This growth, along with the pressure to
continuously drive down the LCoE of the
solutions offered, has led OEMs to follow a
continuous power upgrade, reaching already
12,5MW for offshore wind turbines.
Fromtheconverterpointofview,andinorder
tocomparewhichtechnologyoffersthemost
optimalsolutionineachpowerrate,themain
keycharacteristicsoftheconverterwillbe
analysed.Theseareprice,efficiency,FRT
compliance,THDicompliance,PQV,power
densityandcomplexity.Inordertocoverthe
existingpowerrangesoftoday,thecomparison
willbedividedinto3differentgroups,power
ratesupto6MW,powerratesbetween6MW
and10MWandpowerratesabove10MW.
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3. Wind turbines between 6MW and 10MW
This power range is mainly used in offshore
farms and has experienced a very rapid
development over last years. In fact, turbines
ranging from 6MW to 10MW will cover almost
the 80% of the offshore market up to 2021.
Within this power range, the study provides
the results right:
In terms of initial cost, the FC solution
appears to be the best one, along with the
DFIG solution. Even if the DFIG converter
size is lower, the issues regarding FRT
events make it necessary to introduce
additional cabinets in order to go through
the most restrictive LVRT events (2ph 0%
and 3ph 0%).
As for grid code compliance, DFIG solutions
show some issues that cannot be ignored.
Apart from the FRT fulfilment, the limited
reactive power capability also restricts the
expansion of this technology for higher
power rates. DFIG topologies would require
the introduction of static synchronous
compensators (STATCOM). Nonetheless, the
integration of another VSC, especially in high
power wind turbines, supposes an additional
cost and complexity that in most of the cases
is difficult to justify.
In this way, the first conclusion that can be
taken is the DFIG topology is the less
competitive. Regarding initial cost, the LV FC
solution seems to be more attractive than
the MV FC solution. Nonetheless, the price
increase in the MV FC could be well founded if
the power converter is placed in the base of
the wind turbine, easing the nacelle design,
reducing transportation losses and taking
advantage of the high efficiency of MV
solutions (98%).
Regarding the selection of the number of
PCL, internal studies show that 2PCL will
minimize the LCoE whenever the expected
MTTR is between 1 and 4 weeks.
Regarding voltage, both LV and MV options
are feasible, and the final decision will
depend on the lay-out of the components of
the turbine.
Wind turbines up to 6MW
This power range is mainly used in onshore
farms. It’s the most common power range
and taking into account expert trend
analysis, they will still represent almost the
80% of the installed wind power by 2027.
The onshore market is a mature and reliable
market, with a low LCoE, where efficiency and
costs are the main decision factors. In the new
designs, evolution prevails over revolution.
Within this power range, the study provides
the results right:
In terms of investment, DFIG turbines and LV
FC turbines are the best option due to their
high efficiency at a low cost of the converter.
In order to make the best selection between
them, the site emplacement (wind class) and
the turbine design would be the key factors.
Power density is also relevant. DFIG solution
presents the best value (1,11MW/m3) while
FC LV barely reaches the 75% of this value
and MV only the 50%.
As a conclusion, DFIG and LV FC topologies
would be the most adequate for onshore.
The target market, location and energy cost,
will be the factors which ultimately
determine which one is best.
Fig 3. Topology decision figure for wind turbines up to 6MW
Fig 4. Topology decision figure for wind turbines up to 10MW
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4. Wind turbines between 10MW and 15MW
During last year, some new designs in the
range of 12MW have been announced; mainly
focussed in the offshore market. It is
expected that this power range will
experience a great boom in the next decade.
Within this power range, the study provides
the results below:
Regarding the return of investment (ROI),
MV solutions seem to be the best option,
mainly because of the high efficiency for
partial load (up to 2% better than LV working
at 50% of load).
Concerning power density, about 12MW the
higher currents that the LV solutions have to
manage, necessitates the use of 3 PCL in
parallel configuration. In addition, the volume
of these designs (power cable, power
converter and step up transformer) could
restrict its installation in the nacelle.
No differences are expected between LV FC
and MV FC regarding the grid codes fulfilment.
The manufacturer’s experience is the key
factor when deciding between LV and MV. In
fact, MV technology is wide spread in other
sectors such as traction, industry and
marine, guaranteeing a great reliability to the
MV products.
If we take a look to market trends, MV has
been chosen above 12MW. The current
turbines in the edge of 12MW seem difficult
to be upgraded to higher power rates. Thus,
new technologies could appear and minimize
the LCoE.
Main conclusions
As shown in this analysis, the topology
selection is not directly related to the
converter limits or constraints. From the point
of view of the converter, the technology has
reached maturity in both LV and MV.
Regarding technologies, DFIG is still a very
robust, cheap and efficient topology in
power ranges up to 6MW, and mainly for
onshore applications. Above these power
rates, FC seems to be a better option. In this
case, the voltage selection depends on the
wind turbine manufacturer design.
Nowadays,Ingeteamhasmorethan45GW
installedworldwide.Ourexperiencein
managingdifferentgridevents,severaltypes
ofsitesallovertheworldandwithdifferent
windturbineconfigurations,conferstoour
customersaveryreliablesolution(bothHW
andSW)independentlyofthetopologychosen.
Moreover, the shared knowledge and lessons
learned in other markets such as industry,
marine and traction, make the existing
converters a well proven solution for the
whole power range analysed in this paper.
For more information:
www.ingeteam.com
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Fig 5. Topology decision figure for wind turbines up to 15MW
Figure. 6. 10+ MW MV FC converter
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