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8th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission
Networks for Offshore Wind Farms, held in Bremen, Germany, from 14-15 October 2009. Page 574-578.
Transformers for Offshore Wind Platforms:
Expected Problems and Possible Approaches
Boris Valov (ISET/IWES, GERMANY)
Abstract--In the near future the power of 12 out of 21 wind
farms in Baltic Sea and North Sea of Germany will be from 1
up to 5 GW. For the connection to the power transmission
system on the sea a transformer in this power range will be
required but it is not available due to limitation of production
facilities. Another problem is the transport of such big units. In
this paper the future challenges with power transformers for
offshore wind farms are discussed and possible approaches for
optimization of number, design and performance are
suggested. A new modular design concept and its electrical
operation were investigated and the feasibility has been
verified by calculations. The proposed approaches showed
ways for reduce of weight and volume of transformers as well
as for solution of logistic problems.
Index Terms - Offshore wind farms, transformer, power
transmission.
I. INTRODUCTION
I
N North Sea of Germany 18 offshore wind farms have
been approved (state August 2009) [1]. Further 46 wind
farms are currently in the status of approval procedure [2].
The „ISET-concept“ for the connection of approved and
some planned wind farms is shown on fig. 1 [3]. 12 out of
21 approved wind farms have a rated power between 1000
MW and 5000 MW (table 1). It does not make sense to try
to produce a transformer with this power level because there
is a limit for production regarding the power and for
transport by rail waggon regarding weight and volume. Both
must not be exceeded. As a simple solution of this problem
the installation of several power transformers with lower
power instead of one transformer with high power is
theoretically possible but in this case more space on the sea
platform is required. Thus this solution is technically and
economically not optimal. The calculations showed that the
alternative installation of transformer banks with twowinding or auto transformers instead of usual transformers
with two or three windings shows better technical or
economic characteristics.
For the offshore transmission network according to fig. 1
AC- or DC-technology can be applied. Advantages or
disadvantages of these technologies for offshore are still in
discussion. For this paper calculations and analyses were
carried out just for AC-technology.
Boris Valov is with the Fraunhofer-Institut fuer Windenergie und
Energiesystemtechnik IWES, Koenigstor 59, D-34119 Kassel, Germany (email: [email protected]).
Legend: 1 - Offshore–Bürger–Windpark Butendiek; 2 - Dan Tysk; 3 Sandbank 24; 4 - Nördlicher Grund; 5 - Amrumbank West; 6 - Nordsee
Ost; 7 - ENOVA Offshore Northsea Windpower; 8 - Testfeld "alpha
ventus" ehemals "Borkum West"; 9 - Borkum Riffgrund; 10 - Borkum
Riffgrund West; 11 - Borkum West II; 12 - Gode Wind II;; 13 - Global
Tech I; 14 - Hochsee Windpark Nordsee; 15 - Gode Wind; 16 - BARD
Offshore I; 17 – Meerwind Ost und Meerwind Süd; 18 - Hochsee
Windpark He dreiht; A – Uthland; B - Weiße Bank; C - Vento Tec
Nord I; D - Offshore Windpark Austerngrund; E - Offshore-Windpark
"Deutsche Bucht"; F - Vento Tec Nord II; M - Borkum Riffgat; O Offshore–Windpark Nordergründe;
Fig. 1: „ISET-concept“ for offshore transmission network
II. CAPACITIES FOR PRODUCTION AND TRANSPORT OF
POWER TRANSFORMERS
The biggest power transformer so far has a rated power
of 1100 MVA and a weight of 559 tons [4]. Such a power
transformer exceeds the limit for the weight and the volume
of the biggest rail waggon Uaai 839 which is about 454 tons
and 191 m3 [5]. The biggest sea crane “Wind Luft 1” for
installation of wind turbine generators is not able to carry
more than 500 tons [6]. Therefore a transformer has to be
designed so that the weight of its heaviest unit must not
exceed this value. From these data it follows that the
maximum power of a transformer for offshore sea platforms
is not allowed higher than 1000 MVA. This requirement is
relevant for all wind farms in table 1.
8th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission
Networks for Offshore Wind Farms, held in Bremen, Germany, from 14-15 October 2009. Page 574-578.
TABLE I
GERMAN WIND FARMS WITH TARGETED POWER ABOVE 1000 MW
Wind farm
Targeted
power
(MW)
"Arkona-Becken Südost"
1005
"Godewind"
1100
"BARD Offshore 1"
1200
"Nordsee Ost"
1250
"ENOVA Offshore Northsea Windpower"
1406
"Global Tech I"
1440
"DanTysk"
1500
"Borkum Riffgrund West"
1800
"Nördlicher Grund"
2010
"Hochsee Windpark Nordsee"
2540
"Sandbank 24"
4905
"Arkona-Becken Südost"
1005
From the mentioned numbers basic values for the
relations between power, weight and volume for power
transformers with two windings can be derived which are
significant for designing the substations on sea platforms:
- “weight / power” is about 0,5 t/MVA,
- “weight / volume” is about 2,38 t/m3,
- “power / weight“ is about 2 MVA/t
- “power / volume” is about 5,8 MVA/m3.
The first three basic values are constant for all types of
power transformers. The last basic value is variable and
depends on the transformer´s operation. An increasing of
the relation „power / volume“ should be an important target
with designing power transformers for offshore sea
platforms.
The mentioned limits and relations are also valid for
reactive power compensation units on sea platforms which
should be designed as inductive shunts. They are operated
like a transformer with only one winding. The requirement
for reactive power compensation is determined by the
capacitive characteristics of the sea cable und the level of
the operating voltage. The calculations showed that the
required rated power of the inductive shunts for most wind
farms from table 1 are higher than 1000 MVAr. In general
the same limits for production and transport for these shunts
are valid like for power transformers.
III. PROPOSED TYPE AND POWER OF TRANSFORMERS
The problem with the transport of limit-rating
transformers can be solved by using the following
alternative
types:
A. 3-phase 3-winding transformer with equal medium
voltage windings,
B. 3-phase auto transformer,
C. Transformer bank consisting of three single phase 2winding transformers,
D. Transformer bank consisting of three single phase auto
transformers.
For these types of
characteristics are relevant:
transformers
the
following
A. 3-phase 3-winding transformer with equal medium
voltage windings is cheaper than two 3-phase 2-winding
transformers (basic option today for most of the wind farms)
but it has a worse „power / weight“ relation. Therefore it is
not recommended for wind farms with a power of more than
1000 MVA. However, this type of transformer is explicit
recommended for wind farms with a power of less than
1000 MVA because it can secure very good damping of
opposite system perturbation between the wind farms
connected at its two medium voltage sides.
B. With a 3-phase auto transformer the biggest relation
„power / weight“ compared to a 3-phase transformer with
two windings can be reached because one part of the
winding can be designed just for the differential current and
thereby a winding with higher power can be installed in the
same transformer’s volume.
C. In a transformer bank consisting of three single
phase transformers with two windings the relation „power /
weight“ is further improved. In this case a single phase 2winding transformer can be produced with the same weight
and volume but with a power higher than 1000 MVA. The
power of the transformer bank is the result of adding the
powers of the three single phase transformers and it can
reach approximately 3000-3500 MVA.
D. The desired power of 5000 MVA for the wind farm
"Sandbank 24" can presumably be reached just with a
transformer bank consisting of three single phase auto
transformers.
The further advantage of an auto transformer is the
possibility of integration of another winding for internal
energy supply of the sea platform. Thereby the installation
of an additional transformer is not necessary any more.
8th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission
Networks for Offshore Wind Farms, held in Bremen, Germany, from 14-15 October 2009. Page 574-578.
"Global Tech I"
1440 MW
IV. APPROACH FOR HIGH POWER OFFSHORE WIND FARMS
"DanTysk"
The power of the wind farms in table 1 can be separated
1500 MW
into three groups:
"Arkona-Becken
Two transformers with 500 MVA,
Südost"
ca. 250 tons and 105 m3 each
A. from 1005 to 1800 MW,
1005 MW
B. from 1800 to 2540 MW,
C. more than 2540 MW.
The designing procedure for transformers is different
according to each group. The suggested solutions are shown
in tables II and III.
A. Wind Farms with 1005 - 1800 MW
For this power range the above mentioned limits for
production and transport are not exceeded if two 3-phase 2winding transformers or two 3-phase auto transformers are
used in a substation on a sea platform. Besides the reduction
of weight and volume of a transformer the reliability and
availability of a substation and the whole power
Fig. 3: Substation with two 3-phase auto transformers
transmission system rise in this case.
B. Wind Farms with 1800 - 2540 MW
In this power range the use of a transformer bank with
three single phase 2-winding transformers (fig. 4) or with
three single phase auto transformers (fig. 5) is suggested.
Fig. 2: Substation with two 3-phase 2-winding transformers
The suggested types and rated powers of the transformers
for this power range are shown in table II.
TABLE II
SUGGESTED NUMBER AND POWER OF TRANSFORMERS
Wind farm
"Godewind"
1100 MW
"BARD Offshore
1"
1200 MW
"Nordsee Ost"
1250 MW
"ENOVA Offshore
Northsea
Windpower"
1406 MW
Fig. 4: Substation with a transformer bank with three single phase
transformers
Number and nominal power
(MVA)
Two transformers with 630 MVA,
ca. 315 tons and 132 m3 each
Two transformers with 800 MVA,
ca. 400 tons and 168 m3 each
Fig. 5: Substation with a transformer bank with three single phase auto
transformers
8th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission
Networks for Offshore Wind Farms, held in Bremen, Germany, from 14-15 October 2009. Page 574-578.
The suitable rated powers of the transformers for each parallel operated sea cables.
In opposite to a delta-connection of windings in onshoreoption are shown in table III.
TABLE III
grids a star-connection of windings in offshore-grids can be
SUGGESTED NUMBER AND POWER OF TRANSFORMERS
used at the medium voltage side of transformers because the
compensation of the unbalance of loads is not important any
Number and nominal power (MVA)
Wind farm
more. Generated electricity in generators or inverters of
wind power generators is always symmetrical. With the use
Option due to fig. 4: Transformer bank
of a star-connection at the medium voltage side of a
consisting of three single phase 2"Borkum
transformer the volume of these windings can be reduced,
winding transformers with 630 MVA,
Riffgrund
too. For this reason it is suggested to connect the medium
ca. 315 tons and 132 m3 each.
West"
voltage winding of power transformers in a star (fig. 3, 5
Option due to fig. 5: Transformer bank
and 6). The standard IEC 60076 also allows this option.
consisting of 3 single phase auto
1800 MW
transformers with 630 MVA, ca. 315
tons and 132 m3 each.
Option due to fig. 4: Transformer bank
consisting of three single phase 2"Nördlicher winding transformers with 800 MVA,
ca. 400 tons and 168 m3 each.
Grund"
Option due to fig. 5: Transformer bank
2010 MW
consisting of three single phase auto
transformers with 800 MVA, ca. 400
tons and 168 m3 each.
"Hochsee
Windpark
Nordsee"
2540 MW
Option due to fig. 4: Transformer bank
consisting of three single phase 2winding transformers with 1000 MVA,
ca. 500 tons and 210 m3 each.
Option due to fig. 5: Transformer bank
consisting of three single phase auto
transformers with 1000 MVA, ca. 500
tons and 210 m3 each.
C. Wind Farms with more than 2540 MW
In this case it is about the wind farm "Sandbank 24" with
a targeted power of 4905 MW. Because an installation in
one transformer bank of such high power is not possible, the
substation should consist of two transformer banks (fig. 6).
For this wind farm it is suggested to install three single
phase auto transformers in each transformer bank. Each
auto transformer has a power of 800 MVA, a weight of ca.
400 tons and a volume of ca. 168 m3.
V. NEUTRAL POINT TREATMENT
Power transformers for sea platforms are usually
designed for a high voltage level of 110 - 380 kV and a
medium voltage level of 10 - 35 kV. For the high voltage
side of a transformer the star connection is preferred
because the isolation of this winding is necessary to design
only for phase-to-earth voltage and thereby the winding
volume can be reduced. Low-resistive neutral earthing on
the high voltage side of transformers leads to a fast and
selective disconnection in case of a fault in one of several
Fig. 6: Substation with two banks from three single phase auto
transformers
The solution with a star connection at the medium
voltage side has further advantages, too:
- it is not necessary any more to compensate the big
capacitive short circuit current in medium voltage sea
cables with the help of additional units which need space
on sea platforms,
- unit- and grid protection is simplified and the selectivity
is improved.
In general the economic efficiency rises with the auto
transformer if the value of medium voltage tap nears to
value of the high voltage tap. Thereby it is recommended for
the internal cable connection in the wind farm with auto
transformers not to use 35 kV but 60 kV as the next voltage
level according to the standard IEC 60038. The increasing
of the voltage level causes a reduction of the current, of the
power losses and of copper demand. Thereby the economic
8th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission
Networks for Offshore Wind Farms, held in Bremen, Germany, from 14-15 October 2009. Page 574-578.
alternative types of power transformers has demonstrated
efficiency rises.
that a reduction of space is possible.
VI. POWER LOSSES AND SHORT CIRCUIT POWER
The proposed approaches showed ways for reduce of
weight
and volume of transformers as well as for solution of
The use of the suggested transformers of different types
logistic
problems.
and power levels causes increasing of power losses and
short circuit currents at the medium voltage side in case of
identical feed-in power from wind farms. For example, the
power concentration in one tank of a transformer with a
limit volume of about 190 m3 for one single phase auto
transformer is significant higher than the power
concentration of one 3-phase transformer with the same
volume. Thereby the maximum nominal power and total
power losses of the transformer rise proportionally.
The increase of short circuit currents at the medium
voltage side of the transformer (without feed-in from wind
farm) is determined by the decrease of its impedance. This
causes a network enhancement for the points of common
coupling of the wind turbine generators. The calculated
changing ratios are clarified by the data of table IV.
VIII. ACKNOWLEDGMENT
This contribution has been developed at ISET/IWES
within the project „RAVE – Research at alpha-ventus.
Netzintegration von Offshore-Windparks“ funded by
Forschungszentrum Juelich GmbH - PTJ.
IX. REFERENCES
[1]
[2]
[3]
[4]
TABLE IV
POWER LOSSES AND SHORT CIRCUIT POWER
Type
Change
of power
losses
(p.u.)
Ratio for
increase of
circuit
power (p.u.)
[5]
One 3-phase 2-winding
transformer
1,00
1,00
[6]
One 3-phase auto
transformer
0,92
1,04-1,07
Transformer bank consisting
of three single phase 2winding transformers
1,88
1,60-1,94
Transformer bank consisting
of three single phase auto
transformers
2,13
1,62-2,00
In this table the data of a transformer with two windings
were assumed to 1,0 because today this type is usually
proposed for all offshore wind farms as a basic option.
VII. CONCLUSIONS
The above mentioned limits for weight and volume will
not be exceeded if the proposed alternative types, power
levels and numbers of the transformers are used with the
design of substations on sea platforms. The feasibility of the
proposed options was proved due to network calculations
for the operation of power transmission systems inclusive
power transformers, sea cables and reactive power
compensation units on the sea platform and on the
continent.
The estimation of required space on the sea platform for
Bundesamt für Seeschifffahrt und Hydrographie (BSH): Windparks.
Dokument im Internet, 2009. http://www.bsh.de/de/
Meeresnutzung/Wirtschaft/Windparks/index.jsp.
Windparks in der Nordsee. Dokument im Internet, 2009.
http://www.offshore-wind.de/page/index.php?id=4761.
Valov Boris. Outlook in the future of German North Sea 2020. Power
System for Offshore Wind Power. Wind-Kraft Journal. German
Offshore. 5. Ausgabe 2008. S. 38-39.
RP-Online: Schwertransport zum Braunkohlekraftwerk. Dokument im
Internet, 2009. http://www.rponline.de/public/bildershowinline/regional/niederrheinsued/moenchen
gladbach/nachrichten/moenchengladbach/43423?skip=0&refback=/pu
blic/article/moenchengladbach/693939/Fuenf-Wochen-auf-dem-Wegnach-Sachsen.html|article
Wikipedia – die freie Enziklopedie: Sonderwagen (Güterwagen der
Gattung U). PDF-Dokument im Internet, 2009.
http://de.wikipedia.org/wiki/Sonderwagen_(G%C3%BCterwagen_der
_Gattung_U).
Bard-Magazin. July 2009.
X. BIOGRAPHIES
Dr. Boris Valov was born in Tomsk
in West Siberia (Russia), on July 28,
1950. He studied and gained the title of
PhD at the Polytechnic University
Tomsk, Russia. At this University he
has been working in the field of
mathematical analyses of power quality,
stability in Electrical Power Systems
and Networks for more than 25 years.
During the time from 2000 to 2007 Boris Valov worked as a
scientist at Kassel University at the Institute of Electrical
Power Supply Systems. His was working with analyses and
design of interconnection of Wind Power Plants to
Electrical Power Supply Systems and Networks. Since
March 2007 he is a scientist at the “Institut fuer Solare
Energieversorgungstechnik ISET e.V.” (Fraunhofer-Institut
fuer Windenergie und Energiesystemtechnik IWES)
focussing on interconnection of Renewables into Electrical
Power Supply Systems and Networks. He has more than
120 publications in the area of Power Quality and Grid
Integration of Renewables.