“IWES-Concept 2010” for Offshore Power Transmission System 2020

“IWES-Concept 2010” for Offshore Power Transmission System 2020

“IWES-Concept 2010” for
Offshore Power Transmission
System 2020
B. Valov, P. Strauß, T. Degner, C. Jansen; Fraunhofer-Institut für Windenergie und
Energiesystemtechnik (IWES)
B. Valov
EXTERNAL ARTICLE
Introduction
The German government plans to increase the amount of
renewable energies in electricity production step by step
within the upcoming years, aiming at 30 percent in 2020 [1].
A great part of this production is supposed to be covered by
building offshore wind farms in the North Sea. The first
German offshore wind farm ‘alpha ventus’, that is in operation since the end of 2009, can be seen as an important first
step. Currently, the planning and approval of offshore wind
farms in Germany has a highly dynamic character. An equal
dynamic characterization can be seen in other countries
adjoining the North Sea. The high potential of electricity
production of offshore wind farms is a precondition for realizing the political ambitions. In addition, a realization depends
crucially on providing sufficient absorbing power of the main
grid connection points. Studies performed for this article
show that this is not currently the case in Germany. This
problem may also occur in other European countries. Possible
solutions should be discussed today to ensure feasibility in
2020.
In this article, existing and newly developed plans to ensure
the achievement of the political ambitions will be discussed.
The focus of the work done at Fraunhofer IWES was to
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DEWI MAGAZIN NO. 37, AUGUST 2010
English
develop an offshore transmission system which is able to
transmit the generated power of all offshore wind farms to
the shore, where it is successfully absorbed by the grid connection points. Furthermore, integration into a possible
future ’Supergrid’ is considered.
Current Plans for Future Offshore Wind Farms
According to the ‘Windenergie-Agentur Bremerhaven’ [2]
and the German Federal Maritime and Hydrographic Agency
[3], the current state regarding quantity and power of offshore wind farms in the German North Sea is as follows (Tab.
1) [4, 5]:
Before the power, generated by offshore wind farms, can
reach any customer’s household, several obstacles have to
be overcome:
1. Distances from 50 to 100 km, from the wind power plants
at sea to the substation on shore,
2. Additional distances up to 50 km from the substation at
shore to the main grid connection points,
3. Limited absorption power of the main grid connection
points,
4. Transmission through the main transmission and distribution grid to reach the final customers.
Current status of offshore wind farms
Quantity of offshore wind
farms
Rated power [GW]
Quantity of wind turbines*)
2006
2010
2006
2010
2006
2010
In Operation
0
1
0
1.04
0
208
Approved
15
21
21.96
21.98
4391
4395
Planned
8
48
4.14
19.80
828
3960
Sum
23
70
26.10
42.82
5219
8564
Tab. 1:
Status of offshore wind farms in the German North Sea in 2006 and 2010
*) equivalent to 5 MW wind turbines
Fig. 1:
The „transpower offshore gmbh – Concept“ for the
extension of offshore power transmission system in
the North Sea (with approval of transpower offshore
gmbh)
Fig. 2:
„ISET-Concept 2007“ of the offshore
power transmission system in the
North Sea
Legend: 1 - Offshore–Bürger–Windpark Butendiek; 2 - Dan Tysk; 3 - Sandbank 24; 4 - Nördlicher Grund; 5 Amrumbank West; 6 - Nordsee Ost; 7 - Offshore North Sea Windpower; 8 - Borkum West; 9 - Borkum Riffgrund;
10 - Borkum Riffgrund West; A – Uthland; B - Weiße Bank; C - Vento Tec Nord I; D - Offshore Windpark
Austerngrund; E - Offshore-Windpark “Deutsche Bucht”; F - Vento Tec Nord II; G - Global Tech I; H - Hochsee
Windpark Nordsee; I - Hochsee Windpark Hedreiht; K - BARD Offshore I; L - Gode Wind; M - Borkum Riffgat; O
- Offshore–Windpark Nordergründe; P – Meerwind.
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DEWI MAGAZIN NO. 37, AUGUST 2010
In Germany, the last issue has been discussed in detail in
2005, based on studies of the German Energy Agency:
’Energiewirtschaftliche Planung für die Netzintegration von
Windenergie in Deutschland an Land und Offshore bis zum
Jahr 2020‘‚ [6]. However, the first three questions remain
unsolved. Discussions about possible solutions have begun
[7, 8, 9], but so far no final decisions have been made.
Energy Transmission from Sea to Land
Another big problem remains the transmission of the electricity produced offshore to the main grid connection points
at land, with distances of up to 120 km. While at sea level
only sea cables can be used, the part on land can be developed using land cables, overhead lines, gas insulated transmission lines or a combination of those [10]. Related to the
decision of the used transmission line system, the question
has to be answered which current to use: direct or alternating current [11]. Both technologies are used in the German
North Sea at present: the connection of the first German
offshore wind farm “alpha ventus” uses HVAC, while the connection of the sea substation “BorWin1” was developed
using HVDC. Compared to the huge amount of planned electricity production of 42.82 GW, the installed capacities of 60
MW for the HVAC and 400 MW for the HVDC transmission
line are marginal and thus, these connections can only be
seen as test projects. Experiences made in these testing
projects should initiate productive discussions. From the
authors` point of view, both technologies should be used in
an efficient complementary way.
Nowadays, all operational wind power plants are running and have been connected - using HVAC. As a result, the HVAC
technology for offshore purpose has already proven its suitability and knowledge of use could be acquired. This is why
in the following studies for designing an offshore transmission system only HVAC is considered. In the model used,
unwanted capacitive loading currents which occur in the
cable insulation have been neutralized using reactors. Loadflow calculations have proven the feasibility of offshore
transmission systems based on HVAC only.
A next step for plans towards an offshore transmission system is to set up a design under economic and technical
aspects. A lot of possible designs with lots of variations and
different approaches can be made. Each model will be characterized by its specific costs. These costs rise to around
several billion Euros for offshore transmission systems in giga
watt size. A decision about a preferred optimal design of
such an offshore transmission grid could save some billion
Euros of investment costs by 2020.
Umeå, Sweden
Anti-icing and De-icing technologies
Problem solving
Grid connection
Risk Assessment
transpower offshore gmbh – Concept
Fig. 1 shows the future concept for the grid connection of
offshore wind farms in the German North Sea of the local
grid operator “transpower offshore gmbh”1. Part of this concept is a clustering of reasonable offshore wind farms. The
electricity produced by each cluster will be transmitted with
discrete cable connections. Two traces for sea cable laying
have already been approved, although only one is in use with
one HVAC and one HVDC cable lay. For crossing the island
Norderney, ductwork has been laid and some of the over 30
existing buried empty tubes have been used. The rest of the
ductwork will be used for upcoming cable installations. To
absorb the electricity produced offshore three grid connections points are currently planned: “Diele”, “Dörpen/West”
and “Büttel”.
ISET2-Concept 2007
Concepts for Energy Transmission
According to the law for “speeding up planning processes of
infrastructure projects” [12] the local grid operator has to
plan, build and run connection lines from substations at sea
to the most suited main grid connection point at land. This
will affect all offshore wind farms whose construction will
have started until the end of 2015 and therefore, the amount
of offshore wind farms can hardly be foreseen.
In addition, different concepts of energy transfer from the
offshore sector to the German main grid are presented.
Research concerning the planning of future offshore-transition systems has shown that a radiant system with mainly
separate connections of each offshore wind power plant
with the German grid is preferable. The technical and eco The transmission system operator „transpower stromübertragungs
gmbh“ has the responsibility for operation, maintenance and if necessary the reinforcement of its transmission grid. In charge for the offshore part is the subsidiary „transpower offshore gmbh“.
1
The
2
part of the new Fraunhofer IWES institute in Kassel founded
01.01.2009 arose from the former „Institut für Solare Energieversorgungstechnik - Verein an der Universität Kassel e.V. (ISET)“
DEWI MAGAZIN NO. 37, AUGUST 2010
47
Fig. 3:
„IWES-Concept 2010“ of offshore
power transmission system in the
Nord Sea with an example of an
integration into a trans-European
„Supergrid“
Legend: A - Nordergründe; B - Godewind II; C - Godewind; D - OWP Delta Nordsee I; E - Borkum Riffgrund; F
- alpha ventus (Borkum West); G - Borkum West II; H - Borkum Riffgrund West; I - Meerwind; J - Nordsee Ost
(„Amrumbank“); K - Amrumbank West; L - Butendiek; M - Dan Tysk; N - Nördlicher Grund; O - Sandbank 24;
P - Global Tech I; Q - Hochsee-Windpark Nordsee; R - „He Dreiht“; S - BARD Offshore I; T - Veja Mate; U - OWP
Delta Nordsee II; V - MEG Offshore I; 1 - Borkum Riffgat; 2 - Innogy Nordsee 1; 3 - Diamant; 4 - Borkum
Riffgrund II; 5 - Euklas; 6 - Borkum Riffgrund West II; 7 - OWP West; 8 - Kaskasi; 9 - Hochsee Testfeld Helgoland;
10 - Uthland; 11 - Nordpassage; 12 - Sandbank 24 ext.; 13 - Weiße Bank; 14 - AreaC III; 15 - AreaC II; 16 - AreaC
I; 17 - Skua; 18 - Sea Wind I; 19 - Albatros; 20 - Notos; 21 - Sea Wind II; 22 - He dreiht II; 23 - Bight Power II;
24 - Bight Power I; 25 - Aquamarin; 26 - OWP „Deutsche Bucht“; 27 OWP - „Austerngrund“; 28 - Bernstein; 29
- Citrin; 30 - Sea Storm; 31 - Sea Storm II; 32 - VentoTec Nord I; 33 - VentoTec Nord II; 34 - Aiolos; 35 - Sea
Wind III; 36 - Kaikas; 37 - GAIA I; 38 - GAIA II; 39 - GAIA III; 40 GAIA IV; 41 - Horizont I; 42 - Horizont II; 43 Horizont III; 44 - NSWP 4; 45 - NSWP 5; 46 - NSWP 6; 47 - NSWP 7; 48 - H2-20
nomical disadvantages of this approach are elaborately
exemplified in [13].
In 2007, the „ISET-Concept 2007“ for the construction of a
transmission system was proposed (Fig. 2) [14, 15]. This concept plans the development of a transmission system for a
joint use by all offshore wind farms. The standardized transmission system establishes multiple advantages for the
operation management in normal operation mode, as well
as for grid failures. For example, in normal operation mode,
the losses in the transmission system can be kept at a minimum level by controlling the load flow. With load flows at
giga watt level, this means savings of several million Euros
annually. A new aspect in this concept is the introduction of
interconnections between the wind power plants Global
Tech I - Weiße Bank and the wind power plants Gode Wind
– Meerwind. This allows adjustments of applied load of the
grid connection points on land, because they have different
absorption capacities. Thanks to the interconnections, variations of the feeder line of single wind power plants are
damped and thus dynamic disturbances to the German grid
are reduced. The concept covers optimization of capacity
and placement of compensation reactors, reciprocal support
at black starts, energy support for auxiliary power, reduction
of sea substations, sea cables and transformers. Furthermore
the impact on protected landscape is minimized and recommendations from the “dena study”, regarding the necessity
of a coordinated construction of an offshore transmission
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system, are considered. Load flow calculations performed for
the „ISET-Concept 2007“ proved the feasibility of an offshore
transmission system based on alternating current.
IWES-Concept 2010
The new – advanced – concept, which was updated to development plans of December 2009 (Fig. 3), is a combination of
the two concepts of “transpower offshore gmbh” and “ISETConcept 2007” and the political attempt of Germany and
Europe to connect offshore wind farms to a future TransEuropean “Supergrid”. In comparison to the two other concepts the new concept features:
• 70 wind power plants (Tab. 1) instead of 23 from “ISETConcept 2007”,
• Interconnections introduced in “ISET-Concept 2007”
remain,
• In contrast to the “dena study” and “ISET-Concept
2007”, only 2 out of 4 grid connection points,
• Proposal of possible options for integration of a German
offshore transmission system into a Trans-European
“Supergrid”.
Looking at the geographical location of existing and future
offshore wind farms, it can be observed that their distribution
in space of the Exclusive Economic Zone of Germany (blue
box in Fig. 2 and 3) is uneven: the major part is located in the
southwest while the minor part is located in the northeast of
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the zone. This imbalance is also true for the distribution of the
total offshore generated electric power in 2 pieces, 30.28 GW
(72.8 %) and 11.34 GW (27.2 %) to be absorbed by the 2 grid
connection points. The maximum and minimum capacity of
absorption power of a grid connection point can basically
only be determined by using grid calculations. If the fed in
power exceeds the absorption capacity of the grid connection
point, no convergence after lots of iterations in the grid calculations can be achieved. Studies have shown that the minimum requested absorption power, the sub transient threephase short-circuit apparent power, should be equal to 22
GVA in “Diele” and “16 GVA in “Büttel” to successfully absorb
the total fed in offshore power of 32.6 GW and 13.2 GW,
respectively. The value of the sub transient three-phase shortcircuit apparent power at the grid connection points “Diele”
and “Büttel” is mainly determined by the total fed in power of
power plants in Northern Germany using synchronous generators. Operation of the synchronous generators follows the
current fluctuating demand in electricity and is thus mutable
within certain limits itself. Analysis has shown that for practical use the following limits should be applied (Tab. 2).
Tab. 2 shows, that the initial symmetrical short-circuit power
in the grid connection points can be less than 22 GVA or 16
GVA. In this case, the power generated offshore cannot be
absorbed completely and thus has to be limited. In the long
term, grid connection reinforcements are required. However,
increasing the sub transient three-phase short-circuit appar-
ent power has technical limits, which are caused by protection measures of the grid equipment, insulation coordination,
electromagnetic field strength and allowed current losses.
Therefore, power generated offshore should not only be fed
into a single grid connection point, but into several. As an
alternative to the costly and time-consuming grid reinforcement measures, we propose a new concept of flexible distribution of the total offshore power between the two approved
grid connection points. This can be achieved through an interconnection of the offshore transmission system between the
wind power plants Aiolos - White Bank and AreaC - Sea Wind.
Again, load flow calculations showed the feasibility of the
balancing power of such an interconnection between the grid
connection points. Additional flexibility can be achieved by
connecting the offshore transmission system with a future
offshore Trans-European “Supergrid”.
Such an integration could offer the following benefits:
• Required grid reinforcements reduced to a minimum
because excessive power can be absorbed by the
“Supergrid”,
• Possibility to use the great amount of hydro storage
capacity in other countries, .e. g. Norway,
• Participation in international electricity markets, which
could result in reduced overcharges of the German
electricity grid,
• Improvement of the reliability of the offshore transmission system,
DEWI MAGAZIN NO. 37, AUGUST 2010
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Voltage level
(kV)
420 - 440
350 - 420
193 - 220 - 230 - 245
96 – 110 - 127
Tab. 2:
Sub transient three-phase short-circuit apparent power
(GVA)
minimal
maximal
4,0
40
2,3
35
1,7
20
0,9
10
Relevant values of the sub transient three-phase short-circuit apparent power of different voltage levels according to “Transmission Code 2007”
[16].
• Flexibility in grid operation management.
Some studies considering the design of a future “Supergrid”
are already at hand [17, 18 ,19]. However, they mainly cover
the economic value of interconnection to a future “Supergrid”.
The feasibility of an electric realization remains uncertain
until today [20].
A future “Supergrid” would have a certain absorption power
itself, like any physical grid. While the “Supergrid” would be
set up using HVDC transmission technology, its absorption
power will be limited by the transmission capacity of this
technology. This limit currently add up to 1.5 GW per unit.
The required amounts of HVDC units and converter sea substations have lately been discussed. In the introduced “IWESConcept 2010” 3 converter sea substations are proposed:
“West”, “Center” and “East”. Their absorption power was
estimated at 2 GW each, which is expected to be the future
limit for HVDC units. Presence of the applied 2 grid connection points, 3 converter sea substations for a connection to a
future “Supergrid” and 2 interconnections between the grid
connection points has highly increased the flexibility and reliability of the offshore transmission system.
AC / DC Connections in the Offshore Transmission System
A final decision regarding the choice of transmission technology has not been made yet. There are advantages and disadvantages for each type mentioned. The main disadvantage of
the AC technology is the high capacitance of the sea cables,
which limits the transmission distance at 380 kV voltage level
to about 50 km without compensating measures. For longer
distances, reactive power compensation utilities have to be
installed, which cause additional cost and space requirement. Compared to this disadvantage, the AC transmission
technology provides high reliability, proven long term operation and high durability. Considering the great amount of
potentially more than 40 GW of power generated offshore,
the disadvantages of the HVAC transmission technology
might be considered less important, since the following can
be applied for HVDC transmission technology:
• High complexity,
• Great space requirements at the sea substations,
• Lowered reliability through chain connected use of
power electronics (converter),
• Average life cycles of 10 to 15 years for power electronics,
• High maintenance cost,
• Overall greater amount of current losses compared to
AC technology,
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DEWI MAGAZIN NO. 37, AUGUST 2010
• Possibility of fed in of unwanted harmonics and spreading through the German transmission grid,
• Own grid protection requirements which may affect
the selectivity of the existing grid protection measures.
The information given shows the necessity of one or several
comparative studies for AC / DC technology use to reduce
costs in the amount of multiple millions of Euros. For the
development of the “IWES Concept 2010” only HVAC at 380
kV voltage level was applied. The capacitance of the sea
cables has been offset using compensation reactors. Load
flow calculations have shown the feasibility of the developed
offshore transmission system according to the current grid
operator requirements [21].
Summary
The developed “IWES-Concept 2010” of an offshore power
transmission system supports the current plans for electricity
transmission in the German North Sea and contributes to an
optimized development of offshore wind power integration
into the German Power Transmission System and a TransEuropean “Supergrid”. The proposed solutions aim at increasing the reliability of the German Power Transmission System
and the flexibility in operation management of the offshore
transmission system and at reducing investment cost in the
range of several million Euros. We show the feasibility of the
“IWES-Concept 2010” by appropriate load flow calculations.
Acknowledgement
Parts of this article have been developed in scope of the
project “Windenergieforschung am Offshore-Testfeld”. The
authors thank the Federal Ministry for the Environment,
Nature Conservation and Nuclear Safety and the Project
Management Jülich for their support. Responsibility for this
article is with the authors.
Contact
Dr./OAK Moskau Boris Valov, Fraunhofer Institut für
Windenergie und Energiesystemtechnik IWES, R&D Division
Engineering and Network Integration, Group Electricity Grids,
Koenigstor 59, D-34119 Kassel, Ger­many. Tel.: +49 561 7294125, Fax: +49 561 7294-400.
Mail: boris.valov@iwes.fraunhofer.de;
http://www.iwes.fraunhofer.de
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