Offshore wind farm connections to the grid
Energy to Quality, S.L.
Pº Castellana 114, 9, 7 28046 – Madrid, Spain
0034 915 632 623 – amolina@energytoquality.com
Abstract – In most countries and power systems, wind
power generation covers only a small part of total power
system load. However, there is a tendency to increase
the amount of electricity generated from wind turbines
especially in the use of offshore wind farms. The reason
for offshore wind farm project is that wind speed offshore is potentially higher than onshore, which leads to
a much higher power production. Until now, practical
experience from offshore wind energy projects is limited. The installation and supporting structure of offshore wind turbine is much more expensive that of onshore turbines. Future projects include wind turbines
with a higher rated capacity but also will be significantly large in total size and further away from the
shore. This will require a new concept for the overall
transmission system. Like HVAC connections and
HVDC connections that have two technical options: line
commutated converter (LCC) based on HVDC and
voltage source converter (VSC) based on HVDC technology. This paper presents an overview of offshore
wind farm (in Europe) and transmission system.
Index Terms – Offshore, wind farm, grid connection,
HVAC, HVDC, E2Q.
I.
INTRODUCTION
From the point of view of connection to the grid, offshore
wind farms have specific characteristics with regard to onshore wind farms:
- More powerful wind turbines, to compensate for the
greater costs of foundation,
- More powerful wind farms,
- Greater length of the electric lines in the wind farm, due
to the greater separation between wind turbines,
- Greater distances to the connection to the grid,
- New connection systems to the grid (alternating current
AC or direct DC)
- New problematic with regard to the fulfillment of the
new requirements for the transmission in AC or DC and
the connection to the grid.
All of that entails greater costs of the electric system of
the wind, compared to wind farms onshore. In the following
sections prospects of the sector are described as for European as well as in Spain. Different alternatives of offshore
wind farms connection systems to the grid, as well as their
advantages and inconvenients, are analyzed later.
II. FUTURE OF OFFSHORE WIND ENERGY IN
EUROPE
The EU has stated as an objective for the year 2020, coverage of renewable energy of 20% of the total of primary
energy. Conclusions of the last international congress “Offshore Wind” that took place in Berlin, point that only with
the development of offshore wind energy, these goals will
be achieved. As for European, the following objectives
have been previewed:
Germany: 25.000 MW offshore wind power until 2030,
by contributing a 15% of the total. E.On Netz will build the
first connection of direct current that will let to connect the
offshore wind farms Alpha Ventus (1040MW), Bard Offshore I (400 MW), Hochsee Windpark Nordsee (400 MW)
and Global Tech I (360 MW) to shore. The German regulation of infrastructures of 2006 force the nearest system operator to provide connection to the grid, to those wind farms
situated further than 3 nautical miles (5.556 km) from
shore.
United Kingdom: 33.000 MW offshore wind power are
previewed, and some have obtained the initial authorization
during last months:
- London Array (341 wind turbines of 3 MW, 1.023 MW
in total), situated in the estuary of the Thames, 20 km
from shore. This offshore wind farm is promoted by a
consortium formed by the company DONG Energy and
the electric E.On.
- Walney (42 turbines), situated 15 km from the coast of
Cumbria. This farm is promoted by the Danish DONG
Energy.
The future revision of the Law of Energy (Energy Bill)
will include measures to develop the Renewable Obligation
and finance offshore wind farms. Besides, it will obligate
the operator of the electric transport system to contribute to
the investments in infrastructures to evacuate energy, contributing to improve the economic imbalance that has
blocked the development in the United Kingdom (current
costs of development of offshore wind energy against previewed remunerations in current legislation).
Sweden: 2.500-3.000 MW offshore wind power compared with current 135 MW, with the objective of increase
their wind power production by a factor of 25 in 2020.
Denmark is the country leading offshore sector in the
world. 30% of the national energetic consumption supplied
from renewable energies, thanks to a probable contribution
of offshore wind energy before 2030. Danish companies
optimize projects for a 50 years life (foundation planning,
towers, wrapper of the gondola, and main axis of the turbines). This entails a reduction of the costs of electricity
produced, competitive against onshore installations.
The European Wind Energy Association has presented a
Joint Technology Initiative between the States members, to
join forces. Three key components emphasize in this strategy, based on the Danish model as an example to follow,
“Copenhagen Strategy 2005”:
- Channel of public and private funds towards the international R+D, for the development of the sector.
- Creation of the necessary infrastructures in the sea and
inland to access to the grid.
- Elaboration and coordination of environmental studies.
marized graphically in Figure 1. The offshore wind zones
are classified in:
- Suitable zones (green) or the most adequate because of
its reduced environmental impact. Once the study
is approved, applications can only be presented for
suitable zones.
- Exclusion zones (red) because of conflict with other
uses or because of environmental impact.
- Zones with environmental conditionings (yellow), in
which the development of offshore wind farms is
determined, and its effects or conflicts must be
analyzed in the later phase of design of the project.
III. OFFSHORE WIND ENERGY IN SPAIN
Nowadays, in Spain there is not any offshore installation
in operation, in spite of high wind energy resources are
available in the Spanish coastline. So, the government has
the objective of install offshore wind farms for a power up
to 4.000MW by the year 2030. Spain is in phase of demonstration of the viability and profitability of offshore installations, against the existing problematic as for administrative,
environmental and of evaluation of wind energy resource.
From the electric point of view, which are the points of
difficulty of offshore wind farms against the transport grid
is unknown.
A. Administrative authorizations
The Ministry of Industry, Tourism y Trade announced in
December 2007, that in Spain will not be offshore wind
farms before 2014, alter the publication of the Spanish
Royal Order 1028/2007, in force since August 2007, which
establishes the administrative procedure to obtain the authorization of offshore wind farms.
B. Ecological implications
The Ministries of Industry, Tourism and Trade
(MITYC) and Environment (MMA) have published the
Environmental Strategic Study of the Spanish coast, carried
out with the objective of determining the public knowledge
zones that satisfy favorable conditions for the installation of
offshore wind farms. As a result of the approval of the Law
9/2006, 28 of April, of evaluation of the effects of certain
plans and programs in the environment, must be submit this
Environmental Strategic Study of the Spanish coast for the
installation of offshore wind farms to a process of Strategic
Environmental Evaluation (EAE) which follows the procedure established in the aforementioned law. The Environmental sustainability Report is the result of the firsts phases
of the process of EAE. The study concludes with an Environmental Report.
The results of the Environmental Strategic Study of the
Spanish coast, have been presented publicly, and are sum-
Figure 1: Offshore wind zones suitable (green), excluded (red) and with
environment conditionings (yellow)
In summary, the document establishes that a 43% of the
coastline is available for offshore installations further than 8
km from the coast. At present there are around thirty preapplication, which depend on the environmental impact and
on how the regulation develops. These preapplications add
up more than 7000 MW, being most of them considered in
the Atlantic coast.
C. Costs increase regarding onshore wind energy
Disadvantages of offshore wind farms are well known; in
spite of remuneration of offshore wind energy considered
(0,146 euros/kWh) is greater than current for onshore installations. Investments costs for offshore European projects are generally between 0.06 and 0.11 euros/kWh and
are referred mainly to:
- Foundation and structure costs: These concepts represent approximately 30% of the total cost of the project
in an offshore installation, against the 5% in an onshore
one.
- Connection system to the grid and evacuation costs:
Approximately 30% in the total budget of the offshore
project, against the 5% for the same onshore project.
The risk and uncertainties over the real wind energy resource, contribute to practically duplicate costs regarding an
onshore installation. Not everything is disadvantages. Studies executed in Denmark conclude that, in spite of needing
an initial investment much greater, up to two times than
onshore, electricity production is more stable and a 20%
greater. The lifespan of the farm, by means of a good maintenance, may duplicate.
IV. ALTERNATIVES TO THE TRANSMISSION SYSTEM TO THE GRID
In an offshore wind farm, the electric transmission system to the coast represents approximately the 30% of the
total budget of the project [1]. That is why the importance
of the previous design stage of the electric transmission
system. Main design criteria for the electric transmission
system in offshore wind farm are:
- Equipments must be the most compact as possible,
with great intervals for the maintenance or even designed for not having maintenance.
- High reliability of operation and in critical zones redundancy must be included.
- Installations must be resistant to corrosion caused by
the air of the sea, which is highly humid and contains
salt, so equipments placed inside or even in hermetically closed places must be used.
All existing offshore wind farms have chosen the solution
HVAC in its transmission system. Mainly, this is because
they are located 10-30 km from the coast and with powers
oscillating between 2 MW (Lely in Holland) to 158 MW
(Nysted in Denmark). In zones 50-80 km from the coast
and more powerful farms, it could be not so feasible this
technology HVAC, mainly due to the production of important quantities of reactive power, offering the transmission
technology HVDC more advantages. So, it is expected that
for future developments of further from coast offshore wind
farms and more powerful HVDC technology will be used.
Next, a description of the existing technical solutions for
transmission to the coast of power generated in an offshore
wind faro is presented.
A. High Voltage Alternative Current Transmission System (HVAC)
As it has been discussed, offshore wind farms in operation and projected for a near future have chosen this technology HVAC for the transmission system. The main advantage is its significantly minor initial investment cost that
HVDC technology HVDC and the greater experience. Furthermore, power as well as the distance to the coast is not
too high in current farms, so problems of reactive power
production in cables of HVAC technology are not so important and this would be a good option. However, for greater
power and distances to the coast, AC cables produce great
quantity of reactive power, which not reduces the capacity
of transport of the cable but implies an additional system to
absorb that reactive. This system would be a STATCOM or
SVC connected at the beginning and the end of the AC
network. Even wind turbines with converter at full power
capable to regulate the reactive power produced may be
used. Compensation at both extremes for long distances is
not efficient in the intermediate point of the cable, where
there would be a high capacity charge limiting the transmission capacity. As an example, a XLPE cable of 33 kV produces 100-150 kVAR/km and of 132 kV 1000 kVAR/km
[2].
Figure 2: HVAC technology general schema
B.
High Voltage Direct Current transmission Systems
(HVDC) based in a Line Commutated Converter –
LCC
It is a system based in high voltage with a converter made
up of thiristors and commutated by line (LCC). Its principal
advantages are:
- Possibility of transporting more power to higher distance than HVAC, because DC wires have not the
problem of generating reactive power.
- It is possible to control both active and reactive power,
but not independently.
- Optional to operate in a variable frequency state in the
wind farm internal grid.
- If there is a problem in the grid, the short-circuit current is smaller than the case with HVAC.
The principal disadvantages are:
- There is a higher initial investment.
- Little experience using this technology in marine installations.
- It is necessary to compensate reactive power in both
AC sides of the circuit, due to reactive power consumption by converters.
- It is necessary to use filters to eliminate the harmonic
distortion due to converters.
- It uses a lot of space to install the marine and land substations, converters, external equipment, transformers…
- Auxiliary services are needed in marine substation to
operate with low wind or without generation.
The global efficiency of the conversion, considering both
conversion stations AC-DC-AC, is about 97 and 98 %.
Figure 3: HVDC technology general schema [3]
C.
High Voltage Direct Current transmission Systems
(HVDC) based in a Voltage Source Converter (VSC)
It is a system based in high voltage with a voltage source
converter made up of IGBTs that enables a decoupled
control of both active and reactive power using a Pulse
Width Modulation (PWM) in the converters control. Its
principal advantages are:
- Better stability than HVDC-LCC.
- Control of reactive power on both grid extremes.
- It is possible to operate in weak grids.
- Independent control of active and reactive power.
- It is not necessary to compensate reactive power in
both AC sides of the circuit because it is implemented
in converters- It is not necessary to use filters to reduce the harmonic
distortion.
The principal disadvantages are:
- There is a higher initial investment due to the price of
converters.
- There is a higher limitation of maximum power to be
transported (<400 MW) compared with classical
HVDC thiristors technology.
- There is not experience in offshore wind farms.
- High commutation frequency that provides high losses
(about 2% in each converter station).
Global conversion efficiency is about 94%.
Figure 4: HVDC_VSC Technology general schema [3]
V. COMPARISON BETWEEN DIFFERENT TECHNOLOGIES TO TRANSMIT THE ENERGY TO THE
SHORE
As in AC or DC, cables have different transmission
capacity, depending on distance and the voltage level of the
cable.
System
3 phase cables
of Cu
AC
Isolation
XLPE
Oil
Oil
Impregnated
paper
XLPE
Max
voltage
400
kV
1200
MVA
*
500
kV
1500
MVA
*
600
kV
500 kV
150
kV
2400
MW
2000 MW
500
MW
100
60
80
No limit
No
limit
Max
power
Max
longitude
(km)
2 phase cables
DC
Table 1: maximum characteristics to have a correct operation
* Too much loses at this power
HVAC cables have a maximum of 200 MW nominal
power for voltages about 150 and 170 kV and longitudes
about 100 km. This capability can be increased if the
longitude of the line is decreased or if there are equipments
to compensate the reactive power in both sides of the line.
DC cables used in HVDC solutions have not the problem
of generating reactive power, so the maximum power to be
transported is higher than HVAC solutions. Even though,
the limit of maximum power to be transported with
HVDC_VSC solution is determined by the converter station
(about 300-350 MW). So, for more power, it will be
necessary to use 2 or more converter stations. Both cables
and stations of HVDC-LCC solution do not limit the
transport of power to the grid, and can be reached up to
1000 MW in offshore wind farms.
Load losses
Besides of the capacity of transmission, load losses take a
lot of importance. In HVAC technology it depends on the
cable longitude, the voltage level and its characteristics.
Even though, with HVDC systems, load losses depends less
on the type of cable used and more on efficiency of the
converter stations.
As it is known, HVDC-LCC solutions have better
efficiency (97-98%) than HVDC-VSC solution (96%)
because of the fact that thiristors have lower conduction
losses than IGBTs [4]. In figure 5 a comparative graph of
losses in function to longitude, for HVAC and HVDC
solutions, is presented.
Power
Longitude
Best solution
MW
l>100 km
HVDCVSC
350600
MW
Not important
HVDCLCC
600 900
MW
Not important
HVDCLCC
HVDCVSC
> 900
MW
Not important
HVDCLCC
Figure 4: Losses in function of longitude, for HVAC and HVDC Solutions.
Size of offshore stations
Offshore stations in case of HVDC are a third smaller
than HVAC, because in HVDC stations, in addition to the
transformer, it has to incorporate converters, filters and
capacitors groups. With the HVDC-LCC solution it needs
more room that using the HVDC-VSC one, because the first
is made of auxiliary systems to compensate reactive power
and filter the harmonics.
Characteristics of the connection to the external grid
It is important to take into account existing characteristics
and requirements in offshore farms connection to the land
distribution grid.
HVAC and HVDC-LCC solutions are not indicated to
weak external grids, even though HVDC-VSC one can be
used with this kind of grids.
In addition, some requirements must be fulfilled as for
power range, voltage control, frequency control and voltage
recovery capacity after a fail [5]. In this sense, HVDC
solution has the advantage to contribute less than HVAC
one to fail current. The principal advantage of HVDC-VSC
is its capacity of voltage control, independent control of
active and reactive power injection, and its capacity to
remain connected in voltages dips [6].
Installations costs
In this point a technique-economic comparison between
different solutions in function of power and marine farm
longitude to the shore (table 2) will be done.
Power
Longitude
l<100 km
Until
200
MW
250<l>100
km
l>250 km
200350
l<100 km
Best solution
HVAC
(Vmax=
170kV)
HVDCVSC
HVDCLCC
HVDCVSC
HVDC
Justification
because of voltage level (150245 kV) and
reactive compensation systems.
Transmission
capacity reduction with
HVAC.
HVAC is competitive only in
short longitudes.
In case of
HVDC-VSC it
is necessary to
use double converters and cables both in offshore and onshore stations.
HVDC-LCC is
more competitive, but there is
more risk due to
the absence of
redundant systems.
HVDC-LCC is
more competitive, but there is
more risk due to
the absence of
redundant systems.
Table 1: Estimation of the best technique-economic solution for different
cases
Justification
VI. CONCLUSIONS
No conversion
stations in
HVAC
Excessive losses
in AC
Grid connection of offshore wind farms presents specific
problems respect to onshore farms:
- Calculating the transmission systems to the shore.
- Calculating reactive power compensation systems and
necessary filters.
- It is necessary to have specific grid codes and verify
procedures.
Analyzing considered transmission systems to the shore
(HVAC, HVDC-LCC, HVDC-VSC) it is drawn that the
experience acquired in AC systems is an important point for
this technology, against the advantages of any of DC sys-
AC solution
more expensive
tems presented. The principal advantage of a DC system,
against an AC one, is its capacity of control and the possibility to contribute to the primary and secondary control.
Extra cost associated to these advantages is not justified.
Another associated inconvenient to the capacity of control
in a DC system is how this system establish priorities in
active and reactive power control. The 20 MW DC system
installed in 1954 in Gotland, and actualized in the end of
the 80’s to 260 MW, gives priority to active power generation with the objective of maintaining the voltage level in
DC bus. This characteristic calls into question the system
capacity to react in the reactive power control in the same
way.
E2Q is a pioneer company in Spain doing voltage dip
tests in wind generators to verify that fulfill all
requirements in PO 12.3 of Red Electrica de España (REE),
and another international grid codes. Wind generators
technologies tested by E2Q came from classical machines
adapted to fulfill requirements of operation, to cutting-edge
technology of megawatts.
Energy to Quality, S.L. has received in January 2008 the
accreditation by ENAC (National Entity of Accreditation
and Certification) according to UNE-EN ISO/IEC
17025:2005, to realize test to validate simulation models
according to the conditions established in Procedimiento de
Verificación, Validación y Certificación de los requisitos
del Procedimiento de Operación (P.O.) 12.3 de Red
Eléctrica de España. The laboratory is one of the firsts that
test wind generators that receives the accreditation for
validation of a simulation model according to ISO/IEC
17025 in Spain, and all over the world. This accreditation,
obtained for a test procedure based in simulation, confirms
the technological advance of Spanish companies in wind
sector.
Actually, there are not any studies of the capacity of ridethrough of a HVDC system. The obligation of this
requirement in a lot of grid codes, must get manufacturers
to adequate their systems to not be disconnected before a
voltage dip.
This ride-through capacity during a voltage dip can not
be accredited doing a test because of the high power, so
accredited simulation models are required to realize
simulation studies that permit accredit the capacity of ridethrough of HVDC solutions without interruptions during
voltage dips.
ACKNOWLEDGEMENTS
Marta Hernández Alarcón,
Ana Morales Martínez,
Xabier Robe
and Miguel Eduardo Montilla D’Jesus
REFERENCES
[1] Raul Manzanas, “Parques Marinos”, II Asamblea General de REOLTEC. 25 Septiembre 2007.
[2] Sally D. Wright, Anthony L. Rogers, James F. Manwell,
Anthony Ellis, “Transmission Options for Offshore
Wind Farms in the United States”, American Wind
Energy Association 2002.
[3] Miguel Eduardo Montilla de Jesús, “Estado del Arte y
Tendencia Técnicas de los Parques Eólicos Offshore”,
Trabajo de Investigación Tutelado, Universidad Carlos
III de Madrid, julio 2007.
[4] S. Meier, S. Norrga, H. P. Nee, “New Voltage Source
Converter Topology for HVDC Grid Connection of Offshore Wind Farms”, in proceedings of the 11th International Power Electronics and Motion Control Conference, EPE-PEMC’04, Riga, Latvia, September 2004.
[5] Stephan Meier, “Novel Voltage Source Converter based
HVDC Transmission System for Offshore Wind
Farms”, Royal Institute of Technology Department of
Electrical Engineering Electrical Machines and Power
Electronics, 2005.
[6] Ervin Spahié, Gerd Balzer, “Control Possibility for Offshore Wind Farm”, 15th PSCC, Liege, 22-24 August
2005