Offshore Electricity Infrastructure in Europe - European Wind Energy ...

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4.2.4 Conclusion and discussion on
hubs versus individual connections
Clustering of wind power projects is advantageous in
many cases, depending on the distance from shore
and the concentration of installed capacity in the
same area. A shared hub connection is most advantageous
when the sum of installed wind power capacity
in a small area is relatively large and equal to standard
available HVDC voltage source converter (VSC)
systems.
For distances from the onshore connection point of
up to about 50 km, usually 50 Hertz HVAC technology
is used and hubs do not generally provide much
added value due to limited savings in cable length. For
larger distances, the best solution depends primarily
on the total available wind farm capacity in the area
that could be grouped into the hub. For long distances
to shore a hub connection is mostly beneficial; the
benefit increases with distance.
Integrating a wind farm into a nearby hub is economically
advantageous for wind farm to hub connection
distances of less than 20 km. For greater wind farm
to hub connection distances the benefit will be rapidly
reduced. However, if the hub’s size is large and
if it is located far from shore, this again improves
the economics. For wind farm to hub connection distances
beyond 40 km, no beneficial hub cases were
identified.
In view of the large potential benefits of connecting
wind farm clusters via hubs, political and regulatory
strategies to foster their development are of primary
importance.
Additional considerations
The above assessment is solely based on economic
considerations. In cases where the hub is less beneficial,
decision makers may still opt for hub solutions in
order to mitigate the environmental and social impact
of cable laying through environmentally sensitive or
highly frequented coastal areas. A hub solution may
also allow more efficient logistics during construction.
Time delay costs and stranded
investments
It has to be highlighted that a hub project is in general
more complex. Construction schedules of wind farms
in a certain area seldom coincide. The economic assessment
is based on their connection schedules. If
projects that are supposed to be connected are abandoned,
the overall hub costs increase significantly due
to stranded investments or longer transition times
with oversized cables that are only partly loaded.
However, the case study analysis for a hub connection
of three wind farms with a capacity of 350 MW each
has shown that the costs of such delays or stranded
investments should not be overestimated. In the case
study the hub solution remains economically viable up
to a 10 year time delay for two of the wind farms to
be connected. Even if the one of the wind farms is
not built at all, the hub connection is the preferred
solution.
Where hub connections are clearly shown to be beneficial,
a dedicated regulatory and policy framework to
mitigate this risk will be required. The risk of stranded
investment can be mitigated, e.g., by strategic siting
and concessions with the specific aim of having groups
of projects with a capacity of 1,000 to 2,000 MW per
group concentrated in an area and developed approximately
at the same time.
4.3 Integrated design –
case studies
This section elaborates on the integration of wind
farm hubs and interconnectors, the main idea behind
the offshore grid in the North Sea. The OffshoreGrid
project focuses on two major design concepts. Any offshore
grid design can be broken down into these two
fundamental building blocks:
• Teeing-in of wind farms into interconnectors (jointly
developed with the wind farms or already existing)
as shown in Figure 4.9.
• Interconnection of countries via wind farm hubs on
either side as shown in Figure 4.10.
42 OffshoreGrid – Final Report