IEEE Power & Energy Magazine - July/August 2019 - 70

Traditional OSS Versus OTMs
Transmission infrastructure for an OWF typically accounts
for 10-20% of the project's capital cost. A large portion of
the cost can be directly related to the development, manufacturing, and installation of OSS platforms, which are needed
to convert the array voltage (33 or 66 kV) to a higher level
(115 or 230 kV). If offshore converter platforms are required
for HVdc transmission, the cost will be considerably higher.
A decrease in platform size and weight offers a massive
potential for cost reduction of the OSS. If the total weight of
a topside and substructure can be kept below 1,000 t each,
smaller and less expensive installation vessels can be used,
having a significant effect on the overall cost. The introduction
of a modular concept approach (the OTM) has made inroads
on projects such as the Beatrice OWF, where two OTMs were
installed in 2018, the first of its kind in the offshore wind sector.
The OTM design concept reduces OSS equipment down
to the bare essentials, allowing the substation components to
be placed on a smaller substructure. To create a capacity similar to a traditional OSS, multiple modules are utilized. This
approach then removes the need for large heavy-lift installation vessels to install OSS foundations and their topsides, and it
optimizes the selection and use of more cost-effective vessels.

Integrated OSSs
Another tactic currently being explored in the industry is integrating two HVac substations along with one HVdc converter
platform on a single support substructure. This approach is
geared toward reducing the structure's weight when compared to current HVdc technology. While this technique has
yet to be applied to a live project, studies indicate a significant opportunity for reduced CAPEX and OPEX because of
the leaner cost and service requirements from having one
platform instead of multiple ones. With the introduction of a
66-kV interarray cable in lieu of the current 33 kV, we expect
to see the ac array cables directly coming to a single platform
where ac-to-dc conversion happens.

Interlink of Offshore Platforms
Developers are assessing the risk and availability profiles of
new offshore wind capacity and determining how to best mitigate downtime. Some developers are considering a concept
where they install transmission cables (interlinks) to link the
OSSs of projects that connect to a single common substation.
These submarine cables would be held in open standby in
case of a failure of the cable to shore. With such a failure,
the associated project would still have the capacity to export
some (or all) of its power to shore through the interlink via
the adjacent project's cable to shore (depending on the capacity available on the cable and interlink).
This interlink would essentially provide a redundancy
mechanism in the event of a cable failure and offer a more
cost-effective alternative to using multiple cable connections
to a single common substation from each adjacent offshore
project. The costs would be shared between project owners,
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based on a formula representing each generator's access the
interlink and the availability of cables to shore. Currently, in
the United Kingdom, the transmission tariff methodology
for offshore transmission costs considers only radial cables
to shore and, therefore, does not take into account any interlinks that may be built. This interlink concept presents the
first view of an offshore grid's potentially being created using
the dynamic market forces at play in energy generation.

Technology Considerations
What are the key market disruptors reducing the expense in
today's valuation of the cost of offshore wind? Some leading
technology drivers and changes that are helping the industry
optimize OSS design are discussed next.

Increasing the Interarray Cable Voltage to 66 KV
In the past year, we have seen the adoption of 66-kV cables
on projects, primarily to reduce overall costs. A 66-kV system increases the power transfer capability through the
cables relative to lower-voltage systems, resulting in a more
cost-effective cable system through a more efficient and economical transmission of power from OWFs to shore. Adopting a 66-kV system does incur increased unit costs for highervoltage cables, terminations, and switchgear. However, these
costs are outweighed with benefits, such as
✔ reduced array cable length (approximately 20-30%, depending on site layout), resulting in lower CAPEX for
radial and ring configurations of an interarray design
✔ decreased number of OSSs required for a higher-voltage system
✔ additional design options that can be considered, including connecting all the power cables to a single
platform and the possible use of less-expensive aluminum cables as an alternative to copper.

HVac: Adoption of the Midpoint
Reactive Compensation Platforms
Longer distances to shore drive the need for significant reactive
compensation because of the long lengths of submarine cables
connecting the wind farm to the onshore stations. On projects
such as Hornsea 1, the developer had three collector platforms
and a reactive compensation platform located between the wind
farm and shore. The midpoint reactive compensation structure
facilitates the installation of electrical reactors, which improve
voltage performance and limit electrical losses over the length
of the HVac transmission by reducing reactive power flows.
Midpoint reactive compensation platforms can be avoided with
the adoption of HVdc technology. However, HVdc applications
can bring significant costs and reliability risks.

Adoption of Larger Turbines
The introduction of larger turbines has had an impact on
overall substation design power, resulting in higher voltages
experienced on the system. This, in turn, has affected cable
requirements, thus increasing the cost of the transmission
july/august 2019

IEEE Power & Energy Magazine - July/August 2019

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