IEEE Power & Energy Magazine - July/August 2019 - 69

optimizing the voltage generated offshore, reducing potential electrical losses, and transmitting the electricity to shore
in an economical manner. One of the key design challenges
is identifying the lifecycle cost implication of transmission
and availability losses (i.e., during downtime).
The primary purpose of an OSS (Figure 3) is to reduce
electrical losses on the system by increasing the voltage (i.e.,
stepping up) and then exporting the power to shore. Small
or precommercial projects (lower than 100 MW), nearshore
projects (lower than ~15 km), or projects with a grid connection at collector voltage (i.e., lower than 36 kV) do not
require an OSS. As capacity increases and substations are
sited farther offshore in deeper waters, the requirement
increases, and one or multiple OSSs are needed to meet
project needs. Challenging decisions must be made, such as
those between high-voltage (HV) ac and HVdc, the amount
and type of offshore reactive compensation, and the size of
an OSS. System studies are the starting point to assess the
concept and connection options in the transmission and distribution network.
OSSs typically step up the voltage from the site distribution (30-66 kV) to a higher voltage (115-230 kV), which is
typically the connection voltage. This step-up dramatically
reduces the number of export circuits (subsea cables) between
the OSS and the shore. Typically, each export circuit may be
rated in the range of 150-200 MW. When designing an offshore electrical network, the following elements need to be
taken into consideration during the early development of the
transmission network for a wind farm:
✔ capacity
✔ distance from shore
✔ HVac/HVdc
✔ reactive compensation requirements
✔ the number of export cables to shore
✔ the number of transformers on the OSS (i.e., capacity
dependent)
✔ redundancy
✔ equipment failure rates
✔ traditional OSS or offshore transmission modules
(OTMs)
✔ the power supply for ancillary and low-voltage systems
✔ 33-kV versus 66-kV interarray cables
✔ OSS maintenance strategy
✔ interlinking multiple offshore OSSs and adjacent OWFs
✔ the availability target and requirements
✔ substation installation techniques.
As the offshore wind sector has matured, the project capacity of
projects has increased, and they have moved farther offshore. To
date, the majority of offshore wind projects have been built with
ac transmission (with the exception of a small number of collector hubs in Germany). However, the industry has been successful in delaying the requirement for expensive dc transmission
through the introduction of midpoint compensation platforms.
Such a system requires an ac/dc converter station both offshore
and onshore, which are both large installations. In the next wave
july/august 2019

of U.K. projects in the 2019 contract-for-difference auctions,
the industry is likely to see an introduction of HVdc technology
take its first steps, such as the Dogger Bank area and Vattenfall's Norfolk projects. The industry challenge will be to push
the limits of the technology through innovations while executing
projects cost-effectively through their lifecycles.

When to Use HVdc
When a connection to an OWF from onshore cannot be served
by an ac circuit (e.g., when transmission distance is excessive),
an HVdc system can be used. An HVdc system requires an
offshore HVdc converter station to house the insulated-gate
bipolar transistor valves, filters, and other equipment required
to convert the wind farm's electrical ac output into dc for transmission to shore. To date, such systems have been deployed in
German waters. However, they are now being considered for
farther offshore U.K. Round 3 OWFs. The nature of the installation means that the converter stations employed in these systems are large, with ratings up to 900 MW operating at dc
voltages of 320 kV.
In the German OWF transmission connection model, the
offshore converter is supplied, owned, and operated by the
transmission operator TenneT. In this case, the converters are
located at the wind farm boundary and can support a number
of separately developed wind farms with the converter. In the
United Kingdom, the offshore transmission connection for a
wind farm is typically specific to the OWF that is connected.
Offshore converter stations are generally constructed with
a jacket foundation because of the equipment's considerable
weight. The topside (i.e., the platform and the equipment) can
be in excess of 10,000 t and requires large installation vessels.
Self-installing stations have been used for the projects, such
as the HelWin and DolWin platforms. Self-installing structures can be transported from their fabrication location to the
installation site and negate the requirement for a large installation barge and cranes.

figure 3. The Galloper Offshore Transformer Platform,
United Kingdom. (Source: Atkins; used with permission.)
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IEEE Power & Energy Magazine - July/August 2019

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