IEEE Power & Energy Magazine - July/August 2019 - 71

Adoption of Gas-Insulated
Switchgear Over Air-Insulated Switchgear
The adoption of gas-insulated switchgear (GIS) as an alternative
to air-insulated switchgear (AIS) has led to significant cost
reductions when assessed over the lifetime of the asset. The
classic reason to use a GIS instead of an AIS is that there is
a limited installation footprint available. GIS systems may
be marginally more expensive in terms of their initial cost
(CAPEX), but when considering the total cost (CAPEX plus
OPEX) of a substation during its lifespan, it can be more costeffective because of its smaller offshore footprint.

New Technologies
Several new technologies are being developed for OWFs,
and some are more advanced than others. Low-frequency
ac (LFac) systems are being proposed to transmit power
at a frequency lower than the standard grid frequency to
enable an increased transmission distance of subsea cables.
While such systems have been applied in rail systems across
Europe, no LFac system has been deployed to service offshore wind generation. LFac systems use an onshore frequency converter to reduce the frequency of a 50-60-Hz
grid by one-third for the subsea component of transmission to the OSS and then on to the wind turbine generators
(WTGs). This lower frequency results in lower capacitance
in the cable circuit and a greater ability to deliver power
through the subsea cables.
The principles behind an LFac OSS are the same as those
for a standard 50- or 60-Hz HVac OSS. Both kinds collect
power from the WTG arrays and transform and export it
to the grid through export cabling. The required electrical
equipment is essentially the same: transformers, switchgear,
reactors, auxiliary systems, and emergency systems. However, the lower frequency means that some items will require
modification. In particular, the physical size of transformers on the substation and within the wind turbines become
significantly larger due to the lower magnetic flux linkage
in the transformer cores, which could necessitate the use of
single-phase units as opposed to three phase.
Although the increased transformer size and weight
will impact the design of the OSS structural elements, the
concept of the jacket substructure and topside will remain
july/august 2019

Construction and Operations
Considerations
Moving away from the design side, we provide a high-level
overview of commissioning, installing, and operating an
OSS. We also address considerations for the end of a structure's lifetime.

Transformer 1
HV Switchgear

One of the biggest and most cost-prohibitive issues associated with OSSs was that substations were being custom
designed for each project. This resulted in a higher cost per
substation and provided an avenue to improve efficiencies
with innovative technologies to reduce overall costs. Developing standard designs across a series of substations, optimizations in fabrication processes, and streamlining toward
serial fabrication all contributed to the cost reduction.

MV
Switchgear

Standardization of OSS Structures

as per the standard HVac design. The equipment layout on
the topside structure will also remain as per a typical HVac
design, with the heavy transformers located in the center
of the structure and other equipment around the periphery.
A typical topside would comprise four levels: cable, main,
mezzanine, and roof decks. An example of an LFac OSS layout is shown in Figure 4.
Other concepts or early ideas in industry that could possibly change the dynamic of offshore wind in the decades
ahead are as follows.
✔ Floating substations: These could assist greatly in areas
with environment challenges (e.g., exposure to typhoons
and seismic activity or areas sensitive to piling solutions).
The concept could also have cost advantages around installation, and it may assist in areas such as the United
States where there are installation restrictions regarding
the Jones Act, which inhibits the use of foreign vessels.
✔ An offshore hub development approach: TenneT (an
offshore grid operator in Germany and The Netherlands) is exploring a concept in which an offshore artificial island is developed as a central hub for installation, operation, and maintenance activities.
✔ Utilizing existing offshore infrastructure: This involves transmitting power over existing offshore interconnector runs or connecting into an offshore load,
such as an offshore oil and gas platform.
✔ dc exportation: This would explore whether offshore turbines could export dc directly from the individual turbines.
✔ A submerged substation: Microsoft deployed a submerged data center off the coast of Scotland in 2018.
While a similar approach could be applied to OSSs,
there are challenges around operations and maintenance (O&M) considerations and the cable connection.

MV
Switchgear

system. These costs have been balanced by the increased
electrical output of the bigger turbines.

Transformer 2

figure 4. A typical layout of an LFac OSS. MW: medium
voltage. (Source: Atkins; used with permission.)
ieee power & energy magazine

IEEE Power & Energy Magazine - July/August 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2019