IEEE Power & Energy Magazine - July/August 2019 - 64

is typically quoted as being around
100 km. A dc substation far offshore would need to be housed
in a large protective structure
with a controlled internal environment that shields the equipment against temperature variation, weather, wind, and waves.
(a)
(b)
(c)
Figure 5 shows a typical offshore HVdc housing on a raised
figure 4. An example of busbar topologies and arrangements for breakers and
platform, which is many stories
current-limiting inductors: (a) a single busbar/single breaker, (b) a ring bus, and (c) a
high and the size of a sports field.
breaker-and-a-half scheme. Measurements and disconnectors are not shown.
Substations have been constructed
in the relatively shallow waters
off
the
northwestern
European
coast where engineers use
fault-current-limiting equipment may need careful determination to avoid creating damaging overcurrents in the fixed foundation legs on the seabed. The civil engineering
circuit breakers, for example, because of the discharge of and installation expenses dominate the overall costs of such
the connected cables. Figure 4 provides examples of dc platforms and lead to great demand for compact designs that
busbar arrangements, where care has been taken to avoid a minimize the station size. As a result, all offshore dc substadirect connection between the circuit breaker and the trans- tions have used VSCs and are likely to continue doing so
because of their smaller footprint compared with LCCs. An
mission line.
While it is clear that the design of a dc substation will alternative solution is the construction of artificial islands,
depend on the adopted protection philosophy, it will also but examples of this do not exist yet.
Considerable work is being undertaken to reduce the
hinge on the anticipated growth of an HVdc grid and a potential associated change of philosophy. Small-scale HVdc grids size of the housing (topside), which would further lower
will be protected in a nonselective way, as is done for the its cost. It would also decrease the size and cost of the
existing multiterminal systems, whereas larger-scale HVdc foundation. The size of lifting vessels could be lessened
grids will show an increasing need for dc-side selectivity. DC which, given the limited supply of very large lifting vessubstation layout should provide sufficient space for fault- sels, could also potentially reduce construction lead time
clearing equipment, such as HVdc circuit breakers, in case and costs.
The other major cost factor, maintenance, results from
the HVdc grid is expected to grow.
the challenges of transporting large equipment and personnel to a substation platform a long distance offshore. Several
Substation Technology
The choice of substation technology will influence its lay- factors that must be considered include the following:
out because GIS technologies allow for reduced dimen1) Weather: Transporting large pieces of equipment to a
sions compared with air-insulated switchgear technologies.
platform in poor weather conditions is troublesome.
Although GIS is a mature technology in ac grids, it is not
The delivery of replacement parts will be constrained
widely used for HVdc. The ac GIS designs cannot be transby available weather windows.
ferred straight to dc due to the differences in electric-field
2) Logistics: For the transportation of smaller plant piecdistributions. Currently, GIS technology for dc applications
es, parts, and technicians, a helicopter may be feais under development for voltage levels appropriate for future
sible. Suitable vessels and crew must be procured for
HVdc grids. The first offshore application of dc GIS will be
large equipment. Large equipment is designed to be
for the DolWin6 project in Germany; at present, measurevery reliable; if it fails at all, it may be just once during
ment technology, such as RC dividers for GIS, are also being
the 20-plus-year life of the HVdc system. Despite the
developed and tested.
equipment's high reliability, procuring and managing
special vessels and crews would be required on the
rare occasions of forced outages.
Offshore Substations
HVdc grids are expected to play a key role in future off3) Spare parts: Spare equipment inventories for large
shore grids requiring specific installations on platforms
plants may be prohibitively expensive. Spare parts
where space and volume are precious. Since these substamay need to be manufactured (or existing plant equiptions are located far from land, platform size and system
ment repaired on site).
maintenance dominate design considerations and cost. DC
4) Platform access: The part to be replaced may need to
substations would typically be a long distance from shore
be extracted from the platform, and this may affect the
because ac technology would be used for shorter distances.
layout of items in the substation, complicating electriAlthough the break-even distance varies from site to site, it
cal design.
64

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