IEEE Power & Energy Magazine - July/August 2019 - 66

already seen in some onshore substations and wind turbines,
will undoubtedly transfer to offshore environments.

Summary

figure 7. The interior of the SylWin 1 offshore substation.
(Used with permission of Siemens, www.siemens.com/press.)

Where possible, equipment is not located offshore. Figure 6 shows the onshore location of dc chopper resistors,
which are used to ensure that the dc-connected cables do not
experience overvoltages. The space to house them is not part
of the offshore substation, again reducing costs.
Since multiterminal substations would be more complex
and larger, locating them offshore would increase the size
and cost of offshore substations. The greater complexity
would also increase the potential for component failure. The
application of HVdc breakers, with their large and complex
components, would further increase substation costs. These
increased costs are particularly high when the multiterminal
dc substations are located offshore on a platform, potentially
limiting the development of such substations.
There have been substantial investments in VSC HVdc
reliability. Converters now employ modular multilevel designs
using several submodules. As shown in Figure 7, each submodule is about the size of a large suitcase and can be replaced
individually. A converter can be designed with redundant submodules; should one fail, it can be switched out and replaced
with another online.
The design of offshore converters requires special expertise. Even with 99% efficiency, a 500-MW station still requires
cooling systems to handle up to 5 MW of heat. In addition
to thermal design, electrical-insulation design is a key factor.
Sufficient clearance must be left between components of the
converter and the topside structure to ensure proper electrical isolation. This means that much of the structure inside the
topside is clear air. The authors anticipate the first applications
of DC GIS solutions to be deployed offshore.
While some onshore VSC HVdc designs use voltages up
to 500 kV, present offshore installations are limited to ±320 kV.
This is consistent with the voltage ratings of the XLPE subsea cables that were available at the time of this writing.
Major manufacturers have undertaken a large number of offshore substations, and these have been functioning successfully
for a number of years. The authors anticipate further progress in
reducing station size and cost. In maintenance, the use of robots,
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ieee power & energy magazine

Although the design principles for ac substations, such as substation layout, are, in theory, applicable to the design of dc
substations, fundamental differences exist and depend on the
choices made in system topology, control, and protection. First,
the HVdc grid topology determines the number of transmission lines to route within a dc substation. Second, the primary
and secondary equipment required within the dc substation,
and its characteristics, depends on the adopted control and
protection philosophy. In addition, the location of the substation (for example, on- or offshore) influences the choices made
in construction and insulation technology (for instance, air or
gas insulated). Recent technological developments indicate
that most building blocks for future HVdc grids are in place or
advancing toward practical realization. Nevertheless, the construction of a reliable and cost-effective HVdc grid will require
engineers to consider the many options for system protection,
operation, and control, and the implications that these have on
the practical realization of the underlying dc substations.

For Further Reading
M. Barnes, D. Van Hertem, S. P. Teeuwsen, and M. Callavik,
"HVDC systems in smart grids," Proc. IEEE, vol. 105, no.
11, pp. 2082-2098, Nov. 2017.
CIGRÉ Joint Working Group B4/B5-59, "Local control
and protection of HVDC grids," CIGRÉ, Paris, France,
technical brochure, 2018.
CIGRÉ Joint Working Group A3/B4.34, "Technical requirements and specifications of state-of-the-art HVDC
switching equipment," CIGRÉ, Paris, France, technical brochure, 2017.
D. Van Hertem, O. Gomis-Bellmunt, and J. Liang, HVDC
Grids: For Offshore and Supergrid of the Future. Hoboken,
NJ: Wiley-IEEE Press, 2016.
V. Akhmatov et al., "Technical guidelines and prestandardization work for first HVDC grids," IEEE Trans. Power
Del., vol. 29, no. 1, pp. 327-335, Feb. 2014. doi: 10.1109/
TPWRD.2013.2273978.

Biographies
Dirk Van Hertem is with KU Leuven/EnergyVille, Belgium.
Willem Leterme is with KU Leuven/EnergyVille, Belgium.
Geraint Chaffey is with KU Leuven/EnergyVille, Belgium.
Mudar Abedrabbo is with KU Leuven/EnergyVille, Belgium.
Mian Wang is with KU Leuven/EnergyVille, Belgium.
Firew Zerihun is with KU Leuven/EnergyVille, Belgium.
Mike Barnes is with The University of Manchester, United Kingdom.
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july/august 2019

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IEEE Power & Energy Magazine - July/August 2019

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