IEEE Power & Energy Magazine - May/June 2019 - 37

There is an increasing prevalence of inverter installations at the
distribution and the lower end of transmission voltages associated
with renewable generation, including solar, wind, and energy
storage, not only on the generation side but also on the load side.

of vsc technology are engineered and constructed as independent monopole systems in the form of dual symmetrical monopoles using half-bridge MMc valves at a dc rating
between 600 and 1,000 Mw, at between ±300 and ±320
kvdc. the use of independent monopoles reduces the risk of
common mode failure between the two links, and this allows
the overall link availability to reach levels similar to those in
a traditional lcc bipole. the swl converters use an MMc
vsc topology described earlier in this article. the individual MMc submodules are engineered to achieve much
higher voltage operation; they are similar in appearance to
the valves illustrated in the following project description.

The DolWin3 Offshore System
germany has been implementing a program of installing
a large number of offshore Hvdc transmission systems to
allow new wind generation to connect offshore. the Dolwin3 (Dw3) Hvdc offshore transmission system is one of
these Hvdc links for transmission of offshore wind energy
to the onshore ac network, as seen in Figure 5, from german
waters of the north sea. the Dw3 link has a 900-Mw rating
at ±320 kvdc, configured as a single symmetrical monopole.
complexities are associated with the offshore converter
platform due to the harsh and isolated environment. notable
challenges include implementing personnel operational and
safety provisions and the logistics of getting personnel and
equipment to the platform. extensive use is made of redundant systems, remote control, and monitoring facilities to
avoid permanently staffed operation at the offshore station.
the large number of offshore windfarms in the german
waters of the north sea has enabled the clustering of some offshore converter stations. the platforms are provided with interconnecting cables and a common collector or array cabling
busbar and switchgear arrangements. these offer increased
flexibility to divert wind generation to another converter when
one suffers a fault or is under maintenance outage. also, there
is scope for sharing personnel facilities among platforms by
connecting them with walkways when they are sufficiently
close together.

Summary
the introduction of vsc technology into the Hvdc, Mvdc, and
flexible ac transmission system toolbox for system planners has
stimulated huge growth in interest in using these devices across
transmission and distribution networks. new semiconductor
may/june 2019

technologies will offer improvements in converter ratings and
efficiencies in the near future; this will enable the use of vscs
at higher power levels, which are presently only addressable by
conventional thyristor-based lcc technologies. evolutionary
and revolutionary development work in new converter topologies is ongoing, and new products are anticipated to be released
by manufacturers in the coming years. Much of the necessary
research and development work associated with dc-to-dc converters, dc breakers, and other relevant products and systems
that enable the creation of a dc grid is ongoing across the industry. the field of power electronics has always been an exciting
area in which to work, but the presence of power electronics
across the entire generation, transmission, and distribution
network footprints now and into the future offers an unlimited
scope for engineering challenges and a demand for solutions.

For Further Reading

J. H. baker Jr. et al. (2015, Feb. 10). "growing dc power,"
T&D World. [online]. available: https://www.tdworld.com/
grid-opt-smart-grid/growing-dc-power
n. ahmed, a. Haider, D. van Hertem, l. Zhang, and H.-p.
nee, "prospects and challenges of future Hvdc supergrids with
modular multilevel converters," in Proc. 2011 14th Eur. Conf.
Power Electron. and Appl., birmingham, U.K., pp. 1-10.
amprion, "project description: Ultranet." [online]. available: https://www.amprion.net/grid-expansion/our-projects/
Ultranet/
cigrÉ Joint working group b2.41. (2014). guide to
the conversion of existing ac lines to dc operation. [online].
available: https://e-cigre.org/publication/583-guide-to-theconversion-of-existing-ac-lines-to-dc-operation
Q. Qi, c. long, J. wu, and J. Yu, "impacts of a medium
voltage direct current link on the performance of electrical
distribution networks," Applied Energy, vol. 230, pp. 175-188,
Nov. 2018.
sp energy networks, "angle-Dc." accessed on: Jan.
2019. [online]. available: https://www.spenergynetworks
.co.uk/pages/angle_dc.aspx
tennet, "Dolwin3." accessed on: Jan. 2019. [online]. available: https://www.tennet.eu/our-grid/offshore-projects-germany/
dolwin3/

Biography
Neil Kirby is with general electric, philadelphia.

p&e

ieee power & energy magazine

37


https://www.tdworld.com/grid-opt-smart-grid/growing-dc-power https://www.tdworld.com/grid-opt-smart-grid/growing-dc-power https://www.amprion.net/grid-expansion/our-projects/ultranet/ https://www.amprion.net/grid-expansion/our-projects/ultranet/ https://www.e-cigre.org/publication/583-guide-to-theconversion-of-existing-ac-lines-to-dc-operation https://www.e-cigre.org/publication/583-guide-to-theconversion-of-existing-ac-lines-to-dc-operation https://www.spenergynetworks.co.uk/pages/angle_dc.aspx https://www.spenergynetworks.co.uk/pages/angle_dc.aspx https://www.tennet.eu/our-grid/offshore-projects-germany/dolwin3/ https://www.tennet.eu/our-grid/offshore-projects-germany/dolwin3/

IEEE Power & Energy Magazine - May/June 2019

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