IEEE Power & Energy Magazine - May/June 2019 - 34

VSC technology introduces the realistically achievable
prospect of extending interconnecting individual HVdc
links on the dc side.
are already being used in the generation sector of the electric
power industry and in other industries (e.g., aviation). recently,
these tools have also been applied to Hvdc asset management
and have proven their value not only to asset owners but also
to manufacturers, who can more actively participate in proactively monitoring and maintaining the physical asset by creating a virtual representation using secure remote communications and analytics. now occurring in other industries, this
will offer significant benefits in future Hvdc systems.

dc Grids
Hvdc systems using vsc technology became a mainstream
Hvdc solution approximately 10 years ago, and the international council on large electric systems (Conseil International des Grands Réseaux Électriques, CIGRÉ) organization recognized a need to establish some structure around
the specification of future systems, which would enable the
development of standards and, eventually, dc grids. Multiterminal vsc systems have been installed. but if we consider
a dc grid as a meshed interconnection, there is no working
dc grid anywhere in the world. rather, there is merely an
increasing number of individual vsc dc links. However,
the vsc technologies used in these individual links are

generally specified and implemented within these cigrÉ
guidelines. the future development of new dc links in a particular region, along with dc-dc converters and dc breakers, will allow the interconnection of these individual links
into dc grids in the future, although this future will still
pose significant challenges that should not be trivialized.
Many issues are to be addressed in the meantime, especially
regarding the integration of present vsc Hvdc control and
protection systems into future grid-capable systems.
increasing use is being made of simulation and analytical
tools, including
✔ desktop software tools
✔ real-time simulation and testing
✔ hardware-in-the-loop development
✔ Hvdc control replicas
✔ network replicas.
these will be critical, allowing the development of future
updates to existing Hvdc systems and the specification of
future links for successful integration of the old and the new.

ac-dc Conversion

as long as there has been Hvdc, there has also been interest in converting existing ac lines to dc to take advantage
of the increased power transfer
capacity. while many academic
and utility studies have analyzed
some specific cases around the
world, there has been very little
actual construction of such systems. the major drawback is that,
once the ac line is taken out of
service, the intermediate drop-off
points on the ac line to feed local
loads must be removed, and those
loads must be fed either from local
sources or by additional converters,
which leads to a significant additional cost. Finding the right mix
of line and load parameters that
makes line conversion commercially
attractive is very challenging. However, the flexibility of vsc technology is generating increased interest. Historically, with only lcc
technology available, the available
range of ac line cases that would
benefit from ac-dc line conversion
figure 2. The ±300-kVdc VSCs installed in an HVdc valve hall.

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may/june 2019

IEEE Power & Energy Magazine - May/June 2019

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