IEEE Power & Energy Magazine - September/October 2020 - 59

applications such as the connection of offshore wind power
plants or connection to/between weak grids, VSC-HVdc
often offers the only technoeconomically viable solution.

Using VSC-HVdc to Transform
Power Systems
Today's most prominent application for bulk power transmission with HVdc is still the interconnection of asynchronous
grids (see Figure 1). HVdc interconnectors might deploy
either LCC or VSC technology depending on the project's
needs. Regardless of which technology is used, HVdc is
the technically and economically preferred solution to link
asynchronous grids, cross marine environments, and enable
controllable power exchange. Due to the flexibility, compactness, and dynamic performance of VSC-HVdc, transmission
system operators worldwide tend to favor this technology and
use it in an extended range of applications, such as
✔ the integration of remote offshore wind resources to
the power grid, which requires long submarine cables,
compact offshore substations, and a voltage source at
the remote end
✔ power from shore for offshore installations such as oil
and gas platforms
✔ city center power infeed (for megacities)

✔ the connection of remote islands
✔ flexible, multiterminal dc overlay grids.

In recent years, embedded HVdc systems within synchronous ac systems have also received significant interest and
are being planned and installed all over the world. An embedded HVdc system is defined as
a dc link with at least two ends being physically connected within a single synchronous ac network. With
such a connection, it can perform not only the basic
function of bulk power transmission, but also, importantly, some additional control functions within the ac
network such as power flow control, voltage control,
system stability improvement and the mitigation of
system cascading failure.
(See "CIGRE," Tech. Rep. 536, in the "For Further Reading"
section for the definition.)
The applications of embedded HVdc systems include
✔ back-to-back HVdc schemes in highly meshed systems
with the goal of power flow controllability, e.g., ultrafast
power flow control responding to contingencies, or to
avoid loop flows
✔ parallel HVdc links to ac lines to increase the transfer capability of a combined corridor by operating ac
lines closer to their thermal limit
✔ long-distance HVdc corridors for the integration of
remote renewable generation or to serve remote load
centers within one synchronous grid.
The interesting point about embedded VSC-HVdc is not
only its main functionality of bulk power transfer but also
its additional features, which might increase the overall
system capacity well above the actual capacity added by
the HVdc.

Increasing Grid Capacity and Flexibility
With Embedded VSC-HVdc
As previously discussed, an installed embedded VSC-HVdc
system forms an integral part of the legacy power grid and
is deployed for reasons other than its bulk power transmission capabilities. Power flow controllability, the measures
used to increase power transfer capability, and ancillary
services are of high interest for the secure operation of a
synchronous grid. Some of the key benefits of embedded
VSC-HVdc include the following:
✔ Dynamic/fast power flow controllability could support
preventive and corrective security measures and avoid
widely spreading contingencies in the ac grid.

By Kevin Schönleber,
Alexandre Oudalov,
Athanasios Krontiris, and
Peter Lundberg
©ISTOCKPHOTO.OCM/SERGEI DUBROVSKII

september/october 2020

ieee power & energy magazine

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IEEE Power & Energy Magazine - September/October 2020

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