IEEE Power & Energy Magazine - May/June 2019 - 42

increased penetration of hvdc is expected with the integration of wind, solar, and renewable energy to the grid. it is
therefore necessary to provide supplementary cybersecurity
awareness and design for Scada and WaMpac systems
that interact with hvdc systems.
Utilities often install hvdc lines as a response to load growth
located far away from (potentially new) generation sites. as a
consequence, hvdc lines are long and high capacity and also
designed to have significant overload capacity. Because of the
ac to dc conversion, hvdc lines differ from their ac counterparts in another notable way: more equipment is necessary for
the installation of an hvdc line than a standard high-voltage ac
transmission line, including additional transformers, converter
cooling systems, and filter banks. asset owners must exercise
all preventive measures available to ensure the safety, security,
and resilience of these systems.

HVdc for Resilience
Because of its controllability, rapid response rate, flexibility,
and ability to connect otherwise disjoint asynchronous ac
systems, hvdc has the potential to contribute significantly
to overall bulk power system resilience (i.e., safeguarding
of critical system functionality when subject to perturbation
and restoration after outages). Flexible operation of hvdc
offers several improvements to overall bulk power delivery
security and resilience, ranging from facilitating increases
in renewable penetration to providing actuation for complex
control system strategies for stability.

Point-to-Point and Back-to-Back HVdc
at their core, large interconnected electrical grids provide
redundancy in generation and increased access to loading.
thus, the ability of point-to-point (i.e., a converter located
on either end of a single dc line) and back-to-back hvdc converters to connect multiple, otherwise-incompatible ac systems provides built-in resilience. improved power sharing,
access to additional markets, and the potential for reduced
spinning reserves improve the overall functionality and security of these systems. For example, the 1,400-MW north
Sea network link will interconnect the nordic and British
energy markets, enabling both to better leverage the benefits
of renewable energy. this 520-kv, 730-km vSc hvdc link
will transmit power, mostly underwater, from Blyth, United
Kingdom, and Kvilldal, norway. once commissioned, the
line will be the world's longest subsea power interconnection. When there is excess wind power in the United Kingdom, power will flow via the link to norway, allowing it
to conserve water in its reservoirs. during periods of high
demand and low wind power in the United Kingdom, electricity from norway's hydroelectric plants will flow to the
United Kingdom.
Stability augmentation and the ability for large quantities
of power generated remotely to be transmitted over exceedingly long distances provide additional opportunities for
improved resilience. Given the high degree of controllability
42

ieee power & energy magazine

of active power flow, additional control methods may be
implemented to further increase power system stability. the
ability to arbitrarily transmit current that conforms to a predetermined waveform (i.e., active power modulation) is principally used in high-fidelity system identification methods.
active power may be modulated corresponding to predetermined waveforms, providing engineers with information
that may allow them to better understand the resonant characteristics (i.e., electromechanical oscillatory phenomena)
of large interconnected grids.
armed with accurate models developed via deliberate excitation, sophisticated wide-area damping control
(Wadc) methods are available to reduce undesirable oscillatory behavior in the system. these control systems augment stability while improving system-wide ac power flows
by reducing oscillations due to area-to-area dynamic power
imbalance. hvdc interties have also been proven to reduce
the recovery time from a local blackout, as demonstrated in
Finland in 2015. careful active power injection at the remote
converter station nearest the outage improves the black-start
capabilities of affected generation and loads.

Multiterminal HVdc
Multiterminal hvdc (Mtdc) systems are typically configured to have several (three to five to date) converters connected by multiple hvdc lines. point-to-point hvdc systems
differ from Mtdc systems in their ability to connect more
than two systems simultaneously, while providing services similar to those afforded by a point-to-point scheme. in
addition to the aforementioned improvements in secure bulk
power transmission, Mtdc provides, as a last line of defense,
the forced islanding or emergency network splitting of certain
ac systems during severe events. this may allow for the greatest number of individual ac networks to retain stability independently from one another, decreasing the overall number of
consumers impacted during an event.
an Mtdc system can be configured with all of the converters connected in series, as shown in Figure 2(a), such that
they all see the same current. this scheme lends itself more
to line-current-commutated converter schemes, such as the
Quebec-new england transmission project, which has three
terminals. one terminal controls the dc current, while local
dc voltages are controlled by the other terminals in the system to inject or extract commanded power. vSc hvdc better supports parallel or mesh-connected dc networks, which
could evolve to hvdc grids, shown in FigureĀ 2(b). present
vSc systems require ac circuit breaker (cB) action to clear
dc faults, but ongoing development of dc cBs will provide
increased selectivity for isolating dc faults, thus improving resilience.

What Can Be Done to Ensure
HVdc Security?
Given the wide range of capabilities of hvdc outlined earlier,
it is appropriate to pose the following question: if the future of
may/june 2019



IEEE Power & Energy Magazine - May/June 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2019

Contents
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