IEEE Power & Energy Magazine - May/June 2019 - 76

within a time interval of several milliseconds. compared
to ac circuit breakers, they are more complex because they
must deal with dc currents, which have no naturally recurring zero crossings. to interrupt these nonzero currents,
HVdc circuit breakers consist of several parallel paths that
perform the functions of current interruption themselves
and absorb the energy present in the inductive elements
of the circuit. figure 4 illustrates these paths, where the
upper path typically comprises a mechanical switch and
forms the path for normal conduction, the middle path is
an auxiliary path to aid in interrupting fault currents, and
the lower path is an auxiliary path for energy absorption,
typically consisting of metal-oxide varistors. the HVdc
circuit breaker's operation typically relies on a succession
of actions, which depend on the technology used. resonant circuit breakers make use of mechanical breakers and
resonant circuits that induce current-zero crossings [figure 4(a) and (b)]. Because of their slow clearing time, passive resonant breakers are not appropriate for interrupting
short circuit currents in Vsc-based HVdc systems. HVdc
circuit breakers, based on a combination of power electronics and mechanical breakers in the normal conduction
path, e.g., so-called hybrid HVdc circuit breakers, typically involve using power electronics to avoid arcs in the
mechanical breakers [figure 4(c)]. each of the technologies
recommended by manufacturers has advantages and disadvantages in terms of speed, on-state losses, cost of components, physical dimensions, maximum current interruption
capability, or overall reliability and simplicity of the components used. Although HVdc circuit breakers are not yet
commercially available, prototypes have been tested and
found to perform in the range of expected requirements.
note that these prototypes all require fault-current-limiting equipment.

connects the ac to the dc system. this process enables them to
block the fault current contribution from the ac to the dc side
without needing ac or dc circuit breakers. the most established
converter with fault-blocking capability is the full-bridge
MMc (figure 3), which will see its first application in germany's ultranet project. there are other promising converter
configurations that allow for blocking and controlling dc fault
currents while having reduced losses during normal operation, when compared with a full-bridge MMc. examples of
this are hybrid MMc configurations that use a combination of
half- and full-bridge submodules or alternative topologies such
as the alternate arm converter. fault-blocking capability may
be achieved in a controlled or uncontrolled manner. A major
advantage of controlled blocking is the uninterrupted reactive
power controllability of the converter, which allows the converter to operate in a static synchronous compensator mode
during the entire fault-clearing process.

Fault-Blocking Converters

Fault Current Limiters

DC-DC Converters
Proposed for transforming dc voltage from one level to
another, dc-dc converters may also limit or even interrupt
fault current contribution from one side to the other. the
dc-dc converter's ability to block the transfer of fault currents
depends on its topology. All the topologies that use an intermediate ac stage, thereby providing galvanic isolation between
both sides, can block the propagation of fault current from one
side to the other, irrespective of the fault-blocking capability
of the ac-dc converters used. in direct dc-dc conversion, the
topologies are generally able to block fault-current propagation in one direction, whereas only certain types of topologies
are able to block it in both directions. for instance, to achieve
bidirectional fault-blocking capability with an MMc-type
dc-dc converter, submodules with fault-blocking capability
must be used.

in contrast to converters without fault-blocking capability, converters with this capability are able to block the fault current
contribution from the ac system to the dc fault. these converters rely on converter submodules that are able to insert voltages of positive and negative polarity, unlike the half-bridge
submodule, which can only insert voltages of positive polarity (see figure 3). As such, they can insert a voltage opposing
the ac system voltage contribution in the fault current path that

At present, HVdc grid protection invariably requires equipment that either limits the rate of rise or the magnitude of the
dc fault current. fault-current-limiting equipment may take
various forms, such as inductors in series with the circuit
(typically air-core), active fault-current limiting through converters with fault-blocking capability, HVdc circuit breakers
with fault-current-limiting capability, or superconducting
fault-current limiters. inductors in series with the circuit only
limit the rate of rise of the fault
current and act as a limiter only in
conjunction with equipment that
interrupts the dc fault current, such
as HVdc circuit breakers. these
inductors may be placed directly in
series with the dc circuit breakers
or can also be found in the arms
(a)
(b)
(c)
of the ac-dc converters. the ac-dc
converters with controlled faultfigure 4. Conceptual dc switchgear designs: (a) a passive resonant circuit breaker,
blocking capability can control
(b) an active resonant circuit breaker, and (c) a hybrid circuit breaker.

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