IEEE Power & Energy Magazine - May/June 2019 - 77

HVdc grid protection does not necessarily
implement the same conventional approach used
for ac system protection.
the fault current and limit it to a desired level instead of
reducing the dc fault current entirely to zero. certain types
of HVdc circuit breakers, e.g., the ones making use of controllable power electronic modules, are capable of actively
limiting the current. superconducting fault-current limiters
use a component that is in superconducting mode during
normal operation. therefore, it presents a low impedance to
the circuit. During faults, the superconducting component is
driven out of superconducting mode, resulting in the loss of
superconducting capabilities and, thereby, presents a high
impedance to the circuit. fault-current-limiting equipment
can be installed in series with the transmission lines. in
this manner, they are able to limit both the transmission
line discharge currents as well as the contributions from
the ac-dc converters. fault current limiters that are located
in series with the ac/dc converters do not affect the line
discharge currents.

High-Speed Switches
Dc high-speed switches (Hsss) can be used to quickly isolate a faulted line from the remaining dc network and operate only under near-zero voltage and current conditions;
therefore, these Hsss are not required to interrupt fault currents. However, depending on the application, these switches
may be required to interrupt small residual currents in the
grid, which, e.g., result from passive discharge of capacitive
or inductive grid components. in case residual currents have
zero crossings, these Hsss may make use of traditional ac
circuit breaker technology. Without current-zero crossings,
the Hsss must provide a sufficiently high countervoltage or
have an auxiliary circuit that creates a zero crossing, e.g.,
a passive resonant circuit used in load transfer breakers in
classic point-to-point HVdc connections.

Classification and Characterization of
Fault-Clearing Strategies for HVdc Grids
HVdc grid protection does not necessarily implement the
same conventional approach used for ac system protection.
in the conventional approach to ac system protection, circuit
breakers are placed throughout the system and used to simultaneously interrupt the fault current and isolate the faulted
component. this has led to a fault-clearing strategy that
divides the power system into zones containing grid elements
such as transformers or transmission lines. in case of a fault,
the protection scheme disconnects and de-energizes just the
zone containing the fault. given the different types of equipment available for HVdc grid protection and the characterismay/june 2019

tics of this equipment, alternatives to the selective fault-clearing strategy exist. regarding these alternatives, the protection
zones used for fault current interruption do not necessarily
coincide with the components that should be isolated. these
fault-clearing strategies can be classified in terms of "extent
of the HVdc grid which is deenergized," an approach followed
in the cigre technical brochure (tB) 739, or described in
terms of "action at the protection zone point-of-connection,"
an approach followed by the european committee for electrotechnical standardization (ceneLec).
in cigre tB 739, fault-clearing strategies are divided
into three main philosophies. in the first philosophy, nonselective fault clearing, the entire HVdc grid is completely
de-energized prior to isolation of the faulted component
under near-zero voltage and current conditions. the faulted
component should be identified during or after grid deenergization and may be automatically isolated using Hsss. After
the faulted component is isolated, the remaining part of the
HVdc grid is reenergized before power flow can resume. in
the second philosophy, partially selective fault clearing, the
HVdc grid is subdivided into several protection zones. Here,
the faulted zone is first quickly isolated from the healthy
zones of the grid. this requires that equipment capable of
interrupting dc fault currents is present at all boundaries of
each protection zone. thereafter, the faulted element within
the faulted zone is isolated, as in that of a nonselective philosophy. then the remaining portion of the initially faulted
zone is reenergized and reconnected to the healthy parts of
the grid. the third philosophy, fully selective fault clearing,
adopts an approach to dc fault clearing similar to the conventional approach in ac systems.
in the approach followed by ceneLec, three main concepts applied within a certain protection zone are defined
based on the impact of dc faults within a protection zone on
all ac and dc points of connection, i.e., 1) continuous operation, 2) temporary stop, and 3) permanent stop. for each of
these concepts, a typical "fault separation time," i.e., the
time at which recovery of the active power flow can start, is
defined. these three main concepts are extended to a total
of five when considering the availability of reactive power
during dc fault separation. for a continuous operation, the
exchange of active power with dc systems (connected at
a c point of connection) and active or reactive power with
ac systems (connected at an ac point of connection) must
remain controllable during the entire fault separation process, resulting in fault separation times of a few milliseconds. in the temporary stop concept, the disruption of active
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IEEE Power & Energy Magazine - May/June 2019

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