IEEE Power & Energy Magazine - May/June 2019 - 75

It is conceivable that the desired protection philosophy
provides a higher operation speed and fault-clearing
selectivity as the grid grows in size.
grid design has been determined, the overall objectives for
every HVdc grid-protection scheme are the same: HVdc
grid protection should ensure continuity of secure power
system operation and avoid damage to system components
in case faults occur. for short circuit faults, this means that
the HVdc grid protection should detect the faults, interrupt
the short circuit currents, isolate the faulted component, and
restore the system to a secure operation state. the protection scheme should perform these functions without causing
unwanted ac or dc system instabilities. in addition, it should
avoid damage to equipment and property and guarantee
safety for personnel and the public.
A set of functional requirements for interacting with the
connected ac system can be defined based on the constraints
for its secure operation and its components. each ac system
grid code defines a maximum power imbalance that imposes
a limit on the secure operation of the system. for instance, in
the entso-e (european) grid code, this maximum power
imbalance is specified as 3 gW in continental europe for
an unspecified amount of time and in the united Kingdom
as 1.8 gW, to avoid a frequency deviation outside the range
of 49.5-50.5 Hz for 60 s. HVdc grid protection should thus
prevent loss of the HVDc line from causing a power imbalance in the connected ac system that exceeds that maximum
value. in addition to the maximum power imbalance, the
possible functional requirements for HVdc grid protection
relate to the transient stability of connected ac systems or to
the damage to its components.
Within the HVdc grid, functional requirements for HVdc
grid protection can be based on the continuity of the HVdc
grid operation itself (if required) or from the limits of its
components. At present, there is no HVdc grid code that
defines the constraints within which secure operation is
achieved, and the following suggestions are given for illustration purposes only. HVdc grid protection is required to
keep the voltage at converter terminals within certain limits
so they can remain connected. such a "converter dc fault
ride through," however, may not necessarily imply that every
converter remains in an active switching state or retains the
capability of supporting the ac system during the entire fault
duration. such grid protection should also avoid damage to
the power electronic components and other equipment. At
present, for voltage source converter (Vsc)-based HVdc
point-to-point connections, the most critical components
are the converter power electronic switches, i.e., insulatedgate bipolar transistors. in future HVdc grids, HVdc circuit
breakers (if any) could also become a limiting factor because
may/june 2019

they may be required to absorb a large amount of energy
during fault current interruption.

Technology That Assists With
Clearing dc Faults
several means of interrupting or limiting dc fault currents
are available. Applied technologies include converters, ac
circuit breakers, dc circuit breakers, and combinations of
converter and circuit breakers.

AC Circuit Breakers
in current Vsc HVdc point-to-point schemes, ac circuit
breakers are installed in a converter station as the primary
dc fault-interrupting device given that the ac-dc converter
itself, i.e., either a two-level or half-bridge modular multilevel converter (MMc), has no dc fault-blocking capability.
for these types of converters, a fault current path from the ac
to the dc side always exists via the antiparallel diodes within
the converter's submodules (figure 3). therefore, the ac circuit breakers eliminate the ac contribution to the dc fault
and typically interrupt the ac current associated with the dc
fault within a few cycles of the fundamental frequency. After
eliminating the ac contribution, the dc fault current passively
decays to zero in a time interval that depends on converter
topology and system parameters.

HVdc Circuit Breakers
HVdc circuit breakers are the dc counterparts of ac circuit
breakers and can interrupt both nominal and short circuit
currents up to their short circuit rating. typically, HVdc circuit breakers must interrupt currents in the 5-15-kA range

SM

SM

SM

SM

SM

SM

ac Side

dc Side
SM
SM

SM

SM

SM

SM

SM

Half Bridge
+
-
Full Bridge
+
-

figure 3. The schematic of an MMC comprising half- or
full-bridge submodules. SM: submodule.
ieee power & energy magazine

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IEEE Power & Energy Magazine - May/June 2019

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

Contents
IEEE Power & Energy Magazine - May/June 2019 - Cover1
IEEE Power & Energy Magazine - May/June 2019 - Cover2
IEEE Power & Energy Magazine - May/June 2019 - Contents
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IEEE Power & Energy Magazine - May/June 2019 - Cover3
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