IEEE Power & Energy Magazine - May/June 2019 - 79

In the faulted zones, any approach belonging to the nonselective
protection philosophies can be used to interrupt the dc
fault current and isolate the faulted component.
grid as well as the faulted component isolated under voltages
and currents close to zero; however, it differs significantly
from those strategies in that only a part of the HVdc grid
is deenergized. these strategies must rely on HVdc circuit
breakers or dc-dc converters placed in between the protection zones to isolate the faulted zone from the healthy zones.
in the healthy zones, the dc voltage must be kept within the
boundaries of the ac-dc converters to ensure power flow
continuity. in the faulted zones, any approach belonging
to the nonselective protection philosophies can be used to
interrupt the dc fault current and isolate the faulted component. After fault clearing, the faulted zone is reenergized
and reconnected to the healthy zones. in figure 5(b), a partially selective fault-clearing strategy is adopted to achieve
connection of systems dc1 and dc2 if an outage of the entire
system, i.e., (dc1 + dc2), is, e.g., unacceptable to the connected ac grid, ac1.

Fully Selective With HVdc Circuit Breakers
for a selective fault-clearing strategy, HVdc circuit breakers
are located at the end of each line to interrupt the fault current
and simultaneously isolate the faulted line [(figure 5(c)]. this
strategy is similar to the conventional approach to ac system
protection, in which all of the converters adopt the continuous
operation concept (as defined by ceneLec), which implies
that the fault is cleared sufficiently quickly to avoid collapse
of the dc voltage. if the symmetric monopole configuration
is used, HVdc grid protection must also rebalance the pole
voltages when there are pole-to-ground faults. for pole rebalancing, either special dc chopper circuits or converters that
enable the injection of zero-sequence ac currents into the dc
side can be used.

Discussion
HVdc grid protection can be designed using a wide variety
of fault-clearing strategies, as described previously. choosing
a fault-clearing strategy is not straightforward and depends
on many factors, such as the desired overall reliability of the
power system, the relative power rating of the HVdc grid
compared to the connected ac system, the cost of fault-clearing equipment, or adaptability of the adopted strategy to system expansion. the nonselective strategies only consider the
secure operation of connected ac systems and offer the lowest
cost of all of the components at the dc side. these strategies
require considerable effort to restore the HVdc grid, and grid
restoration times should be adapted to meet the ac system
constraints. the selective strategies consider the protection
may/june 2019

of the HVdc grid itself by keeping the dc voltage within an
acceptable range. these strategies require a higher investment cost in terms of fault-clearing equipment on the dc side
(i.e., the HVdc circuit breakers at the end of each transmission line) but may require less effort in restoring the power
flow, when compared with nonselective strategies. the partially selective strategies face a tradeoff in limiting the extent
of the HVdc grid disconnected against the investment in dcside fault-clearing equipment. the final choice for a certain
strategy will depend on the investment costs associated with
the required fault-clearing equipment, the probability of each
type of fault, and the desired impact of fault clearing on the
HVdc grid itself and the connected ac systems. it is conceivable that the desired protection philosophy provides a higher
operation speed and fault-clearing selectivity as the grid
grows in size. While each fault-clearing strategy has specific
consequences for designing and operating HVdc grids, it is
plausible that systems using multiple strategies will arise,
e.g., for backup and primary protection or for different sections of a single grid.

HVdc Grid-Protection Algorithms
the functional requirements for protection algorithms used
to detect faults and identify their location in HVdc grids are
largely the same as for their ac counterparts. requirements
can be set for parameters such as speed, reliability, or sensitivity. the main challenge is to achieve the desired reliability, in terms of security and dependability, within a very
small amount of time. in particular, protection algorithms
must make the correct decisions on whether to trip faultclearing equipment within the first milliseconds after fault
detection. these decisions are based on the measured voltages and currents. increased operating time of the protection algorithm may lead to a more reliable decision but
also to an increased overall fault-clearing time, which
is unwanted given the quickly increasing dc fault current. When choosing and tuning protection algorithms,
the HVdc grid topology, converter technology, system
grounding, measurements, and operating points are important considerations.
this section focuses on algorithms for primary line protection in HVdc grids. fault detection for selective faultclearing strategies is more challenging than for nonselective
strategies, so the focus is on algorithms for selective fault
detection. However, this does not prevent using the same
algorithms for nonselective fault-clearing strategies. the
protection algorithms can be implemented using principles
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
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