IEEE Power & Energy Magazine - May/June 2019 - 78

power exchange with dc systems (connected at a dc point
of connection) or active and reactive power exchange with
ac systems (connected at an ac point of connection) must
remain short such that the ac or dc system at a point of connection does not enter into an abnormal operating mode. As
such, the temporary stop concept is expected to have a fault
separation time of a few hundred milliseconds. for a permanent stop, the disruption of active or active and reactive
power may cause the transmission systems at the point of
connection to enter an abnormal mode of operation, resulting in fault separation times of seconds.
each fault-clearing strategy results in different consequences for the dc system and the connected ac systems,
and, consequently, the functional requirements for HVdc grid
protection depend on adopted strategy. in the fully selective
philosophy, or in those zones making use of the "continuousoperation" concept, HVdc grid protection should primarily
ensure that the dc voltage remains within acceptable limits,
which keeps the ac-dc converters connected. in nonselective strategies, the dc voltage inherently collapses, and HVdc
grid protection is mainly required to clear the fault and allow
for later system restoration that does not endanger the secure
operation of the connected ac system. in this respect, options
for the protection concepts are either a permanent or a temporary stop. in partially selective strategies, HVdc grid protection must keep the dc voltage in the healthy zones of the system and not endanger the secure operation of the ac system.
therefore, the healthy zones of the system should adopt the
continuous operation concept. the faulted zone takes either
the permanent or temporary stop, depending on the constraints
imposed by the grid codes of the connected ac system(s).

of protection concepts, all of the converter stations adopt the
"permanent stop" concept, as defined by ceneLec. As soon
as a fault is detected using the sensors at the converter terminals, a trip order is sent to the ac circuit breakers to interrupt the fault current. When the ac circuit breakers of all ac/
dc converters in the grid have opened, the ac fault-current
contribution to the dc fault current is eliminated, and the dc
fault current and dc voltage passively decay to a value close
to zero. thereafter, the faulted component is isolated using
disconnect switches or Hsss connected at each end of the dc
lines. reenergizing the ac-dc converters in the new postfault
situation can be achieved by reclosing the ac circuit breakers, restoring the dc voltage to within the normal operation
range, and restoring the power flow to securely operate the
HVdc grid. this strategy is illustrated in the dc1 system in
figure 5(a), where the ac circuit breakers of the half-bridge
MMcs are used to interrupt the fault current, and Hsss or
disconnect switches are used to isolate the faulted equipment.

Nonselective With Fault-Blocking Converters

in this strategy, converters with fault-blocking capability are
used to quickly stop the fault current contribution from the
ac system to the HVdc grid through a joint action by all of
the converters. consequently, the HVdc grid quickly deenergizes, and the fault currents decay to values close to zero.
thereafter, the faulted line is isolated under near-zero voltage and current conditions with a dc Hss that has residual
dc interruption capability. once the faulted line is isolated,
the converters restore the dc voltage and, subsequently, the
power flow. this concept is illustrated in application to the
dc2 system shown in figure 5(a). Actively restoring the dc
voltage from zero requires using converters with controlled
fault-blocking capability. the active voltage restoration can
Fault-Clearing Strategies for HVdc Grids
then occur faster compared with converters without controlled fault-blocking capability or without fault-blocking
Nonselective With ac Circuit Breakers
this strategy relies on the ac circuit breakers on the ac side of capability at all. Hence, with a nonselective strategy that
the ac-dc converters to interrupt the dc fault current. in terms uses converters with controlled fault-blocking capability, dc voltage recovery can occur
faster compared with the nonselective strategy that uses ac cirdc1
dc1
ac1
ac2 ac1
ac2 ac1
ac2
dc1∗
cuit breakers in conjunction with
H
H
H
H
H
H
converters without fault-blocking
H
H
H
H
H
H
capability. With this strategy, the
actions of the converters with condc2
dc2
ac3
ac3
ac3
trolled fault-blocking capability
F
F
F
F
F
F
fall under the protection concept
known as temporary stop.
F
F
F
F
F
F
(a)
H F Half-/Full-Bridge Converter

(b)
Circuit Breaker

figure 5. Example fault-clearing strategies in an extended HVdc grid: (a) a nonselective fault clearing using ac circuit breakers (dc1) or fault-blocking converters (dc2), (b)
a partially selective fault clearing using HVdc circuit breakers or a dc-dc converter,
and (c) a fully selective fault clearing using HVdc circuit breakers.
78

ieee power & energy magazine

Partially Selective With
HVdc Circuit Breakers or
dc-dc Converters
this strategy is similar to the
nonselective fault-clearing strategies in that fault currents are interrupted by deenergizing the HVdc
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