IEEE Power & Energy Magazine - July/August 2019 - 63

In the nonselective case, equipment capable of interrupting
fault currents is only placed at the boundaries of the HVdc grid
(for instance, the ac/dc converters), making it a single protection zone for fault clearing. This results in the entire HVdc grid
being de-energized before the faulted component is isolated
under near-zero voltage and current conditions. After isolating
the component, the rest of the HVdc grid is energized again.
The equipment needed in the transmission line bays to isolate
the faulted component may be mechanical switches, such as
disconnector and fast disconnector switches.
In the fully selective case, which is similar to ac-system
protection, all substation bays require equipment capable of
interrupting fault currents to be
able to individually isolate faults
on every component of the HVdc
grid. Because HVdc circuit breakers are generally larger than their
ac counterparts and require the
use of current-limiting equipment,
the dc substation layout is expected
to differ considerably from ac subA
station layout.
In the partially selective case,
G
the protection zones divide the
HVdc grid into subgrids, encompassing multiple lines and converters. The zones are within the
HVdc gr id bounded by fault
clearing equipment (for example,
HVdc circuit breakers and dc/dc
converters), which quickly isolate
the faulted zone from the healthy
ones. Similar to a nonselectively
protected HVdc grid, the faulted
A
zone is de-energized and the faulted
component isolated using mechanical switches. This protection philosophy relies on two types of
equipment: 1) mechanical switches
without fault-current interruption
capability in transmission line bays
that do not connect protection zones
and 2) equipment with fault-current
interruption capability in bays that
provide the interconnection between
A1
two protection zones.

ment compared with nonselectively protected grids. In the
former case, HVdc circuit breakers in each substation bay
cost more than the disconnectors or switches in the latter case.
Similar to existing ac substation layouts, the dc substation busbar arrangement can adopt a single bus, a double bus-double
breaker, a main-and-transfer arrangement, a double bus-single
breaker, a ring bus or breaker, and a half scheme. The schemes
providing a higher availability generally require a higher
number of switches. The use of HVdc circuit breakers
in the substation bays substantially increases the cost
and the size of the substation. For selectively protected
HVdc grids, the relative location of the circuit breakers and

B
G

G
C
G
(a)

(b)

G
C1

Busbar Arrangements
The protection choice determines
the required equipment within the
substation and affects the busbar
arrangement. In selectively protected HVdc grids, the balance of
availability versus cost will likely
result in a different busbar arrangejuly/august 2019

C2
(c)

figure 3. Configurations and grounding for HVdc grids: (a) asymmetric monopolar,
(b) symmetric monopolar, and (c) bipolar.
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IEEE Power & Energy Magazine - July/August 2019

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