IEEE Power & Energy Magazine - May/June 2019 - 74

+Udc
Udc

+Udc/2

If,ss >>
If,ss = 0

0

If,ss >>

U≈0

0

0

0

-Udc/2

0

-Udc

-Udc

-Udc
(a)

(b)

(c)

U≈0

(d)

(e)

(f)

figure 1. Three possible configurations for HVdc systems that indicate voltages during normal operation and prospective fault voltages and currents during pole-to-ground faults. (a) and (d) A symmetric monopole, (b) and (e) an asymmetric
monopole with metallic return, and (c) and (f) a bipole with metallic return.

complex in HVdc grids compared to ac systems because dc
fault currents have no naturally recurring zero crossings and,
without countermeasures, quickly increase to values that are
unacceptable for power electronic components. conversely,
the technologies used within HVdc grids offer options for
fault clearing beyond the well-known approach of using circuit breakers in existing ac systems.

Functional Requirements
for HVdc Grid Protection

Category 2: Unacceptable Impact
Category 1: Highly Unlikely

Fault Impact

to enable a cost-effective and reliable future power system
with HVdc grids, the design of HVdc grid protection must
begin with determining the likelihood of faults occurring
in the system, their impact on the system, and the type of
equipment available for clearing them. the HVdc gridprotection design is, therefore, closely related to the overall
design of the HVdc grid itself and with the selection of components within the HVdc grid. in this context, we refer to the
overall design of the HVdc grid as the choices made in the
HVdc grid structure, i.e., the number of terminals, a radial or

Category 3: Unacceptable Risk

Category 4: Acceptable Risk
Fault Probability

figure 2. An example classification of power system faults
and impacts based on RTE's "Mémento de la sûreté du
système électrique."
74

ieee power & energy magazine

meshed connection of terminals, the power rating of ac-dc
nodes, and the choice of grid configuration and grounding,
such as high-impedance grounded symmetric monopole or
low-impedance grounded asymmetric monopole or bipole.
for the selection of its components, we refer to the technology used for transmission lines, converters, and faultclearing equipment. the combination of all of these aspects
determines the probability of faults in the system and their
impact without HVdc grid protection.
the impact of the fault on the system in terms of overcurrent or overvoltage depends on the combination of fault
type and the grounding and configuration of the HVdc grid.
for pole-to-pole faults, irrespective of the grounding and
configuration, dc fault currents quickly increase and reach
a high steady-state value in the absence of any protection.
the impact of pole-to-ground faults depends on the type of
grounding: for a low-impedance grounded grid, prospective fault currents are high, whereas for a high-impedance
grounded system, prospective overvoltages on the healthy
poles are high (figure 1).
the faults occurring in the system can be grouped into
several categories indicating the acceptability or unacceptability of their probability and impact, which are combined
to determine risk (figure 2). the resulting design of HVdc
grid protection will largely depend on the outcome of this risk
assessment and determine the final impact of faults on the
system. for instance, it is conceivable that the protection of
a symmetric monopolar cable grid differs considerably from
that of a bipolar grid that makes use of overhead lines. in the
first case, HVdc grid protection may use HVdc circuit breakers with low current interruption capability or may not use
HVdc circuit breakers at all to deal with rare fault occurrences
and the high likelihood of permanent faults. in the second
case, it may make sense to provide HVdc circuit breakers
at both ends of all the lines to quickly isolate the more frequently recurring faults and provide for automatic reclosing of
the HVdc circuit breakers in the event of nonpermanent faults.
Although a set of specific functional requirements for
HVdc grid protection can only be compiled once 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

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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