IEEE Power & Energy Magazine - May/June 2019 - 84

dc voltage, which, for the system under consideration, implies
current of 4 ka and voltage of 320 kV.
in conventional ac transmission systems, generators have
many self-protection controls, but the thresholds are significantly higher relative to the nameplate-rated variables. the
line/cable impedances also have much higher values in ac than
in dc systems, because the frequency, f, contributes to the reactance (the imaginary part of the impedance, 2rfL, where L is
the inductance). these factors lead to the conclusion that, in a
dc system, cBs (and protection systems) must respond much
faster than in ac systems to prevent widespread loss in capacity
(converter blocking) and destructive fault current magnitudes.

not isolated, grid voltage would be low, and currents would
be high under the fault condition. high current would likely
lead to component damage, while low voltage would reduce
power-transfer capability. Fast dc fault isolation is particularly significant for large dc grids, which may have numerous converter stations and transmission lines. the converters
being considered for the european north Sea dc grid are rated
at 1.5-2 Gw, while the total dc grid capacity may exceed
150 Gw in some scenarios.
Figure 1 shows a single-line diagram of a fivenode ±400-kV dc grid, consisting of four ac-dc bipole
converters rated 800 mVa, 2 ka per pole interconnecting
the dc grid (buses 1-4) with corresponding ac systems, one
dc bus without a converter (node 5), eight dc cables, and
16 dc cBs per pole (32 dc cBs in total). the grid topology
has been selected to illustrate how power can be transferred
securely while limiting the number of dc cBs. in comparable
ac systems, the number of ac cBs would be much higher,
according to modern grid development practices. the indicated power and voltage levels are in the highest range of
demonstrated dc cB technology.
For example, a short circuit fault occurring on the line
linking buses 3 and 4 would insert a low impedance at the
point of fault and result in high currents flowing into the fault
from all parts of the network (as indicated by red arrows).
Depending on the impedances of the grid cables and on the
severity of the fault itself, the fault currents would have different magnitudes, but they can easily reach 10-30 times
the rated current (say 20-60 ka) at some points. these are
destructive currents that grid components cannot withstand.
all high-power ac-dc converters have self-protection
control logic, which blocks and disconnects converters in the
event of disturbances. typically, self-protection thresholds
are set at approximately 2 per unit (p.u.) of current and 0.8 p.u. of

1.6 GW 2 kA

Operating Speed
in a dc fault situation, current rises and voltage falls throughout
the dc grid during the delays associated with dc protection and
dc cB operation. once the dc cBs open and isolate the dc fault,
the dc voltage recovers, and dc power flows over the reconfigured grid at a newly established balanced operating point,
which depends on the number of remaining operating converters and their control strategies. the faster the dc cBs operate,
the less likely it is that large parts of the dc grid will be lost.
however, the blocking of nearby converters cannot be
completely avoided, even with application of the fastest available dc cBs. this is because the converters closest to the fault
point may experience high current and thus block before the
dc cBs open. the most recent dc grid studies indicate that
temporary converter blocking for 10-30 ms is possible until
the dc fault is cleared. then the converters can be unblocked
to resume normal operation. the need for blocking converters will depend on the grid topology and the fault location.
many studies indicate that dc grids of various sizes and
topologies can recover from any dc line fault if the total protection time (dc cB opening time plus protection operation

2

1

ac System 1

CB12_1

CB12_2

CB15_1

CB25_2

CB14_1
±400 kV

ac System 4

5

CB

15_

5

_5

45

±400 kV

5
25_
CB
CB
35_
5

3

400 kV

2 kA
CB14_4

CB23_3

CB45_4

CB35_3

CB34_4

CB34_3

±400 kV

Fault

1.6 GW

ac System 2

CB23_2

CB

4

2 kA 1.6 GW

dc-ac
Converter

dc CB
2 kA 1.6 GW

ac System 3

±400 kV

figure 1. A diagram showing a five-node dc grid with four ac-dc converters, eight dc cables, and 16 dc CBs per pole.
84

ieee power & energy magazine

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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