IEEE Power & Energy Magazine - May/June 2019 - 85

Because operators have so little experience with dc CBs, more
study is needed about the internal complexity of dc CBs and
their failure modes to fully understand their reliability.
time) is within approximately 10 ms. this assumes that some
converters may block by self-protection temporarily or permanently, depending on the grid topology and the strategy
used for protecting the network. permanent blocking after
fault clearing occurs if the converter becomes isolated. this
results in grid capacity reduction.

Ratings
in ac systems, ac cBs are rated for the largest available fault
level at their point of connection (the worst case steady-state
fault current). Dc cB prototypes recently brought to market
by manufacturers are designed for only 15-20-ka peak interrupting current. these state-of-the-art dc cBs have such low
ratings because of costs and technological limitations. the
ratings are, for the foreseeable future, lower than expected
fault levels at many dc buses. thus, dc cBs must operate
fast, before dc fault current exceeds their ratings and before
it reaches destructive levels causing many converters to block
and leading to wider dc grid collapse.

Bidirectional Operation
in many applications, dc cBs will be expected to interrupt
fault current in one direction only. Depending on how protection zones and protection systems are configured, some
dc cBs may be expected to operate in both directions. also,
bidirectional operation may be beneficial as a backup protection function. application of a unidirectional dc cB may
suffice in particular projects and result in substantial cost
and size savings for some dc cB topologies.

Fast, Multiple Open/Close Operations
Dc grids with overhead lines can be exposed to frequent
faults, many of which will be transient. as with overhead ac
transmission, reclosing with multiple cB operations within
200-500 ms may reasonably be expected. with underground
or underwater dc cable systems, however, almost all faults
are permanent, and reclosing may not be included in the protection logic. the number of expected operations and timing
of dc cB duty cycles may result in cost and size implications
for some dc cB topologies.

Losses, Size, and Weight
most of the time, dc cBs operate in a closed state and conduct load current. losses in closed-state dc cBs cost grid
operators revenue. also, dc cBs with high losses may require
additional heat-removal equipment. Dc cBs will be noticeably larger and heavier than ac cBs. in europe, dc grid
may/june 2019

studies of offshore environments show that the extra size and
weight of dc cBs could add significantly to platform costs.

Cost and Reliability
the widespread application of ac cBs to ac transmission systems ensures a secure power supply and enables operational
flexibility. the simple dc system in Figure 1 may not provide
the same level of security and flexibility as in ac systems but,
nevertheless, requires 32 dc cBs. the example shows that
future grids are likely to require a considerable number of dc
cBs. these will cost more than ac cBs, and for some dc cB
topologies the costs may be substantially higher.
Because operators have so little experience with dc cBs,
more study is needed about the internal complexity of dc cBs
and their failure modes to fully understand their reliability.
Some dc cBs using semiconductor technologies have self-protection as discussed later in this article. Such features need to be
considered by grid planners and protection system developers.

Standardization of Inputs/Outputs
in large dc grids, the protection system will consist of multiple
relays (microcontroller based) and dc cBs that are interconnected and use adjusted settings controlled by the grid operator.
in the development of protection logic, grid topology changes
and expandability must be considered as important factors.
achieving the desired operation of protection system with components made by different vendors requires that interconnections be standardized and that components be interoperable.

Challenges With dc Circuit Opening
DC Current Commutation
the first developers of dc systems learned that interrupting
dc current is difficult. trying to separate cB contacts under
a current flow creates an arc, which for high-voltage systems
is self-sustaining and generates large amounts of heat caused
by the arc resistance. with ac currents, there is a natural current-zero crossing twice every cycle (every 10 ms in 50-hz
systems). this momentary current interruption-along with
engineering devices that ensure sufficient contact separation
and an arc-extinguishing chamber-enables modern ac cBs
to reliably interrupt very large currents within 20-60 ms.
there are no natural current-zero crossings in dc systems.
at low voltages of 10-30 V (automotive applications), the arc
voltage is larger than the system voltage, and this reduces the
current to zero. at higher voltages, there are two principal
methods to interrupt dc current.
ieee power & energy magazine

85



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
IEEE Power & Energy Magazine - May/June 2019 - 2
IEEE Power & Energy Magazine - May/June 2019 - 3
IEEE Power & Energy Magazine - May/June 2019 - 4
IEEE Power & Energy Magazine - May/June 2019 - 5
IEEE Power & Energy Magazine - May/June 2019 - 6
IEEE Power & Energy Magazine - May/June 2019 - 7
IEEE Power & Energy Magazine - May/June 2019 - 8
IEEE Power & Energy Magazine - May/June 2019 - 9
IEEE Power & Energy Magazine - May/June 2019 - 10
IEEE Power & Energy Magazine - May/June 2019 - 11
IEEE Power & Energy Magazine - May/June 2019 - 12
IEEE Power & Energy Magazine - May/June 2019 - 13
IEEE Power & Energy Magazine - May/June 2019 - 14
IEEE Power & Energy Magazine - May/June 2019 - 15
IEEE Power & Energy Magazine - May/June 2019 - 16
IEEE Power & Energy Magazine - May/June 2019 - 17
IEEE Power & Energy Magazine - May/June 2019 - 18
IEEE Power & Energy Magazine - May/June 2019 - 19
IEEE Power & Energy Magazine - May/June 2019 - 20
IEEE Power & Energy Magazine - May/June 2019 - 21
IEEE Power & Energy Magazine - May/June 2019 - 22
IEEE Power & Energy Magazine - May/June 2019 - 23
IEEE Power & Energy Magazine - May/June 2019 - 24
IEEE Power & Energy Magazine - May/June 2019 - 25
IEEE Power & Energy Magazine - May/June 2019 - 26
IEEE Power & Energy Magazine - May/June 2019 - 27
IEEE Power & Energy Magazine - May/June 2019 - 28
IEEE Power & Energy Magazine - May/June 2019 - 29
IEEE Power & Energy Magazine - May/June 2019 - 30
IEEE Power & Energy Magazine - May/June 2019 - 31
IEEE Power & Energy Magazine - May/June 2019 - 32
IEEE Power & Energy Magazine - May/June 2019 - 33
IEEE Power & Energy Magazine - May/June 2019 - 34
IEEE Power & Energy Magazine - May/June 2019 - 35
IEEE Power & Energy Magazine - May/June 2019 - 36
IEEE Power & Energy Magazine - May/June 2019 - 37
IEEE Power & Energy Magazine - May/June 2019 - 38
IEEE Power & Energy Magazine - May/June 2019 - 39
IEEE Power & Energy Magazine - May/June 2019 - 40
IEEE Power & Energy Magazine - May/June 2019 - 41
IEEE Power & Energy Magazine - May/June 2019 - 42
IEEE Power & Energy Magazine - May/June 2019 - 43
IEEE Power & Energy Magazine - May/June 2019 - 44
IEEE Power & Energy Magazine - May/June 2019 - 45
IEEE Power & Energy Magazine - May/June 2019 - 46
IEEE Power & Energy Magazine - May/June 2019 - 47
IEEE Power & Energy Magazine - May/June 2019 - 48
IEEE Power & Energy Magazine - May/June 2019 - 49
IEEE Power & Energy Magazine - May/June 2019 - 50
IEEE Power & Energy Magazine - May/June 2019 - 51
IEEE Power & Energy Magazine - May/June 2019 - 52
IEEE Power & Energy Magazine - May/June 2019 - 53
IEEE Power & Energy Magazine - May/June 2019 - 54
IEEE Power & Energy Magazine - May/June 2019 - 55
IEEE Power & Energy Magazine - May/June 2019 - 56
IEEE Power & Energy Magazine - May/June 2019 - 57
IEEE Power & Energy Magazine - May/June 2019 - 58
IEEE Power & Energy Magazine - May/June 2019 - 59
IEEE Power & Energy Magazine - May/June 2019 - 60
IEEE Power & Energy Magazine - May/June 2019 - 61
IEEE Power & Energy Magazine - May/June 2019 - 62
IEEE Power & Energy Magazine - May/June 2019 - 63
IEEE Power & Energy Magazine - May/June 2019 - 64
IEEE Power & Energy Magazine - May/June 2019 - 65
IEEE Power & Energy Magazine - May/June 2019 - 66
IEEE Power & Energy Magazine - May/June 2019 - 67
IEEE Power & Energy Magazine - May/June 2019 - 68
IEEE Power & Energy Magazine - May/June 2019 - 69
IEEE Power & Energy Magazine - May/June 2019 - 70
IEEE Power & Energy Magazine - May/June 2019 - 71
IEEE Power & Energy Magazine - May/June 2019 - 72
IEEE Power & Energy Magazine - May/June 2019 - 73
IEEE Power & Energy Magazine - May/June 2019 - 74
IEEE Power & Energy Magazine - May/June 2019 - 75
IEEE Power & Energy Magazine - May/June 2019 - 76
IEEE Power & Energy Magazine - May/June 2019 - 77
IEEE Power & Energy Magazine - May/June 2019 - 78
IEEE Power & Energy Magazine - May/June 2019 - 79
IEEE Power & Energy Magazine - May/June 2019 - 80
IEEE Power & Energy Magazine - May/June 2019 - 81
IEEE Power & Energy Magazine - May/June 2019 - 82
IEEE Power & Energy Magazine - May/June 2019 - 83
IEEE Power & Energy Magazine - May/June 2019 - 84
IEEE Power & Energy Magazine - May/June 2019 - 85
IEEE Power & Energy Magazine - May/June 2019 - 86
IEEE Power & Energy Magazine - May/June 2019 - 87
IEEE Power & Energy Magazine - May/June 2019 - 88
IEEE Power & Energy Magazine - May/June 2019 - 89
IEEE Power & Energy Magazine - May/June 2019 - 90
IEEE Power & Energy Magazine - May/June 2019 - 91
IEEE Power & Energy Magazine - May/June 2019 - 92
IEEE Power & Energy Magazine - May/June 2019 - 93
IEEE Power & Energy Magazine - May/June 2019 - 94
IEEE Power & Energy Magazine - May/June 2019 - 95
IEEE Power & Energy Magazine - May/June 2019 - 96
IEEE Power & Energy Magazine - May/June 2019 - 97
IEEE Power & Energy Magazine - May/June 2019 - 98
IEEE Power & Energy Magazine - May/June 2019 - 99
IEEE Power & Energy Magazine - May/June 2019 - 100
IEEE Power & Energy Magazine - May/June 2019 - 101
IEEE Power & Energy Magazine - May/June 2019 - 102
IEEE Power & Energy Magazine - May/June 2019 - 103
IEEE Power & Energy Magazine - May/June 2019 - 104
IEEE Power & Energy Magazine - May/June 2019 - 105
IEEE Power & Energy Magazine - May/June 2019 - 106
IEEE Power & Energy Magazine - May/June 2019 - 107
IEEE Power & Energy Magazine - May/June 2019 - 108
IEEE Power & Energy Magazine - May/June 2019 - 109
IEEE Power & Energy Magazine - May/June 2019 - 110
IEEE Power & Energy Magazine - May/June 2019 - 111
IEEE Power & Energy Magazine - May/June 2019 - 112
IEEE Power & Energy Magazine - May/June 2019 - 113
IEEE Power & Energy Magazine - May/June 2019 - 114
IEEE Power & Energy Magazine - May/June 2019 - 115
IEEE Power & Energy Magazine - May/June 2019 - 116
IEEE Power & Energy Magazine - May/June 2019 - 117
IEEE Power & Energy Magazine - May/June 2019 - 118
IEEE Power & Energy Magazine - May/June 2019 - 119
IEEE Power & Energy Magazine - May/June 2019 - 120
IEEE Power & Energy Magazine - May/June 2019 - Cover3
IEEE Power & Energy Magazine - May/June 2019 - Cover4
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050621
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030421
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010221
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111220
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091020
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070820
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050620
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030420
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010220
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111219
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091019
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070819
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050619
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030419
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010219
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111218
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091018
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070818
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050618
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030418
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010218
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111217
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091017
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070817
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050617
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030417
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010217
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111216
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091016
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070816
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050616
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030416
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010216
https://www.nxtbook.com/nxtbooks/ieee/powerenergy_010216
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111215
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091015
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070815
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050615
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030415
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010215
https://www.nxtbook.com/nxtbooks/pes/powerenergy_111214
https://www.nxtbook.com/nxtbooks/pes/powerenergy_091014
https://www.nxtbook.com/nxtbooks/pes/powerenergy_070814
https://www.nxtbook.com/nxtbooks/pes/powerenergy_050614
https://www.nxtbook.com/nxtbooks/pes/powerenergy_030414
https://www.nxtbook.com/nxtbooks/pes/powerenergy_010214
https://www.nxtbookmedia.com