IEEE Power & Energy Magazine - May/June 2019 - 86

1) the first uses power electronics to commutate current
into a parallel circuit. with this method, semiconductors
provide solid insulation and can interrupt large currents
very quickly and without arcing. the principal challenges with this method are large component costs and high
losses during normal closed-state operation.
2) the second uses additional circuits, such as commonly
employed inductor-capacitor (lc) resonant systems, to
create current-zero crossings in mechanical switches.
mechanical switches allow almost no closed-state losses
and are generally lightweight, compact, and inexpensive
at high-voltage ratings. the main challenge is arcing,
which suggests the need for heavier and more complex
contacts, leading in turn to slower opening speeds.
Both methods are viable and have been demonstrated on hV
and large-current dc cB units.

DC Current Reduction and Dissipation of Energy

and dc cB operating times, this inductor limits the peak
fault current experienced by dc cBs and other components
in the dc grid. For example, a 400-kV dc bus with a desired
interrupt time (dc cB opening and protection time) of 4 ms
and a maximum peak interrupting current of 16 ka requires
an inductor of around 100 mh. Series inductors with dc cBs
also enable the protection system to differentiate between
faults in inner and outer protective zones. Because of these
multiple requirements for the series inductor, it is not clear
whether the series inductor will be considered part of future
dc cB units or part of the dc protection system.
on the negative side, series inductors store energy and
make voltage control of dc grids less precise. in addition, in
the event of fault interruption, the energy stored in the inductor must be dissipated by dc cBs.

Main dc CB Topologies

Several manufacturers have invested substantially in further
developing dc cBs to voltage and current levels suitable for the
latest dc transmission technology. these manufacturers have
reported results from laboratory tests of dc cB prototypes
in the range of 40-80 kV with peak fault current interruption
capabilities of approximately 15-20 ka. plans call for these
units to become standard modules through series connections to
achieve higher dc voltage levels. reports from manufacturers
and data from recent installations indicate that there are no
substantial obstacles to implementing 300-400-kV dc cBs.
the european union has directed substantial resources
into research on dc cBs, including tests of several full-scale
(70-80 kV) dc cBs at an independent laboratory. there is a
joint effort by multiple manufacturers, grid operators, academics, and consultants that is helping the industry better understand dc cBs and how best to control and test them, know what
their operating limits and failure modes are, and determine
how they interact with dc grid components. considerable effort
is also being directed toward achieving interoperability among
various dc cB technologies and products from different manuSeries Inductor With dc CBs
each dc cB in hVdc grids requires a series inductor on the facturers and also toward initiating standardization.
many different dc cB topologies have been proorder of 50-300 mh. although this inductor cannot reduce
the amplitude of the dc fault current, it can limit the fault posed, with some prototypes of varying ratings undercurrent's rate of rise. Sized for a given level of protection going field tests. many technologies related to dc cBs have
recently been patented, and intensive research will continue in this
Main Branch
Residual Breaker
field. it is possible to group most
Ldc
Idc
VI2
dc Cable
IVI1
VI1
dc cB designs into two main
dc
dc
families: 1) mechanical dc cBs
Load
IS3
Source
VI3a
V
I
dcCB
and 2) hybrid dc cBs that use
LC
+
+
Fault
Vdc
C1a R
semiconductor valves.
1a
-
-
the energy dissipation in a device is the product of the current and voltage across the device, multiplied by time. with
ac cBs, dissipation of energy is minimal because the contacts open when the load current crosses zero. any energy
in the system is conveniently dissipated in the arc chamber.
with dc cBs, nonlinear resistors (surge arresters) are generally used to provide counter dc voltage under the dc load/fault
current at the moment of interruption. these resistors, an integral part of all dc cBs, are also called energy absorbers because
they absorb the energy from the line inductances, which is a
consequence of simultaneous high voltage and high current for a
short period until the dc current is brought to zero. the expected
energy dissipation at a 400-kV dc voltage level is on the order of
10-100 mJ, which affects the thermal ratings and time constants
of the absorbers. this is especially important if repeated dc cB
operations are required. this degree of energy dissipation also
influences the cost, size, and weight of the dc cB.

Current L
1
Injection
Branches

Energy
Absorber

VI3b
C1b R1b

Topology
SA

figure 2. A diagram showing a mechanical dc CB.
86

ieee power & energy magazine

Mechanical dc CBs
Figure 2 shows a typical topology
of a mechanical hVdc cB and
illustrates the following.
may/june 2019

IEEE Power & Energy Magazine - May/June 2019

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