HVDC System a Need for Future Power Transmission

International Journal of Trend in Scientific Research and Development (IJTSRD)
Volume: 3 | Issue: 2 | Jan-Feb 2019 Available Online: www.ijtsrd.com e-ISSN: 2456 - 6470
@ IJTSRD | Unique Reference Paper ID - IJTSRD20318 | Volume – 3 | Issue – 2 | Jan-Feb 2019 Page: 165
HVDC System: a Need for Future Power Transmission
Mohd Liaqat
M.Tech Scholar, Electrical Engineering Department,
Yamuna Institute of Engineering & Technology, Yamunanagar, Haryana, India
ABSTRACT
The continuously increasing demand for electric power and the economic access to remote renewable energysourcessuch as
off-shore wind power or solar thermal generation in deserts have revived the interest in high-voltage direct current (HVDC)
multiterminal systems (networks). A lot of work was done in this area, especially in the 1980s, but only two three-terminal
systems were realized. Since then, HVDC technology has advanced considerably and, despite numerous technical challenges,
the realization of large-scale HVDC networks is now seriously discussed and considered. For the acceptance and reliability of
these networks, the availability of HVDC circuit breakers (CBs) will be critical, making them one of the key enabling
technologies. Numerous ideas for HVDC breaker schemes have been published and patented, but no acceptable solution has
been found to interrupt HVDC short-circuit currents. This paperaimstosummarizethe literature,especiallythatof thelasttwo
decades, on technology areas that are relevant to HVDC breakers. Bycomparingthemainly20+ yearsold,state-of-theartHVDC
CBs to the new HVDC technology,existingdiscrepanciesbecomeevident.Areaswhere additionalresearch anddevelopmentare
needed are identified and proposed. for the couple of well-known applications are discussed.
KEYWORDS: Distributed generation; Renewable energy sources; Fuel cell systems; Power-conditioning units; dc/dc converters;
dc/ac inverters
I. INTRODUCTION
In recent years, the interest in HVDC multiterminal systems
has been revived. The continuously increasing demand for
electricpower and the economicaccess toremoterenewable
energy sources such as off-shore wind power [1]–[2][3] or
solar thermal generation in deserts [4] require an electric
energy transmission system thatbridges very longdistances
with lowlosses. TraditionalHVDCpoint-to-pointsystemscan
help to serve this duty and are available today. Linking more
than two HVDC terminals to form a meshed multiterminal
HVDC system (network) would have several advantages:the
reduction in the number of terminals (reduced costs and
losses), the outage of one dclinedoesnotinterruptthepower
flow at any terminal, each terminal can operate at different
power and current, and the power exchange with all ac
connection points can be fully controlled. It is thus very
attractive to explore therealizationofHVDCnetworks.There
was considerable interest in multiterminal HVDC systems in
the1980s, but only twothree-terminalsystemswererealized
[5], [6]. The advances in HVDC technology also helped to
renew the interest in HVDC networks. In particular, the
voltage-source converter (VSC) HVDC is now available.
However, the acceptance of HVDC networks with respect to
efficiency, reliability, and controllabilitywillstronglydepend
on the availability of HVDC circuit breakers (CBs), making
them one of the key enabling technologies [7]–[8][9][10].
There are significant differences between the requirements
of ac and dc CBs, mainly due to the absence of a natural
current zero crossing in dc systems. DC breakers have to
interrupt short-circuit currents very quickly and need to
dissipate the large amount of energy which is stored in the
inductances in the system. Today, dc CBs are only widely
available for the low- and medium-voltage range. For HVDC
applications, only transfer and load current switches are in
use. Breakers interrupting HVDC short-circuit currents are
not commonly available and have very limited ratings.
Numerous proposals for breaker designs have been
presented in articles and patent applications. All comprise
different series and parallel connections of classical ac
interrupters, resonance circuits with inductors and
capacitors, semiconductors, charging units, varistors, or
resistors. Each of the numerous concepts has certain
advantages and drawbacks. Mostof the publications address
only a few, or even only a single aspect of the many
requirements, butno contribution has tried togiveanoverall
picture.
The main aim of this paper is to give an overview of HVDC
CBs, to identify areas where research and development are
needed, and, by this, to revive the discussion on this subject.
Obviously, this paper cannot discuss each of these identified
needs in detail. But by citing relevant literature, it should
serve as a reference point for others working in this area.
II. HVDC NETWORKS
HVDC systems have two main uses: 1) to connect two ac
networks with different frequencies or different frequency-
control philosophies (back-to-back) or 2) to transmit large
amounts of power via long distances. For large distances, dc
transmission lines have lower losses than ac connections.
“Long” in this context means more than ∼800 km for
overhead lines, and more than ∼40 km for cables. There are
two basic converter technologies in use today for HVDC
power transmission: the “classical” CSC technologybased on
thyristors and the more recent self-commutated VSC
technology based on insulated-gate bipolar transistors
(IGBTs) [11]–[12][13][14][15].
Since thyristors are only turn-on devices, the active power
flow of CSC systems is controlled by adjusting the turn-on
(firing) and the extinction time instant (overlap) prior to
commutation to another valve. Reactive power is consumed
by the rectifier at the sending, and by the inverter at the
receiving end. This has to be compensated for by filters and
additional capacitors on the ac sides. In particular, under
transient conditions, the amount of reactive power

International Journal of Trend in Scientific Research and Development (IJTSRD) @ www.ijtsrd.com eISSN: 2456-6470
consumed varies greatly. The power flow is unidirectional.
The reversal of the power-flow direction requires a change
in polarity of the system, which could be problematic, in
particular, for polymeric cable connections. The technology
is quite mature and two 800-kV systems have been put in
operation recently with power levels of upto6400 MW (800
kV, 4 kA, bipolar) [16]. The losses in one terminal
are ∼0.7% [17] at rated current, of which the converter
transformer contributes ∼50%. The technology is still
advancing and further developments which occurred
recently are within the thyristor switch itself where voltage
and current ratings are continuously increasing, in the
application of capacitor-commutated conversion which
consumes considerably less reactive power [18], [19], or in
the reduction of filter size by using continuously tuned ac
filters and active dc filters [12], [18].
With the advances in power semiconductor devices and the
availability of high-power transistors (IGBTs [20], [21]),itis
also possible to use pulse-width modulation (PWM) or
multilevel concepts for HVDC power transmission. The self-
commutated VSC has a stiff dc voltage, and large capacitors
are used. Due to the use of PWM, only high-frequency
harmonics are present and the filters can be considerably
smaller. VSC HVDC technology transmits active power and
can provide the required amount of reactive power at both
the power sending and the power receiving end. This also
allows a reduction of the filter size. However, the losses in
one VSC terminal are ∼1.6% [17], [22], of which the
converter valves contributealmost70%.Thelargestrealized
VSC-based HVDC system is 150 kV, 400 MW [23] and a 300-
kV, 800-MW system is in the planning phase (BorWin2). The
technology is, in principle, availableforhigher powerstoday,
limited mainly by the voltage constraints of XLPE dc cables.
In case of a dc-side fault, the diodes connected in parallel to
the IGBT modules act as an uncontrolled rectifier, even ifthe
IGBTs are blocked. The short-circuit current is limited only
by the ac system [14]. The small dc-side inductance leads to
a very high rate of rise of dc current. In addition, the dc
capacitors discharge and add to the fault current [24], [25].
Thus, the dc and ac sides have to be decoupled very quickly
to stop the fault current. Since the converter is no longer
controllable in this fault case (in contrast to CSC HVDC
systems), ac CBs are typically used. HVDC CBs could do the
same, but are not available today. Due to thisvulnerability to
dc-side faults, VSC HVDC stations are preferably connected
by cables rather than overhead lines since cables are much
less sensitive to environmental influences.
The key differences between CSC- and VSC-based HVDCsare
summarized in Table I. With respect to HVDC CBs, the most
important ones are the different sizes of capacitances and
inductances (and the resulting different rate of rise of the
fault current) and the loss of control in the VSC HVDC in case
of a dc-side fault.
VSC technology is advancing rapidly. New concepts have
been proposed and partly demonstrated on a small scale.
The aim is to reduce losses to <1%, to reduce the harmonics
content, and even to have the ability to limit and extinguish
the current in case of dc-side faults [26]–[27][28].
Fig.1. Sample comparison of the HVDC network
configuration based on point-to-point systems (left) or
with the use of HVDC CBs (right).
Most of today's HVDC transmission systems are point to
point, connecting only two terminals. In principle, it is
possible to connect a number of ac nodes with many point-
to-point connections in a so-called multiple-pair
configuration (cf. left-hand sideof Fig.1).However,there are
several advantages if the lines were connected on the dc
side, forming a true HVDC network (e.g., like on the right-
hand side of Fig. 1). First, the number of converter stations
could be reduced since only one converter is needed per ac
connection point. Not only does this significantly reduce the
costs, but also the losses of the entire transmission system.
In addition, each station can transmit (send or receive)
power individually and can even change from receiving to
sending power without requiring that anotherstationdothe
opposite. There is more redundancy, and power can be
transmitted even if one line is lost. In order to ensure the
reliable operation of a multiterminal system, it must be
possible to decouple faulty lines or stations from thesystem.
In ac systems, CBs that are able to interrupt the full short-
circuit current are used. For HVDC, realized CBs have only
limited ratings. Compared with ac CBs, they are much larger
in size and considerably more expensive. Thus, other
concepts have been introduced in today's point-to-point
systems. Most commonly, dc linefaults areinterrupted using
converter control [29], which only works for CSC, or the CBs
on the ac side of the converter stations are operated to de-
energize the station and the line [30]. In case of a
nonsustained fault, the stations and the line can be re-
energized. This operation sequence takes at least a few 100
ms [23], [31], [32] up to afewseconds [33],butiswellsuited
for point-to-point connections. De-energizing the entire
HVDC networks has also been suggested [24], [34], [35], but
it is surely limited to small systems with only three or four
terminals [36].
The availability of HVDC CBs is thus a key enabling
technology for the reliable and economic operation of
multiterminalHVDC systems.SupportingHVDC CBswith fast
action of the terminal controls is undoubtedlyadvantageous
and should be investigated further (in particular, also for
VSC topologies).
III. HVDC CIRCUIT BREAKERS
In today's point-to-point HVDC transmission systems, dc
interrupters are used for several different switching duties.
A neutral bus switch (NBS), neutral bus ground switch
(NBGS), metal return transfer breaker (MRTB), ground
return transfer breaker (GRTB), high-speed bypass switch
(HSBS) for parallel lineswitching,andisolation switches also
exist. Interrupters to break dc short-circuit currents have
only been realized in very limited numbers and maximum
ratings are 250 kV, 8 kA or 500 kV, 4 kA, which is not more

than 1.6 times the rated nominal current. The breaking time
is in the order of 35 ms, but as stated before, for CSC-based
systems, the large inductances limit the rate of rise of fault
current, and this time is sufficiently fast. However, the
components of these breakersare verylargeand more costly
than ac CBs with comparable current and voltage ratings. In
point-to-point HVDC transmission systems, the function of
dc breakers has thus typically been substituted by de-
energizing the converter stations (e.g., by acting with the
station control or by operating the breakers on the ac side
and the opening of isolation switches). In low- and medium-
voltage dc applications, short-circuit current interrupting
breakers have been realized based on several different
technologies (e.g., switching arcs or solid-state switches).
The difficulties in realizing HVDC CBs can be attributed to
the demanding requirements onCBsin dcsystems which are
quite different than those of ac CBs. One of the major
differences is the absence of natural current zero crossings
in dc systems. The breakers have to fulfill the basic
requirements (cf. also [54]) as follows.
1. Create a current zero crossing to interrupt the current.
2. Dissipate the energy stored in the system inductance.
3. Withstand the voltage response of the network after
current interruption.
In particular, the first two requirementsleadtoa verystrong
interaction between the breaker and the dc system, an
attribute which is also verydifferentthanAC CBs.Inaddition
to these basic requirements, there are, depending on the
breaker application, additional secondary requirements as
follows.
In VSC-based systems, the HVDC breaker has to be able
to interrupt quickly, as was stated before.
The maximum voltage generatedbythebreakermustbe
low enough to comply with the insulation coordination
of the dc system. This is particularly important for
switching of load currents where the network is at
nominal voltage.
As mentioned before, the interest in meshed HVDC systems
is almost as old as the HVDC technology itself. In parallel to
the interest in HVDC networks, a great amount of research
and development was done on HVDC breakers up to the
1980s and was summarized in 1971, 1983, and partly in
1991. Thus, only the basic dc breaker concept is explained
here. The details of different variants of this concept can be
found in the earlier reviews. After1985,theinterestinHVDC
breakers dropped significantly and only in recent years,
when the interest in HVDC networks has picked up again,
new studies appear. Advances have mainly been made in
related technologies that could helptoimprovetheHVDC CB
design. The focus in this paper will be on reviewingthe more
recent developments, in particular, keeping in mind also the
new VSC technology.
A. Basic Working Principles of HVDC CBs
The current in a dc circuit can be brought to zero by
generating a counter voltage of similar or larger amplitude
than the system voltage. This counter voltage can be
produced by inserting additional resistanceorinductancein
the current path. The energy of the dc system is dissipated
across this device. The larger thecounter voltage, thesmaller
the time needed to interrupt, but thelargertheenergythatis
dissipated in the device. DC breakers with current limiting
and energy dissipating function of anarcare commonlyused
in LV and MV applications. Some proposals for high-voltage
systems have been made, but none of them has proven
efficient and successful in real applications.
An alternative is to have several parallelpathsin thebreaker
and to separate the requirements to different elements. The
simplest is one nominal current path and one parallel path
with a linear or nonlinear resistive element. The nominal
current path typically consists of an interrupter with low
ohmic losses in closed position, which is,sofar,onlypossible
with movable metallic contacts. Upon opening of these
contacts, an arc is established and its arcing voltage is used
to commutate the current to the resistive path where the
energy of the system is then dissipated. The advantage is
that the interrupter in the nominal current path only needs
to produce a voltage sufficient for commutation and not for
counteracting on the full system voltage. In addition, the
breaker does not have to have a large energy dissipating
function, which typically improves its interruption
capability. If the commutation path only consists of a linear
resistor, the arc voltage of the interrupter still has to be very
high. A gradual insertion of resistors or nonlinear resistors
to limit the required commutation voltage would be better.
The commutation process can be eased by adding other
elements, such as a capacitor which temporarily takes the
current flow. More recentdevelopmentsmakeuseof actively
controllable resistances of solid-statedevices(cf. Section III-
B).
Fig. 2. Basic arrangement of the HVDC CB
For most of the practically realized HVDC CBs, separate
commutation and energy absorbing paths have beenused,as
sketched in Fig. 2. The commutation path may then be a
series resonance consisting of a capacitance Cc and
inductance Lc sothatcurrentoscillationbetweenthenominal
and the commutation path can occur at the natural
frequency ω
2
0=1/LcCc. If the amplitude of the oscillating
current in is larger than the system dc current I0, a current
zero crossing occurs in the nominal path, and the
interrupter Sn caninterruptthecurrent.Current I0 continues
to flow, chargingthe capacitor Cc in thecommutationpath.If
the capacitor voltage exceeds a given value, typically chosen
to be the voltage capability of the breaker or the insulation
coordination of the HVDC system,theenergyabsorptionpath
acts, causing the system current I0 to decrease. If the
differential arc resistance dU/dI of the interrupter in the
nominal path is negative, a current oscillation between the
nominal and the commutation path with increasing
amplitudeoccurs, started by the naturalfluctuationinthearc
voltage. Fig. 3 shows the currents in the differentpathsofthe
breaker and the voltage across it. Attime t0,afaultoccursand
the current I0 starts to increase. The interrupter contacts of

the nominal current path separate at t1, and an instable
oscillation starts due to the characteristics of the arc voltage.
At t2, the amplitude of the oscillation is sufficiently large so
that in crosses zero and Sn interrupts. The current quickly
charges Cc until the threshold voltage level of the energy
absorbing elements in the third path is reached at time t3.
This path can consist of energy absorbinglinear or nonlinear
resistors that are inserted with switch Se or benonlinear ZnO
varistors that become partly conductive only above a certain
applied voltage and, thus, do not need an insertion device.
The voltage is limited by these elements, current only flows
through the energy absorbing path, and the current I0 of the
system ceases. In alternative to the self-excited growing
oscillations with purely passive components in the
commutation path is the useofadditionalactivecomponents,
such as the closing switch Sc. If the capacitor Cc is
precharged, a current is injected into the nominal path upon
closing of Sc and a counter-current is induced
Fig. 3. Basic current and voltage development during
interruption
IV. MORE RECENT ACTIVITIES AND RELATED
FIELDS OF TECHNOLOGY
1. Adaptions of the Basic Principle
The basic principle of HVDC CBs, as shown in Fig. 2, and
described in the previous section, has been continuously
optimized. Special focus is on making better use of the most
costly components, the capacitors, and varistors. Methods
have been derived to calculate and select the optimum value
of the capacitor Cc and to extend the current interruption
range. In a configuration with a passive commutation path, a
set of criteria is derived based on the characteristic surge
impedance Lc/Cc and the characteristic frequency ω0.The
criteria are chosen to minimize the interruption time and to
enhance the interruption performance.Iftheconfigurationof
active current injection with the precharged capacitor is
chosen, a large Cc/Lc ratio is advantageous since it leads to
larger injected current with the same energy stored in the
capacitor. Also here, a set of criteria combining the Cc/Lc-
ratio with thenaturalfrequency ω0 ischosentomaximizethe
interruption performance. The current oscillations in the
nominal and commutation path of passive arrangements
grow if the arc characteristic is negative. A fast-growing
oscillation at a high natural frequency leads to faster current
interruption. The ac CBs thataretypicallyusedaselementsin
the nominal path are by no means optimized for this. Thus,
dedicated investigations studying the arc characteristics
under different conditions have been performed and
optimizingthebreaker wastried.Nonetheless,astheclassical
configuration with only passive elements is probably the
most economic and reliable one, further investigation of the
arc behavior under different conditions to optimize the
breaker is surely needed. A dedicated arcing chamber for
optimal excitation of the oscillations could become
interesting if an HVDC network becomes reality and the
number of breakers thatisneeded grows. Thischamberdoes
not necessarily have to have high interruption capability
since another breaker, dedicated for current interruption,
could be placed in series in the nominal current path. In
addition to the passive variant, the growing oscillations can
also be actively excited. In [24], adding a power transistor in
parallel to the interrupter in the nominal current path that is
pulsed at the natural frequency ω0 between conducting and
an appropriate high voltage level is proposed. This pulsing
excites the current oscillation, preferably faster and more
reliably than the instable arc condition can.
The development and optimization of these breakers
requires sophisticatedtesting(cf.alsolatersection)andthere
have been attempts to substitute some of the tests with
simulations. The interaction of the breaker with a real
network is modeled with transient simulation
programs [25], [26], where the arcis represented by a black-
box model [27]with some free parameters that are fitted to
experiments. So far, the focus of these simulations was more
on the behavior of the network rather than optimizing the
breaker itself. As long as the programsarerunwithblack-box
models, these models have to be verified and calibrated by
basicarcmeasurements. An optimization ofthebreakerwith
these type of simulations must always be accompanied by
experiments. Progress has been made in simulating ac
switching arcs of gas CBs and vacuum interrupters using
multiphysics simulations that need less, or even no fitting
parameters at all. Thesetypeofsimulationsshouldbeusedto
optimize the arcing conditions for use in HVDC CBs in
addition to black-box models. Equally important as the
oscillation excitation is the interruption capability of the
interrupter. In contrast to ac CBs, the current
slope dI/dtbefore current zero and the rate of rise of the
transient recovery voltage dU/dt (RRTRV) across Sn after
currentinterruption aredeterminedbythebreakeritselfand
not by the network. The current slope before interruption
depends on the oscillationfrequencyandamplitude(growth)
and may vary over a large range. The switching arc gap is
stressed by the voltage of the capacitor, charged by I0, in the
parallel path. A detailed understanding of the current
interruption process and the interruption limits of the
breaker with different arcingconditions is necessaryandhas
to be gained by experiments and simulations. Vacuum CBs
have been proposedasbuildingelementsforHVDCCBs since
they combine a high interruption performance

(large dI/dt with large dU/dt) with small breaking times
(<5ms). Unfortunately, the maximum voltage withstand of
vacuum interrupters is limited to MV levels (Un≤36 kV) and
only a series arrangements could reach high-voltage levels.
This is mechanically very demandingandneedstobestudied
in further detail.
Depending on the selected HVDC technology, the focus of
future research in optimizing the existing schemes is placed
on one of the directions mentioned before. The requirement
of fast interruption willbeespeciallychallengingtomeetwith
a classical configuration.
Another application of a similar configuration is for the
magnetic coils of the planned fusion experiment ITER, which
are planned to operate with 70-kA dc current. This leads to a
stored magnetic energy of 40 GJ. If a fault in the
superconducting coils is detected, this current needs to be
interrupted and the energy dissipated within ∼11 s. The
interrupter scheme is composedofabypassswitchinparallel
with an interrupter and a counter current injection from a
precharged capacitor. The current is commutated to a
discharge resistor where the energy is dissipated. The
nominal current and energy rating are considerably higher
than for HVDC CBs, but the rated voltage is only 17.5 kV and
the interruption time is 500 ms.
2. Combined Optimization of Topology, Control, and
Breaker
Most of the research on HVDC, multiterminal schemes, and
HVDC CBs focuses only on a particular individual aspect.
Sometimes the breaker is assumed to be known, and the
network control is designed around it. Other times, the
requirements for HVDC CBs are set by the system control
activities and the breaker has to be designed accordingly. No
combined attempts to optimize the system as a whole are
reported. It would be thinkable to adjust the control scheme
to ease the requirements for CBs. Adding additional
inductance to the dc side in VSC-based networks would limit
the rate of rise of the short-circuit current and simplify the
hard breaking time requirement, but would lead to slower
control in normal operation. If high impedance grounding is
used, theshort-circuitcurrentduringdc-line-to-groundfaults
is limited. Focusing on only one aspect is most probably not
optimum as only a combined optimization could lead to the
globally best solution. Another example for combined
optimization is for VSC schemes using multilevel converters.
Voltage levels at a fraction of the total dc line voltage occur
inside the valve and, in principle, it would be possible to
interrupt at these levels. The respective dc breakers would
have a lower voltage rating, but the topology of the valves
needs to be redesigned to incorporate dc breakers. Only a
combined effort of valve and breaker design would lead to a
satisfying result.
3. Standardization
A new multiterminal HVDC system will be built using
components from more than one manufacturer. Standards
and norms for the multiterminal HVDC should be set. This is
particularly important for CBs and their interfaces to the
network and protection system.
4. Solid-State DC Breakers
Also discussed are pure semiconductorswitches,notonlyfor
HVDC, but also for low- and medium-voltage dc and ac. The
clear advantage is that the switching time can be as low as a
fewmicroseconds, comparedtoafew(ten)microsecondsofa
mechanical switch with separating metal contacts. The main
drawbacks are costs and the fact that the resistance in
conducting mode is in the order of a few mΩ and, thus,
considerably higher compared to a few μΩ for a mechanical
switch. The full forward conduction losses of the solid-state
devices are ∼0.10.1–0.4% of the transmitted power. The
application of semiconductor switches was thus typically
limited to applications wherehighPQisofcrucialimportance
and the minimum breaking time is absolutely needed. As
stated before, the rate of rise of short-circuit current in VSC-
based HVDC networks is very high. So far, no other HVDC CB
concept is available with breaking times in the order of 1 ms
and, thus, a solid-state switch is the only feasible solution
today. A single semiconductor device is not abletowithstand
the full voltage and current rating, but a series and parallel
arrangementof several switches is possible to achieve HVDC
CB ratings.
Research and development in the solid-state switching
devices, the semiconductor material itself, and in concepts
where the high on-state losses of solid-state switches can be
avoided, so-calledhybridconceptshavetobeintensified.This
is discussed in the following two sections.
5. Semiconductor Devices
The performance of semiconductor devices continues to
increase constantly, not only in blocking voltage rating, but
also in maximum current rating for a single chip. In the
future, HVDC valves and potentially also HVDC CBs can be
realized with fewer components, which improves the
performance and decreases the losses. Today, all devices are
based on silicon and an even bigger step ahead would be the
change of semiconductor material to a wide bandgap
material. SiC, GaN, and diamond have been discussedandare
intensively investigated.Widebandgapmaterialshavehigher
breakdown field strengths Ec comparedtoSi(Si:0.3,SiC:1.2–
2.4, GaN: 3.3, and diamond: 5.6 MV/cm). For comparable
voltage capability, the chip could be significantly thinner
which decreases the losses of the device. For example, a SiC-
pin-diodewasfabricatedwith4.2-kVbreakdownvoltagewith
5 times lower conduction losses than Si-based diodes with
comparable ratings. Still, no competitive wide bandgap
devices are commercially available, but research and
development is pursued intensively in this region and great
advances can be expected in the following years or decades.
Any progress in this area will influence HVDC technology
significantly, including the possibilityofdesigningsolid-state
HVDC CBs.
6. Hybrid CBs
It was already stated in the previous section that
semiconductors havehigh conductionlossesandarethusnot
optimal as inline switches or CBs. Hybrid switching schemes
are therefore proposed. Here, the nominal current path
contains a mechanical breaker with low-resistive metal
contacts that are separated quickly, causing the current to
commutate to a parallel path with thesemiconductorswitch.
When the current is transferred and the dielectric strength
between the metallic contacts has recovered, the
semiconductor switch is operated. Schemes areproposedfor
ac CBs; accapacitorswitches;fault-currentlimitingunits;and
for dc CBs.
Other hybrid concepts, where gas and vacuum CBs are
connected in series, exist as well. Here, speed is not the
primary aim, but the combination of the high-current and

high-frequency interruption capability of vacuum CBs,
together with the high-voltage withstand capability of gas
CBs. These schemes are continuously under discussion for
both HVDC breakers and ac CBs. These types of hybrid
concepts are of principal interest, but the interaction of
different devices is often complex and makes a detailed
understanding of the working principle and careful
consideration, when coupling, necessary.
7. Fast Switches
One of the key devices for hybrid dc CBs using solid-state
switches is a very fastmechanicalswitchwithlowconduction
losses in the nominal current path. These fast switches have
to operate in <1 ms and to build up sufficient arc voltage to
cause the current to commutate to the interruption path.
Concepts for fast switches based on electromagnetically
driven contacts in air or vacuumCBs havebeendeveloped.In
low-voltage networks, these switches also have a current
limiting function and ratings for CBs have reached 4 kA/1.5
kV with breaking time ∼300 μs. The requirements for fast
switches in hybrid breakers operating in <1 ms are
independent of ac or dc. For high-voltage systems, new
concepts or series arrangements of many switches would be
necessary and research should be carried out in this area.
8. Fault Current Limiters
Many of the concepts to fulfill the basic requirements of an
HVDC CB, as discussed before, are also applicable for fault
current limiters (FCL) in ac and dc systems. The task of fault
currentlimiters is, as the nameimplies,tolimitthemaximum
overcurrent in a power system when a fault occurs. The FCL
thus needs to increase the impedance of the systems, either
self-triggered or externally triggered. The FCL has to be
effective before the peak current is reached, typically 1–3 ms
in 50-Hz ac systems. In addition, the FCL has to handle the
large amounts of energy dissipated during the limitation. In
addition, someFCLs also interrupt the current. If theycannot
do so, a load break switch has to be placed in series to
interrupt the limited current. Some reviewarticleshavebeen
published and the details of FCL will not be repeated here.
Amongst the different operation principles are solid-state
fault current limiters(SSFCLs) andhybridconceptsusingfast
mechanical switches. Both concepts have been discussed
before, but have not been realized for high voltages so far.
Medium-voltage fuses work as self-triggered FCL with
interruption capability, but they are one-shot devices which
have to be replaced manually and are widely available only
up to 10–20 kV. Superconducting fault current limiters
(SCFCLs) of resistive type make use of the intrinsic physical
property that superconductors lose their zero resistance
above a critical current density. These types of fault current
limiters have low conductionlossesinnominaloperation,are
fast and resettable, but require extensive cooling of the
materialtoreach thesuperconductingstate.ResistiveSCFCLs
could be used as fast-acting commutation switches placed in
the nominal current path, but disconnectors have to be
placed in series since the SCFCL has no voltage withstand
capability.
Most fault current limiters have been designed for
distribution voltage levels (<36 kV). For the higher voltages
on the subtransmission and transmission level, only limiting
reactors and resonancelinkswithinlinecapacitorshavebeen
realized. Research and development for high-voltage FCL is
ongoing and any progress in this area is also of benefit for
HVDC CB activities. Even unconventional conceptsasaseries
connection of SCFCL with puffer breaker in LN2 are
investigated.
V. CONCLUSIONS
Throughout the previous sections of the text, several
technological areas where research and development is
needed in order to improve or enable HVDC CBs were
identified and discussed. These areas are summarizedinthe
list below as follows.
Optimization of the existing basic HVDC CB scheme by
optimizing the size of elements, such as capacitors,
inductors, varistors, or charging units. The main goal is a
reduction in size, interruption time, and costs.
Optimization of switching arcs with respect to the growth of
oscillation and capability to interrupt by detailed
investigation of arc characteristics under many different
conditions for gas and vacuum CBs. Derivation and
verification of the parameters are in mathematical arc
models.
Multiphysics simulation of HVDC arcs are for high current
(growing current oscillation) and interruption phase.
Extension of medium-voltage CBs to higher voltagelevels by
either improving the technology, series connection, or by
applying breakers across medium-voltage levels in
multilevel converter topologies.
Fast mechanical switches or disconnectors with high
recovery voltage withstand and low on-state-losses. Ideally,
these switches have sufficient arcing voltage for fast
commutation. Use of such a switch in a hybrid CB.
Pure semiconductor switch with minimal on-state losses.
Use of new wide bandgap power semiconductor devices
(e.g., SiC or GaN).
Fault current limiters for medium and high voltage.
Combinedoptimizationof thewholesystem:breaker-control
protection.
New testing methods for HVDC CBs or its individual
components. Due to the strong breaker-networkinteraction,
power-hardware-in-the-loop techniques would be
advantageous.
Standards and norms for multiterminal HVDC.
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