6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
Economical and Constructional Advantages of HVDC
Prof.Dr.Ing. Horia Balan
Faculty of Electrical Engineering
Technical University of Cluj-Napoca
Cluj, Romania
Ing. Muth Tudor
S.C. Transelectrica – ST Cluj
Cluj-Napoca
130 years ago when the "War of the Currents" began
between Nikola Tesla and Thomas Edison. And is
advancing even more. New materials are beeing
discoverd and used and countries around the world
spend billions of dollars for research and development,
and for construction also. HVDC is gaining ground fast.
Abstract. This paper centers around the
advantages that we can obtain in certain cases if we
use HVDC (High Voltage Direct Current). It brings
arguments to the reasons why this type of voltage is
more feasible now than 100 years ago . Also its
purpose is to bring arguments why this tipe of
electrical energy transportation will be used more
and more in the future.
I.
II.
THE MAIN ADVANTAGES AND USES OF HVDC
It is important to note that not only HVDC systems
carrying power electricity, but have a number of features
that should be solved by other means when using
conventional transport systems in AC. Some of these
issues are:
• there are no limits for transmission distance (this is
true for airlines and cable);
• allow fast and accurate control of power flow,
which causes improvementsin stability, not only for the
HVDC link but also for the system voltage alternative
environment;
• because they need a smaller corridor, reduce
environmental impact and building permission will be
obtained more quickly.
The first application for HVDC systems was to
ensure interconnection point to point between
asynchronous networks of alternative current. Other
applications include:
• energy delivery to remote sources, for example from
hydro to isolated consumption centers;
• energy imports in a congestion area: in areas where it is
impossible to install new generating groups for the
increased consumption.
• Increase the capacity of the existing transmission lines
by converting alternative current to DC. New rights of
way for transmission lines are sometimes impossible to
obtain such that conversion to direct current cables or
adding new DC cables on the same pilons can increase
the power transport capacity of the existing corridors
• control of power flow
INTRODUCTION
The transmission of electrical current was first
designed in d.c. But due to the fact that it wasn't possible
for every home to have it's own generator, or to have a
d.c. generator in every neighborhood it was changed to
alternative current transmission. The main reason was
the fact that the a.c. voltage can be modified and so
electricity can easily be produced in uninhabited areas
and transported to the consumer, limitating the power
losses. All wires currently used have some resistance
(the development of high-temperature superconductors
will probably change this some day). Let's call the total
resistance of the transmission line leading from a power
station to your local substation R. Let's also say the local
community demands a power P=IV from that substation.
This means the current drawn by the substation is I=P/V
and the higher the transmission line voltage, the smaller
the current. The line loss is given by Ploss=I²R, or,
substituting for I, Ploss = P²R/V. Since P is fixed by
community demand, and R is as small as you can make it
line loss decreases strongly with increasing voltage. The
reason is simply that you want the smallest amount of
current that you can use to deliver the power P. This is
the most important factor in favor of the alternative
curent. You can use a "relatively cheep transformer" that
juggles with the voltage using the principle of
electromagnetic induction. But the advantages stop here.
The technology nowadays is much more advanced than
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
• A HVDC line has lower power losses at the same
power circulated.
III.
APPLICATIONS OF HVDC CONVERTERS
The first application for HVDC converters was to
provide point to point interconnections between
asynchronous A.C. networks. There are other
applications for HVDC that include.
Some continental electrical systems are constructed from
asynchronous networks for example the east and west of
Texas, Japan, etc. In order to connect this electricity
systems HVDC can be used.
Power transportation from remote sources
Bringing electricity in very dense populated areas. In
this areas an underground DC. cable would be a very

efficient way of supplying energy.
IV.
ECONOMIC CONSIDERATIONS
The costs for electrical power transmission lines are
not so easily defined. There are many variations due to
many factors such as cost of the line corridor land, labor
costs, terrain difficulty, etc. A simple estimation would
be to say that the costs for a DC transmission line are 80100% of its AC equivalent. The main advantage is that a
long HVDC line can carry almost double the power at
the same voltage.
Figure 1.
Unlike power transformers AC/DC
high voltage
rectifiers are very expensive. This costs don't depend to
the length of the line and for a HVDC system to prove
its worth it must have a so called "equity point". This
point is reached when the combined costs of the
convertion stations + the costs of the line is equal or less
to its AC equivalent. For normal DC lines this length
would be around 7-800 km for underground and
underwater cable the legth would be around 50 km. For
this lengths is more feasible to build HVDC lines.
A three phase AC line (with three cables) de 500 kV is
~1.5 times wider than a 500 kV DC line (with two
cables). Also DC pylons are only about 80% as tall for
the same compared lines. In the case of very long AC
lines when reactive power compensators (capacitive
power reactors (coils)) are needed so their costs are draw
the balance towards HVDC transmission. For the same
transmitted power and considering the power losses for
the same conductor size (diameter), the isolators costs a
HVDC system would be (for long lines) about 87% of its
AC equivalent.
If electricity needs to be transported by underwater or
underground cables then the AC transmission becomes
infeasible because of the cables capacitive effect so the
equity point is reached at only ~50 km.
V.
ENVIRONMENTAL CONSIDERATIONS
The environmental considerations of HVDC lines can
be characterised as electromagnetic field, ionic effect
and corona effect. The electromagnetic field emerges
from the electric current carried by the line and from
the surrounding ionic charged air.. The ions form small
clouds that are carried away by the wind. The corona
effect can produce radio interference, noises and
generate ozone
The electromagnetic field and corona effect are
smaller in DC then in AC. The most important
considerations are:
The pylons are smaller for the same power
transmission line in the case of HVDC compared to AC
so there is less environmental disturbance by the line
corridor.
The magnetic field of an HVDC line nearby the edge
of the corridor will be approximately the same magnitude
as Earths field so the line doesn't cause any disturbance.
Unlike AC electromagnetic field DC field doesn't
have any proven effect on the nearby existing life.There
is no proven theory that can prove how a static
electromagnetic field cand affect the life of humans. The
DC field is almost the same as the electromagnetic field
underneath storm clouds.
The ionic and corona effects made by DC lines
generate small quantities of ozone similar to the natural
generation of ozone during storms.
There are some small possible inconveniences if you
use the return path through the ground in single pole
operations the electromagnetic field cand cause compass
disturbances. Another small problem would be if the
return path is used through ground is that the current may
affect some metallic structures and intensify their
corrosion
THE HVDC CIRCUIT BREAKER
The DC circuit breakers differ from the AC breakers
mainly due to the way they interrupt the electric arc. In
VI.
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
AC voltage drops to zero at the end of every half period,
but in DC an arc extinguishing circuit is needed to
interrupt the current.
breaker’s interruption time and improve the whole
interruption
performance.
Furthermore,
current
oscillations grow when the arc resistance (dU/dt) of the
switch on the nominal path is negative. Growing
oscillations can lead to faster current interruption. At the
same time a large C/L ratio can help maximize the
breaker’s interruption performance
The HVDC Electromechanical circuit breaker
The solid state circuit breaker
Figure 3: Solid State Circuit Breaker
On the figure we can see a basic electromechanical
circuit breaker. The breaker consists of three parts:
 The nominal current path is where DC current
passes through and the switch is closed during
normal operation
 The commutation path consists of a switch and a
resonant circuit with an inductor and a capacitor and
is used to create the inverse current
 The energy absorption path consists of a switch
and a varistor
The commutation path has a series resonance. When
interruption is needed, oscilating current can occur
between the nominal and the commutation path at the
natural frequency (1/LC). If the amplitude of the
oscillating current is larger than that of the input current
then zero crossing takes place and the switch can
interrupt the current in the nominal path. Current
flow(Io) will not be interrupted and will charge the
capacitor. If the capacitor voltage is bigger then a given
value, which is chosen to be the voltage capability of the
circuit breaker, the energy absorption path will act
causing the current to decrease.
This is a basic circuit that would need further
implementations to be efficient in high voltages.
Reduction in cost and better use of the costly
components (varistor, capacitor) will be required. Also,
the optimum capacitance value would minimize the
The second type of circuit breaker that is analyzed is
the solid-state circuit breaker. In picture above we can
see that a solid-state circuit breaker uses gate-commuted
thyristors instead of integrated gate-commuted thyristors
for semiconductor devices, this is due to the fact that in
this topology our immediate concern is lowering the onstate losses.
When there is no circuit failure detected current flows
through the GCTs. Once it is detected, the
semiconductors are switched-off. This leads to the rapid
increase of the voltage until the varistor begins to
conduct. If there is voltage higher than the grid voltage
then it is blocked due to the design of the varistor. This
in turn produces the demagnetization of the line
inductance.
VII. SOLID STATE BREAKER SIMULATION
Below it is simulated a solid-state circuit breaker using
an HVDC model provided by MATLAB. The model
represents a point to point VSC based transmission line
at 230kV. In the attempt to simulate the mallfunction,
both cables were connected to a switch, which when is
opened both cables became grounded. Our breaker
consists of an IGBT and a varistor in parallel. The IGBT
is used instead of a GCT because that was available from
MATLAB and their difference is only on the on state
losses. The model is presented below.
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on-state losses which can be abig disadvantage.The main
advantage is it’s very small interruption time and its
functionality at 230kV is very promising.
Figure 4 : HVDC MATLAB model
First time where tested the power changes when the
mallfunction occurred. The fault occurred at 0.3s while the
IGBT interrupted at 0.5s. From figure 5 it can be seen that
even though power should be zero, during 0.1-0.3s there
are some on state losses because of the use of IGBTs.
Once the DC mallfunction occurs power increases rapidly,
which is normal since current increases until the the circuit
breaker iterrupts. Power though continues to oscillate even
after
the
interruption
has
occurred.
Figure 6: Current vs. Time
VIII. CONCLUSIONS
HVDC is a technology with continuous
advancements and it is going to be more and more
dominant in the next few years. The potential change
from CSC to VSC is going to make HVDC a lot more
efficient, because VSC can offer higher efficiency, faster
switching time and by using fewer components make
systems smaller and cheaper. Also, because that VSC
can make the change in power flow easier, it will allow
HVDC to be created also multi-terminal networks that
are much more performant than point to point HVDC
transmission and AC grids.
The creation of such HVDC grids depends on the
research and development of HVDC circuit breakers that
can efficiently handle fault situations at such high
voltages and current ratings. Electromechanical breakers
work up to a few hundreds of kilo-volts, but some
changes need to be made so that they can cost less
money and interrupt faster than they do now. Solid-state
circuit breakers are a lot faster than the
electromechanical ones, but can work up to 150kV and
they have on-state losses that are too high.
It can be seen that the creation of more performant
circuit breakers can lead to a lot of changes in the area of
energy transmission. Dr Uhlmann’s statement that “It
can be safely stated that a DC circuit-breaker will be
Figure 5: Power vs. time graph
Then it was tested the current response of the model. A
DC fault was moddeled to occur at 0.3s and turned on
the IGBT for current interruption at 0.65s. It is clear that
once the mallfunction takes place the current increase is
rapid, though the breaker response is immediate and
current drops back to its original value. The simulation
showed that solid-state circuit breakers have rather high
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
available at the time the need for such arise” has become
a lot more relevant today, since with the advancements
made in the HVDC field and the uprising need for
energy transportation, circuit-breakers are the only thing
that stop the creation of HVDC grids which can change
the way electricity is transferred all over the world.
IX.
REFERENCES
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