HVDC Light Seminar Report

SEMINAR REPORT ON
“High Voltage Direct Current
Transmission System”
Submitted in partial fulfilment for Bachelor of Technology degree at
Rajasthan Technical University, Kota
(2014-15)
Submitted To: - Submitted By:-
Assit Prof. Anshul Bhati Nadeem Khilji
Assit. Prof. Vikram Rajpurohit B.Tech IV year VIII Sem.
Department of Electrical & Electronics Engineering
VYAS INSTITUTE OF ENGINEERING & TECHNOLOGY,
JODHPUR (RAJ.)

HVDC Transmission System
VYAS INSTITUTE OF ENGINEERING & TECHNOLOGY,
JODHPUR (RAJ.)
Department of Electrical & Electronics Engineering
CERTIFICATE
This is to certify that the student NADEEM KHILJI of IV year VIII Sem EEE
Branch, have successfully completed the Seminar report on titled “High
Voltage Direct Current Transmission System” towards the partial
fulfillment of the degree of Bachelor of Technology (B.TECH). In the
Electrical & Electronics Engineering of the Rajasthan Technical University
during academic year 2014-15.
Guided By Head of the Department
Assist. Prof. Anshul Bhati Prof. Dharmendra Jain
Assist. Prof. Vikram Rajpurohit

Acknowlegment
I would like to take this opportunity to extend my sincere gratitude to Prof.
Dharmendra Jain, Head of Department, Electrical & Electronics Engineering, for
extending every facility to complete my seminar work successfully.
I would like to express my sincere indebtedness to Prof. Anshul Bhati & Prof.
Vikram Singh Rajpurohit, Department of Electrical & Electronics Engineering, for
there valuable guidance, wholehearted co-operation and duly approving the topic as
staff in charge.
I also extend my gratitude towards the staffs, students and parents for their
sincere support and motivation.
Nadeem Khilji
11EVEEX032

ABSTRACT
The development of HVDC (High Voltage Direct Current) transmission system dates
back to the 1930s when mercury arc rectifiers were invented. Since the 1960s, HVDC
transmission system is now a mature technology and has played a vital part in both
long distance transmission and in the interconnection of systems. Transmitting
power at high voltage and in DC form instead of AC is a new technology proven to
be economic and simple in operation which is HVDC transmission. HVDC transmission
systems, when installed, often form the backbone of an electric power system. They
combine high reliability with a long useful life. An HVDC link avoids some of the
disadvantages and limitations of AC transmission. HVDC transmission refers to that
the AC power generated at a power plant is transformed into DC power before its
transmission. At the inverter (receiving side), it is then transformed back into its
original AC power and then supplied to each household. Such power transmission
method makes it possible to transmit electric power in an economic way.
HVDC Light is the newly developed HVDC transmission
technology, which is based on extruded DC cables and voltage source converters
consisting of Insulated Gate Bipolar Transistors (IGBT’s) with high switching
frequency. It is a high voltage, direct current transmission Technology i.e.,
Transmission up to 330MW and for DC voltage in the ± 150kV range. Under more
strict environmental and economical constraints due to the deregulation, the HVDC
Light provides the most promising solution to power transmission and distribution.
The new system results in many application opportunities and new applications in
turn bring up new issues of concern. One of the most concerned issues from
customers is the contribution of HVDC Light to short circuit currents. The main
reason for being interested in this issue is that the contribution of the HVDC Light
to short circuit currents may have some significant impact on the ratings for the
circuit breakers in the existing AC systems. This paper presents a comprehensive
investigation on one of the concerned issues, which is the contribution of HVDC
Light to short circuit currents.

CONTENTS
Chapter 1 INTRODUCTION 1
Chapter 2 HVDC TECHNOLOGY 2
Chapter 3 HVDC LIGHT TECHNOLOGY 16
Chapter 4 SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT 22
Chapter 5 CONCLUSION 29
Chapter 6 REFERENCES 30

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1. INTRODUCTION
The development of HVDC (High Voltage Direct Current) transmission system dates
back to the 1930s when mercury arc rectifiers were invented. In 1941, the first HVDC
transmission system contract for a commercial HVDC system was placed: 60MWwere
to be supplied to the city of Berlin through an underground cable of 115 km in
length. It was only in 1954 that the first HVDC (10MW) transmission system was
commissioned in Gotland. Since the 1960s, HVDC transmission system is now a
mature technology and has played a vital part in both long distance transmission and
in the interconnection of systems.HVDC transmission systems, when installed, often
form the backbone of an electric power system. They combine high reliability with a
long useful life. Their core component is the power converter, which serves as the
interface to the AC transmission system. The conversion from AC to DC, and vice
versa, is achieved by controllable electronic switches (valves) in a 3-phase bridge
configuration.
A new transmission and distribution technology, HVDC Light, makes it
economically feasible to connect small scale, renewable power generation plants to
the main AC grid. Vice versa, using the very same technology, remote locations as
islands, mining districts and drilling platforms can be supplied with power from the
main grid, thereby eliminating the need for inefficient, polluting local generation
such as diesel units. The voltage, frequency, active and reactive power can be
controlled precisely and independently of each other. This technology also relies on
a new type of underground cable which can replace overhead lines at no cost
penalty. Equally important, HVDC Light has
control capabilities that are not present or possible even in the most sophisticated
AC.

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2. HVDC TECHNOLOGY
Electric power transmission was originally developed with direct current. A high-
voltage, direct current (HVDC) electric power transmission system uses direct
current for the bulk transmission of electrical power, in contrast with the more
common alternating current systems. For long-distance transmission, HVDC systems
may be less expensive and suffer lower electrical losses. For shorter distances, the
higher cost of DC conversion equipment compared to an AC system may be
warranted where other benefits of direct current links are useful.
High voltage is used for electric power transmission to reduce the energy lost in
the resistance of the wires. For a given quantity of power transmitted, higher
voltage reduces the transmission power loss. The power lost as heat in the wires is
proportional to the square of the current. So if a given power is transmitted at
higher voltage and lower current, power loss in the wires is reduced. Power loss can
also be reduced by reducing resistance, for example by increasing the diameter of
the conductor, but larger conductors are heavier and more expensive.
High voltages cannot easily be used for lighting and motors, and so transmission-level
voltages must be reduced to values compatible with end-use
equipment. Transformers are used to change the voltage level in alternating
current (AC) transmission circuits. The competition between the direct current (DC)
of Thomas Edison and the AC of Nikola Tesla and George Westinghouse was known as
the War of Currents, with AC becoming dominant. Practical manipulation of DC
voltages became possible with the development of high power electronic devices
such as mercury arc valves and, more recently, semiconductor devices such
as thyristors, insulated-gate bipolar transistors (IGBTs), high
power MOSFETs and gate turn-off thyristors (GTOs).

3
DC transmission now became practical when long distances were to be covered or
where cables were required. The development of HVDC (High Voltage Direct Current)
transmission system dates back to the 1930s when mercury arc rectifiers were
invented. HVDC transmission systems, when installed, often form the backbone of an
electric power system. They combine high reliability with a long useful life. Their
core component is the power converter, which serves as the interface to the AC
transmission system. The conversion from AC to DC, and vice versa, is achieved by
controllable electronic switches (valves) in a 3-phase bridge configuration.
An HVDC link avoids some of the disadvantages and limitations of AC transmission
and
has the following advantages:
 No technical limit to the length of a submarine cable connection.
 No requirement that the linked systems run in synchronism.
 No increase to the short circuit capacity imposed on AC switchgear.
 Immunity from impedance, phase angle, frequency or voltage fluctuations.
 Preserves independent management of frequency and generator control.
 Improves both the AC system’s stability and, therefore, improves the internal
power carrying
capacity, by modulation of power in response to frequency, power swing or
line rating.
2.1 NEED FOR DC TRANSMISSION
The losses in DC transmission are lower. The level of losses is designed into a
transmission system and is regulated by the size of conductor selected. DC and ac
conductors, either as overhead transmission lines or submarine cables can have
lower losses but at higher
expense since the larger cross-sectional area will generally result in lower losses but
cost
more.
When converters are used for dc transmission in preference to ac transmission, it is
generally by economic choice driven by one of the following reasons :

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1. An overhead dc transmission line with its towers can be designed to be less
costly per unit of length than an equivalent ac. line designed to transmit the
same level of electric power. However the dc converter stations at each end
are more costly than the terminating stations of an ac line and so there is a
breakeven distance above which the total cost of dc transmission is less than its
ac transmission alternative. The dc transmission line can have a lower visual
profile than an equivalent ac line and so contributes to a lower environmental
impact. There are other environmental advantages to a dc transmission line
through the electric and magnetic fields being dc instead of ac.
2. If transmission is by submarine or underground cable, the breakeven distance is
much less than overhead transmission. It is not practical to consider ac cable
systems exceeding 50 km but dc cable transmission systems are in service whose
length is in the hundreds of kilometers and even distances of 600 km or greater
have been considered feasible.
3. Some ac electric power systems are not synchronized to neighboring networks
even though their physical distances between them is quite small. This occurs in
Japan where half the country is a 60 Hz network and the other is a 50 Hz
system. It is physically impossible to connect the two together by direct ac
methods in order to exchange electric power between them. However, if a dc
converter station is located in each system with an interconnecting dc link
between them, it is possible to transfer the required power flow even though
the ac systems so connected remain asynchronous.

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2.2 ADVANTAGES OF HVDC OVER AC TRANSMISSION:
The advantage of HVDC is the ability to transmit large amounts of power over long
distances with lower capital costs and with lower losses than AC. Depending on
voltage level and construction details, losses are quoted as about 3% per 1,000 km.
High-voltage direct current transmission allows efficient use of energy sources
remote from load centers.
In a number of applications HVDC is more effective than AC transmission.
Examples include:
 Undersea cables, where high capacitance causes additional AC losses. (e.g.,
250 km Baltic Cable between Sweden and Germany the 600 km Nor Ned cable
between Norway and the Netherlands, and 290 km Bass link between the
Australian mainland and Tasmania)
 Endpoint-to-endpoint long-haul bulk power transmission without intermediate
'taps', for example, in remote areas
 Increasing the capacity of an existing power grid in situations where additional
wires are difficult or expensive to install
 Power transmission and stabilization between unsynchronized AC distribution
systems
 Connecting a remote generating plant to the distribution grid, for
example Nelson River Bipole
 Stabilizing a predominantly AC power-grid, without increasing prospective
short circuit current
 Reducing line cost. HVDC needs fewer conductors as there is no need to
support multiple phases. Also, thinner conductors can be used since HVDC
does not suffer from the skin effect
 Facilitate power transmission between different countries that use AC at
differing voltages and/or frequencies
 Synchronize AC produced by renewable energy sources

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Long undersea / underground high voltage cables have a high electrical capacitance,
since the conductors are surrounded by a relatively thin layer of insulation and a
metal sheath while the extensive length of the cable multiplies the area between
the conductors. The geometry is that of a long co-axial capacitor. Where alternating
current is used for cable transmission, this capacitance appears in parallel with load.
Additional current must flow in the cable to charge the cable capacitance, which
generates additional losses in the conductors of the cable. Additionally, there is
a dielectric loss component in the material of the cable insulation, which consumes
power.
When, however, direct current is used, the cable capacitance is charged only when
the cable is first energized or when the voltage is changed; there is no steady-state
additional current required. For a long AC undersea cable, the entire current-
carrying capacity of the conductor could be used to supply the charging current
alone.
The cable capacitance issue limits the length and power carrying capacity of AC
cables. DC cables have no such limitation, and are essentially bound by only Ohm's
Law. Although some DC leakage current continues to flow through the dielectric
insulators, this is very small compared to the cable rating and much less than with
AC transmission cables. HVDC can carry more power per conductor because, for a
given power rating, the constant voltage in a DC line is the same as the peak voltage
in an AC line. The power delivered in an AC system is defined by the root mean
square (RMS) of an AC voltage, but RMS is only about 71% of the peak voltage. The
peak voltage of AC determines the actual insulation thickness and conductor spacing.
Because DC operates at a constant maximum voltage, this allows existing
transmission line corridors with equally sized conductors and insulation to carry more
power into an area of high power consumption than AC, which can lower costs.
Because, HVDC allows power transmission between unsynchronized AC distribution
systems, it can help increase system stability, by preventing cascading failures from
propagating from one part of a wider power transmission grid to another. Changes in
load that would cause portions of an AC network to become unsynchronized and
separate would not similarly affect a DC link, and the power flow through the DC link
would tend to stabilize the AC network. The magnitude and direction of power flow

7
through a DC link can be directly commanded, and changed as needed to support the
AC networks at either end of the DC link. This has caused many power system
operators to contemplate wider use of HVDC technology for its stability benefits
alone.
2.3 DISADVANTAGES:
The disadvantages of HVDC are in conversion, switching, control, availability and
maintenance..HVDC is less reliable and has lower availability than AC systems,
mainly due to the extra conversion equipment. Single pole systems have availability
of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault
redundant bipole systems provide high availability for 50% of the link capacity, but
availability of the full capacity is about 97% to 98%.
The required static inverters are expensive and have limited overload capacity.
At smaller transmission distances the losses in the static inverters may be bigger
than in an AC transmission line. The cost of the inverters may not be offset by
reductions in line construction cost and lower line loss. With two exceptions, all
former mercury rectifiers worldwide have been dismantled or replaced
by thyristor units. Pole 1 of the HVDC scheme between the North and South Islands
of New Zealand still uses mercury arc rectifiers, as does Pole 1 of the Vancouver
Island link in Canada. Both are currently being replaced – in New Zealand by a new
thyristor pole and in Canada by a three-phase AC link. In contrast to AC systems,
realizing multi-terminal systems is complex, as is expanding existing schemes to
multi-terminal systems.
Controlling power flow in a multi-terminal DC system requires good communication
between all the terminals; power flow must be actively regulated by the inverter
control system instead of the inherent impedance and phase angle properties of the
transmission line. Multi-terminal lines are rare. Another example is the Sardinia-
mainland Italy link which was modified in 1989 to also provide power to the island of
Corsica.
High voltage DC circuit breakers are difficult to build because some mechanism must
be included in the circuit breaker to force current to zero, otherwise arcing and

8
contact wear would be too great to allow reliable switching. Operating a HVDC
scheme requires many spare parts to be kept, often exclusively for one system as
HVDC systems are less standardized than AC systems and technology changes faster.
2.4 RECTIFYING AND INVERTING:
2.4.1 Components
Most of the HVDC systems in operation today are based on Line-Commutated
Converters. Early static systems used mercury arc rectifiers, which were unreliable.
Two HVDC systems using mercury arc rectifiers are still in service (As of 2008).
The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a
solid-state semiconductor device similar to the diode, but with an extra control
terminal that is used to switch the device on at a particular instant during the AC
cycle. The insulated-gate bipolar transistor (IGBT) is now also used, forming
a Voltage Sourced Converter, and offers simpler control, reduced harmonics and
reduced valve cost.
Because the voltages in HVDC systems, up to 800 kV in some cases, exceed
the breakdown voltages of the semiconductor devices, HVDC converters are built
using large numbers of semiconductors in series. The low-voltage control circuits
used to switch the thyristors on and off need to be isolated from the high voltages
present on the transmission lines.
This is usually done optically. In a hybrid control system, the low-voltage control
electronics sends light pulses along optical fibers to the high-side control electronics.
Another system, called direct light triggering, dispenses with the high-side
electronics, instead using light pulses from the control electronics to switch light-
triggered thyristors. A complete switching element is commonly referred to as
a valve, irrespective of its construction.
2.4.2 Rectifying & Inverting Systems
Rectification and inversion use essentially the same machinery. Many substations
(Converter Stations) are set up in such a way that they can act as both rectifiers and
inverters. At the AC end a set of transformers, often three physically separated
single-phase transformers, isolate the station from the AC supply, to provide a local

9
earth, and to ensure the correct eventual DC voltage. The output of these
transformers is then connected to a bridge rectifier formed by a number of valves.
The basic configuration uses six valves, connecting each of the three phases to each
of the two DC rails. However, with a phase change only every sixty degrees,
considerable harmonics remain on the DC rails.
An enhancement of this configuration uses 12 valves (often known as a twelve-pulse
system). The AC is split into two separate three phase supplies before
transformation. One of the sets of supplies is then configured to have a star
secondary, the other a delta secondary, establishing a thirty degree phase difference
between the two sets of three phases. With twelve valves connecting each of the
two sets of three phases to the two DC rails, there is a phase change every 30
degrees, and harmonics are considerably reduced.
In addition to the conversion transformers and valve-sets, various passive resistive
and reactive components help filter harmonics out of the DC rails.

10
2.5 CONFIGURATIONS OF HVDC SYSTEM:
2.5.1 Monopole And Earth Return
In a common configuration, called monopole, one of the terminals of the rectifier is
connected to earth ground. The other terminal, at a potential high above or below
ground, is connected to a transmission line. The earthed terminal may be connected
to the corresponding connection at the inverting station by means of a second
conductor.
If no metallic conductor is installed, current flows in the earth between the earth
electrodes at the two stations.
Figure 1: Block diagram of a monopole system with earth return
Therefore it is a type of single wire earth return. The issues surrounding earth-return
current include:
 Electrochemical corrosion of long buried metal objects such as pipelines.
 Underwater earth-return electrodes in seawater may produce chlorine or
otherwise affect water chemistry.
 An unbalanced current path may result in a net magnetic field, which can
affect magnetic navigational compasses for ships passing over an underwater
cable.
These effects can be eliminated with installation of a metallic return conductor
between the two ends of the monopolar transmission line. Since one terminal of the
converters is connected to earth, the return conductor need not be insulated for the
full transmission voltage which makes it less costly than the high-voltage conductor.

11
Use of a metallic return conductor is decided based on economic, technical and
environmental factors. Modern monopolar systems for pure overhead lines carry
typically 1,500 MW. If underground or underwater cables are used, the typical value
is 600 MW. Most monopolar systems are designed for future bipolar expansion.
Transmission line towers may be designed to carry two conductors, even if only one
is used initially for the monopole transmission system. The second conductor is
either unused or used as electrode line or connected in parallel with the other (as in
case of Baltic-Cable).
2.5.2 Bipolar
In bipolar transmission a pair of conductors is used, each at a high potential with
respect to ground, in opposite polarity. Since these conductors must be insulated for
the full voltage, transmission line cost is higher than a monopole with a return
conductor.
Figure 2: Block diagram of a bipolar system that also has an earth return.
However, there are a number of advantages to bipolar transmission which can make
it the attractive option.
 Under normal load, negligible earth-current flows, as in the case of monopolar
transmission with a metallic earth-return. This reduces earth return loss and
environmental effects.
 When a fault develops in a line, with earth return electrodes installed at each
end of the line, approximately half the rated power can continue to flow using
the earth as a return path, operating in monopolar mode.

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 Since for a given total power rating each conductor of a bipolar line carries
only half the current of monopolar lines, the cost of the second conductor is
reduced compared to a monopolar line of the same rating.
 In very adverse terrain, the second conductor may be carried on an
independent set of transmission towers, so that some power may continue to
be transmitted even if one line is damaged.
A bipolar system may also be installed with a metallic earth return conductor.
Bipolar systems may carry as much as 3,200 MW at voltages of +/-600 kV. Submarine
cable installations initially commissioned as a monopole may be upgraded with
additional cables and operated as a bipole.
2.5.3 Back to Back
A back-to-back station (or B2B for short) is a plant in which both static inverters and
rectifiers are in the same area, usually in the same building.
The length of the direct current line is kept as short as possible. HVDC back-to-back
stations are used for:
 Coupling of electricity mains of different frequency (as in Japan; and the GCC
interconnection between UAE [50 Hz] and Saudi Arabia [60 Hz] under
construction in ±2009–2011).
 Coupling two networks of the same nominal frequency but no fixed phase
relationship (as until 1995/96 in Etzenricht, Dürnrohr, Vienna, and the Vyborg
HVDC scheme).
 Different frequency and phase number (for example, as a replacement
for traction current converter plants).
The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-
back stations because of the short conductor length. The DC voltage is as low as
possible, in order to build a small valve hall and to avoid series connections of

13
valves. For this reason at HVDC back-to-back stations valves with the highest
available current rating are used.
2.6 SYSTEMS WITH TRANSMISSION LINES
The most common configuration of an HVDC link is two inverter/rectifier stations
connected by an overhead power line. This is also a configuration commonly used in
connecting unsynchronized grids, in long-haul power transmission, and in undersea
cables.
Multi-terminal HVDC links, connecting more than two points, are rare. The
configuration of multiple terminals can be series, parallel, or hybrid (a mixture of
series and parallel).
Parallel configuration tends to be used for large capacity stations, and series for
lower capacity stations. An example is the 2,000 MW Quebec - New England
Transmission system opened in 1992, which is currently the largest multi-terminal
HVDC system in the world.

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2.7 CORONA DISCHARGE
Corona discharge is the creation of ions in air by the presence of a strong electric
field. Electrons are torn from neutral air, and either the positive ions or the
electrons are attracted to the conductor, while the charged particles drift. This
effect can cause considerable power loss, create audible and radio-frequency
interference, generate toxic compounds such as oxides of nitrogen and ozone, and
bring forth arcing.
Both AC and DC transmission lines can generate coronas, in the former case in the
form of oscillating particles, in the latter a constant wind. Due to the space
charge formed around the conductors, an HVDC system may have about half the loss
per unit length of a high voltage AC system carrying the same amount of power. With
monopolar transmission the choice of polarity of the energized conductor leads to a
degree of control over the corona discharge.
In particular, the polarity of the ions emitted can be controlled, which may have an
environmental impact on particulate condensation. (particles of different polarities
have a different mean-free path.) Negative coronas generate considerably more
ozone than positive coronas, and generate it further downwind of the power line,
creating the potential for health effects. The use of a positive voltage will reduce
the ozone impacts of monopole HVDC power lines.
2.8 AREAS FOR DEVELOPMENT IN HVDC CONVERTERS
The thyristor as the key component of a converter bridge continues to be developed
so that its voltage and current rating is increasing.
Gate-turn-off thyristors (GTOs) and insulated gate bipole transistors (IGBTs) are
required for the voltage source converter (VSC) converter bridge configuration. It is
the VSC converter bridge which is being applied in new developments . Its special
properties include the ability to independently control real and reactive power at
the connection bus to the ac system. Reactive power can be either capacitive or
inductive and can be controlled to quickly change from one to the other.
A voltage source converter as in inverter does not require an active ac voltage
source to commutate into as does the conventional line commutated converter. The
VSC inverter can generate an ac three phase voltage and supply electricity to a load

15
as the only source of power. It does require harmonic filtering, harmonic
cancellation or pulse width modulation to provide an acceptable ac voltage wave
shape.
Two applications are now available for the voltage source converter. The first is for
low voltage dc converters applied to dc distribution systems. The first application of
a dc distribution system in 1997 was developed in Sweden and known as “HVDC
Light”. Other applications for a dc distribution system may be:
1. In a dc feeder to remote or isolated loads, particularly if underwater or
underground cable is necessary.
2. For a collector system of a wind farm where cable delivery and optimum and
individual speed control of the wind turbines is desired for peak turbine efficiency.
The second immediate application for the VSC converter bridges is in back-to-back
configuration. The back-to-back VSC link is the ultimate transmission and power flow
controller. It can control and reverse power flow easily, and control reactive power
independently on each side. With a suitable control system, it can control power to
enhance and preserve ac system synchronism, and act as a rapid phase angle power
flow regulator with 360 degree range of control.
There is considerable flexibility in the configuration of the VSC converter bridges.
Another option is to use multilevel converter bridges to provide harmonic
cancellation. Additionally, both two level and multilevel converter bridges can
utilize pulse width modulation to eliminate low order harmonics. With pulse width
modulation, high pass filters may still be required since PWM adds to the higher
order harmonics. As VSC converter bridge technology develops for higher dc voltage
applications, it will be possible to eliminate converter transformers. This is possible
with the low voltage applications in use today. It is expected the exciting
developments in power electronics will continue to provide exciting new
configurations and applications for HVDC converters.

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3. HVDC LIGHT TECHNOLOGY
A new transmission and distribution technology, HVDC Light, makes it economically
feasible to connect small-scale, renewable power generation plants to the main AC
grid. Vice versa, using the very same technology, remote locations as islands, mining
districts and drilling platforms can be supplied with power from the main grid,
thereby eliminating the need for inefficient, polluting local generation such as diesel
units. The voltage, frequency, active and reactive power can be controlled precisely
and independently of each other. This technology also relies on a new type of
underground cable which can replace overhead lines at no cost penalty. Equally
important, HVDC Light has control capabilities that are not present or possible even
in the most sophisticated AC systems.
As its name implies, HVDC Light is a dc transmission technology. However, it is
different from the classic HVDC technology used in a large number of transmission
schemes. Classic HVDC technology is mostly used for large point-to-point
transmissions, often over vast distances across land or under water. It requires fast
communications channels between the two stations, and there must be large
rotating units - generators or synchronous condensers - present in the AC networks at
both ends of the transmission.
HVDC Light consists of only two elements: a converter station and a pair of ground
cables. The converters are voltage source converters, VSC’s. The outputs from the
VSC’s are determined by the control system, which does not require any
communications links between the different converter stations. Also, they don’t
need to rely on the AC network’s ability to keep the voltage and frequency stable.
These feature make it possible to connect the converters to the points bests suited
for the ac system as a whole.
The converter station is designed for a power range of 1-100 MW and for a dc voltage
in the 10-100 kV range. One such station occupies an area of less than 250 sq. meters
(2 700 sq. ft), and consists of ust a few elements: two containers for the converters
and the control system, three small AC air-core reactors, a simple harmonics filter
and some cooling fans.
The converters are using a set of six valves, two for each phase, equipped with high
power transistors, IGBT (Insulated Gate Bipolar Transistor). The valves are controlled

17
by a computerized control system by pulse width modulation, PWM. Since the IGBTs
can be switched on or off at will, the output voltages and currents on the AC side
can be controlled precisely.
The control system automatically adjusts the voltage, frequency and flow of active
and reactive power according to the needs of the AC system. The PWM technology
has been tried and tested for two decades in switched power supplies for electronic
equipment as computers. Due to the new, high power IGBTs, the PWM technology
can now be used for high power applications as electric power transmission. HVDC
Light can be used with regular overhead transmission lines, but it reaches its full
potential when used with a new kind of dc cable. The new HVDC Light cable is an
extruded, single-pole cable. The easiest way of laying this cable is by plowing.
Handling the cable is easy, despite its large power-carrying capacity. It has a specific
weight of just over 1 kg/m. Contrary to the case with AC transmission; distance is
not the factor that determines the line voltage. The only limit is the cost of the line
losses, which may be lowered by choosing a cable with a conductor with a larger
cross section. Thus, the cost of a pair of dc cables is linear with distance.
A dc cable connection could be more cost efficient than even a medium distance AC
overhead line, or local generating units such as diesel generators. The converter
stations can be used in different grid configurations. A single station can connect a
dc load or generating unit, such as a photo-voltaic power plant, with an AC grid.
Two converter stations and a pair of cables make a point-to point dc transmission
with AC connections at each end. Three or more converter stations make up a dc
grid that can be connected to one or more points in the AC grid or to different AC
grids. The dc grids can be radial with multi-drop converters, meshed or a
combination of both. In other words, they can be configured, changed and expanded
in much the same way AC grids are.

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3.1 HVDC LIGHT INSTALLATION
HVDC light system mainly consists of transformers, converter units, phase reactors
and filters.
Figure 4: HVDC Light transmission System
The transformers are used to step-up/step-down voltages and the converters units
converts AC to DC and vice versa. HVDC cables are used to carry currents and the
filters are used for filtering unwanted signals.
3.2 HVDC LIGHT CHARACTERISCTICS
An HVDC Light converter is easy to control. The performance during steady state and
transient operation makes it very attractive for the system planner as well as for the
project developer. The benefits are technical, economical, environmental as well as
operational.
The most advantageous are the following:
• Independent control of active and reactive power
• Feeding of power into passive networks (i.e.
network without any generation)
• Power quality control
• Modular compact design, factory pre-tested
• Short delivery times
• Re-locatable/Leasable

19
• Unmanned operation
• Robust against grid alterations
3.1.1 Control Of Active & Reactive Power
The control makes it possible to create any phase angle or amplitude, which can be
done almost instantly. This offers the possibility to control both active and reactive
power independently. As a consequence, no reactive power compensation equipment
is needed at the station, only an AC-filter is installed. While the transmitted active
power is kept constant the reactive power controller can automatically control the
voltage in the AC-network. Reactive power generation and consumption of an HVDC
Light converter can be used for compensating the needs of the connected network
within the rating of a converter. As the rating of the converters is based on
maximum currents and voltages the reactive power capabilities of a converter can
be traded against the active power capability.
3.1.4 Robust Against Grid Alterations
The fact that a Light converter can feed power into a passive network makes it very
robust and can easily accommodate alterations in the AC-grid to where it is
connected. This is a very valuable property in a deregulated electricity market
where AC-network conditions in the future will change more frequently than in a
regulated market.
3.2 THE CABLE SYSTEM
The HVDC Light extruded cable is the outcome of a comprehensive development
program, where space charge accumulation, resistivity and electrical breakdown
strength were identified as the most important material properties when selecting
the insulation system. The selected material gives cables with high mechanical
strength, high flexibility and low weight. Extruded HVDC Light cables systems in
bipolar configuration have both technical and environmental advantages. The cables
are small yet robust and can be installed by plowing, making the installation fast and
economical.

20
3.3 APPLICATIONS
3.3.1 Overhead Lines
In general, it is getting increasingly difficult to build overhead lines. Overhead lines
change the landscape, and the construction of new lines is often met by public
resentment and political resistance. People are often concerned about the possible
health hazards of living close to overhead lines. In addition, a right-of-way for a high
voltage line occupants valuable land. The process of obtaining permissions for
building new overhead lines is also becoming time-consuming and expensive. Laying
an underground cable is a much easier process than building an overhead line. A
cable doesn’t change the landscape and it doesn’t need a wide right-of-way.
Cables are rarely met with any public opposition, and the electromagnetic field from
a dc cable pair is very low, and also a static field. Usually, the process of obtaining
the rights for laying an underground cable is much easier, quicker and cheaper than
for an overhead line. A pair of HVDC Light cables can be plowed into the ground.
Despite their large power capacity, they can be put in place with the same
equipment as ordinary, AC high voltage distribution cables. Thus, HVDC Light is
ideally suited for feeding power into growing metropolitan areas from a suburban
substation.
3.3.2 Replacing Local Generation
Remote locations often need local generation if they are situated far away from an
AC grid. The distance to the grid makes it technically or economically unfeasible to
connect the area to the main grid. Such remote locations may be islands, mining
areas, gas and oil fields or drilling platforms. Sometimes the local generators use gas
turbines, but diesel generators are much more common. An HVDC Light cable
connection could be a better choice than building a local power plant based on fossil
fuels. The environmental gains would be substantial, since the power supplied via
the dc cables will be transmitted from efficient power plants in the main AC grid.
Also, the pollution and noise produced when the diesel fuel is transported will be
completely eliminated by an HVDC line, as the need for frequent maintenance of the
diesels. Since the cost of building an HVDC Light line is a linear function of the
distance, a break-even might be reached for as short distances as 50-60 kilometers.

21
3.3.3 Connecting Remote Power Grids
Renewable power sources are often built from scratch, beginning on a small scale
and gradually expanded. Wind turbine farm is the typical case, but this is also true
for photovoltaic power generation. These power sources are usually located where
the conditions are particularly favorable, often far away from the main AC network.
At the beginning, such a slowly expanding energy resource cannot supply a remote
community with enough power. An HVDC Light link could be an ideal solution in such
cases. First, the link could supply the community with power from the main AC grid,
eliminating the need for local generation. The HVDC Light link could also supply the
wind turbine farm with reactive power for the generators, and keeping the power
frequency stable.
When the power output from the wind generators grows as more units are added,
they may supply the community with a substantial share of its power needs. When
the output exceeds the needs of the community, the power flow on the HVDC Light
link is reversed automatically, and the surplus power is transmitted to the main AC
grid.
3.3.4 Asynchronous Links
Two AC grids, adjacent to each other but running asynchronously with respect to
each other, cannot exchange any power between each other. If there is a surplus of
generating capacity in one of the grids it cannot be utilized in the other grid. Each of
the networks must have its own capacity of peak power generation, usually in the
form of older, inefficient fuel fossil plants, or diesel or gas turbine units. Thus, peak
power generation is often a source of substantial pollution, and their fuel economy is
frequently bad. A DC link, connecting two such networks, can be used for combining
the generation capacities of both networks. Cheap surplus power from one network
can replace peak power generation in the other. This will result in both reduced
pollution levels and increased fuel economy. The power exchange between the
networks is also very easy to measure accurately.

22
4. SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT
The HVDC Light transmission system mainly consists of two cables and two converter
stations. Each converter station is composed of a voltage source converter (VSC)
built up with IGBTs, phase reactors, ac filters and transformer. By using pulse width
modulation (PWM), the amplitude and phase angle (even the frequency) of the
converter AC output voltage can be adjusted simultaneously.
Since the AC side voltage holds two degrees of control freedom, independent active
and reactive power control can be realized. Regarding the active power control, the
feedback control loop can be formulized such that either tracks the predetermined
active power order, or tracks the given DC voltage reference. This gives two
different control modes, i.e., active power control mode (Pctrl) and DC voltage
control mode (Udc ctrl). If one station is selected to control the power, namely, in
Pctrl mode, the other station should set to control the DC voltage, namely, in Udc
ctrl mode.
Regarding the reactive power control, the feedback control loop can be formulized
such that it either tracks the predetermined reactive power order, or tracks the
given AC voltage reference. This also gives two control modes, i.e., reactive power
control mode (Qctrl) and AC voltage control mode (Uac ctrl). The two control modes
can be chosen freely as desired in each station.
Under the normal operation condition, the VSC can be seen as a voltage source.
However, under abnormal operation conditions, for instance, during an ac short-
circuit fault, the VSC may be seen as a current source, as the current capacity of the
VSC is limited and controllable.

23
4.1 INVESTIGATION OF SHORT CIRCUIT CURRENTS
4.1.1 Studied AC System
The studied AC system has a mixture structure in radial and mesh connection. It
includes high, medium and low voltage buses. The AC transmission lines are modeled
with p-link. The loads are constant current loads. Three types of fault, namely, the
close-in fault; the near-by fault and the distant fault, are applied at bus A, B and C,
respectively. A 3-ph close-in fault results in a voltage reduction of almost 100%,
whereas a 3-ph near-by fault and distant fault result in voltage reduction on CCP bus
of about 80% and 20%, respectively. In the following discussion, the short circuit
ratio (SCR) is defined as the short circuit capacity of the AC system observed at CCP
divided with the power rating of the converter.
Figure 5: SLD of studied AC system

24
4.1.2 The Impact of Strength of AC Networks
The possible maximum relative short circuit current increment (∆Imax) is determined
by the short circuit ratio (SCR). Supposing that the ∆Imax is defined as (1), it is found
that the ∆Imax is inversely in proportional to the SCR as the solid curve shown in
Figure 6.
Figure 6: Characteristic showing the impact of AC network strength.
where, Isc is the short-circuit current of the original AC system alone at a 3-ph fault
and I SC_HVDC_L , is the short-circuit current of the AC system with converter station
connected and in operation at the same fault. It should be noticed that the solid
curve in Figure 6 is valid if there is no tap-changer, or the tap-change is at the
position corresponding to the nominal winding ratio. If there is a tap changer
in transformer, the AC network will observe a different current although the
maximum current of the
converter is a fixed value. Therefore, the maximum possible short circuit current
increment is in the boundary defined by the two dashed curves. AC networks with
SCR equal to 1.85, 3.14 and 12 have been simulated and the results are also
shown in figure 6 with black dots.

25
Different control modes and different operation points may change the short circuit
current contribution from the VSC. However, it will not be higher than the ∆Imax. For
instance, the
short circuit current contribution from the VSC will not exceed 12% if the SCR is 10
and voltage tap-change range is ± 20%.
4.1.3. The Impact of Control Modes
The current is mainly limited by the impedances of transmission lines and
transformers when a short circuit occurs. Since the impedance of lines and
transformers is dominated by the inductive impedance, the short circuit current is
mainly consisted of reactive current.
Because of that, the choice of different control modes in respect of the active power
control does not give any impact to the short circuit current. Therefore, the
following discussion will focus on the choice between the control modes Qctrl and
Uac ctrl.
It is important to notice that the change of short circuit current and the variation of
bus voltages usually go hand in hand. The increase of short circuit current, namely,
the increase of short circuit capacity, will improve the voltage stability and minimize
the reduction of bus voltage due to faults. Inversely, the reduction of short circuit
current may leads to voltage instability and voltage collapse during faults, in
particular in weak AC systems. With Uac ctrl control mode, the reactive current
generation will be automatically increased when the AC voltage decreases.
Therefore, the Uacctrl control mode provides the possibility of improving the voltage
stability and minimizing the reduction of bus voltage due to faults. On contrast, with
Qctrl control mode it has the potential risk of getting voltage instability or voltage
collapse during faults if the AC system is weak and no control protection action is
taken. One way to avoid this potential risk is that the control is automatically
switched to Uac ctrl if the AC voltage is detected out of the specified range
(Umin~Umax, for instance, 0.9~1.1 per-unit). The other way is that the maximum
value for the current order should be decreased with the AC voltage decreasing
during faults. If the current from the VSC is reduced, its contribution to the short
circuit current will also be reduced. Therefore, with Qctrl control mode the
contribution of VSC to the short circuit current is almost neglectable independent of

26
operation points, or load level. It will then be only interesting to discuss the Uac ctrl
control mode in respect of different operation points.
4.1.4 The Impact of Operation Points
As it has been discussed, the maximum possible short circuit increment (∆Imax) due
to HVDC Light is determined by the SCR. It will occur if the VSC is operating at zero
active power, namely, it is operating as an SVC or STATCOM. Figure 7 shows the
characteristic of short circuit current contribution versus the load level. The two
dashed curves are the result by taking into account the transformer winding ratio
variation due to the tap-changer.AC networks with SCR equal to 3.14 has been
simulated. For different load levels the observed short circuit currents, during a 3-
phase close fault, are marked with black dots in Figure 7. It should be noted that the
short circuit current would be also reduced if the current order is also limited with
the Uac ctrl. The black dot with a circle in Fig. 4 shows the result when the current
order is limited to 35% of the rated current during the AC fault.

27
4.1.5 The Impact of Fault Type and Location
If the fault current is evaluated in per unit with the base value equal to the 3-phase
fault current at the corresponding fault location and without HVDC Light connected,
it turns out that the impact of the fault location seems to be insignificant. Under the
same load and operation condition, the 1-ph fault current is usually smaller than the
3-ph fault current. This is because the average voltage reduction is smaller for 1-
phase fault, thereby the required reactive power generation is smaller during a 1-
phase fault. In addition, the VSC only generates balanced 3-phase currents, even if
the AC bus voltage is unbalanced due to 1-phase faults. As an example, Figure 8
shows 1-phase and 3-phase fault currents at different locations (bus B and bus C in
Figure 5) under the same operation condition (SCR=3.14, P=-0.8 and Uac ctrl).
Currents in plot (a) and (b) have one base value, and currents in plot (c) and (d) have
another base value. Plot (b) shows that the peak value is slightly higher than 1,
which means the short circuit current with HVDC Light is slightly higher than that
without the HVDC Light for the same fault. It should be noticed that when a close-in
short-circuit fault occurs the connected converter station will only feed the fault
current. This implies that the current during the fault in the rest AC lines will be the
same as the original AC network alone. In other words, the close-in fault isolates the
HVDC Light terminal from the AC network. If it is the circuit breakers in the AC
network to be mainly concerned, this type of fault will be less significant. This is
why that the performed studies do not focus on this type of faults.
4.1.6 Line Current during Faults
It is seen that the contribution from the HVDC Light makes the difference between
the current of health lines and faulted lines larger, which may have a positive impact
in distinguishing the faulted
and health line. When a short circuit occur in the AC network, the sudden AC bus
voltage variation may result in over current to the converter due to the
measurement and control delay. As soon
as the over current in the converter is detected, the protection will trigger a
temporary blocking of converter.. It is obvious that the transient and steady state
current contribution from the HVDC Light is different. Nevertheless, it should be
noted that usually the circuit breakers do not react to the over current

28
spontaneously, and it often has a delay time of about 60 ~100 ms. Therefore, it is
the steady state current during the fault that should be considered.

29
5. CONCLUSION
From detailed analysis it is seen that HVDC system is used for long distance
transmission and its more reliable and best method for power transmission when
compared to ac power transmission.
A comprehensive investigation on the issue regarding the contribution of HVDC Light
to short circuit current has also been performed. The studies lead to the following
conclusions; The HVDC Light, in contrast to the conventional HVDC which does not
contribute any short circuit current, may contribute some short circuit current. The
possible maximum short circuit current contribution is determined by the SCR. It is
inversely in proportional to the SCR and it occurs when the transmission system is
operating at zero active power. Hence, it is comparable to the STATCOM as long as
the maximum short circuit current contribution is concerned. The amount of
contribution depends on control modes, operation points and control strategies. With
the reactive power control mode, the short circuit current contribution will be
limited due to the current order limit decreasing with the voltage.
With the AC voltage control mode, the short circuit current contribution will be
increased with the decreasing of active power, if the current order limit is not
changed. If the current order limit is decreasing with voltage, the short circuit
current contribution will be small even if the load level is low. The contribution to
the short circuit current is irrelevant to the fault location if the fault current is
evaluated in per unit with the base value equal to the 3-phase fault current at the
corresponding fault location and without HVDC Light connected. Under the same
load and operation condition, the 1-phase fault current is usually smaller than the 3-
phase fault current. Finally, it should be noticed that in associated with higher short-
circuit current the voltage stability and performance is likely to be improved. If the
HVDC Light contributes a higher short-circuit current, the voltage dip due to distant
fault is possibly reduced and thereby the connected electricity consumers may suffer
less from disturbances.

30
6. REFERENCES
1.) DC Transmission based on voltage source converters, Gunnar Asplund, Kjell
Eriksson and Kjell Svesson,1997.
2.) The ABCs of HVDC transmission technologies, IEEE Power and Energy Magazine,
2006.
3.) A Course in Electrical Power, J.B. Gupta.
4.) On the Short Circuit Current Contribution of HVDC Light, IEEE , Y. Jiang-Hafner,
M. Hyttinen, and B. Paajarvi.
5.) www.wikipedia.org

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