high voltage direct current transmission system report
TRANSCRIPT
SEMINAR REPORT ON
“High Voltage Direct CurrentTransmission 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 KhiljiAssit. Prof. Vikram Rajpurohit B.Tech IV year VIII Sem.
Department of Electrical & Electronics EngineeringVYAS 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
HVDC Transmission System
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
HVDC Transmission System
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.
HVDC Transmission System
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
HVDC Transmission System
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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HVDC Transmission System
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).
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HVDC Transmission System
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.
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HVDC Transmission System
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 :
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.
4
HVDC Transmission System
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
5
HVDC Transmission System
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
6
HVDC Transmission System
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
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
7
HVDC Transmission System
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 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
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HVDC Transmission System
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 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.
9
HVDC Transmission System
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.
10
HVDC Transmission System
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.
11
HVDC Transmission System
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.
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).
12
HVDC Transmission System
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 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.
13
HVDC Transmission System
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.
14
HVDC Transmission System
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 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.
15
HVDC Transmission System
16
HVDC Transmission System
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.
17
HVDC Transmission System
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 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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HVDC Transmission System
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
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HVDC Transmission System
• 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.
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HVDC Transmission System
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HVDC Transmission System
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
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HVDC Transmission System
Light line is a linear function of the distance, a break-even might be reached
for as short distances as 50-60 kilometers.
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
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HVDC Transmission System
both reduced pollution levels and increased fuel economy. The power
exchange between the networks is also very easy to measure accurately.
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HVDC Transmission System
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.
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HVDC Transmission System
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
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HVDC Transmission System
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
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HVDC Transmission System
shown in figure 6 with black dots.
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
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HVDC Transmission System
reduced. Therefore, with Qctrl control mode the contribution of VSC to the
short circuit current is almost neglectable independent of 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.
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HVDC Transmission System
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 30
HVDC Transmission System
should be noted that usually the circuit breakers do not react to the over
current 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.
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HVDC Transmission System
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.
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HVDC Transmission System
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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