High Voltage Direct Current (HVDC)Transmission Systems Electronics and Communication Engineering (ECE) Notes | EduRev

Electronics and Communication Engineering (ECE) : High Voltage Direct Current (HVDC)Transmission Systems Electronics and Communication Engineering (ECE) Notes | EduRev

 Page 1


1
Presented at Energy Week 2000, Washington, D.C, USA, March 7-8, 2000
High Voltage Direct Current (HVDC)Transmission Systems
Technology Review Paper
Roberto Rudervall J.P. Charpentier Raghuveer Sharma
ABB Power Systems World Bank ABB Financial Services
Sweden United States Sweden
Synopsis
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC)
transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It
then reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a
design, construction, operation and maintenance points of view. The paper then discusses the recent
developments in HVDC technologies. The paper also presents an economic and financial comparison of
HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC
systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s
electricity system development.
In today electricity industry, in view of the liberalisation and increased effects to conserve the environment,
HVDC solutions have become more desirable for the following reasons:
• Environmental advantages
• Economical (cheapest solution)
• Asynchronous interconnections
• Power flow control
• Added benefits to the transmission (stability, power quality etc.)
Historical Perspective on HVDC Transmission
It has been widely documented in the history of the electricity industry, that the first commercial electricity
generated (by Thomas Alva Edison) was direct current (DC) electrical power. The first electricity
transmission systems were also direct current systems. However, DC power at low voltage could not be
transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC
power at high voltages and over long distances, giving rise to HVDC transmission systems. Some
important milestones in the development of the DC transmission technology are presented in Box 1.
Box 1: Important Milestones in the Development of HVDC technology
• Hewitt´s mercury-vapour rectifier, which appeared in 1901.
• Experiments with thyratrons in America and mercury arc valves in Europe before 1940.
• First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
• First solid state semiconductor valves in 1970.
• First microcomputer based control equipment for HVDC in 1979.
• Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984.
• First active DC filters for outstanding filtering performance in 1994.
• First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998
• First Voltage Source Converter for transmission  in Gotland, Sweden ,1999
HVDC Installations in the world today
Since the first commercial installation in 1954 a huge amount of HVDC transmission systems have been
installed around the world. Figure 1 shows, by region, the different HVDC transmissions around the world.
(picture at the end of the document)
Page 2


1
Presented at Energy Week 2000, Washington, D.C, USA, March 7-8, 2000
High Voltage Direct Current (HVDC)Transmission Systems
Technology Review Paper
Roberto Rudervall J.P. Charpentier Raghuveer Sharma
ABB Power Systems World Bank ABB Financial Services
Sweden United States Sweden
Synopsis
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC)
transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It
then reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a
design, construction, operation and maintenance points of view. The paper then discusses the recent
developments in HVDC technologies. The paper also presents an economic and financial comparison of
HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC
systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s
electricity system development.
In today electricity industry, in view of the liberalisation and increased effects to conserve the environment,
HVDC solutions have become more desirable for the following reasons:
• Environmental advantages
• Economical (cheapest solution)
• Asynchronous interconnections
• Power flow control
• Added benefits to the transmission (stability, power quality etc.)
Historical Perspective on HVDC Transmission
It has been widely documented in the history of the electricity industry, that the first commercial electricity
generated (by Thomas Alva Edison) was direct current (DC) electrical power. The first electricity
transmission systems were also direct current systems. However, DC power at low voltage could not be
transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC
power at high voltages and over long distances, giving rise to HVDC transmission systems. Some
important milestones in the development of the DC transmission technology are presented in Box 1.
Box 1: Important Milestones in the Development of HVDC technology
• Hewitt´s mercury-vapour rectifier, which appeared in 1901.
• Experiments with thyratrons in America and mercury arc valves in Europe before 1940.
• First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
• First solid state semiconductor valves in 1970.
• First microcomputer based control equipment for HVDC in 1979.
• Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984.
• First active DC filters for outstanding filtering performance in 1994.
• First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998
• First Voltage Source Converter for transmission  in Gotland, Sweden ,1999
HVDC Installations in the world today
Since the first commercial installation in 1954 a huge amount of HVDC transmission systems have been
installed around the world. Figure 1 shows, by region, the different HVDC transmissions around the world.
(picture at the end of the document)
2
Rationale for Choosing HVDC
There are many different reasons as to why HVDC was chosen in the above projects. A few of the reasons
in selected projects are:
• In Itaipu, Brazil, HVDC was chosen to supply 50Hz power into a 60 Hz system; and to
economically transmit large amount of hydro power (6300 MW) over large distances (800
km)
• In Leyte-Luzon Project  in Philippines, HVDC was chosen to enable supply of bulk
geothermal power across an island interconnection, and to improve stability to the Manila AC
network
• In Rihand-Delhi Project in India, HVDC was chosen to transmit bulk (thermal) power (1500
MW) to Delhi, to ensure: minimum losses, least amount right-of-way, and better stability and
control.
• In Garabi, an independent transmission project (ITP) transferring power from Argentina to
Brazil, HVDC back-to-back system was chosen to ensure supply of 50 Hz bulk (1000MW)
power to a 60 Hz system under a 20-year power supply contract.
• In Gotland, Sweden, HVDC was chosen to connect a newly developed wind power site to the
main city of Visby, in consideration of the environmental sensitivity of the project area (an
archaeological and tourist area) and improve power quality.
• In Queensland, Australia, HVDC was chosen in an ITP to interconnect two independent grids
(of New South Wales and Queensland) to: enable electricity trading between the two systems
(including change of direction of power flow); ensure very low environmental impact and
reduce construction time.
Details about the above projects are provided elsewhere (under Details of Selected HVDC Applications).
The HVDC technology
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to
DC (rectifier) at the transmitting end, and from DC to AC (inverter) at the receiving end. There are three
ways of achieving conversion:
• Natural Commutated Converters. Natural commutated converters are most used in the HVDC
systems as of today. The component that enables this conversion process is the thyristor,
which is a controllable semiconductor that can carry very high currents (4000 A) and is able
to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is
possible to build up a thyristor valve, which is able to operate at very high voltages (several
hundred of kV).The thyristor valve is operated at net frequency (50 hz or 60 hz) and by means
of a control angle it is possible to change the DC voltage level of the bridge. This ability is the
way by which the transmitted power is controlled rapidly and efficiently.
• Capacitor Commutated Converters (CCC). An improvement in the thyristor-based
commutation, the CCC concept is characterised by the use of commutation capacitors inserted
in series between the converter transformers and the thyristor valves. The commutation
capacitors improve the commutation failure performance of the converters when connected to
weak networks.
• Forced Commutated Converters. This type of converters introduces a spectrum of advantages,
e.g. feed of passive networks (without generation), independent control of active and reactive
power, power quality. The valves of these converters are built up with semiconductors with
the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source
Converters). Two types of semiconductors are normally used in the voltage source converters:
the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of
them have been in frequent use in industrial applications since early eighties. The VSC
commutates with high frequency (not with the net frequency). The operation of the converter
is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase
Page 3


1
Presented at Energy Week 2000, Washington, D.C, USA, March 7-8, 2000
High Voltage Direct Current (HVDC)Transmission Systems
Technology Review Paper
Roberto Rudervall J.P. Charpentier Raghuveer Sharma
ABB Power Systems World Bank ABB Financial Services
Sweden United States Sweden
Synopsis
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC)
transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It
then reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a
design, construction, operation and maintenance points of view. The paper then discusses the recent
developments in HVDC technologies. The paper also presents an economic and financial comparison of
HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC
systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s
electricity system development.
In today electricity industry, in view of the liberalisation and increased effects to conserve the environment,
HVDC solutions have become more desirable for the following reasons:
• Environmental advantages
• Economical (cheapest solution)
• Asynchronous interconnections
• Power flow control
• Added benefits to the transmission (stability, power quality etc.)
Historical Perspective on HVDC Transmission
It has been widely documented in the history of the electricity industry, that the first commercial electricity
generated (by Thomas Alva Edison) was direct current (DC) electrical power. The first electricity
transmission systems were also direct current systems. However, DC power at low voltage could not be
transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC
power at high voltages and over long distances, giving rise to HVDC transmission systems. Some
important milestones in the development of the DC transmission technology are presented in Box 1.
Box 1: Important Milestones in the Development of HVDC technology
• Hewitt´s mercury-vapour rectifier, which appeared in 1901.
• Experiments with thyratrons in America and mercury arc valves in Europe before 1940.
• First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
• First solid state semiconductor valves in 1970.
• First microcomputer based control equipment for HVDC in 1979.
• Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984.
• First active DC filters for outstanding filtering performance in 1994.
• First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998
• First Voltage Source Converter for transmission  in Gotland, Sweden ,1999
HVDC Installations in the world today
Since the first commercial installation in 1954 a huge amount of HVDC transmission systems have been
installed around the world. Figure 1 shows, by region, the different HVDC transmissions around the world.
(picture at the end of the document)
2
Rationale for Choosing HVDC
There are many different reasons as to why HVDC was chosen in the above projects. A few of the reasons
in selected projects are:
• In Itaipu, Brazil, HVDC was chosen to supply 50Hz power into a 60 Hz system; and to
economically transmit large amount of hydro power (6300 MW) over large distances (800
km)
• In Leyte-Luzon Project  in Philippines, HVDC was chosen to enable supply of bulk
geothermal power across an island interconnection, and to improve stability to the Manila AC
network
• In Rihand-Delhi Project in India, HVDC was chosen to transmit bulk (thermal) power (1500
MW) to Delhi, to ensure: minimum losses, least amount right-of-way, and better stability and
control.
• In Garabi, an independent transmission project (ITP) transferring power from Argentina to
Brazil, HVDC back-to-back system was chosen to ensure supply of 50 Hz bulk (1000MW)
power to a 60 Hz system under a 20-year power supply contract.
• In Gotland, Sweden, HVDC was chosen to connect a newly developed wind power site to the
main city of Visby, in consideration of the environmental sensitivity of the project area (an
archaeological and tourist area) and improve power quality.
• In Queensland, Australia, HVDC was chosen in an ITP to interconnect two independent grids
(of New South Wales and Queensland) to: enable electricity trading between the two systems
(including change of direction of power flow); ensure very low environmental impact and
reduce construction time.
Details about the above projects are provided elsewhere (under Details of Selected HVDC Applications).
The HVDC technology
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to
DC (rectifier) at the transmitting end, and from DC to AC (inverter) at the receiving end. There are three
ways of achieving conversion:
• Natural Commutated Converters. Natural commutated converters are most used in the HVDC
systems as of today. The component that enables this conversion process is the thyristor,
which is a controllable semiconductor that can carry very high currents (4000 A) and is able
to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is
possible to build up a thyristor valve, which is able to operate at very high voltages (several
hundred of kV).The thyristor valve is operated at net frequency (50 hz or 60 hz) and by means
of a control angle it is possible to change the DC voltage level of the bridge. This ability is the
way by which the transmitted power is controlled rapidly and efficiently.
• Capacitor Commutated Converters (CCC). An improvement in the thyristor-based
commutation, the CCC concept is characterised by the use of commutation capacitors inserted
in series between the converter transformers and the thyristor valves. The commutation
capacitors improve the commutation failure performance of the converters when connected to
weak networks.
• Forced Commutated Converters. This type of converters introduces a spectrum of advantages,
e.g. feed of passive networks (without generation), independent control of active and reactive
power, power quality. The valves of these converters are built up with semiconductors with
the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source
Converters). Two types of semiconductors are normally used in the voltage source converters:
the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of
them have been in frequent use in industrial applications since early eighties. The VSC
commutates with high frequency (not with the net frequency). The operation of the converter
is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase
3
angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be
done almost instantaneously. Thus, PWM offers the possibility to control both active and
reactive power independently. This makes the PWM Voltage Source Converter a close to
ideal component in the transmission network. From a transmission network viewpoint, it acts
as a motor or generator without mass that can control active and reactive power almost
instantaneously.
The components of an HVDC transmission system
To assist the designers of transmission systems, the components that comprise the HVDC system, and the
options available in these components, are presented and discussed. The three main elements of an HVDC
system are: the converter station at the transmission and receiving ends, the transmission medium, and the
electrodes.
The converter station: The converter stations at each end are replica’s of each other and therefore consists
of all the needed equipment for going from AC to DC or vice versa. The main component of a converter
station are:
Thyristor valves: The thyristor valves can be build-up in different ways depending on the
application and manufacturer. However, the most common way of arranging the thyristor valves is
in a twelve-pulse group with three quadruple valves. Each single thyristor valve consists of a
certain amount of series connected thyristors with their auxiliary circuits. All communication
between the control equipment at earth potential and each thyristor at high potential, is done with
fibre optics.
VSC valves: The VSC converter consists of two level or multilevel converter, phase-reactors and
AC filters. Each single valve in the converter bridge is built up with a certain number of series-
connected IGBTs together with their auxiliary electronics. VSC valves, control equipment and
cooling equipment would be in enclosures (such as standard shipping containers) which make
transport and installation very easy.
All modern HVDC valves are water-cooled and air insulated.
~~~~~~
Converter  
AC Filters   
Shunt capacitors or other reactive equipment   
~~~
Control system    
AC Bus     
DC Filter   
Smoothing reactor
 
Converter Station
Transmission 
line or cable 
(excluded if   
Back-to-Back) 
Page 4


1
Presented at Energy Week 2000, Washington, D.C, USA, March 7-8, 2000
High Voltage Direct Current (HVDC)Transmission Systems
Technology Review Paper
Roberto Rudervall J.P. Charpentier Raghuveer Sharma
ABB Power Systems World Bank ABB Financial Services
Sweden United States Sweden
Synopsis
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC)
transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It
then reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a
design, construction, operation and maintenance points of view. The paper then discusses the recent
developments in HVDC technologies. The paper also presents an economic and financial comparison of
HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC
systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s
electricity system development.
In today electricity industry, in view of the liberalisation and increased effects to conserve the environment,
HVDC solutions have become more desirable for the following reasons:
• Environmental advantages
• Economical (cheapest solution)
• Asynchronous interconnections
• Power flow control
• Added benefits to the transmission (stability, power quality etc.)
Historical Perspective on HVDC Transmission
It has been widely documented in the history of the electricity industry, that the first commercial electricity
generated (by Thomas Alva Edison) was direct current (DC) electrical power. The first electricity
transmission systems were also direct current systems. However, DC power at low voltage could not be
transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC
power at high voltages and over long distances, giving rise to HVDC transmission systems. Some
important milestones in the development of the DC transmission technology are presented in Box 1.
Box 1: Important Milestones in the Development of HVDC technology
• Hewitt´s mercury-vapour rectifier, which appeared in 1901.
• Experiments with thyratrons in America and mercury arc valves in Europe before 1940.
• First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
• First solid state semiconductor valves in 1970.
• First microcomputer based control equipment for HVDC in 1979.
• Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984.
• First active DC filters for outstanding filtering performance in 1994.
• First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998
• First Voltage Source Converter for transmission  in Gotland, Sweden ,1999
HVDC Installations in the world today
Since the first commercial installation in 1954 a huge amount of HVDC transmission systems have been
installed around the world. Figure 1 shows, by region, the different HVDC transmissions around the world.
(picture at the end of the document)
2
Rationale for Choosing HVDC
There are many different reasons as to why HVDC was chosen in the above projects. A few of the reasons
in selected projects are:
• In Itaipu, Brazil, HVDC was chosen to supply 50Hz power into a 60 Hz system; and to
economically transmit large amount of hydro power (6300 MW) over large distances (800
km)
• In Leyte-Luzon Project  in Philippines, HVDC was chosen to enable supply of bulk
geothermal power across an island interconnection, and to improve stability to the Manila AC
network
• In Rihand-Delhi Project in India, HVDC was chosen to transmit bulk (thermal) power (1500
MW) to Delhi, to ensure: minimum losses, least amount right-of-way, and better stability and
control.
• In Garabi, an independent transmission project (ITP) transferring power from Argentina to
Brazil, HVDC back-to-back system was chosen to ensure supply of 50 Hz bulk (1000MW)
power to a 60 Hz system under a 20-year power supply contract.
• In Gotland, Sweden, HVDC was chosen to connect a newly developed wind power site to the
main city of Visby, in consideration of the environmental sensitivity of the project area (an
archaeological and tourist area) and improve power quality.
• In Queensland, Australia, HVDC was chosen in an ITP to interconnect two independent grids
(of New South Wales and Queensland) to: enable electricity trading between the two systems
(including change of direction of power flow); ensure very low environmental impact and
reduce construction time.
Details about the above projects are provided elsewhere (under Details of Selected HVDC Applications).
The HVDC technology
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to
DC (rectifier) at the transmitting end, and from DC to AC (inverter) at the receiving end. There are three
ways of achieving conversion:
• Natural Commutated Converters. Natural commutated converters are most used in the HVDC
systems as of today. The component that enables this conversion process is the thyristor,
which is a controllable semiconductor that can carry very high currents (4000 A) and is able
to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is
possible to build up a thyristor valve, which is able to operate at very high voltages (several
hundred of kV).The thyristor valve is operated at net frequency (50 hz or 60 hz) and by means
of a control angle it is possible to change the DC voltage level of the bridge. This ability is the
way by which the transmitted power is controlled rapidly and efficiently.
• Capacitor Commutated Converters (CCC). An improvement in the thyristor-based
commutation, the CCC concept is characterised by the use of commutation capacitors inserted
in series between the converter transformers and the thyristor valves. The commutation
capacitors improve the commutation failure performance of the converters when connected to
weak networks.
• Forced Commutated Converters. This type of converters introduces a spectrum of advantages,
e.g. feed of passive networks (without generation), independent control of active and reactive
power, power quality. The valves of these converters are built up with semiconductors with
the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source
Converters). Two types of semiconductors are normally used in the voltage source converters:
the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of
them have been in frequent use in industrial applications since early eighties. The VSC
commutates with high frequency (not with the net frequency). The operation of the converter
is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase
3
angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be
done almost instantaneously. Thus, PWM offers the possibility to control both active and
reactive power independently. This makes the PWM Voltage Source Converter a close to
ideal component in the transmission network. From a transmission network viewpoint, it acts
as a motor or generator without mass that can control active and reactive power almost
instantaneously.
The components of an HVDC transmission system
To assist the designers of transmission systems, the components that comprise the HVDC system, and the
options available in these components, are presented and discussed. The three main elements of an HVDC
system are: the converter station at the transmission and receiving ends, the transmission medium, and the
electrodes.
The converter station: The converter stations at each end are replica’s of each other and therefore consists
of all the needed equipment for going from AC to DC or vice versa. The main component of a converter
station are:
Thyristor valves: The thyristor valves can be build-up in different ways depending on the
application and manufacturer. However, the most common way of arranging the thyristor valves is
in a twelve-pulse group with three quadruple valves. Each single thyristor valve consists of a
certain amount of series connected thyristors with their auxiliary circuits. All communication
between the control equipment at earth potential and each thyristor at high potential, is done with
fibre optics.
VSC valves: The VSC converter consists of two level or multilevel converter, phase-reactors and
AC filters. Each single valve in the converter bridge is built up with a certain number of series-
connected IGBTs together with their auxiliary electronics. VSC valves, control equipment and
cooling equipment would be in enclosures (such as standard shipping containers) which make
transport and installation very easy.
All modern HVDC valves are water-cooled and air insulated.
~~~~~~
Converter  
AC Filters   
Shunt capacitors or other reactive equipment   
~~~
Control system    
AC Bus     
DC Filter   
Smoothing reactor
 
Converter Station
Transmission 
line or cable 
(excluded if   
Back-to-Back) 
4
Transformers: The converter transformers adapt the AC voltage level to the DC voltage level and
they contribute to the commutation reactance. Usually they are of the single phase three winding
type, but depending on the transportation requirements and the rated power, they can be arranged
in other ways
AC Filters and Capacitor Banks: On the AC side of a 12-pulse HVDC converter, current
harmonics of the order of 11, 13, 23, 25 and higher are generated. Filters are installed in order to
limit the amount of harmonics to the level required by the network.. In the conversion process the
converter consumes reactive power which is compensated in part by the filter banks and the rest
by capacitor banks.
In the case of the CCC the reactive power is compensated by the series capacitors installed in
series between the converter valves and the converter transformer. The elimination of switched
reactive power compensation equipment simplify the AC switchyard and minimise the number of
circuit-breakers needed, which will reduce the area required for an HVDC station built with CCC.
With VSC converters there is no need to compensate any reactive power consumed by the
converter itself and the current harmonics on the AC side are related directly to the PWM
frequency. Therefore the amount of filters in this type of converters is reduced dramatically
compared with natural commutated converters.
DC filters: HVDC converters create harmonics in all operational modes. Such harmonics can
create disturbances in telecommunication systems. Therefore, specially designed DC filters are
used in order to reduce the disturbances. Usually no filters are needed for pure cable transmissions
as well as for the Back-to-Back HVDC stations. However, it is necessary to install DC filters if an
OH line is used in part or all the transmission system
The filters needed to take care of the harmonics generated on the DC end, are usually considerably
smaller and less expensive than the filters on the AC side. The modern DC filters are the Active
DC filters. In these filters the passive part is reduced to a minimum and modern power electronics
is used to measure, invert and re-inject the harmonics, thus rendering the filtering very effective.
Transmission medium
For bulk power transmission over land, the most frequent transmission medium used is the overhead line.
This overhead line is normally bipolar, i.e. two conductors with different polarity. HVDC cables are
normally used for submarine transmission. The most common types of cables are the solid and the oil-filled
ones. The solid type is in many cases the most economic one. Its insulation consists of paper tapes
impregnated with a high viscosity oil. No length limitation exists for this type and designs are today
available for depths of about 1000 m. The self –contained oil-filled cable is completely filled with a low
viscosity oil and always works under pressure. The maximum length for this cable type seems to be around
60 km.
The development of new power cable technologies has accelerated in recent years and today a new HVDC
cable is available for HVDC underground or submarine power transmissions. This new HVDC cable is
made of extruded polyethylene, and is used in VSC based HVDC systems.
Design, Construction, Operation and Maintenance considerations
In general, the basic parameters such as power to be transmitted, distance of transmission, voltage levels,
temporary and continuous overload, status of the network on the receiving end, environmental requirements
etc. are required to initiate a design of an HVDC system.
For tendering purposes a conceptual design is done following a technical specification or in close
collaboration between the manufacturer and the customer. The final design and specifications are in fact the
result of the tendering and negotiations with the manufactures/suppliers. It is recommended that a turnkey
approach be chosen to contract execution, which is the practice even in developed countries.
Page 5


1
Presented at Energy Week 2000, Washington, D.C, USA, March 7-8, 2000
High Voltage Direct Current (HVDC)Transmission Systems
Technology Review Paper
Roberto Rudervall J.P. Charpentier Raghuveer Sharma
ABB Power Systems World Bank ABB Financial Services
Sweden United States Sweden
Synopsis
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC)
transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It
then reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a
design, construction, operation and maintenance points of view. The paper then discusses the recent
developments in HVDC technologies. The paper also presents an economic and financial comparison of
HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC
systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s
electricity system development.
In today electricity industry, in view of the liberalisation and increased effects to conserve the environment,
HVDC solutions have become more desirable for the following reasons:
• Environmental advantages
• Economical (cheapest solution)
• Asynchronous interconnections
• Power flow control
• Added benefits to the transmission (stability, power quality etc.)
Historical Perspective on HVDC Transmission
It has been widely documented in the history of the electricity industry, that the first commercial electricity
generated (by Thomas Alva Edison) was direct current (DC) electrical power. The first electricity
transmission systems were also direct current systems. However, DC power at low voltage could not be
transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC
power at high voltages and over long distances, giving rise to HVDC transmission systems. Some
important milestones in the development of the DC transmission technology are presented in Box 1.
Box 1: Important Milestones in the Development of HVDC technology
• Hewitt´s mercury-vapour rectifier, which appeared in 1901.
• Experiments with thyratrons in America and mercury arc valves in Europe before 1940.
• First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
• First solid state semiconductor valves in 1970.
• First microcomputer based control equipment for HVDC in 1979.
• Highest DC transmission voltage (+/- 600 kV) in Itaipú, Brazil, 1984.
• First active DC filters for outstanding filtering performance in 1994.
• First Capacitor Commutated Converter (CCC) in Argentina-Brazil interconnection, 1998
• First Voltage Source Converter for transmission  in Gotland, Sweden ,1999
HVDC Installations in the world today
Since the first commercial installation in 1954 a huge amount of HVDC transmission systems have been
installed around the world. Figure 1 shows, by region, the different HVDC transmissions around the world.
(picture at the end of the document)
2
Rationale for Choosing HVDC
There are many different reasons as to why HVDC was chosen in the above projects. A few of the reasons
in selected projects are:
• In Itaipu, Brazil, HVDC was chosen to supply 50Hz power into a 60 Hz system; and to
economically transmit large amount of hydro power (6300 MW) over large distances (800
km)
• In Leyte-Luzon Project  in Philippines, HVDC was chosen to enable supply of bulk
geothermal power across an island interconnection, and to improve stability to the Manila AC
network
• In Rihand-Delhi Project in India, HVDC was chosen to transmit bulk (thermal) power (1500
MW) to Delhi, to ensure: minimum losses, least amount right-of-way, and better stability and
control.
• In Garabi, an independent transmission project (ITP) transferring power from Argentina to
Brazil, HVDC back-to-back system was chosen to ensure supply of 50 Hz bulk (1000MW)
power to a 60 Hz system under a 20-year power supply contract.
• In Gotland, Sweden, HVDC was chosen to connect a newly developed wind power site to the
main city of Visby, in consideration of the environmental sensitivity of the project area (an
archaeological and tourist area) and improve power quality.
• In Queensland, Australia, HVDC was chosen in an ITP to interconnect two independent grids
(of New South Wales and Queensland) to: enable electricity trading between the two systems
(including change of direction of power flow); ensure very low environmental impact and
reduce construction time.
Details about the above projects are provided elsewhere (under Details of Selected HVDC Applications).
The HVDC technology
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to
DC (rectifier) at the transmitting end, and from DC to AC (inverter) at the receiving end. There are three
ways of achieving conversion:
• Natural Commutated Converters. Natural commutated converters are most used in the HVDC
systems as of today. The component that enables this conversion process is the thyristor,
which is a controllable semiconductor that can carry very high currents (4000 A) and is able
to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is
possible to build up a thyristor valve, which is able to operate at very high voltages (several
hundred of kV).The thyristor valve is operated at net frequency (50 hz or 60 hz) and by means
of a control angle it is possible to change the DC voltage level of the bridge. This ability is the
way by which the transmitted power is controlled rapidly and efficiently.
• Capacitor Commutated Converters (CCC). An improvement in the thyristor-based
commutation, the CCC concept is characterised by the use of commutation capacitors inserted
in series between the converter transformers and the thyristor valves. The commutation
capacitors improve the commutation failure performance of the converters when connected to
weak networks.
• Forced Commutated Converters. This type of converters introduces a spectrum of advantages,
e.g. feed of passive networks (without generation), independent control of active and reactive
power, power quality. The valves of these converters are built up with semiconductors with
the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source
Converters). Two types of semiconductors are normally used in the voltage source converters:
the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of
them have been in frequent use in industrial applications since early eighties. The VSC
commutates with high frequency (not with the net frequency). The operation of the converter
is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase
3
angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be
done almost instantaneously. Thus, PWM offers the possibility to control both active and
reactive power independently. This makes the PWM Voltage Source Converter a close to
ideal component in the transmission network. From a transmission network viewpoint, it acts
as a motor or generator without mass that can control active and reactive power almost
instantaneously.
The components of an HVDC transmission system
To assist the designers of transmission systems, the components that comprise the HVDC system, and the
options available in these components, are presented and discussed. The three main elements of an HVDC
system are: the converter station at the transmission and receiving ends, the transmission medium, and the
electrodes.
The converter station: The converter stations at each end are replica’s of each other and therefore consists
of all the needed equipment for going from AC to DC or vice versa. The main component of a converter
station are:
Thyristor valves: The thyristor valves can be build-up in different ways depending on the
application and manufacturer. However, the most common way of arranging the thyristor valves is
in a twelve-pulse group with three quadruple valves. Each single thyristor valve consists of a
certain amount of series connected thyristors with their auxiliary circuits. All communication
between the control equipment at earth potential and each thyristor at high potential, is done with
fibre optics.
VSC valves: The VSC converter consists of two level or multilevel converter, phase-reactors and
AC filters. Each single valve in the converter bridge is built up with a certain number of series-
connected IGBTs together with their auxiliary electronics. VSC valves, control equipment and
cooling equipment would be in enclosures (such as standard shipping containers) which make
transport and installation very easy.
All modern HVDC valves are water-cooled and air insulated.
~~~~~~
Converter  
AC Filters   
Shunt capacitors or other reactive equipment   
~~~
Control system    
AC Bus     
DC Filter   
Smoothing reactor
 
Converter Station
Transmission 
line or cable 
(excluded if   
Back-to-Back) 
4
Transformers: The converter transformers adapt the AC voltage level to the DC voltage level and
they contribute to the commutation reactance. Usually they are of the single phase three winding
type, but depending on the transportation requirements and the rated power, they can be arranged
in other ways
AC Filters and Capacitor Banks: On the AC side of a 12-pulse HVDC converter, current
harmonics of the order of 11, 13, 23, 25 and higher are generated. Filters are installed in order to
limit the amount of harmonics to the level required by the network.. In the conversion process the
converter consumes reactive power which is compensated in part by the filter banks and the rest
by capacitor banks.
In the case of the CCC the reactive power is compensated by the series capacitors installed in
series between the converter valves and the converter transformer. The elimination of switched
reactive power compensation equipment simplify the AC switchyard and minimise the number of
circuit-breakers needed, which will reduce the area required for an HVDC station built with CCC.
With VSC converters there is no need to compensate any reactive power consumed by the
converter itself and the current harmonics on the AC side are related directly to the PWM
frequency. Therefore the amount of filters in this type of converters is reduced dramatically
compared with natural commutated converters.
DC filters: HVDC converters create harmonics in all operational modes. Such harmonics can
create disturbances in telecommunication systems. Therefore, specially designed DC filters are
used in order to reduce the disturbances. Usually no filters are needed for pure cable transmissions
as well as for the Back-to-Back HVDC stations. However, it is necessary to install DC filters if an
OH line is used in part or all the transmission system
The filters needed to take care of the harmonics generated on the DC end, are usually considerably
smaller and less expensive than the filters on the AC side. The modern DC filters are the Active
DC filters. In these filters the passive part is reduced to a minimum and modern power electronics
is used to measure, invert and re-inject the harmonics, thus rendering the filtering very effective.
Transmission medium
For bulk power transmission over land, the most frequent transmission medium used is the overhead line.
This overhead line is normally bipolar, i.e. two conductors with different polarity. HVDC cables are
normally used for submarine transmission. The most common types of cables are the solid and the oil-filled
ones. The solid type is in many cases the most economic one. Its insulation consists of paper tapes
impregnated with a high viscosity oil. No length limitation exists for this type and designs are today
available for depths of about 1000 m. The self –contained oil-filled cable is completely filled with a low
viscosity oil and always works under pressure. The maximum length for this cable type seems to be around
60 km.
The development of new power cable technologies has accelerated in recent years and today a new HVDC
cable is available for HVDC underground or submarine power transmissions. This new HVDC cable is
made of extruded polyethylene, and is used in VSC based HVDC systems.
Design, Construction, Operation and Maintenance considerations
In general, the basic parameters such as power to be transmitted, distance of transmission, voltage levels,
temporary and continuous overload, status of the network on the receiving end, environmental requirements
etc. are required to initiate a design of an HVDC system.
For tendering purposes a conceptual design is done following a technical specification or in close
collaboration between the manufacturer and the customer. The final design and specifications are in fact the
result of the tendering and negotiations with the manufactures/suppliers. It is recommended that a turnkey
approach be chosen to contract execution, which is the practice even in developed countries.
5
In terms of construction, it can take from three years for thyristor-based large HVDC systems, to just one
year for VSC based HVDC systems to go from contract date to commissioning.  The following table shows
the experience for the different HVDC technologies:
Natural commutated HVDC 3 years
CCC based HVDC 2 years
VSC based HVDC 1 year
To the extent that the term operation denotes the continual activities that are aimed at keeping the system
availability at designed levels, modern HVDC links can be operated remotely, in view of the semiconductor
and microprocessor based control systems included. There are some existing installations in operation
completely unmanned. Moreover, modern HVDC systems are designed to operate unmanned. This feature
is particularly important in situations or countries where skilled people are few, and these few people can
operate several HVDC links from one central location.
Maintenance of HVDC systems is comparable to those of high voltage AC systems. The high voltage
equipment in converter stations is comparable to the corresponding equipment in AC substations, and
maintenance can be executed in the same way. Maintenance will focus on: AC and DC filters, smoothing
reactors, wall bushings, valve-cooling equipment, thyristor valves. In all the above, adequate training and
support is provided by the supplier during the installation, commissioning and initial operation period.
Normal routine maintenance is recommended to be one week per year. The newer systems can even go for
two years before requiring maintenance. In fact in a bipolar system, one pole at a time is stopped during the
time required for the maintenance, and the other pole can normally continue to operate and depending on
the in-built overload capacity it can take a part of the load of the pole under maintenance.
In addition, preventive maintenance shall be pursued so that the plants and equipment will achieve
optimally balanced availability with regard to the costs of maintenance, operating disturbances and planned
outages. As a guideline value, the aim shall be to achieve an availability of 98 % according to Cigré
protocol 14-97.
While HVDC systems may only need a few skilled staff for operation and maintenance, several factors
influence the number of staff needed at a station. These factors are: local routines and regulations, working
conditions, union requirements, safety regulations, and other local rules can separately or together affect the
total number of personnel required for the type of installed equipment.
 Cost structure
The cost of an HVDC transmission system depends on many factors, such as power capacity to be
transmitted, type of transmission medium, environmental conditions and other safety, regulatory
requirements etc. Even when these are available, the options available for optimal design (different
commutation techniques, variety of filters, transformers etc.) render it is difficult to give a cost figure for an
HVDC system.  Nevertheless, a typical cost structure for the converter stations could be as follows:
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