high_voltage_direct_current

1
High Voltage Direct Current
(HVDC)Transmission Systems An Overview.
2

• What is HVDC?
• HVDC stands for High Voltage Direct Current and
  is today a well-proven technology employed for
  power transmission all over the world. In total
  about 70,000 MW HVDC transmission capacity is
  installed in more than 90 projects.
• The HVDC technology is used to transmit
  electricity over long distances by overhead
  transmission lines or submarine cables.
• It is also used to interconnect separate power
  systems, where traditional alternating current
  (AC) connections can not be used.
• There are three different categories of HVDC
  transmissions 1. Point to point
  transmissions2. Back-to-back stations3.
  Multi-terminal systems

3

• The development of the HVDC technology started in
  the late 1920s, and only after some 25 years of
  extensive development and pioneering work the
  first commercially operating scheme was
  commissioned in 1954.
• This was a link between the Swedish mainland and
  the island of Gotland in the Baltic sea.
• The power rating was 20 MW and the transmission
  voltage 100 kV.
• At that time mercury arc valves were used for the
  conversion between AC and DC, and the control
  equipment was using vacuum tubes.
• A significant improvement of the HVDC Technology
  came around 1970 when thyristor valves were
  introduced in place of the mercury arc valves.
  This reduced the size and complexity of HVDC
  converter stations substantially.
• The use of microcomputers in the control
  equipment in today's transmissions has also
  contributed to making HVDC the powerful
  alternative in power transmission that it is
  today.

4

• Historical Perspective on HVDC Transmission
• The first commercial electricity generated (by
  Thomas Alva Edison) was direct current (DC)
  electrical power.
• The first electricity transmission systems were
  also DC 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.
• Important Milestones in the Development of HVDC
  technology
• Hewitts 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

5

• The Classic HVDC Transmission
• Using HVDC to interconnect two points in a power
  grid, in many cases is the best economic
  alternative, and furthermore it has excellent
  environmental benefits.
• The HVDC technology (High Voltage Direct Current)
  is used to transmit electricity over long
  distances by overhead transmission lines or
  submarine cables.
• It is also used to interconnect separate power
  systems, where traditional alternating current
  (AC) connections can not be used.
• In a high voltage direct current (HVDC) system,
  electric power is taken from one point in a
  three-phase AC network, converted to DC in a
  converter station, transmitted to the receiving
  point by an overhead line or cable and then
  converted back to AC in another converter station
  and injected into the receiving AC network.
• Typically, an HVDC transmission has a rated power
  of more than 100 MW and many are in the 1,000 -
  3,000 MW range.

6
Aerial overview of the 3000MW HVDC converter
station at Longquan, China( Three
Gorges-Changzhou HVDC transmission)

• HVDC transmissions are used for transmission of
  power over long or very long distances, because
  it then becomes economically attractive over
  conventional AC lines.
• With an HVDC system, the power flow can be
  controlled rapidly and accurately as to both the
  power level and the direction. This possibility
  is often used in order to improve the performance
  and efficiency of the connected AC networks.

7
Why HVDC? Its Advantages Power stations
generate alternating current, AC, and the power
delivered to the consumers is in the form of AC.
Why then is it sometimes more suitable to use
direct current, HVDC, for transmitting electric
power? The vast majority of electric power
transmissions use three-phase alternating
current. The reasons behind a choice of HVDC
instead of AC to transmit power in a specific
case are often numerous and complex. Each
individual transmission project will display its
own set of reasons justifying the choice of HVDC,
but the most common arguments favoring HVDC are
1. Lower investment cost2. Long distance water
crossing3. Lower losses4. Asynchronous
interconnections5. Controllability6. Limit
short circuit currents7. Environment
8

• In general terms the different reasons/ADVANTAGES
  for using HVDC can be divided in two main groups,
  namely
• HVDC is necessary or desirable from the technical
  point of view (i.e. controllability).
• HVDC results in a lower total investment
  (including lower losses) and/or is
  environmentally superior.

The Baltic Cable HVDC Link, Overview of Herrenwyk
station
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• 1) HVDC transmission for lower investment cost.
• A HVDC transmission line costs less than an AC
  line for the same transmission capacity. However,
  the terminal stations are more expensive in the
  HVDC case due to the fact that they must perform
  the conversion from AC to DC and vice versa. But
  above a certain distance, the so called
  "break-even distance", the HVDC alternative will
  always give the lowest cost.

• The break-even-distance is much smaller for
  submarine cables (typically about 50 km) than for
  an overhead line transmission. The distance
  depends on several factors (both for lines and
  cables) and an analysis must be made for each
  individual case.

• The importance of the break-even-distance concept
  should not be over-stressed, since several other
  factors, such as controllability, are important
  in the selection between AC or HVDC.

10
Typical investment costs for an overhead line
transmission with AC and HVDC.
11
Relative Cost of AC versus DC

• For equivalent transmission capacity, a DC line
  has lower construction costs than an AC line
• A double HVAC three-phase circuit with 6
  conductors is needed to get the reliability of a
  two-pole DC link.
• DC requires less insulation ceteris paribus.
• For the same conductor, DC losses are less, so
  other costs, and generally final losses too, can
  be reduced.
• An optimized DC link has smaller towers than an
  optimized AC link of equal capacity.

12
Typical tower structures and rights-of-way for
alternative transmission systems of 2,000 MW
capacity.
Source Arrillaga (1998)
13
AC versus DC (continued)

• Right-of-way for an AC Line designed to carry
  2,000 MW is more than 70 wider than the
  right-of-way for a DC line of equivalent
  capacity.
• This is particularly important where land is
  expensive or permitting is a problem.
• HVDC light is now also transmitted via
  underground cable the recently commissioned
  Murray-Link in Australia is 200 MW over 177 km.
• Can reduce land and environmental costs, but is
  more expensive per km than overhead line.

14
AC versus DC (continued)

• Above costs are on a per km basis. The remaining
  costs also differ
• The need to convert to and from AC implies the
  terminal stations for a DC line cost more.
• There are extra losses in DC/AC conversion
  relative to AC voltage transformation.
• Operation and maintenance costs are lower for an
  optimized HVDC than for an equal capacity
  optimized AC system.

15
AC versus DC (continued)

• The cost advantage of HVDC increases with the
  length, but decreases with the capacity, of a
  link.
• For both AC and DC, design characteristics
  trade-off fixed and variable costs, but losses
  are lower on the optimized DC link.
• The time profile of use of the link affects the
  cost of losses, since the MC of electricity
  fluctuates.
• Interest rates also affect the trade-off between
  capital and operating costs.

16
Increased Benefits of Long-Distance Transmission

• Long distance transmission increases competition
  in new wholesale electricity markets.
• Long distance electricity trade, including across
  nations, allows arbitrage of price differences.
• Contractual provision of transmission services
  demands more stable networks.
• Bi-directional power transfers, often needed in
  new electricity markets, can be accommodated at
  lower cost using HVDC

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2) HVDC cable transmissions for long distance
water crossing. There are no technical limits
for the length of a HVDC cable. In a long AC
cable transmission, the reactive power flow due
to the large cable capacitance will limit the
maximum possible transmission distance. With HVDC
there is no such limitation, why, for long cable
links, HVDC is the only viable technical
alternative. The 580 kilometer-long NorNed link
will be the longest underwater high-voltage cable
in the world in 2007 and thereby surpassing the
present longest, the Baltic Cable transmission
between Sweden and Germany with its 250 km.
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• 3)HVDC transmission has lower losses.
• HVDC transmission losses come out lower than the
  AC losses in practically all cases.
• An optimized HVDC transmission line has lower
  losses than AC lines for the same power capacity.
  
• The losses in the converter stations have of
  course to be added, but since they are only about
  0.6 of the transmitted power in each station,
  the total HVDC transmission losses come out lower
  than the AC losses in practically all cases.
• HVDC cables also have lower losses than AC
  cables.

An optimized DC line has lower
losses than an AC line
19
Comparison of the losses for overhead line
transmissions of 1200 MW with AC and HVDC.
20
4)HVDC link for asynchronous interconnections
Many HVDC links interconnect incompatible AC
systems

• Several HVDC links interconnect AC systems that
  are not running in synchronism with each other.
• For example the Nordel power system in
  Scandinavia is not synchronous with the UCTE grid
  in western continental Europe even though the
  nominal frequencies are the same. And the power
  system of eastern USA is not synchronous with
  that of western USA.
• The reason for this is that it is sometimes
  difficult or impossible to connect two AC
  networks due to stability reasons. In such cases
  HVDC is the only way to make an exchange of power
  between the two networks possible. There are also
  HVDC links between networks with different
  nominal frequencies (50 and 60 Hz) in Japan and
  South America.
• For smaller asynchronous interconnections HVDC
  Light is the proper choice.

• A HVDC link can be a firewall against cascading
  disturbances

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• 5) HVDC transmission for controllability of power
  flow.
• Controllability One of the fundamental
  advantages with HVDC is that it is very easy to
  control the active power in the link.
• In the majority of HVDC projects, the main
  control is based on a constant power transfer.
  This property of HVDC has become more important
  in recent years as the margins in the networks
  have become smaller and as a result of
  deregulation in many countries.
• An HVDC link can never become overloaded!
• In many cases the HVDC link can also be used to
  improve the AC system performance by means of
  additional control facilities.
• Normally these controls are activated
  automatically when certain criteria are
  fulfilled. Such automatic control functions could
  be constant frequency control, redistribution of
  the power flow in the AC network, damping of
  power swings in the AC networks etc.
• In many cases such additional control functions
  can make it possible to increase the safe power
  transmission capability of AC transmission lines
  where stability is a limitation.
• Today's advanced semi-conductor technology,
  utilized in both power thyristors and
  microprocessors for the control system, has
  created almost unlimited possibilities for the
  control of the HVDC transmission system.
• Different software programs are used for
  different kind of studies.

22

• Normally a positive sequence program for example
  ABBs SIMPOW (now transferred to STRI AB) or
  PTIs PSS/E program is used for load-flow and
  stability studies.
• For more detailed investigations of the
  performance of the inner control loops of the
  converter and its interaction with nearby network
  is simulated in a full three-phase representation
  program such as PSCAD/EMTDC.

PSCAD/EMTDC is used for detailed investigations
of the performance of a HVDC link.
23
Control room with VDU displays and          
 mimic board at Talcher,India.
FennoSkan HVDC Station, Control room, Rauma,
Finland.
24

• 6)An HVDC transmission limits short circuit
  currents.
• An HVDC transmission does not contribute to the
  short circuit current of the interconnected AC
  system.
• When a high power AC transmission is constructed
  from a power plant to a major load center, the
  short circuit current level will increase in the
  receiving system.
• High short circuit currents is becoming an
  increasingly difficult problem of many large
  cities.
• They may result in a need to replace existing
  circuit breakers and other equipment if their
  rating is too low.
• If, however, new generating plants are connected
  to the load center via a DC link , the situation
  will be quite different.
• The reason is that an HVDC transmission does not
  contribute to the short circuit current of the
  interconnected AC system.

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• 7) Environmental benefits
• Positive effects on the power systems Many HVDC
  transmissions have been built to interconnect
  different power systems by overhead lines or
  cables. By means of these links the existing
  generating plants in the networks more
  effectively so that the building of new power
  stations can be deferred. This makes economic
  sense, but it is also good for the environment.
• There is an obvious environmental benefit by not
  having to build a new power station, but there
  are even greater environmental gains in the
  operation of the interconnected power system by
  using the available generating plants more
  efficiently.
• The greatest environmental benefit is obtained by
  linking a system, which has much hydro generation
  to a system with predominantly thermal
  generation. This has the benefit of saving
  thermal generation ( predominately at peak demand
  ) by hydro generation.
• Also the thermal generation can be run more
  efficiently at constant output and does not have
  to follow the load variations. This can be done
  easily with the hydro generation.

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• A DC line can carry more power than an AC line of
  the same size.

The figure above compares two 3,000 MW HVDC lines
(for the Three Gorges - Shanghai transmission,
China) to five 500 kV AC lines that would have
been used if AC transmission had been selected
27
HVDC technology. The conceptual design of the
classic HVDC converter stations of today dates
back from the mid 1970's, when thyristor valves
were taking over in place of the mercury arc
valves. But there has been a dramatic development
in the performance of HVDC equipment and systems.
28

• A HVDC converter station uses thyristor valves to
  perform the conversion from AC to DC and vice
  versa.
• The valves are normally arranged as a 12-pulse
  converter.
• The valves are connected to the AC system by
  means of converter transformers.
• The valves are normally placed in a building and
  the converter transformers are located just
  outside.
• The 12-pulse HVDC converter produces current
  harmonics (11th, 13th, 23rd, 25th, 35th, 37th
  etc.) on the AC side.
• These harmonics are prevented from entering into
  the connected AC network by AC filters, i.e.
  resonant circuits comprising capacitors,
  inductances (reactors) and resistors.
• The filters also produce a part of the reactive
  power consumed by the converter.
• The HVDC converter also gives rise to voltage
  harmonics on the DC side (12th, 24th, 36th etc.).
  
• A large inductance (smoothing reactor) is always
  installed on the DC side to reduce the ripple in
  the direct current.
• In addition, a DC filter is also normally needed
  to reduce the level of harmonic currents in the
  DC overhead line.
• The harmonics may otherwise cause interference to
  telephone circuits in the vicinity of the DC
  line.

29

• The power transmitted over the HVDC transmission
  is controlled by means of a control system.
• It adjusts the triggering instants of the
  thyristor valves to obtain the desired
  combination of voltage and current in the DC
  system.
• Several other apparatus are needed in a converter
  station, such as circuit breakers, current and
  voltage transducers, surge arresters, etc.
• The conceptual design of the classic HVDC
  converter stations remained unchanged until 1995,
  when ABB introduced HVDC with Capacitor
  Commutated Converters (CCC).

30
Aspects on HVDC Classic performance
1.Reliability and availability 2.Losses
3.Disturbances 4.Fault performance

• 1.HVDC Classic reliability and availability
• Transmission configuration
• The Reliability and availability requirements on
  a particular HVDC transmission are particularly
  high for links supplying major parts of a load
  (e.g. a city or an island) or evacuating a major
  power plant. Where it is essential to have at
  least 50 power if an outage occurs and for large
  size transmissions, a bipolar HVDC transmission
  is the natural choice. For network
  interconnections of moderate size often a
  monopolar configuration is chosen.

• Bipolar HVDC converter stations are designed such
  that there shall be no risk of having a forced
  outage of both poles at the same time.
• The most probable type of line fault a ground
  fault due to lightning, affects only one pole.
• Bipolar HVDC line faults only happens in case of
  a fallen line tower.
• Since bipolar faults are very rare, one can
  regard a HVDC bipole as being equivalent to a
  double circuit AC line from the reliability point
  of view.

31

• B. Maintenance Spares.
• Modern HVDC converter stations require little
  maintenance. Most HVDC stations schedule an
  annual maintenance period at a time when the
  utilization of the transmission is low.
• For a bipolar link, one pole can be serviced
  while the other pole is live.
• Maintenance can also be performed on redundant
  equipment, such as ABBs MACH 2TM , when the link
  is in full operation.
• The majority of equipment in a converter station
  is normal high-voltage and low-voltage equipment
  ( breakers, disconnectors, transformers,
  capacitors, reactors, low-voltage power
  distribution and motor control systems, etc) that
  require normal service.
• To further reduce the scheduled, and forced,
  outage time a facility for Remote Fault Tracing
  and Maintenance is included where the station can
  be monitored from virtually any remote location.

• Spares are normally provided based on experience.
  
• For some items which are essential for the
  operation and which may cause extensive downtime
  if failure happens, a complete unit is normally
  provided for each station. This is normally the
  case for converter transformers, smoothing
  reactors, wall bushings, instrument transformers,
  filter reactors and resistors, etc

32

• 2.HVDC Classic transmission losses
• DC Line
• An optimized HVDC transmission line has lower
  losses than AC lines for same power capacity.
• DC cables
• HVDC cables also have lower losses than AC
  cables.
• One reason for this is that there is no
  dielectric losses in an DC cable as there are in
  AC cables.
• Also the full current capacity can be used for
  the power transmission as there is no 50 or 60 Hz
  charging current that causes conductor losses
  without any contribution to the active power.

• Converter station
• The losses, in the HVDC Classic converter
  stations amount to about 0.6 - 0.7 of the rated
  HVDC transmission capacity (per station) at rated
  load.
• The no-load (standby) losses are about 0.1 . The
  main contributors to these losses are the
  converter transformers ( 50 ) and the thyristor
  valves ( 30 ). The rest comes from the AC
  filters, the smoothing reactor, the station
  service power and the DC filter.
• Loss minimization in AC network
• If the HVDC link is operated in parallel with AC
  lines, there is a possibility to adjust the power
  on the HVDC link to minimize the total grid
  losses.

33

• 3.Disturbances in HVDC Classic transmissions
• The AC/DC conversion process gives rise to
  electromagnetic harmonics of various frequencies.
  These harmonics must be dealt with in order not
  to cause disturbances with communication
  equipment. A converter station also has equipment
  that generates acoustic noise that can be
  disturbing to people in the neighborhood.
• Telephone interference
• Frequencies between 100 Hz and up to say 3 kHz,
  i.e. harmonics within the audible range, can
  cause telephone interference to people close to
  the DC and AC lines coming from the converter
  station.
• The disturbance is then magnetically induced in
  the telephone cable (or wires) running at some
  distance from the high voltage line.
• In order to prevent this AC filters and DC
  filters that suppresses these frequencies are
  included in the station.
• Telephone interference from HVDC stations are
  relatively rare.
• PLC interference
• If power line carrier communications (in the
  range from 20 - 40 kHz up to about 200 kHz) are
  used in the AC grid (or on the DC line) high
  frequency noise from the HVDC converter might
  cause interference. To prevent this, a PLC filter
  can be installed.

34

• Radio interference (RI)
• High frequency noise from the HVDC converter
  might also cause radio interference in the AM
  bands (150 kHz - 30 MHz) in the vicinity of the
  converter station.
• FM radio, TV and mobile phones occupy higher
  frequencies and are not disturbed.
• The way to avoid radio interference is proper
  screening of the valve buildings (or outdoor
  valves).
• In addition small RI-filters are normally
  provided that take care of the RI noise that
  escape from the building via the AC and DC
  bushings.
• Audible noise
• The audible noise that a HVDC converter station
  emits to the surroundings comes mainly from the
  converter transformers, the valve cooling fans,
  the smoothing reactors and the AC and DC filters.
  
• There are a number of methods to mitigate the
  noise 1) orient disturbing equipment away from
  the most sensitive sound direction, 2) use of low
  noise level equipment, 3) screening or enclosing
  equipment .

35
4.HVDC Classic fault performance

• DC overhead line faults
• When a fault (flash-over) occurs on a AC line,
  there are circuit breakers that disconnects the
  line. It is then normally automatically
  re-connected again.
• There are no DC breakers in the HVDC converter
  stations, so when a fault occurs on a DC line
  another method must be applied.
• The fault is detected by the DC line fault
  protection. This protection orders the rectifier
  into inverter mode and this discharges the line
  effectively. After some 80 - 100 ms the line is
  charged again by the rectifier. If the fault was
  intermittent, due to e.g. a lightning strike,
  then normally the line can support the voltage
  and the power transmission continues. Full power
  is then resorted in about 200 ms after the fault.
• But if the fault was due to contaminated line
  insulators there is a risk that re-charging of
  the line results in a second fault.
• Many HVDC transmissions are designed such that
  after a number of failed restart attempts the
  following attempts are made with reduced voltage
  (80 ).

• It should be pointed out that the DC line fault
  clearing does not involve any mechanical action
  and therefore is faster than for an AC line.
• The DC fault current is also lower than the AC
  fault current and therefore the dead time before
  the restart is shorter than for an AC line!
• The reduced voltage restart is also unique for
  HVDC.

36

• DC cable faults
• Cable faults are very rare. They are as a rule
  caused by mechanical damage. Therefore submarine
  DC cables are often buried (except in deep
  waters) to prevent damage from anchors and
  trawls. The same protection action occurs as for
  a DC line but without the restart attempt.
• AC network faults
• When a temporary fault occurs in the AC system
  connected to the rectifier, the HVDC transmission
  may suffer a power loss. Even in the case of
  close single-phase faults, the link may transmit
  up to 30 of the pre-fault power. As soon as the
  fault is cleared, power is restored to the
  pre-fault value.
• When a fault occurs in the AC system connected to
  the inverter, a commutation failure can occur
  interrupting power flow.
• If the AC-fault is temporary the power is
  restored as soon as the fault is cleared.
• A distant fault with little effect on the
  converter station voltage (lt 10 percent) does not
  normally lead to a commutation failure.
• A CCC (Capacitor Commutated Converter) HVDC
  converter can tolerate about twice this voltage
  drop before there is a risk of commutation
  failure.
• Converter station faults
• HVDC converter stations are provided with an
  elaborate protection system that is designed to
  detect fault conditions or other abnormal
  conditions that might expose equipment to hazard
  and/or cause unacceptable disturbances. The
  faulty equipment is taken out of service by the
  protection system.

37
Special Applications of HVDC

• HVDC is particularly suited to undersea
  transmission, where the losses from AC are large.
• First commercial HVDC link (Gotland 1 Sweden, in
  1954) was an undersea one.
• Back-to-back converters are used to connect two
  AC systems with different frequencies as in
  Japan or two regions where AC is not
  synchronized as in the US.

38
Special Applications (continued)

• HVDC links can stabilize AC system frequencies
  and voltages, and help with unplanned outages.
• A DC link is asynchronous, and the conversion
  stations include frequency control functions.
• Changing DC power flow rapidly and independently
  of AC flows can help control reactive power.
• HVDC links designed to carry a maximum load
  cannot be overloaded by outage of parallel AC
  lines.

39
Renewable Energy HVDC

• HVDC seems particularly suited to many renewable
  energy sources
• Sources of supply (hydro, geothermal, wind,
  tidal) are often distant from demand centers.
• Wind turbines operating at variable speed
  generate power at different frequencies,
  requiring conversions to and from DC.
• Large hydro projects, for example, also often
  supply multiple transmission systems.

40
HVDC Solar Power

• HVDC would appear to be particularly relevant for
  developing large scale solar electrical power.
• Major sources are low latitude, and high altitude
  deserts, and these tend to be remote from major
  demand centers.
• Photovoltaic cells also produce electricity as
  DC, eliminating the need to convert at source.

41
Transcontinental Energy Bridges

• Siberia has large coal and gas reserves and could
  produce 450-600 billion kWh of hydroelectricity
  annually, 45 of Japanese output in 1995.
• A 1,800 km 11,000MW HVDC link would enable
  electricity to be exported from Siberia to Japan.
• Siberia could also be linked to Alaska via HVDC.
• Zaire could produce 250500 billion kWh of
  hydroelectricity annually to send to Europe
  (5-6,000 km) on a 30-60,000 MW link.
• Hydroelectric projects on a similar scale have
  been proposed for Canada, China and Brazil.

42
HVDC Installations in the world today
43
New Technologies Needed?

• For transfers of 5,000 MW over 4,000 km, the
  optimum voltage rises to 1,0001,100 kV.
• Technological developments in converter stations
  would be required to handle these voltages.
• Lower line losses would reduce the optimum
  voltage.
• However, environmentalist opposition and unstable
  international relations may be the biggest
  obstacle to such grandiose schemes.

44
HVDC links in India
45
AC harmonic filter area  at Talcher
Converter transformers of one pole at  Talcher,
in front of the valve hall.
46
(No Transcript)
47
Rihand HVDC station, India
Rihand HVDC station, Valve Hall interior, India
48
Vindhyachal HVDC 2250MW Back to back station,
India
Rihand HVDC station, Control room, India