1. High Voltage Direct Current (HVDC)Transmission Systems: An Overview.
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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 transmissions 2. Back-to-back stations 3. 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
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

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 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?: It’s 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 cost 2. Long distance water crossing 3. Lower losses 4. Asynchronous interconnections 5. Controllability 6. Limit short circuit currents 7. 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

9. 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

17. 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.

18. 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

21. 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 ABB’s SIMPOW (now transferred to STRI AB) or PTI’s 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.

25. 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.

26. 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. Spares are normally provided based on experience.
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 ABB´s 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 % 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 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 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 (< 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 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 250–500 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 1,000–1,100 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.

47. Rihand HVDC station, India
Rihand HVDC station, Valve Hall interior, India

48. Vindhyachal HVDC 2*250MW Back to back station, India
Rihand HVDC station, Control room, India