HVDC transmission has advantages over long distance HVAC transmission. The first HVDC link was set up between Sweden and Gotland in 1954 using mercury arc valves. Since then HVDC technology has advanced with the development of thyristors. HVDC systems use converter stations with transformers at each end to convert between AC and DC and allow bidirectional power flow by changing the firing angle of the thyristors. They have benefits like reduced transmission losses, improved voltage profile and increased transmission capacity.

2.  Now a days large blocks of power are needed to be
transmitted.
 There arises some technical problems of transmitting power to
such a long distance using ac.
 In the view of the draw backs of ac the HVDC transmission
has come into picture.

3. HISTORY:
 IN the initial stages dc used to be employed for generation
,transmission and distribution of electric power.
 The first 110V D C central electric station was installed by
Edison in New York in1882.with the introduction of
transformer and 3-phase ac system the situation changed
in favours of ac.

4. HISTORY
 for many reasons ,such as facility of transformations of
voltages from one level to another ,better performance of ac
motors and superiority of ac generators in comparison to dc
generator ,the power was generated ,transmitted, distributed
and utilized in the form of ac.
 But the supporters of DC carried out extensive research on dc
field And found the advantages of dc in high and extra high
voltage of transmission for long distance.

5.  The first dc link was set up in 1954 in between
Swedish main land and the island of Gotland.
 The use of an HVDC link in an ac system requires
converter stations at each end of the line.

6.  with the successful development of high power thyristors the
HVDC transmission become technically and commercially
viable alternative to EHV/UHV-AC transmission.
 The first dc link was setup in 1954 between Swedish mainland
and island of got land .This was a Monopolar
,100KV,20MW,cable system making use of sea return .the
England –France cross Channel dc link was commissions
in1961.this was bipolar +-100KV,160MW,Cable system over a
distance of 65 KM .since then more and more HVDC system
have been set up in the world.

7.  HVDC Transmission has also been introduced in India .A +-
500kv ,1500mw,810km bipolar HVDC line has already been
set up between Rihand and Delhi. it is designed to operate in
bipolar ,Monopolar ground return ,and Monopolar metallic
return mode.
 Vindhyachal HVDC back to back system for exchanging power
between northern and western region in either direction .

8.  Chandarpur-padghe bipolar system (-
+500kv,1500mw,736km),Jeypore back to back
project(2*250mw),Ana Mau back to back project
(2*250mw)are also proposed .
 At present the world has over 50 HVDC schemes in operation
for a total capacity of more than 50,000MW and the capacity is
increasing by about 2000MW every year.

9. why high voltage for transmission? :
 why high voltage for transmission?
 Because with increase in transmission voltage with
same power supply the current in the conductor
decreases. so size of the conductor decreases and
since current in the conductor decrease so losses
are decreases and hence efficiency of the line
increases. so high voltage is used in transmitting the
power .also the voltage regulation will improve.
 Reduction of electrical losses, increase in
transmission efficiency ,improvement of voltage
regulation and reduction in conductor material
requirement.
 Line losses are reduced since line losses are
inversely proportional to transmission voltage .

10.  Transmission efficiency increases because of
reduction in line losses .
 Voltage regulation will improve because of reduction
of percentage of line drop .
 Size of the conductor material required is inversely
proportional to the square of supply voltage .
 Flexibility for future system growth.
 Increase in transmission capacity of the line .
 Increase of surge impedance loading(load carrying
capability of it’s supply line ).

11. Disadvantage of HVAC?
 with increase in distance and voltage the problems
associated with ac systems increase. these problems are
of
 Corona loss and radio interference.
 Heavy supporting structure and erection difficulties .
 Insulating problem .
 Stability problem.
 Current carrying capacity.
 Ferranti effect.
 Surge impedance loading.
 Mechanical vibrations and oscillations.
 Radio interference.

12. PRINCIPLE OF OPERATION OF HVDC LINE
 It required two ac system which are to be connected by
dc link.
 Step-up transformer at sending end and step-down
transformer at receiving end.
 Two Converter station one at sending end for converting
ac to dc and other at receiving end for converting dc to
ac.
 By changing the firing angle it act as rectifier(0 to 90)
deg. And inverter (90 to 180)deg. .so it can operate in
bidirectional flow of power.
 The dc out-put voltage magnitude can be controlled by
changing the firing angle of the converter.
 In practical HVDC converter station three-phase bridge
converter are employed at both end .

14. Advantages of
dc
transmission
Technical
Advantages
Economic
Advantages

15. Technical Advantages
 Reactive power requirement.
 System stability.
 Short Circuit Current.
 Independent Control of ac system.
 Fast change of energy flow.
 Lesser Corona Loss and
 Radio interference Greater Reliability.
 No limits in transmitted distance Direction of power
flow can be changed very quickly.

16. Economic advantages:
 DC lines and cables are cheaper than ac lines or
cables.
 The towers of the dc lines are narrower, simpler and
cheaper compared to the towers of the ac lines.
 Line losses in a dc line are lower than the losses in
an ac lines.

17. Comparison between the prices of AC & DC
Transmission

18. Types of DC
links
Monopolar
Bipolar
Homopolar

19. Monopolar link

20. Bipolar Link

21. Incorporating HVDC into AC systems:
 Two terminal DC link point to point transmission.
 Back to Back DC link DC line in Parallel with AC link.
 Multi-Terminal DC link.

22. ESSENTIAL PARTS OF HVDC SYSTEM:
 AC sub station and HVDC sub station at each
terminal.
 Interconnecting HVDC lines.
 Electrode lines and earth electrodes.

23. MAIN PARTS OF HVDC TERMINAL
SUBSTATION
 AC switchyard.
 Thyristor valves.
 Converter Transformer.
 DC Reactor.
 Harmonics Filtering Equipment.
 Control Equipment.
 Reactive power compensation.
 HVDC yard.
 Electrical and mechanical auxiliaries.

24. GROUND RETURN
 Most dc transmission lines use ground return for
reasons of economy and reliability Ground return are
used by the Monopolar and the bipolar link for
carrying the return current.
 The ground path has a low resistance and, therefore
low power loss as compared to a metallic conductor
path provided the ground electrodes are properly
designed.
 The resistance of the ground path is independent of
the depth of the line.

25. PROBLEMS
 The Design of grounding electrodes for low cost of
installation and maintenance.
 Location and screening of electrodes so that ground
currents cause negligible electrolytic corrosion of
buried and immersed metallic structures.

26. EARTH ELECTRODE
 HVDC system requires a properly designed earth
electrode at each station.
 The electrode is situated at a safe distance (5 to 30
km) from the station.
 The earth electrode at one of the station acts as a
anode and at the other end acts as a cathode.

27. RECENT ADVANCES:
 GTO’s have come into use.
 Use of active ac and dc filters.
 Advanced fully digital control systems using optical
fibres.

28. CONCLUSION
 Recent studies indicate that HVDC systems are very
reliable.
 The data collected from 31 utilities says that forced
unavailability of energy due to the converter station
is 1.62%.
 The scheduled unavailability of energy is about
5.39%.

29. By
L.A.G.K.Koundinya