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.

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.

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.

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.

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.

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 .

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.

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 .

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 ).

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.

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 .

Advantages of

dc

transmission

Technical

Advantages Economic

Advantages

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.

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.

Comparison between the prices of AC & DC

Transmission

Types of DC

links

Monopolar

Bipolar

Homopolar

Monopolar link

Bipolar Link

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.

ESSENTIAL PARTS OF HVDC SYSTEM:

AC sub station and HVDC sub station at each

terminal.

Interconnecting HVDC lines.

Electrode lines and earth electrodes.

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.

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.

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.

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.

RECENT ADVANCES:

GTO’s have come into use.

Use of active ac and dc filters.

Advanced fully digital control systems using optical

fibres.

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%.

By

L.A.G.K.Koundinya