HVDC TRANSMISSION

PRESENTED BY:

J.R.MANIKANTA

P.S.PRAKASH BABU

3rd B.Tech EEE

NARSAPUR INSTITUTE OF TECHNOLOGY

ABSTRACT:

The HVDC transmission systems

have led the way in providing assistance

from a source other than a generator. This

transmission made a modest beginning in

1954. But, since 1972 when the Nelson river

transmission entered service in Canada. The

D.C. System controls have been used to

modulate the D.C. power to help stabilize

the power frequency at either end of the link

and dampen the power oscillations between

weakly connected areas of the A.C. System.

In India D.C. connections between

separate electrical areas have been built to

encourage energy trading. The systems they

interconnect are relatively weak. This paper

explains the effective use of HVDC over

A.C., frequency and system damping

assistance and stability control through

HVDC links.

INTRODUCTION:

• Now a day’s large blocks of power

are needed to be transmitted.

• There arise 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.

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 .

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(loadcarryingcapabilit

y of it’s supply line ).

TYPES OF DC LINKS:

MONOPOLAR

BIPOLAR

BACK TO BACK

MONOPOLAR LINK:

One rectifier terminal is connected to

earth ,the other higher or lower

potential is connected to the

transmission line

ADVANTAGES OF

MONOPOLAR LINK:

Most common type for moderate

power HVDC

Modern versions of monopole carry

1500MW for overhead transmission

lines

600MW for underground or

underwater systems

Simple and cheap

Only require two convertors and one

high-voltage insulated cable

BIPOLAR LINK:

Two opposite polarity, high potential

conductors are used.

More expensive than monopole due

to required full insulation of the

lines.

ADVANTAGES OF

BIPOLAR LINK:

Carry up to 3200MW at 600 kv

Negligible earth current flows under

normal load, which reduces

environmental effects and return loss

Conductors may be on separate

transmission towers to prevent both

being damaged at once by harsh

conditions

BACK-TO-BACK LINK:

1. A short dc line where static inverters

and rectifiers operate together

2. Dc voltage along the intermediate

circuit may be selected

ADVANTAGES OF BACK-

TO-BACK LINK:

Couple different frequency

electricity mains

Couple two networks with varying

phase relationship but the same

frequency

Change frequency and phase number

like traction convertor plant

ESSENTAIL PARTS OF HVDC

SYSTEM:

AC substation and HVDC substation

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.

ADVANTAGES OF DC

TRANSMISSION:

THECHNICAL ADVANTAGES

ECONOMICAL

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:

RECENT ADVANCES:

GTO’s have come into use.

Use of active ac and dc filters.

Advanced fully digital control

systems using optical fibers.

Future implications:

HVDC may be the super grid

solution

1. European super grid

2. Trans America grid

Possible conversion of existing 3-

phase ac lines into bipolar dc with

neutral return

Potential improvements in

Convertor

HVDC circuit breaker

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%.