HIGH VOLTAGE D.C. TRANSMISSION SYSTEM

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WHAT IS HVDC ?

In a HVDC (High voltage D.C.) transmission system, electricity is taken from an AC power network, converted to DC in a converter station and transmitted to the receiving point by a transmission line (overhead) or cable(underground). It is then converted back to AC in another converter station and injected

into the receiving AC network. HVDC enables the power flow

to be controlled rapidly and accurately, and improves the performance, efficiency and economy of the connected AC networks.

POINTS TO BE DISCUSSED

Introduction. History of HVDC. HVDC goes global. India adopts HVDC. Why HVDC ? Advantages of HVDC over HVAC:• Disadvantages of HVDC.• Cost of HVDC.• Components of HVDC.• Types of HVDC links.• Application of HVDC.• Recent advances in HVDC technology.• Conclusion.

INTRODUCTION

• Now a days large blocks of power are needed to be transmitted over very long distances.•There arises some technical problems of transmitting power to such a long distance using A.C.•In the view of the drawbacks of a.c system the HVDC transmission has come into picture.•It is useful for bulk power transmission and interconnecting incompatible grids.

History of HVDC

Firstly concept was developed by a swiss engineer ‘Rene Thury’ during the end of 19th century.

Earlier mercury-arc valves were used in Converters which later get replaced by Thyristor-valves.

In 1941 a 60MW 115km ‘Elbe-project’ was designed in Berlin(Germany),but couldn’t be completed. Above equipments were moved to Soviet Union & in 1951, first HVDC link was established between Moscow and Kashira. However in 1954 first successful HVDC 20MW 96km commercial installation was done between the Gotland island and Sweden.

This transformer converted the first HVDC transmission in Gotland. This transformer converted

the first HVDC transmission in gotland

HVDC goes Global

Currently there are some more than 100 of HVDC systems across the whole world ,out of which 6 are in India.

Some of them are as follows: -Between Sweden & Denmark. -Between south & north islands of New Zealand. -Between Italy & Sardinia. -Between Los Angeles & Oregon. -Between north African deserts & central Europe. -Between UK & western Europe.Proposal of a.c transmission across the English

channel was abandoned and get replaced by HVDC system

HVDC IN INDIA

In India presently 6 HVDC systems are in operation.India- Pioneer developer of HVDC since 1990 with the first HVDC

set up of 1000MW 814Km Rihand-Dadri line between U.P & Rajasthan.

2000MW 1440Km Talcher-Kolar link is biggest so far connecting four states-orissa,andhra pradesh,tamilnadu and karnataka.

In march 2000,Power-grid(largest power transmission utility of India) awarded a new long distance HVDC transmission project to SIEMENS,Germany.

More greater plans are cooking to add 1lakh MW of power by 2012 in which HVDC will play vital role to transmit these powers to all corners of India.

Main data

Commissioning year: 1990

Power rating: 1 500 MW

No. of poles: 2

AC voltage: 400 kV (both ends)

DC voltage: ±500 kV

Length of overhead DC line:

814 km

Main reason for choosing HVDC:

Long distance, network stability

   

 

FACTS & FIGURES

Data of Rihand-Dadri line Data of Talcher-Kolar line Commissioned year 1990

Power rating 1500MWNo. of poles 2AC voltage 400KV(both

ends)DC voltage 500KVLength of overhead DC line

814Km

Reason for choosing HVDC

Long distance, Network stability.

Commissioned year

14th Feb 2003

Power rating 2000MW

No. of poles 2

No. of towers 5651

Amount of steel 10,000 metric tone

Amount of cement 80,000 metric tone

Length of line 1400Km

Cost of project Rs.700 crore

Comparison between the prices of AC & DCTransmission

ADVANTAGES

Advantages of HVDC can be classified as:

Advantages of dc transmission

Technical Advantages

EconomicAdvantages

Technical Advantages

Reactive power requirementSystem stabilityShort Circuit Current Independent Control of ac systemFast change of energy flowLesser Corona Loss and Radio interferenceGreater 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.

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 linkDC line in Parallel with AC link.Multi-Terminal DC link.

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.

CONVERTER STATION EQUIPMENT

Thyristor valvesConverter TransformerDC ReactorHarmonics Filtering EquipmentControl EquipmentReactive power compensation

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.

ADVANCES RECENT

GTO’s have come into use.Use of active ac and dc filters.Advanced fully digital control systems using

optical fibers.

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%.