HVDC TRANSMISSION

By

Gautam Prabhakar

Ishaan Malhotra

Soham Banerjee

INTRODUCTION

A high-voltage, direct-current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common ac systems.

For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses.

For shorter distances, the higher cost of DC conversion equipment compared to AC system works against HVDC.

History

The modern form of HVDC developed in 1930s in Sweden.

Earlier commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira.

A 10-20 MW system between Gotland and Sweden in 1954.

The longest HVDC link currently is Inga-Shaba,1700 kms,600 MW in Congo.

HVDC IN INDIA

Pioneer Developer of HVDC since 1990 First major project-1000 MW Rihand - Dadri line in

92’ 2000 MW Talcher - Kolar link is the biggest

developed by power grid - spans four states: Orissa, Andhra Pradesh, Tamil Nadu and Karnataka

International Projects-a 400 KV HVDC cable being laid from the Indian sea coast to the Sri Lankan sea coast (39 km-undersea).

OTHER PROJECTS

HVDC back-to-back links in Sasaram, vizag and vindhayachal

Chandrapur - Padghe 500KV, 1500MW line ballia to bhiwadi 2500 MW line

Dadri Project

Types of HVDC Configuration

Monopolar Bipolar Homopolar Back to Back

Other Projects

Monopolar

Monopolar link has one conductor (ground as return).

Metallic return can also be used where concerns for harmonics and corrosion exist.

It is normally operated in negative polarity

Bipolar

The bi polar arrangement uses two poles, one positive pole and other negative pole at each conversion substation.

The mid-points of converter are earthed, the current carried by the ground, is however less.

If one of the poles is out of service, the bi polar arrangement can be used as a mono polar arrangement.

Bi polar arrangement is universally used for bulk power HVDC overhead transmission and also for overhead lines for interconnection.

Homopolar

In this, two conductors having same polarity can be operated with ground or metallic return.

It has the advantage of reduced installation cost.

It has the disadvantage of earth return due to which bipolar is usually preferred.

Back to Back

A Back to Back arrangement is used when two asynchronous ac systems need to be interconnected for bulk power transmission or for ac stabilisation reasons.

In it, no overhead lines separating rectifier and inverter are there. Hence, transmission electrical losses on the DC side can be neglected.

The DC voltage is kept low, the DC current high to optimize the costs of thyristor valves as well as DC equipments.

Analysis of HVDC Converters

INTRODUCTION

A HVDC system requires an electronic converter for converting electrical energy from AC to DC and vice versa.

There are basically two configurations types of three phase converters:– Current Source Converter (CSC)– Voltage Source Converter (VSC)

Cont…

During the period (about) 1950-1990s, HVDC systems used the CSC configuration almost exclusively.

The traditional CSC utilized mercury-arc valve from the early 1950s to the mid-1970s, and thereafter, the thyristor valve became the fundamental switching device.

From 1990 onwards, VSC became economically viable due to the availability of new self-commutating high-power switches such as GTOs and IGBTs.

Comparison of VSC and CSC

On AC sideCURRENT SOURCE CONVERTER

Acts as a constant voltage source. Require a capacitor as the energy storing

device Requires large AC filters for harmonic

elimination Requires reactive power supply for pf

correction.

On AC sideVOLTAGE SOURCE CONVERTER

Acts as a constant current source. Require a inductor as the energy storing

device Requires only a small AC filter for harmonic

elimination Reactive power supply is not required.

On DC sideCURRENT SOURCE CONVERTER

Acts as a constant current source. Require a inductor as the energy storing

device Requires DC filters. Provides inherent fault current limiting

features.

On DC sideVOLTAGE SOURCE CONVERTER

Acts as a constant voltage source. Require a capacitor as the energy storing

device The capacitor provides DC filtering at no

extra cost.

Switches CSC

Line commutated or forced commutated with series capacitor

Switching occurs at line frequency Lower switching losses

Switches VSC

Self commutated Switching occurs at high frequencies High switching losses

ADVANTAGES and APPLICATIONS

There is no limit for transmission distance both for OH and underground cables.

Very fast control of power flow, which implies more stability in the system.

Used in undersea cables, where high capacitance causes additional losses in AC sys.

Facilitates synchronism, i.e. power transmission between different countries that use AC at differing voltages or frequencies.

Cont..

Connecting a remote generating plant to the distribution grid,

thinner conductors can be used as there is no skin effect

DISADVANTAGES

For the same rating, the investment for a HVDC converter station is several times higher than reqd. for an AC substation. Thus it isn’t economical for smaller transmission distance.

A converter acts as a source of harmonic currents and voltages, thus distorting current and voltage waveforms.

Without current zero-crossing point, DC circuit breakers are difficult to manufacture

Cont..

It requires many spare parts to be kept, as HVDC systems are less standardized than AC.

In contrast to AC systems, realizing multiterminal systems is complex and costly as a control station is reqd. wherever power is to be tapped.

THANK YOU!!!