HIGH VOLTAGE DIRECT CURRENT

Transmitting power at high voltage and in DC form instead of AC is a new technology proven to be economic and simple in operation which is HVDC transmission. The HVDC (High Voltage Direct Current) technology is used to transmit electricity over long distances by overhead transmission lines or submarine cables. It is also used to interconnect separate power systems. A further development in this technology is HVDC light where HVDC light converters are used for faster and efficient conversion of power.

HVDC Light is a fundamentally new power transmission technology developed recently. It is particularly suitable for medium to small-scale power transmission applications. This new transmission and distribution technology, HVDC Light provides an important role to today’s requirements on our network systems and opens up new opportunities for both investors and environmentalist alike.

HVDC light technology for transmission of electric power is introduced in this paper. Its features, advantages and applications are pointed out. HVDC cables, their design, underground laying, their advantages and applications are also added. The paper also gives a note on the advantages of HDVC light cables over AC underground cables.

INTRODUCTION

Competition in the electricity power industry, coupled with continued load growth requires that the existing transmission system assets are utilized more effectively and sometimes closer to their technical limits. As the existing AC lines become loaded closer to their thermal capacity with increasing losses and reduced power quality we face the risk of declining network stability. One solution would be to simply build new, more powerful AC lines.

But, it is getting increasingly difficult to obtain permits to build new high voltage Overhead transmission lines, the right-of-way occupies valuable land. An overhead line change the landscape, causes public resentment and are often met by political resistance. People are increasingly concerned about the possible health hazards of living close to overhead lines.

There are many examples today of public agitation against overhead power lines and the call for them to be buried. Media reports which link living close to power lines with higher cancer risks and leukemia in children don’t help the situation. On the other hand laying an underground cable is an easier process than building an overhead line. A cable doesn't change the landscape and it doesn't need a wide right-of-way. Cables rarely meet with public opposition. There are technical constraints, which limit the distance of traditional AC underground cables to around 80km.And, even though the cost of laying AC cables is rapidly reducing it still costs more than equivalent over head lines.

Currently there is little incentive for putting high voltage lines underground particularly when the Network Service provider is predominantly driven by cost to provide performance-based transmission services at a competitive price. So what is the solution?

HVDC Light technology has the potential to play an important role in achieving this solution. It provides improved power quality and power flow control as well as Introducing extruded DC-cables which have no technical limit to distance which can be installed, and can provide an alternative to overhead lines particularly when the total capital and environmental costs are considered.In Australia, at Direct Link and Murray link, we have two such examples where HVDC Light technology with underground DC-cables has been implemented in a competitive, market-oriented network service.

ADVANTAGES

Flexibility to be expanded

Limits the short circuit power contribution

Supplies reactive power to wind generator, independently to active power it receives.

A meshed DC grid can be built which connects the wind farms

CONCLUSION

HVDC Light is a new technology that has been specifically developed to match the requirements of the new competitive electricity markets. It provides the ability to connect renewable generation to the AC grid. It allows us to supply power to remote locations and islands replacing local diesel generation. It is an ideal vehicle for privately funded developers to link different regions and trade energy. The technical merits are that by virtue of their standardized prefabricated modular constructions which lead to short delivery times, it is relocatable and can be expanded to meet growing demand. Moreover, a key advantage is that it provides accurate control of the transmitted active power and independent control of the reactive power in the connected AC networks.

A pair of lightweight DC cables can be laid direct in the ground in a cost-effective way which is comparable to or less than a corresponding total life cycle cost of AC overhead line. As opposed to an overhead line, an underground cable pair has no visual impact on the landscape. Usually it’s much easier to obtain permission and public approval for an underground cable transmission compared with an overhead line, especially in residential areas. For these reasons HVDC Light provides an important role as a business concept and opens up new opportunities for both investors and environmentalist.