23-12-2010, 04:37 PM
SUBMITTED BY:B.VYSHNAVI
HVDC LIGHT.ppt (Size: 1.12 MB / Downloads: 177)
INTRODUCTION
Competition in the electricity power industry, coupled with continued load growth require that the existing transmission system assets are utilised more effectively and some times closer to their technical limits.
One solution would be to simply build new, more powerful AC lines.
It is getting increasingly difficult to obtain permits to build new high voltage overhead transmission lines, the right-of-way occupies valuable land.
People are increasingly concerned about the possible health hazards of living close to overhead lines.
EVOLUTION OF HVDC LIGHT TECHNOLOGY
The reforms that have or are being introduced in the electricity sector in many countries have radically changed the business environment of the electricity supply industry .
Recent development efforts in transmission technology have focussed on compact, small weight and cost-effective, so-called voltage source converters (VSC ).
By combining the advances made in VSC and DC cables, a new breed of electricity transmission and distribution technology emerges: The "HVDC Light" technology.
Transmission of electricity over long distances using underground DC cables is both economical and technically advantageous .
Laying an underground cable is an easier process than building an overhead line.
Hvdc light technology is also known as “the invisible power transmission” since it is based on underground cables.
Possible application fields include the feeding of distant loads and the connection of distant generation plants.
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.
HVDC LIGHT TECHNOLOGY
HVDC Light is a high voltage, direct current transmission technology and is well suited to meet the demands of competitive power market for transmission up to 330MW and for a DC voltage in the ± 150kV range.
It is a well-proven and environmentally-friendly way to design a power transmission system for a submarine link, an underground cable system or network interconnection.
HVDC Light® can rapidly control both active and reactive power independently of each other, to keep the voltage and frequency stable.
An HVDC Light transmission system can do much more than just transmit power between two points – it can also improve conditions in the connected AC networks.
HVDC Light converter station design is based on a modular concept.
It consists of two AC to DC converter stations and a pair of underground cables interconnecting each converter station .
The site areas needed for converter stations is also minimal.
The AC to DC converters employ the latest in power semiconductor technology, the IGBT (Insulated Gate Bipolar Transistor).
This technology provides the HVDC Light converter with a switching speed 27 times faster than a traditional HVDC, thyristor controlled converter.
This fast control makes it possible to create any phase angle or amplitude which can be done almost instantaneously providing independent control of both active and reactive power.
While the transmitted active power is kept constant the HVDC Light converter can automatically control the voltage of the connected AC network by compensating the generation and consumption of reactive power within the capacity of its rating .
HVDC LIGHT CABLE
The HVDC Light cable is a new design triple extruded, polymeric insulated DC-cable, which has been successfully type tested to150kV DC, following a comprehensive R & D program .
The cables are operated in bipolar mode, one cable with positive polarity and one cable with negative polarity.
By operating the cables with anti-parallel currents, the overall magnetic field of the cables is nearly eliminated, which is another positive aspect of HVDC Light technology.
The main advantage of ‘HVDC Light’ cables over their HVAC counterparts is their reduced weight and dimensions, which result in a higher power density.
This strength and flexibility make the HVDC Light cables perfect for severe installation conditions:
The land cables can be installed less costly with ploughing technique.
The submarine cables can be laid in deeper waters and on rough seabeds.
HVDC cables can also be installed as overhead cables.
HVDC LIGHT CONVERTER TECHNOLOGY
Conventional HVDC converter technology is based on the use of line-commutated or phase-commutated converters (PCC). With the appearance of high switching frequency components, such as IGBT’s (Insulated Gate Bipolar Transistor) it becomes advantageous to build VSC (Voltage Source Converters) using PWM (Pulse Width Modulation) Technology.
HVDC Light uses Pulse Width Modulation to generate the fundamental voltage.
It controls the magnitude and phase of the voltage freely and almost instantaneously and allows independent and very fast control of active and reactive power flows.
HVDC LIGHT CONVERTER SYSTEM
The key part of the HVDC Light converter consists of an IGBT valve bridge.
No special converter transformers are necessary between the valve bridge and the AC-grid
A converter reactor can separate the fundamental frequency from the raw PWM waveform.
If the desired DC voltage does not match the AC system voltage, a normal AC transformer may be used in addition to the reactor.
A small shunt AC-filter is placed on the AC-side of the reactor.
On the DC-side there is a DC capacitor that serves as a DC filter.
ACTIVE AND REACTIVE POWER CONTROL AND BLACKOUT CAPABILITY
The fundamental voltage across the converter reactor defines the power flow between the AC and DC sides.
Changing the phase angle between the fundamental frequency voltage generated by the converter and the voltage on the AC bus controls the active power flow between the converter and the network.
The reactive power flow is controlled by the width of the pulses from the converter bridge.
A VSC transmission system will be a very valuable asset during a grid restoration which is exposed to the blackout
It will be available almost instantly after the blackout and does not need any short circuit capacity in order to become connected to the grid.
ADVANTAGES OF DC UNDERGROUND CABLES COMPARED WITH OVERHEAD POWER LINES
Once it's installed the cable route can be replanted with Native vegetation.
Reduced environmental impact.
Underground cables rarely meet with public opposition and often receive political support.
The system reliability is enhanced with reduced risk of damage from natural causes such as storms, wind, earthquakes and fire. You simply bury it and forget it.
Operation and maintenance costs of the transmission easement are virtually eliminated.
The width of the corridor to install the underground cable can be as narrow as 4 metres, which will give greater flexibility with the selection of a transmission route.
APPLICATIONS
Connection to a wind farm
Connection of small dispersed electricity generators to a grid
Delivery of electricity to islands
Feeding electric power to large and rapidly growing cities
Feeding of electric power to remotely located loads