01-03-2013, 03:03 PM
High Voltage Direct Current (HVDC)Transmission Systems: An Overview.
High Voltage.ppt (Size: 1.61 MB / Downloads: 259)
What is HVDC?
HVDC stands for High Voltage Direct Current and is today a well-proven technology employed for power transmission all over the world. In total about 70,000 MW HVDC transmission capacity is installed in more than 90 projects.
The HVDC 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, where traditional alternating current (AC) connections can not be used.
There are three different categories of HVDC transmissions: 1. Point to point transmissions2. Back-to-back stations3. Multi-terminal systems
Historical Perspective on HVDC Transmission
The first commercial electricity generated (by Thomas Alva Edison) was direct current (DC) electrical power.
The first electricity transmission systems were also DC systems.
However, DC power at low voltage could not be transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
Nevertheless, with the development of high voltage valves, it was possible to once again transmit DC power at high voltages and over long distances, giving rise to HVDC transmission systems.
The Classic HVDC Transmission
Using HVDC to interconnect two points in a power grid, in many cases is the best economic alternative, and furthermore it has excellent environmental benefits.
The HVDC technology (High Voltage Direct Current) is used to transmit electricity over long distances by overhead transmission lines or submarine cables.
It is also used to interconnect separate power systems, where traditional alternating current (AC) connections can not be used.
In a high voltage direct current (HVDC) system, electric power is taken from one point in a three-phase AC network, converted to DC in a converter station, transmitted to the receiving point by an overhead line or cable and then converted back to AC in another converter station and injected into the receiving AC network.
Typically, an HVDC transmission has a rated power of more than 100 MW and many are in the 1,000 - 3,000 MW range.
HVDC transmission for lower investment cost.
A HVDC transmission line costs less than an AC line for the same transmission capacity. However, the terminal stations are more expensive in the HVDC case due to the fact that they must perform the conversion from AC to DC and vice versa. But above a certain distance, the so called "break-even distance", the HVDC alternative will always give the lowest cost.
Relative Cost of AC versus DC
For equivalent transmission capacity, a DC line has lower construction costs than an AC line:
A double HVAC three-phase circuit with 6 conductors is needed to get the reliability of a two-pole DC link.
DC requires less insulation ceteris paribus.
For the same conductor, DC losses are less, so other costs, and generally final losses too, can be reduced.
An optimized DC link has smaller towers than an optimized AC link of equal capacity.
Increased Benefits of Long-Distance Transmission
Long distance transmission increases competition in new wholesale electricity markets.
Long distance electricity trade, including across nations, allows arbitrage of price differences.
Contractual provision of transmission services demands more stable networks.
Bi-directional power transfers, often needed in new electricity markets, can be accommodated at lower cost using HVDC