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INTRODUCTION
HVDC transmission is widely recognized as being advantageous for long-distance, bulk power delivery, asynchronous interconnections and long submarine cable crossings. The number of HVDC projects committed or under consideration globally has increased in recent years reflecting a renewed interest in this mature technology. New converter designs have broadened the potential range of HVDC transmission to include applications for underground, offshore, economic replacement of reliability-must-run generation, and voltage stabilization.
HVDC APPLICATIONS
HVDC transmission applications can be broken down into different basic categories. Although the rationale for selection of HVDC is often economic, there may be other reasons for its selection.
A. Long Distance Bulk Power Transmission
B. Cable Transmission
C. Asynchronous Ties
D. Offshore Transmission
E. Power Delivery to Large Urban Areas
LONG DISTANCE BULK POWER TRANSMISSION
HVDC transmission systems often provide a more economical alternative to ac transmission for long distance , bulk-power delivery from remote resources such as hydroelectric developments, mine-mouth power plants or large scale wind farms. Higher power transfers are possible over longer distances using fewer lines with HVDC transmission than with ac transmission. Bipolar HVDC lines are comparable to a double circuit ac line since they can operate at half power with one pole out of service but
require only one-third the insulated sets of conductors as a double circuit ac line.
CABLE TRANSMISSION
Unlike the case for ac cables, there is no physical restriction limiting the distance or power level for HVDC
underground or submarine cables. Underground cables can be used on shared ROW with other utilities without impacting reliability concerns over use of common corridors. For underground or submarine cable systems there is considerable savings in installed cable costs and cost of losses when using HVDC transmission. Depending on the power level to be transmitted, these savings can offset the higher converter station costs at distances of 40 km or more.
ASYNCHRONOUS TIES
With HVDC transmission systems, interconnections can be made between asynchronous networks for more economic or reliable system operation. The asynchronous interconnection allows interconnections of mutual benefit while providing a buffer between the two systems. Often these interconnections use back-to-back converters with no transmission line. Asynchronous HVDC links act as an effective “firewall” against propagation of cascading outages in one network from passing to another network.
OFFSHORE TRANSMISSION
Self-commutation, dynamic voltage control and black-start capability allow compact VSC HVDC transmission to serve isolated loads on islands or offshore production platforms over long distance submarine cables. This capability can eliminate the need for running expensive local generation or provide an outlet for offshore generation such as that from wind. The VSC converters can operate at variable frequency to more efficiently drive large compressor or pumping loads using high voltage motors.
POWER DELIVERY TO LARGE URBAN AREAS
Power supply for large cities depends on local generation
and power import capability. Local generation is often older and less efficient than newer units located remotely. Often, however, the older, less-efficient units located near the city center must be dispatched out-of-merit because they must be run for voltage support or reliability due to inadequate transmission. Air quality regulations may limit the availability of these units. New transmission into large cities is difficult to site due to right of way limitations and land use constraints.