It is well recognized that direct current and direct voltageoffer special advantages for both land and sea cable systems,both with regard to power transmission capability, losses, aswell as possible transmission length due to no capacitivecurrents. As cable systems were used very early in largecities, one of the first applications considered for HVDC wasto use it for city infeed and some schemes were also built.However, it turned out that the cost for the stations was toohigh and that the savings on the cable part were not highenough to justify the high costs of the converter stations,even considering other possible benefits of the HVDC techniquessuch as fast control of active power and almost no contributionto fault currents.
During the 1990s new HVDC Voltage Source Converters, VSC,and new HVDC cables with solid insulation have been developedand the relative cost for the converters has been steadilydecreasing. It was, therefore, found justifiable to reexaminethe feasibility of using HVDC, especially based on the new VSCtechnique, for feeding electrical power to large cities. It wasalso decided, that the study should clarify the specialrequirements that had to be considered in the planning of cityinfeed systems as no good survey could be found. Because ofthis the study has been performed in close co-operation with anumber of utilities responsible for the power supply of somemedium sized and large cities. One such requirement, that alsojustified the study, was that it is expected that in the futuremore overhead lines in the cities or close to the cities haveto be replaced by cables.
Although the transmission and distribution of electricalpower will be preferably made with conventional AC technique,but HVDC transmission would offer special advantages for longtransmission cable, systems with especial requirements withregard to power flow control, systems with restrictions toshort circuit currents, and other relevant issues related tocity center infeed. The use of HVDC transmission to feed powerinto city centers will also be preferable when severerestrictions exist in the system that would require significantadditional measures to mitigate using conventional ACtechnique. In those cases, the cost of these additionalmeasures can be significant enough to justify the use of analternative technique. Or, the implementation of those measureswill make the system too complex to operate. In these cases,HVDC transmission would have advantages over the conventionalAC solution, simplifying the operation of the system orresulting in a more economical solution.
Keywords:HVDC (High Voltage Direct CurrentTransmission), Line Commutated Converters, Voltage SourceConverters, City Center Infeed, Underground CableTransmission.
Stockholm: Elektrotekniska system , 2003. , 170 p.