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A recently proposed concept of simultaneous ac-dc power transmission enables the long extra high-voltage ac lines to be loaded close to their thermal limits. The conductors are allowed to carry a certain amount of dc current superimposed on usual ac. This paper presents the feasibility of small power tapping from composite ac-dc power transmission lines which would pass over relatively small communities/rural areas having no access to a major power transmission network. The proposed scheme is digitally simulated with the help of a PSCAD/EMTDC software package. Simulation results clearly indicate that the tapping of a small amount of ac power from the composite ac-dc transmission line has a negligible impact on the normal functioning of the composite ac-dc power transmission system. [ABSTRACT FROM AUTHOR]

Long extra high voltage (EHV) ac lines cannot be loaded to their thermal Limits in order to keep sufficient margin against transient instability. With the scheme proposed in this paper, it is possible to load these lines very close to their thermal limits. The conductors are allowed to carry usual ac along with dc superimposed on it. The added dc power flow does not cause any transient instability. This paper presents the feasibility of converting a double circuit ac line into composite ac-dc power transmission line to get the ad- vantages of parallel ac-dc transmission to improve stability and damping out oscillations. Simulation and experimental studies are carried out for the coordinated control as well as independent control of ac and dc power transmissions. No alterations of conductors, insulator strings, and towers of the original line are needed. Substantial gain in the loadability of the line is obtained. Master current controller senses ac current and regulates the dc current orders for converters online such that conductor current never exceeds its thermal limit. [ABSTRACT FROM AUTHOR]