Power upgrading by simultaneous AC-DC power transfer in a double circuit AC line

Long EHV AC lines cannot be loaded to their thermal limits to keep sufficient margin against transient instability. With proposed simultaneous AC-DC transmission it is possible to load these line very close to their thermal limits. The conductors are allowed to carry superimposed DC current along with AC. The added DC power flow does not cause any transient instability. This paper presents the feasibility of converting a double circuit AC line into simultaneous AC-DC power transmission line to enhance power transfer as well as to get the advantages of parallel AC-DC transmission. Simulation studies are carried out for the coordinated control as well as independent control of AC and DC power transmission. 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 measures AC current and on-line regulates DC current orders for converters such that conductor current never exceeds its thermal limit