Feasibility Study of Conversion of Double Circuit ac Transmission Line for Simultaneous ac-dc Power Transmission

Long EHV ac lines cannot be loaded to its thermal limit to keep sufficient margin against transient instability. EHV lines may be loaded to a very high value if the conductors are allowed to carry superimposed dc current along with ac current. The added dc power flow does not cause any transient instability. In HVDC line the capacitive VAR of the line cannot be tapped to compensate a part of the lagging VAR consumed by the converters. Present paper studies the feasibility of converting a double circuit ac line for simultaneous ac-dc power flow to get the advantages of parallel ac-dc transmission to improve stability and damping oscillations as well as for 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. Unlike HVDC line, capacitive VAR of the line is available to compensate a part of the lagging VAR.