Enhanced Loadability of Existing Ehvac Line by Combined Ac-dc Transmission System

Now-a-days, demand for electric power has shown steady but geographically uneven growth.The wheeling of the available energy through existing long ac lines to load centers has a certain upper limit due to stability considerations. Thermal limit is the major limit to be considered while increasing loadability of existing EHV long transmission lines. Thus, these lines cannot be loaded to their thermal limit to keep sufficient margin against transient instability. This paper presents the concept of composite ac-dc power transmission. The conductors are allowed to carry usual ac along with dc superimposed on it. This will load the transmission lines close to its thermal limit thereby increasing the stability of the system. This paper presents the feasibility of converting a double circuit ac line into composite ac–dc power transmission line to get the advantages of parallel ac–dc transmission to improve stability and damping out oscillations. No alterations of conductors, insulator strings, and towers of the original line are needed. The proposed scheme is digitally simulated with the help of MATLAB software package. Simulation results clearly indicate the substantial increase in loadability of existing EHV AC lines by combined ac-dc transmission.