Power Upgrading of Transmission Line by Injecting DC Power in to AC Line with the help of ZIG-ZAG Transformer

We never loaded a long extra high voltage to their thermal limits in order to keep sufficient margin against transient instability. With this scheme proposed in this paper, it is possible to load these lines very near to their thermal limits. Here we inject dc power in to dc transmission line to carry usual ac along with dc superimposed on it. The added 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 advantages of parallel ac–dc transmission to improve stability and damping out oscillations. Simulation is 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.