ANALYSIS OF POWER FLOW IN AC-DC TRANSMISSION SYSTEM WITH COMMUNICATION LINK BETWEEN REMOTE TERMINALS

The transient instability of a transmission line imposes severe limits on the amount of power being transferred in EHV ac systems at any given power angle. As the power transfer increases the line tends to become unstable. A combined ac-dc transmission line increases the power transfer capability of the line by considerable margin without causing any transient instability. The transmission line is allowed to carry a superimposed dc along with the ac current without any major changes in the existing line. The additional advantage is that the capacitive VAR of the line can be used to compensate for the leading VAR consumed by the converters. This paper presents the analysis of a combined ac-dc system with respect to the power flow in a converted and an unconverted line. The advantages of parallel ac-dc transmission is achieved with damping of the oscillations and independent control of ac and dc power transmissions. Substantial gain in the loadability of the line is obtained. The master current controller senses the ac current and regulates the dc current orders for the converters on line such that conductor current never exceeds its thermal limit.