Power transfer capability of long transmission lines with midpoint sited FACTS and HVDC

Abstract

Long transmission lines are required to transmit power from remote generation sites to the population centers. When transmission is by AC, the large reactances of the long lines limit the power transmissibility. By siting FACTS controllers or HVDC stations, which have strong voltage support, at the midpoint of the lines, the transmission distance between voltage supported nodes is halved and so is the line reactance. Thus, the maximum steady-state power can be twice that of the original lines. Digital simulation comparison of a realistic AC transmission system shows that the gain factor, based on the steady-state stability limit, is only 1.59 and not 2. This paper reports on analysis undertaken to account for the discrepancy. The analysis shows that the distributed capacitance in the original long transmission line is already providing voltage support. For this reason, the voltage support by the midpoint sited FACTS controllers or HVDC stations becomes less significant. Furthermore, the FACTS controllers or HVDC stations are inserted at the midpoint through transformers, whose leakage reactances add to the line impedance.