Voltage stability analysis in conversion of double three-phase to six-phase transmission line

In recent years, problems regarding the voltage stability have been of vital importance at EHV and UHV level because of its sensitivity to active and reactive power changes. Beside, conversion of three-Phase Double-Circuit (3PDC) to six-Phase Single-Circuit (6PSC) line is known as a well method to increase the power transfer capability. The voltage stability problem has received little attention in case of 6PSC systems. In this paper, an investigation of voltage stability has been carried out in conversion of an existing 3PDC to a 6PSC transmission line considering conversion transformers reactance. The results show that by this conversion, improvement in power transfer capacity is gained for long 6PSC lines, but a 3PDC line has the better performs from reactive power view for voltage stability and the minimum line length at which the power transfer capability is limited by voltage stability concern is dramatically decreased in 6PSC line. Moreover, the reactance increase of conversion transformers worsens the aforementioned problems. A comparison of 3PDC and 6PSC systems presented in the paper and other economical and practical factors can be studied for planning, development and design of future transmission network.