Stability aspect of a six phase transmission system

Recently, there is a growing interest on the concept of six phase transmission as an alternative to the voltage upgradation of a line, to increase the power transfer capability, within the existing right of way. Allegheny Power System has been investigating the conversion of some of their 138 kV three phase double circuit lines to 138 kV six phase lines as an alternative to 230 kV three phase double circuit lines. So far, the stability aspect of a six phase system has been only estimated approximately from the possible power transfer capability under fault. This paper deals with the stability aspect of a six phase system in greater detail, defining a new transformation and calculation of critical clearing angles under fault and comes to the conclusion that the six phase system is more stable compared to the original and upgraded three phase double circuit lines.