This paper is Part I of a two part paper set  dealing with the investigation of the effects of differing investment programs and macroeconomic parameters-such as interest rate and inflation rate-on long-term power transmission system expansion planning. The uncertainties associated with the fixed capital costs is also discussed. For this purpose, the rates based an various development models were used for comparing alternative capital investment plans. The obtained results provide quantitative evaluation and comparison of different economic and financial models. Case studies with a reduced Turkish Western Anatolian interconnected System are presented.