I. Introduction
The ability to accurately forecast the load is an important requirement in electric power system planning and operating. It is impossible, however, to exactly forecast the load at some future time and therefore some degree of load forecast uncertainty will always exist. This is particularly true in the case of long-range planning which looks into conditions extending over the next decade. Considerable work has been done on the incorporation of load forecast uncertainty in generating capacity adequacy evaluation [1]–[3] and in unit commitment risk assessment [4]. Load forecast uncertainty can have a significant effect on the calculated reliability indices in a generating capacity study and in general it requires a higher capacity reserve to satisfy a future uncertain load than it does to meet a future known load at a specified level of reliability [2]. Load forecast uncertainty considerations can be extended to incorporate the power demand variations at the individual load points in a bulk electric system. in these cases, the conventional generation system analysis used in a Hierarchical Level I (HLI) study is extended to include the transmission facilities. This is known as composite generation and transmission or Hierarchical level II analysis [2]. There has been relatively little work on incorporating load forecast uncertainty in this area [5] relative to that done at HLI. The inclusion of load forecast uncertainty in bulk electric system reliability evaluation is of practical importance and it is necessary to consider both the system load forecast uncertainty and the correlation between the individual buses [5].