© 2017 IEEE. In this paper, a new formulation is introduced in order to solve the DC-based optimal power flow (DCOPF) problem as well as the security-constrained generation capacity expansion planning (SC-GCEP) problem. The proposed methodology is accomplished taking advantage of the generalized generation distribution factors (GGDF) which are used to transform the transmission limit constraints. With this approach, it is possible to compute the power flows using these linear factors and the power generation of each unit. The preventive security-constrained analysis is included in the optimization problem solving simultaneously both the pre- and post-contingency constraints. In addition, the optimization model considers only one equality equation for the pre- and the post-power balance equation. The obtained mathematical formulation is compared using the partial transmission distribution factor (PTDF) formulation. In contrast to the PTDF-based formulation, the GGDF formulation obtains the same linear distribution factors independent of using any reference (slack) bus. Several examples for solving both the DCOPF and the SC-GCEP problems are figured out in order to determine the performance of the methodology introduced in this study. Different electric power systems are used to demonstrated that this methodology could be applied to model medium- and large-scale power systems.