Voltage Stability is one of the factors considered in determining the reliability and efficiency of the operation of the power grid. Unstable voltage variation and voltage collapse can be caused by variation of demand and line outage which might lead to shortage of active and reactive power. Integration of distributed generation (DG) in power systems to improve voltage stability is a commonly used method in the operation of power networks. For efficient utilization of DGs, the investigation of optimal allocation of grid connected DG units is vital research challenge to evaluate the impact of DG on the grid voltage stability. This paper presents a comparative study of the optimal placement for squirrel cage induction generator (SCIG) and doubly fed induction generator (DFIG) based distributed generators with an objective function of enhancing voltage profile. The probabilistic load flow that accounts for load variation has been considered. In addition, the impact of line outage on the system voltage is tackled in the study.