This paper presents a voltage sag assessment program. The program provides various functions for stochastic assessment of voltage sags such as short-circuit analysis, the determination of the area of vulnerability and the calculation of expected sag frequency(ESF). Effective data visualization functions based on computer graphics and animation were also implemented in the developed program. In this paper, the concept of voltage sag assessment and the assessment method considering generator scheduling and time-varying fault rates are presented. The influence of generator scheduling and time-varying fault rates on voltage sag prediction is also described by performing case studies using the developed program.