Additions to the method of critical distances for stochastic assessment of voltage sags

The method of critical distances has been introduced in earlier papers as a fast way of (stochastically) predicting the number of voltage sags at a certain power supply point. The basic expressions are based on a number of assumptions: all impedances are reactive; and only three-phase faults are considered. In this paper, it is shown that the expression used is accurate in almost all cases. The exact expression and a more accurate approximation are derived. Finally, the paper shows how the method can be applied to voltage sags due to single-phase and phase-to-phase faults.