Stochastic prediction of voltage sags in a large transmission system

In this paper we discuss two stochastic assessment methods for voltage sags and apply them to a 98-bus model of the 400 kV National Grid of England and Wales. The method of fault positions is most suitable for implementation in a software tool. It has been used to get exposed areas and sag frequencies for each bus. The results are resented in different ways, including a so-called voltage sag map showing the variation of the sag frequency through the network. The method of critical distances is more suitable for hand calculations, as both the amount of data and the complexity of the calculations are very limited. It has been used to obtain sag frequencies for a number of buses. A comparison with the results obtained by using the method of fault positions, shows that the method of critical distances is an acceptable alternative were software or system data are not available for a more accurate analysis.