A failure model for distribution systems

When distribution systems have multiple sources of supply for key loads there can be a significant increase in the overall reliability. For key systems a common design has been that no loss of supply should be experienced for any single contingency. This is referred to as N-1 reliability. When the peak load is only experienced for a short time there can be gains in capital expenditure from delaying investment in additional equipment even when peak loading is above N-1 levels. If the load dependence of subsequent failure is included in the analysis there can be a significant rise in the probability of cascading failure particularly if emergency loading levels are high. This analysis indicates that if leaving N-1 design safety careful analysis of the possibility of cascading failures can be critical.