Investigation on the suitability of equivalent models of subtransmission level for security analyses at transmission level

•Extended Wards can be exactly adopted by the developed process to other power flows.•To cover (n−0) power flows in the distribution system one Extended Ward is needed.•Significant high-voltage line outages increase the number of needed equivalents.•High-voltage line outages in one corridor can be represented by one Extended Ward.•For most high-voltage grids a lower two-digit number of equivalents is sufficient. This paper deals with methodical investigations about the suitability and needed demand of equivalent models of the subtransmission level for security analyses in the transmission network. Based on available public data the investigations are carried out in a detailed and cross-voltage network model of the German transmission system with underlaying realistic distribution grids. The examined equivalents are built with the Extended Ward approach. As the results show, the (n−1) system state of the transmission grid is accurately achievable by the equivalents even for different grid structures and power flow cases. A created equivalent can also be adapted to all (n−0) power flow cases in the distribution grid reducing the number of needed equivalents. Moreover, for representing (n−1) distribution line outages with potential impact on the transmission system a low two-digit number of equivalents is needed.