Security boundary characterizes the operations range of a distribution system satisfying N-1 security, and this paper proposes three judgment theorems for it. Firstly, the relevant concepts of the security boundary are introduced. The security boundary represents the set of all the critical secure operating points in the state space of a distribution system. Based on the degree of criticality, it can be classified into two types, i.e., strict and non-strict boundaries. Secondly, the load relationships between inte…