An optimal strategy of relaxing bindings first

then expanding lines one by one is proposed by using a transmission planning model considering both static security risk and dynamic security risk with probabilistic N-1 safety criteria. A global optimization framework is developed to resolve the problems caused by the multi-object mathematical model and remarkably reduces the computational burden. Based on the extended equal area criterion (EEAC)

the proposed method can calculate the stability margin and dynamic risks

identify the grid bottleneck

and can provide guidance to the line expansion. A numerical example is presented to illustrate the rationality and validity of the method.