Transmission system planning in the electricity market environment has to be confronted with much more uncertain factors

such as the locations and capacities of new power plants

and the electric power demand. In order to ensure a flexible transmission system

this paper extends the transmission planning model

which considers both static security risk and dynamic security risk to deal with uncertain injection power at the buses. A global optimization framework is developed to resolve the problems caused by the multi-object mathematical model and uncertain power injections. The resolving method based on Monte Carlo simulation and the optimal strategy of relaxing bindings first then expanding lines one by one is given. A numerical example is presented to illustrate the validity of the method.