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The use of the Transient Energy Function (TEF) method in transient stability analysis is extended to include an ac/dc system with a two terminal HVDC. A simple, yet accurate, dc model which is valid for first swing transient stability studies is developed and tested. The model used includes the steady-state converter model with the static dc line representation, the fast changing dc power, and voltage dependent current order limit (VDCOL); while the dynamics of the dc internal controls are neglected. With this model, the dc operation mode is determined by an inference method, and the corresponding dc variables are computed. Using the simplified dc model, the Transient Energy Function is modified to include the effect of the two terminal HVDC. A procedure for first swing transient stability assessment of an ac/dc system is proposed and is tested on a 20-generator test system. Transient stability assessment with the new method is presented and compared with results obtained with the EPRI transient stability program.