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Transient stability constrained optimal power flow (TSC-OPF) is originally a non-linear optimisation problem with variables and constraints in time domain, which is not easy to deal with directly because of its huge dimension. This study presents an efficient approach to realise TSC-OPF by introducing an independent dynamics simulation algorithm into the optimisation procedure. In the new approach, the simulation algorithm is used to realise the dynamics constraints and to deduce the transient stability constraint, whereas the optimisation algorithm verifies the steady state and the transient stability constraints together. The new TSC-OPF has just one more constraint than that of a conventional OPF and can be solved by a conventional OPF algorithm with small modification. Moreover, the new approach makes it easy to improve the accuracy and efficiency of TSC-OPF, because of its flexibility in choosing machine models and simulation methods, which is important for large power systems. In the study, the proposed approach is tested with two small power systems, where the two-axis machine model and a mixed time step simulation method are used to assess system transient stability with a 1-ms time resolution.