A method of pre-feedback to formulate the generalised forced Hamiltonian system model for speed governor control systems is proposed. Furthermore, passivation controllers are designed based on the scheme of Hamiltonian structure for single machine infinite bus and multimachine power systems. In particular, in the case of multimachine systems, all the variables in the control law are only relevant to the state variables of the local generator, which means that a decentralised controller is achieved. Simulation results of a four-machine system show that the controller can enhance power system transient stability.