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The Heffron-Phillips model of a synchronous machine has successfully been used for investigating the low frequency oscillations and designing power system stabilisers. The parameters of the model are usually calculated using the synchronous generator parameters and some system variables at steady-state conditions. As the parameters of synchronous generators are not easily measurable or accurately available and steady-state calculations may have some errors, the identification of such parameters using online measurements is investigated by the authors. A generating unit is a multivariable system and is well defined in a state space structure. The sub-space state space (4SID) identification method is very suitable for the identification of such a system. This method is used to identify the parameters. Since the synchronous generators are nonlinear, the parameters of the identified Heffron-Phillips model would depend on the operating conditions. To follow such changes, a look-up table model is developed. The proposed method is first applied on a simulated nonlinear model of a synchronous generator with saturation effect and then it is applied on a micro-machine system. Simulation and experimental results show the good accuracy of the identified models.