Alternative approaches for linear analysis and prediction of a synchronous generator under partial- and full-load rejection tests

This work introduces the so-called linear hybrid state model of a synchronous machine based on the superposition principle of the two-port representation of the d-axis (two-port circuits) and the single-port structure of a q-axis equivalent circuit. The detailed formulation of the linear hybrid state model is developed in terms of the equivalent circuit parameters of the machine. As the terminal voltages and the field current recorded after full- and partial-load rejection tests are the generator's time-variant responses for a step change of armature currents with a constant field voltage, exact analytical expressions of the terminal voltages and the field current following the generator tripping (load rejection) tests are derived in terms of physical machine parameters using the state model solution of the linear system theory. Two load rejection tests (50% and 100%) of purely inductive and capacitive load are performed and comparisons are made between simulation results and actual data obtained from a four-pole, 1.5-kVA, 208-V, 60-Hz laboratory machine.