Identification of coherent generators for power system dynamic equivalents using unstable equilibrium point

A new technique for identifying coherent generators in dynamic equivalencing of large power systems is presented. Coherency of generators in the early part of the transient period is determined by directly checking the relative rotor angle deviations. This requires the prefault angles, faulted angles and the angles at the unstable equilibrium point corresponding to the expected mode of system instability. The faulted angles are estimated by the Taylor series expansion and the unstable equilibrium point is computed by the Davidon-Fletcher-Powell method in the centre of angle reference frame. The proposed method was tested on two power networks and the results obtained were perfectly matched with those found from direct examination of swing curves