Harmonic interaction in generator pre-sunchronous operation

The pre-synchronous condition of generators operating at normal speed, with voltage on manual control, while connected to open circuited step up transformers, can last several minutes. This is particularly true if plant problems delay the synchronization process. Under these conditions the generator absorbs all the positive and negative sequence harmonics due to transformer saturation. Instances of unexplained excursions in voltage and erratic excitation system operation have come to the author´s attention over several years. Some involving over-voltages and step up transformer insulation failures. This paper suggests that interactions between the harmonic currents in the generator stator propagating, through induction, to the field current and to the diode-bridge excitation system may be the cause. The dynamics of this process are explored through time domain simulations, in attempts to explain the nature of the problem. Recommendations are given on measures to avoid the occurrence of such interactions.