Induced field-system transients in synchronous generators having unidirectional forms of excitation supply

Synchronous-generator excitation supply systems that are based on a.c. exciters operating in conjunction with rectifiers, as is now widely the practice, give rise to particular modes of generator operation that do not occur when d.c. exciters are used. In the case of a d.c. exciter, the field current can reverse, if it is required so to change by stator operating conditions, whereas an a.c. exciter and a rectifier system provide an essentially unidirectional form of supply. A preclusion of field-current reversals by the rectifiers introduces a constraint into the interaction between the rotor and stator circuits of the generator for which the excitation system provides a supply. For a wide range of generator operating conditions, the constraint is inactive, and so no adverse effects arise, but credible circumstances can combine to cause the constraint to become active. When this is the case, the transition of the constraint from inactive to active is accompanied by a pulse of induced voltage in the field system. The paper reports a study of field-system overvoltages that might be generated in this way for the particular conditions encountered in synchronising. A mathematical model suitable for use in computer studies in this area of applied analysis in power systems is summarised, and the validity of the model is checked by comparing computer study results with those available from fullscale tests on a 120 MW turbogenerator unit. Numerous synchronising studies have been based on this model, and representative results from them are collected together in the paper, together with the synchronising conditions that can give rise to overvoltages. When synchronising is achieved within the close limits typical of British practice, the hazard of induced field voltages of substantial magnitude should be avoided altogether, but the continuously recurring nature of synchronising generator units with a parent system suggests a general study of the conditions. It is a general study of this kind to which the paper is devoted.