Capacitive Load Induced Field Stresses on Generators Connected into DC Systems

Sometimes, a synchronouis machine experiences a sudden increase. in capacitive loading, due for example to a load rejection by the blocking of an hvdc rectifier, or to the tripping of the synchronous machines when connected to a predominantly capacitive load. This sudden increase in capacitive loading can initiate oscillations in the stator circuit, which are reflected as sudden oscilldtions in the machine´s field current. In machines with solid state exciters without reverse current capability, severe oscillations may momentarily try to reduce the field current to zero, and result in excessive field overvoltages. It is these overvoltages, that this paper attempts to study, through detailed simulations and also through demonstration on a laboratory setup. The situation discussed here relates to such phenomena occuring on machines connected to hvdc rectifier stations, although similar situations could also exist in other systems.