Reduction of Induced Shaft Voltage and Bearing Current in Turbo Generators: Modeling, Compensation, and Practical Test

The static excitation system of a turbo generator, supplied by a rectifier, can cause induced voltages on the generator's shaft. This article proposes a complete distributed model of a turbo generator, regulated by a static excitation system, to investigate the shaft voltage and bearing current. The complete modeling of all components is proposed by considering the couplings between them in a static excitation system. When shaft voltages exceed the dielectric breakdown voltage of the lubricating oil film in journal bearings, electrical discharge current flows through the bearings. This will further reduce the life expectancy of the dielectric gradually. Two different approaches are explained and discussed in detail to keep the shaft voltages within the safe region: a passive filter and a dc-dc converter solution. Both methods along with the initial system are simulated and compared accordingly. Additionally, frequency-domain analysis has been performed for evaluating the compensating solutions. Moreover, experimental tests were carried out on a 200-MVA turbo generator, and results of the suggested passive filter with those of the proposed compensation method are compared. Both experiments and simulations confirm the mentioned issue and show the effectiveness of the proposed compensation approaches in this regard.