Influence of Converter Topology and Carrier Frequency on Airgap Field Harmonics, Magnetic Forces, and Vibrations in Converter-Fed Hydropower Generator

This paper investigates the effects of converter-fed operation on magnetic forces and vibration in a hydrogenerator. The variable-speed hydrogenerator (in a pumped-storage power plant) is a large synchronous machine with fractional-slot windings. In this paper, the generator is connected to either a resistive load or voltage source converter. Loading cases include two-level converter and three-level neutral-point-clamped converter, and the carrier frequency is equal to 450 and 1050 Hz. Using electromagnetic finite element analysis, airgap flux density harmonics are analyzed in different loading cases. In addition, electromagnetic torque and variations in the rotor field current are also studied. Using Maxwell stress tensor, radial force density distribution in the airgap is investigated and the effect of converter-fed operation on different radial force harmonics is studied. Moreover, structural analysis and electromagnetic vibration synthesis are presented to investigate the effect of converter harmonics on the stator vibration.