Investigation of magnetic field and radial force harmonics in a hydrogenerator connected to a three-level NPC converter

This paper investigates spatial and time harmonics of flux density and radial force distributions in the airgap of a hydrogenerator with fractional-slot windings. Hydrogenerator is connected to a neutral-point-clamped (NPC) converter, representing a converter-fed synchronous machine (CFSM) in pumped-storage hydropower plant. Magnetic field in the hydrogenerator is computed using time-stepping finite element (FE) analysis, coupled with an external circuit (where PWM converter is modelled). Radial forces are calculated analytically based on Maxwell stress tensor. Three loading conditions are studied (i.e. resistive loading and converter-fed operation with two different carrier frequencies). In the paper, it is investigated how converter-fed operation could change the characteristics of the radial magnetic forces. It is also discussed how vibration of the hydrogenerator is expected to be affected by additional harmonics generated by the PWM converter.