Finite-Element Analysis of Unbalanced Magnetic Pull in a Large Hydro-Generator Under Practical Operations

The rotor eccentricity of a large hydro-generator could cause a huge radial magnetic force, known as an unbalanced magnetic pull (UMP), exerting on the rotor. In this paper, Maxwell stress tensor (MST) method is used to compute the UMP based on the 2-D finite-element (FE) calculation, and the accuracy of UMP FE calculations is investigated. This paper presents a new method for the FE calculation of UMP due to rotor eccentricity in large hydro-generators. This method significantly overcomes difficulties of loading over thousand current sources for the UMP FE calculation of large hydro-generators with hundred stator slots operating under loads. In this paper, the UMP on a 240 MW, 88 poles, 720-slot hydro-generator, for various degrees of rotor eccentricity, are calculated and compared for operations with no load, half rated, and rated load. Also, the UMP on this hydro-generator is calculated when the rotor windings are subjected to a short-circuited fault condition. The method proposed in this paper was successfully used to study the vibration of Three-Gorge generation units in China.