Transient Roebel Bar Force Calculation in Large Salient-Pole Synchronous Machines

The radial and tangential force components acting on each sub-conductor of a Roebel bar in a large generator lead to mechanical vibration of the stator bars in the slot. This double-frequency vibration results in additional mechanical stress on fixation materials in the slot and causes insulation deterioration and looseness of stator bars. Particularly during short circuits, the stator bars are subjected to immense radial forces. Understanding the origin of these vibrations and developing models to anticipate them is essential to increase the reliability and expected life time of large generators. This paper presents an analytical method for the estimation of Roebel bar forces during symmetrical and asymmetrical short circuits. For this purpose, an analytical calculation method is developed to estimate the transient three-phase currents during different short circuits. These currents induce tangential and radial flux density components along the slot width and length, respectively. An analytical approach is introduced to estimate these flux densities as a function of the slot length. Subsequently, an analytical method for calculating the Roebel bar forces considering three-dimensional transpositions is developed and verified with finite element calculations. Beyond the state of the art, the impact of the rotor field on the radial and tangential components of the stray fluxes are analytically estimated. The result is a comprehensive analytical tool for the estimation of Roebel bar forces during different short circuits in large generators.