Impact of Real Air-Gap Nonuniformity on the Electromagnetic Forces of a Large Hydro-Generator

Abstract

In this paper, an accurate two-dimensional time-stepping finite-element model of a large hydro-generator considering a nonuniform air gap for computing the magnetic-flux density and the electromagnetic forces is developed and presented. The 74-MVA industrial hydro-generator is modeled using an actual air gap obtained from a monitoring system having eight capacitive sensors installed on its stator teeth. First, the method consisted in creating geometrically the real air gap in the finite-element model. Then, the latter was experimentally validated. The study showed that the impact of the real air gap on the hydro-generator performance could be found in the electromagnetic force density and in the radial magnetic-flux density as well. Furthermore, the considered real air-gap shape contributes to induce many higher magnitude forces at rotating frequency and its multiples that can excite the natural frequencies of the generator structure. Moreover, the computed dynamic rotating net force considering poles displacement is evaluated as 29% of the average value of the static net force.