Computation and validation against measurements of stator end-region fields in synchronous generators

Stator end-region fields in turbine-generators are calculated using the finite element method. Representation of the nonlinearity of the stator iron and the eddy currents induced in the stator core back are features of the model. Computed results for axial flux are compared with measurements taken from a 660 MW machine running on load at leading power factor. The 3-dimensional electromagnetic field problem which requires solution in the end region of the machine is simplified by assuming that all quantities vary in a first harmonic sinusoidal manner in the circumferential direction. To represent the field, a combination of electric vector potential and magnetic scalar potential is used. Because the problem is nonlinear the finite element equations must be solved iteratively. A combination of Newton¿Raphson and chord iterations is used. The first stage employs Newton¿Raphson iterations assuming the problem to be magnetostatic, then, with this solution as a starting approximation, chord iterations are used to include eddy currents into the nonlinear model.