Modeling of machine coils under fast front excitation using a non-uniform multiconductor transmission line approach

AC rotating machines are exposed to fast-front pulses when fed from frequency converters. This type of excitation produces dielectric stress in the machine's insulation system that can result in premature deterioration or even failure. A nonuniform transmission line model is described and applied in this paper to predict the transient voltage distribution in a machine coil under steep-fronted surge conditions. The non-uniform transmission line model is a function not only of frequency, but also of space to represent the variation of electrical parameters in the overhang and slot regions of the coil. These parameters are computed using the finite element method. The model allows the computation of transient overvoltages in a turn-by-turn basis. A test case is included to analyze the effect of different rise times of the excitation on the voltage distribution along the coil.