End-Windings Influence in the Transient Voltage Distribution in Electrical Machine Windings Using Finite Elements Method

Transient voltage distribution due to steep-fronted surges must be taken into account during machine's design once it can lead main and turn insulations to failure. In medium voltage machines, the end-windings region has a considerable percentage of the coil length and it has to be considered during transient studies. This paper presents a set of numerical simulations with the purpose of verifying the influence of the end-windings in transient voltage distributions due to a voltage surge applied in the line end coil of a large electrical machine. The simulations are done using a model in which transient magnetic fields and electric circuit elements are coupled. The transient simulation and the calculation of the lumped parameters used in the model are done by means of Finite Elements Method (FEM). Inductance and resistance of the end-windings are calculated by using an equivalent 2D geometry and capacitances between turns and from turns to ground are obtained by using a 3D geometry of the coil. To verify the influence in the results, simulations for different surge rise times are performed considering and neglecting the end-windings lumped parameters. As main results the coil-to-coil, coil-to-ground, turn-to-turn and turn-to-ground voltages are analyzed. It is concluded that the end-windings, indeed, affect the transient voltage distribution for both, interturn and intercoil voltages.