MTL model and FEM package for the evaluation of steep-front surges distribution in machine windings

The numerical evaluation of the electrical stress in the line-end coil of the stator winding of a medium voltage motor fed by a PWM inverter seems to be indispensable for a rational design of the machine. In order to fulfill such a task, many computer models have been developed, in which the system is generally modeled by connecting an ideal ramp generator (the inverter) with a single transmission line (the cable connecting the inverter to the motor) in series with a Multiconductor Transmission Line (MTL) (the motor winding). Whichever technique is used for solving the MTL equations, one of the main problems consists in the determination of the per-unit-length electrical parameters of the winding, so that simplified methods are often used for their calculation. In the present paper the calculation of capacitance (C) and inductance (L) matrices is performed by means of a Finite Element Method (FEM) package, so taking into account the effective field distribution in the region of interest. We will show how the use of the simplified methods can result in a wrong prediction of the stresses in the insulation. In order to validate the proposed model, the simulated results are compared with experimental data.