Modeling of large AC motor coils for steep-fronted surge studies

Interturn insulation failure is now recognized as a common cause of failure in large AC machines. The line-end coil is the most likely site of such failure, since under steep-fronted surge conditions it bears 100% of the surge amplitude for several microseconds. During this period, measurements show that the interturn voltage distribution in the line-end coil is nonlinear, with up to 40% of the surge voltage appearing between the last two turns of the coil. To predict the voltage distribution in the line-end coil under such conditions, a multiconductor transmission line model of the coil has been developed. This model can then be used in a general-purpose electromagnetic transients program (EMPT) or in a special-purpose EMPT. Results are presented for measured and simulated waveforms on a large AC motor. The simulations have been done on a special- purpose EMPT. The latter has also been used to compute the impulse response of the coil which can, then be convolved with the applied surge waveforms to derive the coils response to a variety of surge waveshapes. This is a much cheaper method of solution if responses to a variety of surge waveforms are required since it only requires running the PT program once, followed by a number of runs of the much simpler and faster convolution program