General theory of fast-fronted interturn voltage distribution in electrical machine windings

Motor-coil failure caused by high interturn stressing can occur because of the application of fast-fronted switching surges to the stator winding. It is shown that a new method of analysis of surge propagation in coils is necessary. A generalised method of analysis, which is capable of predicting voltage distribution in coils due to fast-fronted surges, is developed. The analysis, omitting losses, gives results which are significantly more accurate than those previously published. Further development of the analysis to include losses is also given. A comparison of measured results and results predicted by the lossy model shows the model to be extremely accurate. It is demonstrated that surges propagate through coils in both a series and a parallel manner and that each mode of propagation cannot be treated separately. (See also WRIGHT, M.T., YANG, S.J., and McLEAY, K.: `The influence of coil and surge parameters on transient interturn voltage distribution in stator windings¿, IEE Proc. B, Electr. Power Appl., 1983, 130, (4), pp. 257¿264.