The influence of coil and surge parameters on transient interturn voltage distribution in stator windings

Steep-fronted switching surges can cause undesirable transient voltage distributions in stator windings, resulting in severely stressed interturn insulation. The effects of coil and surge parameters on transient voltage distribution are not well understood. The paper reports the results of an investigation into the effects of these parameters. The investigation was carried out using a new coil model which has been shown to have good accuracy. The voltage distributions on coils of various shapes, for a range of surge risetimes, were found. In addition, the variation of voltage distribution with the dimensions of the coil insulation were computed for the range of surges. The variation between the proportion of the surge which appears between turns and the surge risetime is given. It is demonstrated that an increase in p.u. interturn voltage is the result of an increase in interturn insulation thickness. The shape and size of the coil are shown to have a decreasing effect on the magnitude of the interturn stresses as the surge risetime is reduced. The effects of the parameters on winding surge impedance are illustrated and discussed, although it is shown that the concept of a machine surge impedance is of limited value. (See also WRIGHT, M.T., YANG, S.J., and McLEAY, K.: `General theory of fast-fronted interturn voltage distribution in electrical machine windings¿, IEE Proc. B, Electr. Power Appl., 1983, 130, (4), pp. 245¿256.