Models incorporating different levels of sophistication are presented for predicting the surge distribution in motor stator windings. Results predicted by these models have been validated against experimental results. Simple models, which provide insight into the relevant mechanisms, are presented for estimating the interturn voltage and the line-end coil voltage, and they are used to derive guidelines for evaluating alternate winding designs. A detailed model which has been solved using the Electromagnetic Transients Program (EMTP) software package is presented for more accurate prediction of interturn voltages. The influence of coil design parameters on the surge distribution is discussed. >

Measurement and analysis of surge distribution in motor stator windings

An analytical model (a frequency-dependent transients program) was developed to predict turn and interturn voltages across the line-end coil under steep-fronted surge conditions. The results of simulation for laminated and solid slot cases showed a close agreement with the corresponding experimental results. This investigation shows the error in replacing the laminated slot core by a solid slot core in the study of steep-fronted surge propagation in large a.c. motor coils. For the laminated slot case, the peak interturn voltages are greater (by a factor of about 1.5) than those for the solid slot model for the coil used in this study. The steeper the surge front impinged, the greater the peak interturn voltages. >

http://ieeexplore.ieee.org/iel2/565/3783/00140797.pdf

A multiconductor transmission line model for surge propagation studies in large a.c. machine windings

To date, the modeling of machine windings for steep fronted surge propagation has been based on an assumption that the slot walls act as flux barriers at the frequencies of interest (1 to 10 MHz). This assumption led to the use of a line end coil structure embedded in a solid slot core for experiments, and excellent agreement between measured and computed waveforms has been reported by P.G. McLaren et al. (1988). The assumption is valid for a solid slot environment as the magnetic flux cannot penetrate beyond the skin depth, delta , which is on the order of 10/sup -6/ m for the frequencies concerned. However, this is not the case for a laminated slot environment as it can allow magnetic flux penetration deeper into the core that the skin depth, delta . In the present study, the effects of such a flux penetration on steep fronted surge propagation in large AC motor coils are studied. The results obtained demonstrate that the solid slot models cannot accurately replace the actual laminated environments in the study of steep fronted surge propagation in machine windings. Maximum interturn voltages observed for the solid slot model are smaller (by a factor of about 2/3) than those for the laminated slot environment. >

The effects of laminations on steep fronted surge propagation in large AC motor coils

Large medium-voltage electric machine stators are usually equipped with form wound coils made of flat conductors (strands) and embedded in open (rectangular) slots. Air-gap magnetic flux lines can enter the slot and, sweeping the strands placed nearest the slot opening, induce eddy currents in them. Such eddy currents cause additional losses which can be much higher than usual skin-effect and proximity losses. In order to avoid dangerous overheating and hot spots, the additional losses in question need to be carefully predicted in the design stage. Time-stepping finite-element analysis (TSFEA) can be used for the purpose, which however implies a large computational burden and requires the machine geometry to be modeled in detail. This article proposes alternative methods based on time-harmonic finite-element analysis (THFEA) simulations performed on highly simplified machine models and with no need to take rotor motion into account. The proposed methods are shown to produce very accurate results, compared to TSFEA, but with very significant time and computational savings.

Fast Computation Method for Stator Winding Skin-Effect Additional Losses in Synchronous Machines With Open Slots and Arbitrary Rotor Geometry

The modeling of machine windings for steep-fronted surge propagation has been based on an assumption that the slot walls act as flux barriers at the frequencies of interest (1 to 10 MHz). The assumption is valid for a solid slot environment, because the magnetic flux cannot penetrate beyond the skin depth, delta , which is of the order of 10/sup -6/ m for the frequencies concerned. However, this is not the case for a laminated slot environment, which can allow magnetic flux penetration on steep-fronted surge propagation in large AC motor coils are examined. Experimental data are reported showing that solid-slot models cannot accurately replace the laminated environments. Maximum interturn voltages observed for the solid slot model are smaller (by a factor of about 2/3) than those for the laminated slot environment. The differences between solid and laminated environments is attributed to the effect of laminations, which can be approximately modeled using the concept of surface impedance. >

Modeling of effects of laminations on steep fronted surge propagation in large AC motor coils

The authors review the response of the windings of high-voltage AC rotating machines to system-generated transient voltages and also impulse test techniques and withstand levels of form-wound stator coils. Design tests on coils with a vacuum pressure impregnation (VPI) insulation system have confirmed very high turn-to-turn insulation strength well above actual system transient voltages. Because of this and since the amplitude of line-to-ground overvoltages to which windings are exposed in service can be controlled to some extent, there appears to be no need for impulse testing of complete machines. Also, the line-to-ground withstand capability of a winding would be adequately verified by the standard AC applied potential test. If, under special conditions, the actual turn-to-turn insulation strength must be demonstrated experimentally, such tests should be performed on additionally fabricated sacrificial coils which are manufactured with and identical to those in the winding. >

K. Scherer

Papers

Practical considerations concerning impulse voltage testing of form-wound coils for rotating machines