Interturn Voltage Distribution Research Articles

Analysis of Turn-to-Turn Transient Voltage Distribution in Electrical Machine Windings

The interturn overvoltage during transient events in stator windings is an important feature to be considered during the machine’s design. In the literature there is no consensus about which turn is subjected to the highest voltage, once there are results of simulations and measurements that found the highest values across the last turns of the first coil, although, there are other that verifies the highest overvoltages across the first turns. In this context, this paper aims at verifying the transient interturn voltage distribution (turn-to-turn and turn-to-ground) in the stator windings of a large motor, using a method in which transient magnetic fields and electric circuit elements (resistance, inductance and capacitance) are coupled. The transient simulation and the calculation of the electric circuit elements used in the model are performed by means of Finite Elements Method (FEM). The end-windings parameters are calculated based on a 3D geometry. The model used only needs of the machine’s geometry, that is, the studies can be done at design phases of the machine. Moreover, the simulations consider all coils of one winding group from the 3 phases, which is different from the other simulation models found in the literature that consider only one or two coils in series of a single phase. To verify the interturn voltages, different numerical simulations are done by changing the surge rise time. According to the results, for the particular motor studied, the last turns were the ones submitted to the highest voltages.

脉冲变压器充电过程中次级线圈匝间电压的分布

脉冲变压器充电过程中次级线圈匝间电压的分布

Application of Hybrid Modeling for Calculating Interturn Voltages in Transformer Windings

This letter deals with the hybrid modeling of transformers and the determination of interturn voltage distribution in transformers with layer-type windings. The hybrid model uses a combination of a multiconductor transmission-line model and a single transmission-line model. The good agreement between computed and measured voltage waveforms shows that this model can be used for the computation of interturn voltages with full success. Analysis is performed by making use of a step function with 100-ns rise time.

Voltage Distribution of Steep Fronted Transients in Stator Windings–Modelling, Simulation and Experimental Verification

Steep fronted surge voltages are produced at H.T. motor terminals due to various switching operations. These are distributed within the stator winding coils and turns of the phases in a highly nonuniform manner leading to frequent insulation failures. There has been considerable interest in the development of the computational methods for analysing such phenomenon. This paper presents the development of a circuit model to predict the interturn voltage distribution and its validation through comparison of the results of simulation using EMTP with those obtained experimentally on sample coils for a specific three phase 6.6 kV, 400KW H.T. motor. The correlation between the experimental and simulation results is found to be good.

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.

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.

Steep-fronted switching voltage transients and their distribution in motor windings. Part 2: Distribution of steep-fronted switching voltage transients in motor windings

Severe interturn insulation stresses may be produced in motor windings by steep-fronted switching voltage transients. To evaluate the severity of such stresses, transient-voltage distribution tests must be carried out under conditions closely simulating those obtained in service. In a complementary paper, system tests were described which were carried out to determine the characteristics of the transients produced by circuit-breaker closure operations. The steepness and magnitude of the transients were determined and the system parameters defining them evaluated. Based on this information, a series of interturn voltage distribution tests were carried out on a motor winding under conditions closely simulating practical system conditions. From these tests, the magnitude of the interturn voltages in the winding, produced by normal switching operations, were estimated and shown to be severe. The effect that system parameters, external to the motor winding, and also the use of series inductors or shunt capacitors used as surge-protection devices, have on the voltage distribution are also studied. Impulse voltage tests to breakdown were undertaken on a number of the motor coils and the breakdown levels shown to be only marginally above the transient levels produced by normal switching operations. The impulse voltage wave used for these tests was closely correlated to that which might be produced by steep-fronted switching transients.