Analysis of operating conditions of ground-fault protection schemes for generator stator winding

This paper presents results of simulation studies of the influence of methods of grounding the stator winding of the unit-connected generator neutral on the operating conditions of a 100-percent ground-fault protection excited by the third-harmonic voltage ratio in the generator neutral and at its terminals. The analysis was performed for a generator with the neutral grounded through a ground-fault neutralizer, a third-harmonic stop filter, a resistor, and a capacitive voltage divider. The influence of the parameters of the particular elements of the grounding circuits on the voltage-feeding measuring element of the protection was determined. The capacitance to ground of the generator breakers was taken into account. The simulations were run for 1110-MVA unit-connected generator at ground faults along the whole length of the generator stator winding. It has been found that the method of grounding the generator neutral and the parameter of the grounding elements significantly influences the operating conditions of this protection. It is shown that through the proper selection of these parameters, the ground-fault protection of whole stator winding, counting from the generator neutral to the terminals is ensured.     

Performance evaluations of 50 MVA airgap winding stator forsuperconducting generator

Structural features of a trial-manufactured air-gap winding stator for a 50 MVA superconducting generator and results of various characteristic tests are described. The stator has an improved winding section, strengthened against electromagnetic force and smaller due to use of a four-layer water-cooled helical air-gap armature winding. The axial flux through the stator core, assumed to be generated by the circumferential current of the helical winding, is too low to pose any practical problems, as shown by a three-dimensional magnetic-field analysis taking into account the magnetic anisotropy due to the laminated iron core. The air-gap armature winding electromagnetic force in the radial and axial directions (which does not contribute to the generator output) increases when the running power factor decreases. The helical winding is shown to be applicable to the armature winding of large superconducting generators     

A thermal cycling type test for generator stator winding insulation

A test has been designed and implemented which cycles the temperature between 40°C and 150°C in about 80 min by the use of circulating currents and cooling air. The test has been successfully used to evaluate the relative performance of similar stator bars made by three different manufacturers. The thermal cycling test was able to duplicate the insulation delamination process and presumably those insulation systems which performed well in the thermal cycling test could last longer in service. In addition to being used for pumped-storage generators, the thermal cycling test may be useful to evaluate the insulation system in combustion turbine generators     

Diagnosis of stator winding insulation of large generator based on partial discharge measurement

The Stator winding insulation is aged by exposure to a combination of thermal, electrical, mechanical, and environmental stresses. Diagnosis of stator winding insulation is an important measure of ensuring the safe operation and extending the remaining life of large generator. In the paper, two new parameters are proposed for the diagnosis of stator winding insulation based on the test results of multistress accelerating aging test of stator bars. The test results show that the skewness of partial discharge phase resolved distribution and the high-frequency crest of partial discharge (PD) impulse change much more remarkably with the aging time than do the conventional parameters. Therefore, the two parameters could be considered as characteristic parameters for the diagnosis of stator winding insulation of the large generator. With these characteristic parameters, authors successfully assessed the insulation condition of actual stator bars removed from several large generators.     

An evaluation of model-based stator resistance estimation forinduction motor stator winding temperature monitoring

In this paper, the feasibility of using an estimate of the stator resistance (R_s) as an indicator of stator winding temperature (T_s) is evaluated. The advantages of resistance-based temperature monitoring over conventional thermal model-based methods are presented. Since obtaining an accurate estimate of R_s is critical for this approach, an investigation of existing R_s estimation schemes, and an analysis showing the sensitivity of model-based R_s estimation due to the uncertainties in motor parameters and variables, are given. It is shown that R_s estimation is difficult during high-speed operation, because estimated R _s becomes sensitive to errors in motor electrical parameters and variables, as the input excitation frequency (speed) increases. A new R_s estimation scheme for the purpose of monitoring the temperature in steady state is proposed. Experimental results on a line-connected induction machine verify the validity of the proposed method and the analysis     

Analysis of the voltage distribution in a motor stator winding subjected to steep-fronted surge voltages by means of a multiconductor lossy transmission line model

In this paper, the effect of steep-fronted voltage waveshapes infringing on a pulse-width-modulated (PWM) inverter fed induction motor is studied. The system, composed of a feeder cable and a stator winding, is modeled and simulated by using multiconductor transmission line theory in order to predict the voltage distribution among the coils of the stator winding. A recently developed time-domain equivalent circuit is used; it allows one to correctly describe the dielectric losses and the skin-effect in the conductors. The relationship among the voltage distribution inside the electrical insulation and parameters like the rise time of the applied voltage, the cable length, and the distributed losses is deeply discussed. Good agreement has been found among experimental and numerical results.     

Relation between generator geometry and resistance in armature winding

Resistive losses constitute a big part of the total loss in a generator, especially in low speed generators where frequency-dependent losses are low. The resistive losses in the winding are proportional to the length of the winding. In this paper, a computer-based program for generator design, using a two-dimensional finite-element method, is used to verify a theoretically-derived result, that the length of the armature winding is minimal for a specific generator geometry. A shorter armature winding might imply only a negligible improvement of the overall efficiency. The result of the computer-based design program is compared with the theoretically-derived results. A rough estimation of the relation between the length of the armature winding and the overall efficiency is included in the paper, as well as data from two existing generators.     

变工况对带涡流发生器的PEMFC性能的影响

质子交换膜燃料电池(PEMFC)是一种高效的能量利用装置。为了提高其工作性能,在其中加装涡流发生器,并研究工况对其性能的影响,对其进行了单因素和多因素影响分析,利用响应面法建立数学关系式。结果表明：加装涡流发生器,可以明显改善PEMFC温度均匀性、提高燃料利用率、改善通道排水,对燃料电池的性能有明显提升,电压为0.4 V时电流密度提升了55.4%;在所选区间内温度为65℃,相对压力为100 kPa,湿度为60%,化学计量比为2.50时,燃料电池的功率密度最大。     

Estimation of peak transient voltages at wind turbine generator terminals

This paper proposes equivalent circuits and a method for the estimation of the peak transient voltages at generator terminals in wind turbines equipped with back‐to‐back converters. Equivalent circuits as well as a way of phase‐to‐phase and phase‐to‐ground voltages estimation in back‐to‐back converter are presented. Proposed theoretical background is well confirmed by small‐scale measurements. It is shown and proven that phase‐to‐ground voltages represent a dominating threat for the wind turbine generators insulation. Copyright © 2014 John Wiley & Sons, Ltd.     

Power factor correction of induction motors using PWM inverter fedauxiliary stator winding

A scheme for power factor correction of an induction machine by using an auxiliary stator winding in conjunction with a PWM voltage inverter and a capacitor on the DC side of the inverter is described in this paper. The operating principle of the proposed scheme is explained and a criterion is developed to select the optimum value of the power factor at which the scheme gives best performance. Simulation and experimental studies show that the harmonic components in the PWM output voltage can be reduced to obtain nearly sinusoidal current     

Stability of doubly-fed induction generator under stator voltage orientated vector control

The stability of a doubly-fed induction generator (DFIG) under vector control in stator voltage orientation (SVO) is investigated. Prior art has tended to assume that the inner current loop dynamics can be neglected when an SVO is employed. As a result, the poorly damped poles of the DFIG system were considered unaffected by the inner current loop tuning. The state-space model of the machine including the inner current closed loop dynamics is developed for schemes where different feed-forward compensation terms are used. The interaction between inner current loop dynamics and damping of the critical poles of the system is illustrated through analysis and simulation. The main outcome of the analysis is that the stability of the machine system in an SVO depends solely on the parameters of the proportional-integral controllers. Erroneous tuning can lead to instability, irrespective of the particular feed-forward compensation scheme, which could cause the disconnection of the machine as a result of rotor current oscillations of unacceptable magnitude in an actual case. The main contribution is to provide the necessary methodology in order to ensure the stable operation of a DFIG under SVO vector control.     

齿槽形状对永磁风力发电机温度场分布的影响

以影响发电机温度场分布的齿槽结构为研究对象,基于变工况下的实测值,采取有限元数值模拟法,开展功率为400 W的不同齿槽结构的永磁风力发电机温度场研究。首先,按照测试方案并运用FLIR红外热成像仪采集梨形槽、扇形槽、斜肩圆底槽3种结构发电机在不同工况下的动态温度场,将测试温度值整理后,通过meshgrid(x,y)函数和surf(x,y,z)函数相结合,分别绘制出不同槽型的定子沿径向随负载值大小和位置变化的三维曲面;其次,以所测温度值为计算载荷和边界条件,模拟出不同齿槽结构的温度场;最后,对比分析测试和计算温度场的分布规律,以二者一致且有效的规律为依据,找出产热低、散热强的最优齿槽结构。     

Effectiveness of stress grading coatings on form wound stator coil groundwall insulation under fast rise time pulse voltages

In this paper, the effectiveness of stress grading coatings (SGCs) on form wound stator coil groundwall insulation under fast rise time pulses is analyzed. The combined performance of the stress grading coating and the semiconductive slot coating is studied under fast rise time transient voltages. During the rapid rise of voltage, the high stress moves from the stress grading coating to the semiconductive coating at the end of the slot. According to the results, it is evident that the design of both the stress grading and semiconductive slot coatings must be done together for effective stress relief. The use of materials with varistor behavior in the stress grading coating, combined with a high conductivity semiconductive slot coating, is analyzed as a solution to the high stress that develops at the slot end of form wound stator coil groundwall insulation of inverter fed drive (IFD) motors.     

Improved transient simulation of salient-pole synchronous generators with internal and ground faults in the stator winding

An improved model for simulating the transient behavior of salient-pole synchronous generators with internal and ground faults in the stator winding is established using the multi-loop circuit method. The model caters for faults under different ground conditions for the neutral, and accounts for the distributed capacitances of the windings to ground. Predictions from the model are validated by experiments, and it is shown that the model accurately predicts the voltage and current waveforms under fault conditions. Hence, it can be used to analyze important features of faults and to design appropriate protection schemes.     

Transient performance study of a brushless doubly fed twin stator induction generator

This paper presents theoretical, simulation, and experimental study of the brushless doubly fed twin stator induction generator (BDFTSIG) dynamics under vector control based on the orientation on the power machine stator flux. A complex transfer function is derived which links the control current and power winding current space vectors in the field coordinates. Based on this result, the transient response of the BDFTSIG to step changes in the control current is examined theoretically. The oscillatory transients are explained in detail and linked to control flux transients triggered whenever operation point of the generator is changed. Furthermore, BDFTSIG operation with closed loop control of the power machine active and reactive powers is examined theoretically and experimentally. It is shown that in the closed loop operation, the system damping may be reduced so that the PI controller gains must be properly selected to achieve a good transient response.     

发电机定子铁芯叠片粘连处理及铁损试验

介绍发电机大修中定子铁芯叠片粘连损伤处理工艺及定子铁损试验，可有效降低发电机的能量损耗，对同类型机组检修有一定的参考价值和指导意义。     

Proposition of a new method for in-service monitoring of the aging of stator winding insulation in AC motors

A new on-line, nonintrusive monitoring system, able to observe the aging of an ac machine winding insulation is described. The principle of this system consists of detecting small variations of the turn-to-turn capacitances due to the dielectric aging. The authors have performed experiments on accelerated insulation aging as well as on a system able to measure machine winding high-frequency resonance, which depends on turn-to-turn capacitance. First, twisted pairs of magnet wires were submitted to thermal and electrical stresses, in order to establish a connection between the turn-to-turn capacitance with the breakdown voltage and the probability of failure. Then, the changes in the resonance frequency of a simplified stator winding, the "motorette," were measured. Measurements of the currents or of the magnetic field in the vicinity of the machine can be made. The analysis of the experimental data yields information on the turn-to-turn capacitance variations, hence on the insulation aging.     

A fault warning for inter-turn short circuit of excitation winding of synchronous generator based on GRU-CNN

Synchronous generators are important components of power systems and are necessary to maintain its normal and stable operation.To perform the fault diagnosis of mild inter-turn short circuit in the excitation winding of a synchronous generator,a gate recurrent unit-convolutional neural network (GRU-CNN) model whose structural parameters were determined by improved particle swarm optimization (IPSO) is proposed.The outputs of the model are the excitation current and reactive power.The total offse...     

基于快速事故筛选的发电机负荷组合模型的研究

采用电势恒定发电机模型和动态负荷模型组合,通过改变负荷模型中恒阻抗模型和感应电动机模型的组合比例,模拟了发电机6阶模型和静负荷模型(60%功率+40%恒阻抗)组合下的暂态稳定计算,并通过PSASP仿真两个算例,验证了该算法的可行性。     

李家峡水电站4#发电机定子下线的监理

介绍了大型水轮发电机的新型冷却方式——蒸发冷却方式在李家峡水电站4#发电机定子绕组中的原理、应用及优点,对李家峡水电站4#发电机定子下线的整个工艺过程、定子下线安装过程中发现的问题及时分析、总结,以及解决的办法。