Rotor Slot Wedges Research Articles

Influence of Rotor Slot Wedge Materials on Rotor Losses of Turbogenerator During Loss of Excitation

A short period of steady-state asynchronous operation after loss of excitation can improve the reliability of turbogenerator. The rotor damping structure of the turbogenerator can provide asynchronous torque and, thus, play an important role in improving the asynchronous operating ability. The rotor slot wedges as a part of rotor damping structure often made of aluminum, beryllium bronze, or stainless steel. The different materials of the rotor slot wedges can affect not only the asynchronous operating ability of the turbogenerator but also the rotor losses. The selection of material for the rotor slot wedges is therefore a key factor that should be carefully considered in the design of turbogenerator. In this article, the effect of the rotor slot wedges made of different materials on the asynchronous operating ability, rotor loss densities, and loss distributions of the turbogenerator after loss of excitation is investigated by time-stepping finite element model. The variations of the asynchronous operating ability and the rotor losses are revealed along with the rotor slot wedge conductivity. The rotor regions where the losses are significantly affected by the rotor slot wedge material are obtained. The results provide theoretic basis for improvement of the asynchronous operating ability and reduction of the rotor loss of turbogenerator.

Rotor Loss and Temperature Field of Synchronous Condenser Under Single-Phase Short-Circuit Fault Affected by Different Materials of Rotor Slot Wedge

A synchronous condenser (SC) can provide the dynamic reactive power for the ultra-high-voltage direct current converter station during the system voltage drop caused by the single-phase short-circuit fault, which is one of the prevalent faults. However, the single-phase short-circuit fault can also result in the increase of the rotor loss and temperature and thus restrict the transient operating ability of the SC. A key factor that affects the rotor loss and temperature rise is the material of the rotor slot wedges. For a 300-MVar SC with the rotor slot wedges made of the aluminum, beryllium bronze, and stainless steel, the rotor loss and temperature distribution under the single-phase short-circuit are calculated by coupling the electromagnetic and temperature field models of the SC with the models of the power grid. The variations of the rotor losses along with the rotor slot wedge conductivity are revealed. The rotor regions, where the highest temperature rise appears, are obtained. According to the maximum permissible temperature of the rotor material, the transient operating ability of the SC withstanding the single-phase short-circuit fault is obtained. This article can provide the theoretic basis for improving the transient operating ability of the SC.

Rotor Loss and Thermal Analysis of Synchronous Condenser Under Single-Phase Short-Circuit Fault in the Transmission Line

A synchronous condenser (SC) can provide the reactive power for the Ultra High Voltage Direct Current (UHVDC) converter station to keep voltage stability and prevent commutation failure. The single-phase short circuit at a transmission line is prevalent and it can result in not only the voltage drop but also the increase of the rotor loss and temperature of the SC. In order to study the ability of the SC to withstand a single-phase short-circuit fault, the rotor loss and temperature field of a 300-MVar SC are studied by coupling the electromagnetic and temperature field models of the SC with the models of the electric circuit and power grid. The loss distributions in the rotor core and slot wedges are calculated under a single-phase short-circuit fault at the different positions of the transmission line. The rotor temperature distributions of the SC under over- and under-excitations are calculated and verified by the experiments. The influence of short-circuit duration on the rotor temperature of the SC is calculated under the single-phase short circuit. The study can provide a theoretic basis for the improvement of the operating ability of the SC under the single-phase short-circuit faults.

Predicting the eddy current loss of a large nuclear power turbo generator using a fuzzy c-means deep Gaussian process regression model

The distribution of the rotor slot wedge eddy current loss in large nuclear power turbo generators is complex and is influenced by many factors. Excessive eddy current loss leads to severe rotor heating, potentially leading to thermal accidents; therefore, the design precision of large generators must be improved. In this paper, a Fuzzy C-Means Deep Gaussian Process Regression (FCM-DGPR) method is proposed to predict the eddy current loss of a large generator in order to solve the problem of the insufficient accuracy of deep Gaussian process regression (DGPR) with increasing number of the data samples. First, the original dataset is obtained by the finite element method (FEM) and then normalized to construct the samples of the eddy current loss of a large nuclear power generator. Second, the training set is automatically clustered into different subsets by the fuzzy c-means algorithm, and each subset is used to train the DGPR model to obtain different sub models. The membership degree of each data point in the test set is calculated and used to evaluate the sub model of the data. Then, the sub model is used to predict the eddy current loss. Finally, the result is obtained by concatenating the results of each sub model. The results show that the goodness of fit (R2) is 0.9809, the root mean square error (RMSE) is 0.0271, the prediction error is small, and the model exhibits good prediction performance. Further experimental results show that the FCM-DGPR method is superior to the existing DGPR models and other models and is more suitable for predicting the eddy current loss of large generators.

Influence of Rotor Damping Structures on Damping Torque Coefficient of Turbo-Generator During Low-Frequency Oscillation Caused by Large Disturbance

The damping structures of turbo-generators, such as rotor damping bars, core, and slot wedges, can provide positive damping effect for the low-frequency oscillation of power system. However, the magnetic saturation, distortion, and skin effect of generators make nonlinear damping effects during the low-frequency oscillation caused by large disturbance. In order to quantitatively describe the damping effect of the three damping structures, the damping torque coefficient (DTC) during the low-frequency oscillation is calculated by coupling time-stepping finite element method with the polynomial approximation method. The DTCs of turbo-generator with different rotor damping structures are obtained by comparing the individual and combined effect of the three damping components. Since there are several material options for the rotor slot wedges and damping bars, the influence of these materials on DTC is studied. A rotor damping structure that can not only improve the symmetry of rotor damping structure, but also enhance the DTC of turbo-generator is proposed. The results provide theoretical basis for power system stability enhancement by improving the rotor damping structures of turbo-generator.

Influence of Rotor Damping Structure on Speed Fluctuation and Asynchronous Operating Ability of Turbogenerators With Loss of Excitation

The asynchronous operation of turbogenerators with the loss of excitation can avoid equipment damage and improve the reliability of power systems. The rotor damping that provides asynchronous torque plays an important role in asynchronous operation. In order to study the influence of rotor damping structures—comprised of damping bars, slot wedges, and core—on asynchronous operating ability of turbogenerators, a time-stepping finite-element model is established and verified by the experiment on a model machine. By using this model, the relationship between the rotor damping structures and speed fluctuation during asynchronous operating process is studied. The asynchronous operating ability is compared in terms of the separate and combined effects of the rotor damping structures. Considering the material options for rotor slot wedges, the influence of aluminum, beryllium bronze, and stainless steel slot wedges on the speed fluctuation and the asynchronous operating ability is also studied. The study shows that the speed fluctuation of turbogenerators with the loss of excitation depends on the asymmetry of the rotor winding structure and the asynchronous operating ability depends on the average damping torques. The results provide theoretical basis for an approach to the enhancement of power system stability by improving rotor damping structures of turbogenerators.

Design and performance analysis of magnetic slot wedge application in double‐fed asynchronous motor‐generator by finite‐element method

In this study, an improvement method for double-fed asynchronous motor-generators (DFAMG) design with open slots is proposed to reduce the capacity demand of rotor converter for the first time. The proposed method is achieved by decreasing the air-gap length (AGL) and applying conductive and magnetic rotor slot wedges. The design method of magnetic slot wedges and AGL in large DFAMG is presented. The mechanical analysis of a large DFAMG prototype reveals the necessity of using metal rotor wedges. To precisely investigate the influence of magnetic wedges in DFAMG, a two-dimensional field-circuit coupled model is proposed to consider the effects of rotor converter. Several wedge configurations of commercially available products are designed and analysed. The relationships between the electromagnetic properties of rotor wedges and prototype performance are investigated. The feasibility and advantage of the proposed method in DFAMG design are verified. A guidance for applying the proposed method in design improvement of large DFAMG is developed.

The influence of turbine generator rotor damping structure and material on first swing stability

In this paper, we study the influence of rotor damping structure and material on First Swing Stability (FSS) of turbine generator connected to power system after three-phase short circuit. Considering the complex rotor damping structure of turbine generator, we make crucial improvements in finite element discrete strategy reflecting skin effect of eddy current and establish field-circuit-network coupled time-step finite element model. We test this model by the experiment of 7.5kW model machine. Based on this model, we take a 300MW turbine generator as an example to study the influence of damping bar, rotor iron core and conductive slot wedge on FSS. We investigate the importance of the rotor damping structure to FSS and find its impact is not negligible. By comparing the FSS limits affected by three components of rotor damping structure respectively, we conclude that the effect of rotor slot wedge and iron core is not linear superposition but Coupling relationship. Since we have several rotor slot wedge materials to choose from and the conductivity of these materials is different, we establish the relationship between FSS limit and conductivity of rotor slot wedge. The result shows that the FSS limit gradually increases as the conductivity of rotor slot wedge changes from σal (conductivity of aluminum alloy) to 0.05σal.

Transient electromagnetic field, losses and forces in a synchronous turbogenerator rotor

PurposeThe paper seeks to present a methodology of computer simulation of 3D transient electromagnetic fields, losses and forces due to negative sequence currents in fragments of large synchronous turbogenerator rotors. The methodology allows for the preparation of initial data for further computations of thermal and mechanical behaviour of rotors.Design/methodology/approachThe governing equations for 3D negative sequence transient electromagnetic fields with the Coulomb gauge using magnetic vector potential and scalar electric potential A, V – A are solved by the nodal finite element method in a Cartesian coordinate system moving synchronously with the rotor.FindingsThe presented methodology of 3D transient electromagnetic phenomena computation seems to be effective because the electromagnetic field in the rotor of a synchronous generator is generally three dimensional, and therefore 2D field‐computation approaches and software are not able to simulate intrinsically 3D electromagnetic processes in turbogenerator rotors.Research limitations/implicationsCurrently it is difficult to carry out accurate numerical simulation of 3D transient electromagnetic fields and therefore losses and forces within the whole structure of the rotor because of the resulting huge computational expenses. This paper is devoted to the finite element analysis of electromagnetic fields, losses and forces in separate structural parts of the rotor. As an example of practical utilization of the developed technique, the computer simulation of electromagnetic phenomena in junctions of nonmagnetic rotor slot wedges of a 300 MVA class synchronous turbogenerator is carried out.Practical implicationsThe methodology can successfully be used during the design process of modern large synchronous turbogenerators.Originality/valueThis paper presents numerical analysis of intrinsically 3D transient electromagnetic phenomena in large turbogenerator rotors.

Influence of rotor slot wedges on stator currents and stator vibration spectrum of induction machines: a transient finite-element analysis

The stator currents and stator vibration spectrum of a 45-kW four-pole induction machine is computed using a transient two-dimensional finite-element analysis. The analysis is performed for two different rotor geometries: with open and with closed rotor slots. Open rotor slots increase the harmonic content of the air gap field and also the harmonic content of the stator currents. Open rotor slots thus increase the stator vibration level for frequencies higher than 1 kHz.

Material and Microslip Damping in a Rotor Taking Gravity and Anisotropic Bearings Into Account

The paper is concerned with material and microslip damping in a rotor. The horizontal rotor is carried by anisotropic bearings, which means that the shaft feels three different frequencies, the rotational frequency and the difference and the sum of the rotational and vibrational frequencies. When material damping is studied, these three frequencies lead to three different equivalent viscous damping constants and the dissipated energy can be solved analytically. The rotor slot wedges in a turbine generator are used as an example of microslip damping. In this case the damping is nonlinear and the results are obtained through numerical simulations. The results show that these two different internal damping sources give both similarities and dissimilarities. The sign change and different magnitude of the dissipated energy running sub- or supercritical are the same. However the dissipated energy for material damping is not affected by gravity which microslip damping is.

Dynamic Instability of a Turbine Generator due to Microslip

The present paper is concerned with dynamic instability of a turbine generator due to friction between rotor slot wedges and the rotor. When the normal force on the wedge is constant the dissipated energy is of the same type as hysteretic material damping in the sense that for a circular motion excluding gravity it is independent of the rotational frequency, but changes sign when the rotational frequency exceeds the vibrational frequency. The magnitude of the dissipated energy will however depend on the rotational frequency as the normal force does. The transferred energy due to friction is a nonlinear phenomenon and approximately proportional to the amplitude cubed and may be much larger than material damping. It is also shown that when gravity is included or the motion is elliptical the energy transferred is larger than for a simple circular motion.

OPTIMAL DESIGN TO REDUCE DYNAMIC INSTABILITY OF A TURBINE GENERATOR DUE TO MICROSLIP

This paper is concerned with dynamic instability of a turbine generator to friction between rotor slot wedges and the rotor. The energy transferred due to friction can be reduced by increasing wedge length, coefficient of friction or normal force or by reducing the cross-section area of the wedge. The bearing damping is normally sufficient to master the energy input from the wedges. This bearing damping becomes more powerful the weaker the bearings are. The size of the rotor cross-section area affected by the wedge is difficult to estimate but it is shown to be of minor importance for the results, provided it is larger than the wedge-cross-section area.

The influence of relative slip range and contact material on the fretting fatigue properties of 3.5NiCrMoV rotor steel

The influence of relative slip range and contact material on the fretting fatigue properties of 3.5NiCrMoV rotor steel has been investigated. Two generator rotor slot wedge materials, 1CrMo steel and 2014A aluminium alloy, were employed as contact materials. The results indicate that the observed relationships between both relative slip range and contact material and the fretting fatigue strength of 3.5NiCrMoV steel can be fully rationalized in terms of surface fretting contact stresses. Relative slip range and contact material are themselves of secondary importance. Fretting fatigue tests have been interrupted and specimens broken open, demonstrating that small cracks can initiate by fretting at stresses considerably below the fretting fatigue limit. The size of these nonpropagating cracks is related to the frictional and normal forces at the fretting contact and is independent of externally applied stresses and relative slip range, at least over the range of conditions investigated. The maximum non-propagating crack size is approximately 0.25 mm. Fretting cracks initially grow at a shallow angle to the specimen surface, probably by a shear mode of crack extension, and are highly extended along the specimen surface. The results of the investigation indicate that fretting fatigue strength may best be assessed by a method which uses fracture mechanics to model the continued propagation of these small cracks; models which relate fretting fatigue strength simply to relative slip range (such as the Engineering Sciences Data Unit route) are inadequate.

Computation of current flow distribution in rotor-slot wedges of turbo-alternators

The use of the 3-D electromagnetic program, TRIFOU, for the solution of an eddy-current problem is described. The emphasis is on the facilities offered by the model (formulation) and by the program to solve a 3-D industrial problem with a reasonable number of elements. Displays of eddy-current distributions are shown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Friction coefficients of graphite over the temperature interval 25°C to 2450°C : Allen R. Driesner and Paul Wagner. Journal of applied physics, v. 29, June 1958, p. 901–903

The influence of relative slip range and contact material on the fretting fatigue properties of 3.5NiCrMoV rotor steel has been investigated. Two generator rotor slot wedge materials, 1CrMo steel and 2014A aluminium alloy, were employed as contact materials. The results indicate that the observed relationships between both relative slip range and contact material and the fretting fatigue strength of 3.5NiCrMoV steel can be fully rationalized in terms of surface fretting contact stresses. Relative slip range and contact material are themselves of secondary importance.Fretting fatigue tests have been interrupted and specimens broken open, demonstrating that small cracks can initiate by fretting at stresses considerably below the fretting fatigue limit. The size of these nonpropagating cracks is related to the frictional and normal forces at the fretting contact and is independent of externally applied stresses and relative slip range, at least over the range of conditions investigated. The maximum non-propagating crack size is approximately 0.25 mm. Fretting cracks initially grow at a shallow angle to the specimen surface, probably by a shear mode of crack extension, and are highly extended along the specimen surface.The results of the investigation indicate that fretting fatigue strength may best be assessed by a method which uses fracture mechanics to model the continued propagation of these small cracks; models which relate fretting fatigue strength simply to relative slip range (such as the Engineering Sciences Data Unit route) are inadequate.