Large Turbogenerators Research Articles

Mechanism of magnetic losses variation in stator‐end structures with windings extensions using space vectors

Electromagnetic and heat issues are more complex in the end region of a generator than in the line segment area. In this study, the electromagnetic field and related challenges are discussed. Stepped cores are considered in the performed calculations. Eddy-current losses variation mechanisms for structures in the end region and iron losses of end stepped cores are investigated under rated load condition, using space vectors. The effects of the end windings stretching out the end cores from the rotor and stator on the electromagnetic losses of the end structures studied in detail. The obtained results can be employed for developing large turbo-generators, and reveal a novel approach for reducing eddy currents and iron losses.

Numerical analysis of end part temperature in the turbogenerator end region with magnetic shield structure under the different operation conditions

Due to the serious overheating problem of end parts in the large turbogenerator end region, accurate calculation of end-part temperature has become a key design factor. In this paper, 3-D fluid and thermal coupling model in the end region of 1250 MW turbogenerator are given. The loss values gained from 3-D transient electromagnetic field calculation are applied to the solving end region as heat sources. Pressure value of fan outlet and fluid velocities of end region inlets from flow network calculation are applied to the solving end region as boundary conditions in the 3-D fluid and thermal coupling analysis model. The fluid flow and temperature distribution of end parts in the end region of large turbogenerator with magnetic shield structure are studied under the different operation conditions. The distribution of fluid velocity in the ventilation duct of stator end core is obtained. The temperature distribution laws of stator-end copper coils, screen plate, finger plate, press plate, and magnetic shield are researched under the different operation conditions. The location of the highest temperature point in the turbogenerator end region with magnetic shield structure is determined. The accuracy of numerical calculation is validated by experimental measured values.

Numerical calculation of CHTC on end metal parts and flow in end region of a turbogenerator

The convection heat transfer coefficient (CHTC) distribution of surfaces of end metal parts directly affects the temperature distribution of end metal parts. In this study, a large turbogenerator is analysed. To study the CHTC distribution of surfaces of end metal parts in detail, a three-dimensional (3D) fluid and thermal analysis model of whole turbogenerator end region is established. The losses of end metal parts from 3D transient electromagnetic field calculation are provided to end metal parts as heat sources. Pressure values of end-region outlets and fluid velocity of fan outlet from flow network calculation are provided to end region as boundary conditions in the 3D fluid and thermal analysis model. CHTC distribution laws of the surfaces of the clamping plate, finger plate, and copper screen are studied in detail by the finite volume method. The fluid flow and temperature distribution of end metal parts are determined in the end region of the turbogenerator. The calculated results are compared with measured data.

Analytical Algorithm of Calculating Circulating Currents Between the Strands of Stator Winding Bars of Large Turbo-Generators Considering the Air Gap Magnetic Field Entering Stator Slots

The accurate and fast calculation of circulating currents between strands for different transposition types is the important premise of the engineering design for stator bars in large turbo-generators. The air gap magnetic field partly enters into the stator slot, also generating circulating currents. However, there is a lack of fast simulation method for calculating the air gap magnetic field entering the stator slot in the existing calculation method for circulating currents. In this paper, the analytical algorithm of calculating circulating currents considering the air gap magnetic field entering stator slots is proposed, in which the equivalent circuit model of transposed strands in stator bars is established based on the superposition principle, the analytical algorithm of calculating the induced electromotive force is proposed based on the discrete integral method, and the analytical algorithm of calculating the air gap magnetic field entering stator slots is proposed based on the conformal mapping method. Then, a 1400-MW turbo-generator is taken as an example, circulating currents between transposed strands in stator bars under no-load and rated condition are calculated by the proposed analytical algorithm. Finally, the proposed analytical algorithm is validated by the two-dimensional finite element method.

Calculation of Temperature Distribution in End Region of Large Turbogenerator Under Different Cooling Mediums

In order to study the influence of different cooling mediums on the fluid velocity and the temperature distribution of end parts in the turbogenerator end region, a 330-MW turbogenerator is analyzed. Three-dimensional fluid-thermal coupling analysis model of the turbogenerator end region is given. When the different cooling mediums are used, fluid velocity of fan inlet and pressure values of end-region outlet from the flow network calculation are applied to the end region as boundary conditions, and the losses from 3-D transient electromagnetic field calculation are applied to the end parts as heat sources in the fluid-thermal coupling field. The fluid field and temperature field in the turbogenerator end region are calculated using the hydrogen and air as cooling medium, respectively. Fluid velocity and temperature of end parts in the turbogenerator end region are compared when the different cooling mediums are used. When the hydrogen is used as the cooling medium, comparing the calculated temperature results with the test values, the calculated temperature results match well with test values. These will provide a reference for the ventilation cooling design of large turbogenerator.

Optimal Scheme for Structural Design of Large Turbogenerator Stator End Winding

A three-dimensional (3-D) electromagnetic model of the end region of a 600 MW turbogenerator is set up. The variations of electromagnetic force densities of the stator end winding under rated load and three-phase short circuit from full load are obtained by numerical simulation. Then, the electromagnetic forces varying with time and space on the end winding are applied to the 3-D fine finite-element model established in ABAQUS/CAE for structural analysis, and the deformation and stress distributions of the stator end winding during rated load and three-phase short circuit are numerically analyzed by means of the ABAQUS software. Furthermore, the influences of structural and material parameters of the end winding on its dynamic responses under rated load are numerically investigated. Based on the numerical results, an optimal scheme for the design of the turbogenerator stator end winding is proposed.

Erratum to “Influence of Cooling Fluid Parameter on the Fluid Flow and End Part Temperature in End Region of a Large Turbogenerator” [Jun 16 466-476

Erratum to “Influence of Cooling Fluid Parameter on the Fluid Flow and End Part Temperature in End Region of a Large Turbogenerator” [Jun 16 466-476

Anti-vibration optimization of the key components in a turbo-generator based on heterogeneous axiomatic design

With the increasing demand for the cleaner production of electric energy, many countries around the world strive to develop new turbo-generators with advanced parameters and large capacities. In the design of a new turbo-generator, it is important to optimize the anti-vibration performance of its key components in order to ensure its safe and reliable operation. The anti-vibration optimization of the key components in a turbo-generator when a set of alternative schemes are provided for selection is a heterogeneous multi-attribute decision making (HMADM) problem involving deviation attributes, which cannot be solved by present HMADM approaches. To solve such a HMADM problem, a novel heterogeneous axiomatic design (HAD) method is developed by introducing the distance measure into axiomatic design for computing the information contents of the alternative schemes. With the attribute data in various mathematical forms unified as triangular fuzzy numbers, their probabilities of success can be conveniently calculated by a unified formula once their corresponding positive and negative ideal solutions are determined. As a result, the complexity and computational cost involved in computing the information contents of alternatives can be greatly reduced, which enables the efficient solution of the HMADM problem involving deviation attributes. A case study on the fixture scheme optimization for the stator end windings in a large turbo generator demonstrated the feasibility and effectiveness of proposed HAD. The ninth fixture scheme with the structural manufacturability being very good (VG), the natural frequencies at the turbine and exciter ends being [68.2, 78.5]Hz and [72.5, 83.2]Hz, the maximum amplitudes at the turbine and exciter ends being 67 μm and 72 μm was chosen as the optimal scheme from nine alternatives according to the HMADM results.

Influence of Cooling Fluid Parameter on the Fluid Flow and End Part Temperature in End Region of a Large Turbogenerator

In order to study the influence of the cooling fluid parameter on the fluid flow and end part temperature in the end region of the large turbogenerator, a 330-MW water–hydrogen–hydrogen cooled turbogenerator is analyzed. The fluid velocity and pressure values from the flow network calculations are applied to the end region as boundary conditions and the losses obtained from 3-D transient electromagnetic field calculations are applied to the end parts as heat sources in the fluid and thermal coupling analysis. After solving the fluid and thermal equations of fluid–solid conjugated heat transfer, the fluid velocity and end part temperature in the turbogenerator end region are gained under the different cooling fluid parameters. The influence of different fluid velocities and fluid temperatures in the water pipe inlet, and in the fan inlet on the fluid flow and end part temperature in the turbogenerator end region is researched. The calculation results of copper shield temperature are compared with the measured values. The calculation results coincident well with the measured values. These provide the important reference for better cooling turbogenerator end region.

Calculation and analysis of complex fluid flow in the multistage fan of a large turbogenerator end region

In order to study the distributions of complex fluid velocity and fluid pressure in the multistage fan of a large turbogenerator end region, the multistage fan in the 1407 MVA turbogenerator is analyzed. Characteristic curve of pressure and fluid rate of the multistage fan is determined. Static pressure distribution of dynamic and static fan blade surfaces and fluid velocity distribution around dynamic and static fan blade surfaces in the multistage fan are researched. The distribution laws of radial velocity, tangential velocity, and axial velocity between dynamic fan blade and static fan blade are given. The change laws of fluid static pressure and fluid total pressure are analyzed after fluid passes through dynamic and static fan blades. The calculated results match well with measured data. This research can provide an important reference for the optimal design and research of multistage fan in a large turbogenerator.

Research on the Impact of Screen Properties on Eddy Current and Flux in End Region of a Large Air-Cooled Turbo-Generator

Large turbo-generators are an important energy conversion device in power stations. Once a failure occurs in the turbo-generator, it not only affects the reliability of the power system but also causes enormous economic losses. Nowadays, large air-cooled turbo-generators have developed. The safe and steady operation of turbo-generator is important for the electric power system. Leakage magnetic field is a potential hazard in end region of a turbo-generator. Leakage magnetic flux could cause eddy current to be induced in the metal component of end region, which leads to heat generation. In this paper, aiming at the complex end structure of turbo-generator, a three dimensional (3-D) mathematic model used for electromagnetic field calculation in end region of a 150-MW air-cooled turbo-generator is established. Finite element method is used to calculate the 3-D magnetic field in end region of a turbo-generator. Magnetic flux leakage testing is conducted under noload condition. It shows that there is a good agreement between the theoretical simulation and the experimental results. Eddy current loss of end parts is gained based on the electromagnetic induction theory.

Instrumental Inspection of the Repair Quality of Stator Core Suspension in Turbogenerators

Results of instrumental inspection of the condition of the elastic suspension of the stator core in a high-power turbogenerator are considered. The effectiveness of repair is assessed by analyzing the dynamic characteristics of suspension elements during repair. Integrated diagnostic inspections regularly reveal (1 - 3) loosening of the joints between upper keybars and lamina- tions in large turbogenerators (165 MW and higher) with axial suspension of the stator core. The occurrence and de- velopment of such a defect is accompanied by increased vibration and sign-variable mechanical stresses in structural elements of loose sections of the suspension, which shortens their fatigue life. Moreover, impacts of the dovetail of a loose keybar on the core locally intensify the wear of the "dove- tails" of the keybar and the core. In the most critical cases, upper laminations of the stator core break and spall (Fig. 1). Fragments of laminations are direct danger to the insula- tion of the stator winding because they can pass through the ventilating passage and reach the surface of bars, where they, vibrating in variable magnet field, can cut the frame insula- tion to copper and, thus, cause earthing. Theoretical and full-scale studies show that a cause of in- creased local vibration is a decrease of the natural frequen- cies of loose keybars toward the resonance at the frequency of "magnetic" vibration of the stator core (100 Hz) caused by the radial attraction by the rotor poles. Currently, such defects are detected using an instrumen- tal method for testing the joints between the keybars and the core. The method involves measurement and spectral analy- sis of the free vibrations of certain sections of keybars after shock excitation (1, 2). Unlike visual inspections required by the Scope and Standards of Testing Electrical Equipment, the method allows us to quantitatively assess the actual condi- tion of the suspension. This substantially improves the reli- ability of detection and the sensitivity to incipient defects. Preserving and extending the residual life and ensuring the high operational reliability of high-power heavy-duty generators is a challenge which can be resolved with an inte- grated approach involving not only timely and reliable detec- tion of failures, but also development of repair measures and control of repair quality. Here we will analyze the experience of using the shock- pulse method to assess the effectiveness of repair measures intended to restore the vibration-isolation properties of the elastic suspension of the stator core of a high-power turbo-

Direct optimization of uncertain structures based on degree of interval constraint violation

A constrained interval optimization model is proposed for the optimization of uncertain structures with their mechanical performance indices described as the objective and constraint functions of the design vector and interval uncertain parameters. Present indirect approaches for solving such interval optimization models by converting them into deterministic ones will result in the loss of uncertainty information and deviate from the original intention of realistically modeling engineering optimization problems. To overcome these shortcomings, a novel optimization algorithm is proposed for directly solving the nonlinear constrained interval optimization models based on a novel concept of the degree of interval constraint violation (DICV) and the DICV-based preferential guidelines. A nested genetic algorithm (GA) is developed to realize the direct interval ranking of various design vectors. The outer layer GA locates the optimal solution based on direct interval ranking. The inner layer GAs integrated with Kriging technique compute the intervals of the mechanical performance indices of every design vector in the current population of the outer layer GA. The validity and superiority of the proposed direct interval optimization algorithm was verified by three numerical examples. Finally, the proposed direct interval optimization method was applied to the optimization of the cone ring fixture with uncertain material properties in a large turbo generator aimed at moving its natural frequencies away from the exciting one. The results demonstrated its feasibility and effectiveness in optimizing practical engineering structures under uncertainties.

Numerical calculation for circulating current in stator transposition bars of large water-cooled turbo-generators

Accurate calculation of circulating current is one of the key problems for stator transposition bars in the design of turbo-generators. Aimed at limitation that analytical algorithm of circulating current could not reflect the local electromagnetic field distribution and difficulty that overlaps easily exist in solid modeling process of stator transposition bars, a simplified physical model of transposition bars is established. A three-dimensional (3-D) numerical method for circulating current in stator transposition bars of large water-cooled turbo-generators is investigated, which is combined with field-circuit coupling method. Taking stator bars less than 540° transposition with void model of a 600-MW water-cooled turbo-generator as the research object, the magnetic flux density distribution, current density distribution and circulating current distribution of transposition strands are obtained by numerical calculation. Compared with calculation results of the improved analytical algorithm, the correctness of the numerical calculation for circulating current is demonstrated, the calculation value difference for the maximum current of strands is obtained. The numerical calculation for circulating current will provide an appropriate basis for the reasonable calculation of local overheating of stator transposition bars and the design of safety margin for turbo-generators.

Calculation and Analysis of the Surface Heat-Transfer Coefficient and Temperature Fields on the Three-Dimensional Complex End Windings of a Large Turbogenerator

With increased turbogenerator capacity and electromagnetic load, overheating of the complex end parts has become one of the main problems affecting safe and stable turbogenerator operation. In this research, a flow network was built representing the structural and ventilation characteristics of a 330-MW turbogenerator. The fan inlet velocity and pressures (boundary conditions) of each end-region outlet were obtained by the flow network method. The 3-D transient electromagnetic field in the turbogenerator end was calculated, and the eddy current losses (heat sources) of the end parts were obtained by the finite-element method. To study the surface heat-transfer coefficient distribution on the stator-end winding surface, fluid and thermal mathematical and geometric models of the whole turbogenerator end region were given. Using the finite-volume method, the surface heat-transfer coefficient distribution on the complex 3-D stator-end winding surface, fluid-flow distribution, and temperature distribution of the end parts were investigated under rated-load conditions. The calculated temperature results match well with measured data. This research can provide a theoretical basis for calculating the heat-transfer coefficients of the outer surfaces of large turbogenerators.

Robust optimization of structural dynamic characteristics based on adaptive Kriging model and CNSGA

Dynamic characteristics greatly influence the comprehensive performance of a structure. But they are rarely included as objectives in traditional robust optimization of structures. In this study, a robust optimization model including both means and standard deviations of dynamic characteristic indices in the objective and constraint functions is constructed for improving the structural dynamic characteristics and reducing their fluctuations under uncertainty. Adaptive Kriging models are employed for the efficient computation of dynamic characteristics. An intelligent resampling technology is proposed to save computational costs and accelerate convergence of Kriging models, which takes full advantage of the test points for precision verification, the sample points within the local region of the biggest relative maximum absolute error and the near-optimal point to improve the global and local precision of Krigings. The high efficiency of proposed intelligent resampling technology is demonstrated by a numerical example. Finally, an efficient algorithm integrating adaptive Kriging models, Monte Carlo (MC) method, constrained non-dominated sorting genetic algorithm (CNSGA) is proposed to solve the robust optimization model of structural dynamic characteristics. Kriging models are interfaced with MC method to efficiently compute the fitness of individuals during CNSGA. The implementation of proposed methodology is explained in detail and highlighted by the robust optimization of a cone ring fixture with lots of circumferentially distributed holes in a large turbo generator aimed at moving its natural frequencies away from the exciting one. The comparison of the optimized design with the initial one demonstrates that the proposed methodology is feasible and applicable in engineering practice.

Multidisciplinary Computer Simulation of Transient Electromagnetic-Thermal Phenomena in a Turbogenerator Rotor

A finite element technique and computer code for the numerical analysis of 3D transient electromagnetic fields and temperature distribution due to negative sequence currents in large synchronous turbogenerator rotors are developed. Electromagnetic-thermal phenomena in a 300 MVA class turbogenerator rotor during a line-to-line short circuit on two phases of the machine are numerically investigated. Effect of the various conditions of the rotor cooling on its thermal state is studied. Numerical results obtained can be used for the computer simulation of the thermo-mechanical behaviour of the rotor.

Numerical Calculation and Analysis of Three-Dimensional Transient Electromagnetic Field in the End Region of Large Water–Hydrogen–Hydrogen Cooled Turbogenerator

Due to the complexity of the structures and magnetic field distribution in the end region of large turbogenerators, using a 330-MW water-hydrogen-hydrogen cooled turbogenerator as an example, the 3-D mathematical and geometry models of the nonlinear transient eddy current field were given. Taking the nonlinearity of the core material and the influences of noncontact between the copper screen and the clamping plate, as well as the shape of stator end windings into consideration, the 3-D transient electromagnetic field was calculated, and the losses of different metal parts were obtained by the finite-element method. The calculated power losses were applied to the thermal field as heat sources. Temperatures of the copper screen were gained. The calculated results of copper screen were well coincident with the test data. Hence, the calculated results are accurate, and the method of calculation is effective.

Robust Optimization for Dynamic Characteristics of Mechanical Structures Based on Double Renewal Kriging Model

Present optimization approaches for the mechanical structures usually neglect the uncertainty of material characteristics and the influence of their dynamic characteristics on the overall performance. Moreover, the algorithms for solving the optimization problems are often inefficient. To overcome the above shortcomings, the robust optimization method of the dynamic characteristics of mechanical structures based on double renewal Kriging models is proposed. A robust optimization model including both the means and variances of the mechanical structures' dynamic characteristics in the objectives is established. The model includes the deformation of mechanical structures as constraints. The kriging models that can accurately and efficiently obtain the values of objective and constraint functions are constructed based on the Latin hypercube experimental design, double update strategy, and the sample point encryption technology in both the local area of maximum error and the neighborhood of the approximate optimal solution. And then all the Pareto optimal solutions to the robust optimization problem of the mechanical structures' dynamic characteristics are located by integrating a neighborhood cultivation-based genetic algorithm with the double renewal Kriging model. The robust optimal design of the dynamic characteristics of the cone-shape fixed structure of a large turbogenerator's stator end windings verifies the validity of the proposed method as well as its superiority over the deterministic ones.

Influence of Copper Shield Structure on 3-D Electromagnetic Field, Fluid and Temperature Fields in End Region of Large Turbogenerator

A new kind of the copper shield structure called the empty solid copper shield structure (ESCSS) in the large turbogenerator is proposed in this paper. Based on the complex structure characteristics of a 330-MW water-hydrogen-hydrogen cooled turbogenerator, the flow network within a half of the generator is established, and the total flow rate, pressure, flow rates (boundary conditions) of the various ventilation ducts and the chambers in the generator are separately obtained after solving the equations of the flow network when the traditional copper shield structure and the ESCSS are adopted. The 3-D transient electromagnetic field of the generator end region is calculated by using the time-stepping FEM, and the electromagnetic loss distributions (heat sources) are determined separately. Then, the fluid and thermal analysis model for the whole end region is established. Through numerical calculating, the whole end region 3-D fluid and temperature distributions are obtained separately. The results show that the new copper shield structure makes the copper shield temperature much lower. Meanwhile, the copper shield material is saved. The obtained conclusions may provide useful reference for the optimal design and research of the large turbogenerator.