Field In End Region Research Articles

Influence of Metal Shield Materials on Electromagnetic Field and Losses in End Region of Synchronous Condenser

A synchronous condenser (SC) used in an ultrahigh-voltage direct current system often operates under overexcitation and underexcitation conditions, which can dramatically change the field in the end region. A metal shield plays an important role in the end region of SC, and different metal shield materials have an effect on the distribution of the magnetic flux and loss. The selection of the metal shield is, therefore, a key factor that should be carefully considered in the design of SC. For a 300-Mvar SC with different metal shields, this article presents the mathematical models of the three-dimensional (3-D) transient electromagnetic field in the end region. The flux densities and losses of the end structural components are compared under the rated excitation and loss of excitation. From the detailed performance evaluations by the 3-D finite-element analysis, the variation laws of the flux densities at the key points and the losses of the end structural components along with the conductivity of the metal shield are revealed. The regions of the maximum loss in the end structural components are obtained. The results provide the theoretical basis for the improvement of the loss distribution in the end region of the SC.

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.

Finite-Element Calculation of 3-D Transient Electromagnetic Field in End Region and Eddy-Current Loss Decrease in Stator End Clamping Plate of Large Hydrogenerator

The stator clamping plate is an important component of hydrogenerators used to fasten the stator end core. Due to the complex leakage electromagnetic field in the end region, eddy-current loss in the stator clamping plate is usually very large, which will lead to partial overheat. Evaluation of eddy-current losses in stator clamping plates and reasonable structure design are the premise of ensuring the safe and stable operation of a large hydrogenerator. A 278-MVA hydrogenerator is taken as an example in this paper, and the calculation model of 3-D transient electromagnetic field of end region is established and validated by temperature test value. Then, a new arch-shaped clamping plate structure is presented. The transient electromagnetic field and eddy-current losses of end regions with traditional fan-shaped and new arch-shaped stator clamping plate structures are calculated by time-stepping finite-element method and are compared. Calculation results indicate that the new arch-shaped clamping plate structure can effectively reduce the eddy-current loss of stator clamping plate.

Influence of clamping plate permeability and metal screen structures on three-dimensional magnetic field and eddy current loss in end region of a turbo-generator by numerical analysis

A significant problem of turbogenerators on complex end structures is overheating of local parts caused by end losses in the end region. Therefore, it is important to investigate the 3-D magnetic field and eddy current loss in the end. In end region of operating large turbogenerator at thermal power plants, magnetic leakage field distribution is complex. In this paper, a 3-D mathematical model used for the calculation of the electromagnetic field in the end region of large turbo-generators is given. The influence of spatial locations of end structures, the actual shape and material of end windings, clamping plate, and copper screen are considered. Adopting the time–step finite element (FE) method and taking the nonlinear characteristics of the core into consideration, a 3-D transient magnetic field is calculated. The objective of this paper is to investigate the influence of clamping plate permeability and metal screen structures on 3-D electromagnetic field distribution and eddy current loss in end region of a turbo-generator. To reduce the temperature of copper screen, a hollow metal screen is proposed. The eddy current loss, which is gained from the 3D transient magnetic field, is used as heat source for the thermal field of end region. The calculated temperatures are compared with test data.

Validation of a Time-Harmonic Numerical Model for Solving Magnetic Field in End Region of a Radial-Flux Machine

We present an analysis of the magnetic field in the end region of a radial-flux rotating machine. In numerical simulations, we used three familiar boundary conditions to replace the modeling of the end shields and frame. We made measurements for comparison, and the simulation results were quite consistent with the measurements. Our analysis shows that the eddy current in the end shields and frame influences the magnetic field in the end region slightly and that the use of a homogeneous Neumann boundary condition or a standard impedance boundary condition (SIBC) to replace the end shields and frame can solve the magnetic field in the end region more accurately than a homogeneous Dirichlet boundary condition. Validation by the measurements demonstrates that 3-D current-driven time-harmonic model with suitable boundary conditions can be used to solve the magnetic field in the end region quite accurately.

3D nonlinear transient finite element analysis of eddy currents in the stator clamping system of large hydro generators

PurposeAn accurate calculation of eddy current losses in the stator clamping parts of large hydro generators is a matter of particular interest with the initial design and the design optimization because they can reach high values and produce local thermal hot‐spots due to the non‐linear magnetic behaviour of the clamping plate.Design/methodology/approachWith a fully 3D approach of the generator pole pitch, both time‐harmonic and non‐linear transient finite element analyses are carried out for the eddy currents using a magnetic vector potential formulation.FindingsWith the introduction of a novel modelling strategy for the non‐linear clamping plate, the total eddy current losses evaluated from both analysis methods show a good agreement. Nevertheless, the time‐harmonic solution in comparison with the non‐linear transient solution yields different local eddy current distributions in particular with the clamping plate.Research limitations/implicationsThe presented analyses use only the fundamental harmonic in the end region field. Further research will need to be carried out for the influence of the higher harmonics in the end region field and again the comparison of both analysis methods.Practical implicationsWith the intention of including the numerical analyses with design review and design optimization of the generators, the results obtained from both analysis methods are compared regarding the total eddy current losses as well as their local distributions.Originality/valueWith a fully 3D approach of the generator pole pitch, second order pentahedral and hexahedral edge elements are introduced with both time‐harmonic and non‐linear transient eddy current finite element analyses.

A cross-corner effect in a rectangular sputtering magnetron

Electron trajectories, ionization distribution, and magnetic field in a conventional rectangular sputtering magnetron cathode are simulated in order to understand the mechanism of a cross-corner effect, which is a common phenomenon associated with rectangular magnetron cathodes and which limits the target utilization. It is found that once the magnetic field in end region of the cathode is different from that in straightway, the cross-corner effect exists. Using a fourth-order Runge–Kutta method, the electron trajectories are simulated, showing that the electrons may drift much faster in the end region than in the straightway and pass quickly to cross-corner region. A Monte-Carlo method is employed to simulate ionization distribution and to quantitatively predict target erosion. The results show denser ionization in the cross-corner region, causing more intensive erosion in that area. We demonstrate that by properly modifying the magnet field in the end region, the electron drift velocity and ionization distribution can be controlled and the cross-corner effect may be significantly reduced.

Finite element analysis of electric machinery

A nonmathematical description of the finite element analysis method is presented. Application of the method to determining the excitation requirement of a large turbine generator is discussed. The handling of short-circuit behavior and end-region fields is considered. An extension of the finite element technique is described that allows one to find simultaneously the electromagnetic fields, the winding currents, and the rotor motion in order to understand the effect of harmonic current and voltage on large squirrel-cage induction motors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of rotor currents on end-winding forces in cage motor

The paper proposes a simple model that enables the influence of rotor currents to be included in the calculation of the electromagnetic forces acting on the end windings of cage induction motors, using the Biot–Savart method. The inclusion of the rotor currents is shown to have greatest influence close to core, where the end-region fields are also highest. The validity of the model is established using experimental measurements of flux density made on a two pole machine. Calculated force distributions show the effect of the rotor currents to be significant enough to merit their inclusion in the model.

A Method to Calculate Turbogenerator End Region Fields and Losses and Validation using Measured Results

Modern turbogenerators are designed with high specific loadings. One of the consequences of this is the requirement to design the machine to minimise the size and effects of stator end region leakage fields. This paper describes a finite element method, including both iron saturation and eddy current reaction, to calculate the losses induced in the stator by these fields. The calculations compare favourably with loss densities measured within the stator core of a full size machine. Axial flux densities calculated by the method also show good agreement with flux densities measured on open and short circuit and at full load leading power factor whilst the machine was in service.

A Method to Calculate Turbogenerator End Region Fields and Losses and Validation Using Measured Results

A Method to Calculate Turbogenerator End Region Fields and Losses and Validation Using Measured Results

Computation and validation against measurements of stator end-region fields in synchronous generators

Computation and validation against measurements of stator end-region fields in synchronous generators

A comparison of finite element and boundary element formulations for three-dimensional magnetostatic problems

Several numerical methods are currently in vogue for solving two-dimensional magnetic field problems [1,2,3]. Some of the techniques that have been efficiently applied to model 2D and axisymmetric fields are the boundary element method [4] and the finite element method [3]. However, engineering problems are truly three-dimensional, such as the end-region fields of electrical machinery, fringing fields of accelerator magnets, and others. There is now a growing interest in three-dimensional calculations for evaluating the field distribution. This paper presents the results of a three-dimensional magnetostatic problem solved by both the boundary element method and a hybrid finite element method. The formulations are described with the aim of illustrating the relative merits of each. The results are compared for a test case.

Circulating-current loss within Roebel-bar stator windings in hydroelectric alternators

Circulating currents are produced within stator windings of Roebel bars by endregion leakage fields, causing voltages to arise between the strands or subconductors. A method for calculating the endregion fields arising from both rotor and stator is given, and values of voltage for a simplified bar representation are obtained. The impedance of a Roebel bar to a circulating current is discussed, and values of circulating current are obtained for one machine. It is shown that the circulating currents in each bar are substantially independent of those in other bars, but that the reactance within each bar is of the same order as the resistance. The circulating-current loss is a small component of the total loss in the machine considered (10% of the stator I2R loss), but varies with the load and the power factor. Its significance is that two-thirds of it occurs in the top layer of the stator winding, increasing the temperature rise there by 13%. The theoretical analysis from which the above conclusions are drawn is confirmed by search-coil measuremnets which show an average agreement within 18%.

Calculation of the end-region field of a.c. machines

The rapid increase in electric loadings of turboalternators has demanded improved methods of predicting the end-zone field distribution. The paper describes an analytical method for calculating the flux distribution in the end zone of a.c. machines based on the solution of Laplace&apos;s equation for the scalar magnetic potential. All end-zone boundaries are taken into account, although simplifications of shape and of magnetic behaviour are made. Stator end turns are assumed to lie on a cylindrical surface. The case of an infinitely permeable core end surface is given special consideration, and equations are derived for the field components. The equations are in the form of double Fourier or Fourier-Bessel series, and convergence is discussed. An equation is also derived for stator-end-winding inductance.The validity of the equations has been checked by measurements on a simple model machine wound with a single coil per pole, which is the basic element in the analytical work. Agreement is encouraging. Calculated results for a large turboalternator are given. Some possible improvements in the method are briefly discussed.