Search

Purpose This paper aims to develop a methodology for progressive finite element (FE) modeling of transformers, from simple to complex models of both magnetic cores and windings. Design/methodology/approach The progressive modeling of transformers is performed via a subproblem (SP) FE method. A complete problem is split into SPs with different adapted overlapping meshes. Model refinements are performed from ideal to real flux tubes, one-dimensional to two-dimensional to three-dimensional models, linear to nonlinear materials, perfect to real materials, single wire to volume conductor windings and homogenized to fine models of cores and coils, with any coupling of these changes. Findings The proposed unified procedure efficiently feeds each SP via interface conditions (ICs), which lightens mesh-to-mesh sources transfers and quantifies the gain given by each refinement on both local fields and global quantities, with a clear view on its significance to justify its usefulness, if any. It can also help in education with a progressive understanding of the various aspects of transformer designs. Originality/value Models of different accuracy levels are sequenced with successive additive corrections supported by different adapted meshes. The way the sources act at each correction step, up to the full models with their actual geometries, is given a particular care and generalized, allowing the proposed unified procedure. For all the considered corrections, the sources are always of IC type, thus only needed in layers of FE along boundaries, which lightens the required mesh-to-mesh projections between subproblems.

Due to differences in the manufacturing process, centrifuged rotors have higher efficiency and interbar resistance than those produced by the high-pressure die-casting process. Therefore, the annealing applied to high-pressure die casted rotors to improve the insulation of the cage is not traditionally applied by manufacturers to centrifuged rotors. This study evaluates the effect of the traditional 450 °C heat treatment on centrifuged rotors, as well as the feasibility and effects of 520 oC rotor annealing, temperature not recommended for high-pressure die-casted rotors. The efficiency measurements were performed with a dynamometer, using small single-phase motors, usually applied in domestic refrigeration compressors, with a standard stator kept under controlled conditions. A structured test was performed to assess the effects in a mass production scenario, aiming to reflect the variation of the production parameters within the manufacturer specifications.

The need for models that represent the behaviour of transformers operating in steady state or submitted to transient events is of great importance in studies involving power systems. In this context, this study presents a model used to represent the impedance frequency response of transformers at no load and under nominal conditions. In the model, networks of parameters with RLC cells are employed to reproduce the resonances present in the frequency responses and also RL network sections are used to represent the frequency-dependent effects in the core, which aid in the current damping during transient events. The parameter values of these networks are determined with the least-squares numerical method. To evaluate the model as well as the methodology to determine the parameter values, data originating from measurements of the frequency responses with frequency range from 10 Hz to 2 MHz of two single-phase transformers (16 kVA and 55 MVA) are used. Validations of the model are carried out in the frequency and time domains. The model can be used to simulate transformer operation under nominal conditions of voltage and frequency and also for a broad frequency spectrum.

This paper presents an equipment dedicated to the monitoring of synchronous generator through the measurements of the external magnetic field time derivative. The equipment uses a methodology which can be an additional technique to those applied currently on the faults detection, mainly when monitoring the incipient faults type. Based on frequency spectra (magnetic signatures), the faults detection is performed by the analysis of the components evolution. As it is not possible to test the ability of the equipment in detecting several types of faults of generators faults in a power plant, a testbench was constructed for this purpose. Faults inserted deliberately in the generators of the testbench are detected by the equipment, showing its capacity. This equipment has been also installed in power plants.

A subproblem finite element method is developed for modeling the transformer core joints. It applies magnetostatic and magnetodynamic models on progressive geometries and different components of the solution, supported by different meshes. It allows an efficient and robust analysis of magnetic circuits in any frequency range, with an accurate calculation of flux density, losses, reluctance, and impedance in transformer core joint zone. The models of the study properly account for the effects of core design parameters such as length of air gaps and overlap length in stacked-lamination cores. The proposed models, which include saturation, are applied to grain-oriented silicon steel and two types of step-lap joints are considered: single-step-lap (SSL) joints and multistep-lap (MSL) joints. The values of magnetic reluctance, impedance, and Joule losses obtained with SSL joint are bigger than with MSL joint.

This study has analyzed the performance of an alternating electric current measuring system using a Rogowski coil as a transducer, applied to magnitudes of milliamps to hundreds of amperes. It has been noticed that for low amplitudes, as well as for low frequencies, noise interferes with precision. For operation levels with a better signal-to-noise rate, the results presented agreements with those from a Hall effect current probe. Although prototypes have been carefully constructed, performance can be improved by the use of more appropriate winding techniques as well as through improvement in electronic circuits.

This paper deals with the electromagnetic modeling and design aspects of brushless doubly fed reluctance machines (BDFRM). It presents the development and the experimental validation of a semi-analytical model (SAM) used for sizing new machine designs. The SAM is an optimization-oriented model implemented using analytical approaches and a deterministic optimization algorithm to find an optimal machine by solving iteratively an objective function, whereas satisfying several output constraints simultaneously. It is fast and accurate for pre-design stages, premises that are confirmed by both simulation and experimental results.

In this paper we propose an original system for replacing batteries or feeding cables used to feed monitoring equipments exposed to stray magnetic fields. The main elements of this system are a coil intended to capture the energy from magnetic field and an electronic circuit for voltage controlling. Two voltage control systems are presented: a DC–DC converter yielding a regulated voltage and a voltage tripler circuit carrying a load capacitor. This system furnishes approximately 10 mW depending on the field magnitude and the coil core material. It is shown that the low consumption feeding circuit is effective and the use of core material may increase the furnished power to up to 25 %.

This paper presents a method to analyse electrical machines considering simultaneously the electromagnetic field, electric circuit, control loop, movement and skewing effects. The major contribution of this work leans on its generality, i.e. it can be applied to electrical machines connected to static converters submitted to any control laws, avoiding an a priori analysis. Simulation results of a three‐phase Brushless AC (BLAC) motor fed by a PWM converter is presented as well as a comparison of simulation and experimental results obtained using a two‐phase‐on converter were also presented.

An original and easy‐to‐implement method to take into account movement (motion) in the 2D harmonic balance finite element modelling of electrical machines is presented. The global harmonic balance system of algebraic equations is derived by applying the Galerkin approach to both the space and time discretisation. The harmonic basis functions, i.e. a cosine and a sine function for each nonzero frequency and a constant function 1 for the DC component, are used for approximating the periodic time variation as well as for weighing the time domain equations in the fundamental period. In practice, this requires some elementary manipulations of the moving band stiffness matrix. Magnetic saturation and electrical circuit coupling are considered in the analysis as well. As an application example, the noload operation of a permanent‐magnet machine is considered. The voltage and induction waveforms obtained with the proposed harmonic balance method are shown to converge well to those obtained with time stepping.

International audience; Purpose – The purpose of this paper is to develop a subproblem method (SPM) for progressive modeling of inductors, with model refinements of both source conductors and magnetic cores.Design/methodology/approach – The modeling of inductors is split into a sequence of progressive finite element (FE) SPs. The source fields (SFs) generated by the source conductors alone are calculated at first via either the Biot-Savart (BS) law or FEs. With a novel general way to define the SFs via interface conditions (ICs), to lighten their evaluation process, the associated reaction fields for each added or modified region, mainly the magnetic cores, and in return for the source conductor regions themselves when massive, are then calculated with FE models. Changes of magnetic regions go from perfect magnetic properties up to volume linear and nonlinear properties, and from statics to dynamics.Findings – For any added or modified region, the novel proposed ICs to define the SFs appear of general usefulness, which opens the method to a wide range of model improvements.Originality/value – The resulting SPM allows efficient solving of parameterized analyses thanks to a proper mesh for each SP and the reuse of previous solutions to be locally corrected, in association with novel SF ICs that strongly lighten the quantity of BS evaluations. Significant corrections are progressively obtained for the fields, up to nonlinear magnetic core properties and skin and proximity effects in conductors, and for the related inductances and resistances.

An efficient and robust time discretization procedure of theta type is proposed in the frame of the finite element‐circuit equation coupling for electronic circuits with switches, i.e. the use of diodes, thyristors and transistors. This procedure enables the use of the Crank‐Nicolson scheme whatever the circuit and its working conditions by eliminating the undesirable oscillations of the solution peculiar to this scheme. It is based on the accurate determination of the switching instants and on a local modification of the theta parameter.

International audience; The Brushless Doubly-Fed Reluctance Machine (BDFRM) is a potential candidate to compete with induction and permanent magnet machines in variable speed application such as wind power. It is in research phase and much effort, mostly on its electromagnetic design, is still needed so that this machine can be used in large scale. This paper focuses on rotor design by studying the impact of geometric parameter variations on its global performance by using Finite Element Analysis (FEA). Issues with the flux barrier ending's geometry, number of rotor slots, existence of an additional interpolar flux barrier, rotor slot arc angle and duct ratio are investigated. The results show potentially good practices that should be considered on BDFRM rotor design. They may also be used as initial step for a more refined optimization process using specialized tools

Considering the importance of modeling the electri- cal power system behavior under high frequency transients, this paper aims to present a practical methodology to obtain a RL lumped parameters model of an air-cored reactor. Firstly, the RL frequency responses are obtained using the Finite Element Method (FEM). Thus, the frequency response is converted into a RL circuit in accordance with the classical series Foster model and its parameters values are calculated through Least Square (LS) fitting algorithm. Finally, the lumped parameters circuit is converted to an ATP network model. For validation purposes a well-known air-cored reactor is modeled for a frequency range from 10 Hz to 100 kHz and the frequency responses obtained with FEM, LS fitting and ATP are compared. Index Terms—Frequency response, Finite Element Method, Least Square, ATP, Air-cored reactor

This paper presents the implementation of the control loop in the simulation of the association converter/electromagnetic structure represented by two-dimensional finite element method. The state variable equations of the converter are constructed entirely in an automatic manner. These equations are then coupled to the electromagnetic structure equations, and finally solved simultaneously using time stepping procedure. In order to validate the proposed method, a simulation of an inverter connected to an electromagnetic structure including an hysteresis current control scheme is performed.

This paper presents an experimental comparative study of iron losses obtained by means of the Epstein frame, Ring Core and dynamometer tests with non-oriented electrical steel samples. The comparisons are qualitative and they are intended to show the importance of carrying out other tests to evaluate electrical steels in addition to the standard Epstein frame method. In industry, complex or costly tests are avoided, whenever it is possible. This study shows that the ring core test can be a low cost solution since it is effective. For the manufacturer point of view, comparing different commercial steels at purchase time with a simple process is an advantage.

This paper deals with the development of an experimental workbench to evaluate stator cores of fractional induction motors. The bench is able to evaluate the quality of the stator core in terms of the magnetic material, dimensions as well as manufacture faults like short-circuit between laminations. It is shown the steps to develop the equipment and the first results.

This paper presents a comparative study of iron losses obtained by means of the Epstein frame and Ring Core tests with non-oriented electrical steel samples. The comparisons are qualitative and they are intended to show the importance of carry out other tests to evaluate electrical steels in addition to the standard Epstein frame method.

In this paper the perturbation technique is developed for modeling of the magnetic shunt and an aluminum screen in large power transformers. The problem is first studied using either a simplified analytical or a finite element model. A more elaborated model is then defined to take into account its structural details. The calculation of iron losses and of the minimum clearance between bus bars leading current and magnetizable steel parts for a frequency range is performed. The modeling takes into account the eddy currents in magnetic shunt laminations due to the magnetic field in the shunt. © 2010 IEEE. ispartof: Digests of the 2010 14th Biennial IEEE Conference on Electromagnetic Field Computation, CEFC 2010 ispartof: Biennial IEEE Conference on Electromagnetic Field Computation (CEFC) location:Chicago, Illinois, USA date:9 May - 12 May 2010 status: published

This paper presents an analysis of a 75 kW brushless doubly fed induction generator (BDFIG) prototype. Experimental results are presented for the prototype in the cascade mode that enables a comparison in terms of efficiency and power factor with an equivalent wound-rotor doubly fed induction generator. The generator's control system is based on vector control principles which enable to decouple active and reactive power control. The power flow in the two stator windings (power and control windings) and in the grid is analyzed with and without the reactive compensation capacitor bank in the grid using a MatLab/Simulink simulation model. Four cases were studied: (a) the reactive power compensation of the machine is realized only by the control-side converter; (b) the reactive power compensation is realized by both converters (control-side and grid-side converters); (c) the reactive power compensation of the machine is performed only by the control- side converter and a capacitor bank connected at the grid terminals and (d) the reactive power compensation is performed by both converters and a capacitor bank is connected at the grid terminals. The main advantage is obtained with the last case resulting in a significant improvement of the overall system efficiency.

In this paper a performance comparison between a classic wound rotor double-fed induction generator and a 75 kW brushless double-fed induction generator prototype is presented. The main objective of this comparison concerned the respective efficiencies at three distinctive points of operation. A simulation using MATLAB/SIMULINK/SimPowerSystems was made that extends this comparison to the whole speed operation range. The results show that at rated speed, the efficiency of the brushless generator is only about 1% lower than the generator fitted with brushes. The cost of operation of both machines was compared taking into account the difference of generated energy due to the relative efficiency as well as the maintenance costs. Also, an analytical and experimental analysis of the generators vibration performance was performed and the results are presented. These results show that the design criteria adopted led to a category "A" vibration behavior according to standard ISO 2372.

This paper analyses performance aspects of a brushless double fed cage induction motor as a new driving alternative to the classical wound rotor induction motor used in a car shredder drive. It focuses in a real application of 3150 kW - 4160 V - 60 Hz - 514 rpm wound rotor induction motor. Firstly it analyses the car shredder duty and compares the performance of the two motors without a controller. The starting condition is analyzed and a Matlab/Simulink simulation is used to show the advantages of the brushless double fed cage induction motor over the classical wound rotor induction motor, in rated operation condition. The performance of the two drives is compared using the simulation results

PurposeThe paper seeks to develop dual 3D finite element (FE) formulations for modeling both inductive and capacitive effects in massive inductors, in particular micro‐coils. The paper aims to build circuit relations relating the voltages and the currents in such inductors to be used in circuit coupling.Design/methodology/approachA circuit relation involving a unique voltage and complementary inductive and capacitive currents is defined for each inductor. The inductive circuit relation is first classically obtained by a FE magnetodynamic model. Then, the capacitive relation is obtained through a FE electric model, using sources evaluated from the first model. The conformity is defined on one hand for the magnetic flux density and the electric field, and on the other hand for the magnetic field and the electric flux density. Mixed FE, i.e. nodal, edge and face elements, are used to satisfy each chosen conformity level for the unknown fields and to naturally define the involved global quantities, i.e. the voltages, currents and charges.FindingsThis contribution points out the interest of satisfying conformity properties for the coupled magnetic and electric problems. An accurate computation of these effects is obtained in the critical frequency range of their strong interaction. In addition, the complementarity of dual solutions gives the possibility to estimate the discretisation error.Originality/valueThe mathematical and discretisation tools for any wished conformity level are unified for naturally coupling magnetic and electric problems. The global quantities basis functions involved in the FE circuit relations benefit from a significant support reduction, which facilitates their evaluation and gives them direct physical interpretations.

The single phase equivalent circuit is largely used to model the three-phase induction motors in steady-state operation and under sinusoidal balanced voltages. Depending on the desired application, the circuit may or may not represent core losses, a double cage or even the variation of parameters due to skin effect and saturation. However, the determination of the circuit parameters through standard methods, such as those described in IEEE Standard 112, may not be possible in many situations given the lack of the necessary resources. This paper presents initially a survey on the determination of circuit parameters from alternative methods, i.e., non-standard tests. Special focus is given to methods which employ only data usually provided by manufacturers on catalogs and nameplates. Six analytical methodologies used in the context of efficiency estimation at steady-state operation are assessed, compared and then combined in order to improve results. The assessment is based on the closeness of the resulting parameter values to reference values and on the inexistence of absurd results, such as negative electrical resistances. The combination of methods has improved the accuracy of calculations for the studied motors.