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This paper presents a three-dimensional coupled approach for transient simulation of electromagnetic devices. The approach is based on the A-V-A formulation, in which the eddy current field and the electric circuit equations are solved simultaneously. The Newton-Raphson procedure has been used to take the magnetic non-linearity into account. The transient short-circuit state of a 240000 kVA power transformer is simulated by the coupled approach.

The paper presents a suitable functional in the form of a convolution of the magnetic vector potential in the space-time domain for transient nonlinear eddy problem. And then, appropriate element equations have been derived following the usual finite element variational procedure. Furthermore, the element equations are improved. Finally, the improved space-time finite element method (STFEM) has been used to compute the transient nonlinear electromagnetic field and performances of a permanent magnet synchronous motor (PMSM). Numerical illustrations have indicated that the improved space-time finite element method is a valid method to simulate performances of PMSM.

Intelligent simulated annealing (ISA) algorithm for the optimal design of electromagnetic devices is presented in this paper. The algorithm is implemented by assembling fuzzy inference into the improved simulated annealing algorithm, which gives the SA algorithm the ability to reject infeasible solutions prior to objective function computation. The algorithm is applied to the optimal design of a brushless DC motor, and about 60 percent of CPU time of an SA algorithm can be saved.

A suitable functional in the form of a convolution of the magnetic vector potential in the space-time domain for transient nonlinear eddy current problem has been derived. Appropriate element equations have thus been derived following the usual finite element variational procedure. The space-time finite element method has then been used to compute the transient nonlinear electromagnetic field in a practical transformer. Numerical illustrations have indicated that the computed results are good agreement with the tested ones.

The optimization of pole shoes in harmonic exciting synchronous generators is effectively performed using the combined strategy of improved simulated annealing and genetic algorithms with some new improvements made in this paper. The improvements include a rule for the generation of new states, an adaptive regulation schedule for the length of the interval of the pseudo-random parameter and the addition of geometrical constraints. Numerical results on different machines reveal that the iterative number used by executing the combined strategy with improvements is only about 50% of that used by executing the original method, whilst that the optimization results obtained using different algorithms are nearly the same.

The eddy current field due to both windings and heavy current leads (HCL) in large power transformers is analyzed by using the improved T-/spl Omega/ method, in which T and /spl Omega/ can be solved separately in different subregions. Thus, storage capacity and CPU time can be saved to a great extent. In addition, the local overheating on the tank wall near the magnetic shield gap is calculated and analyzed, and improvement measures are proposed. On the basis of this work, a composite shield is analyzed and optimized. The numerical results show that the eddy current losses are reduced and the local overheating is removed by using this composite shield.

A combined strategy of an improved simulated annealing (SA) algorithm and genetic algorithm is presented, with the goal of reducing the computational expenses. The improvements made on the SA algorithm include two parts, i.e., the adaptive regulating for the step vector, and the dynamic testing for the equilibrium of the Metropolis process. The proposed strategy has both the advantage of SA algorithm, the ability to avoid being trapped in a local optimum, and that of genetic algorithm, the ability to use the information about the searched states for the next iteration. A practical application on geometry optimization of pole shoes in large salient pole synchronous generators is effectively implemented using the strategy. The numerical results show that the number of iterations used by executing the combined strategy are only about 75% of those by executing basic SA algorithm.

The stray field of heavy current carrying conductors (HCCC) in large power transformers shows marked differences as compared to that of windings. New types of measures, separated magnetic steel plates welded on a nonmagnetic steel plate and closed loop shield, are presented to reduce the eddy current losses and to prevent the tank cover and walls from local overheating due to HCCC. Furthermore, the influences on the bolts and bushings are discussed. The BEM and FEM are employed to analyze the three dimensional eddy current field. Both numerical and experimental results show that no local overheating appears even if the phase current is up to 16 kA.

Large complex problems, eddy current fields and local overheating due to heavy current carrying conductors (HCCC) including leads and connecting bus bars in a 360 MVA/500 kV power transformer, are analyzed by the boundary element method. To improve numerical accuracy, this paper chooses triangle polar coordinate mapping and addition-substraction techniques to evaluate singular integrals. The distributions of magnetic flux and eddy current densities in metallic parts of different materials are discussed. According to the criterion of overheating, permissible lead current and limiting distance between HCCC and metallic parts and arrangements of HCCC are suggested. >

The improved T-/spl Omega/ method is applied to the computation of the three dimensional eddy current field in a 360 MVA/500 kV large power transformer, the structure of which is complicated. The different cases of tank wall with magnetic shunt, aluminium screen or without any shield are analyzed. The relations between eddy current losses and the materials, structure and size of the tank shield are presented. Compared with a parallel magnetic shunt arrangement, the vertical magnetic shunt reduces stray losses by 63.8%. The optimal thickness of the vertical magnetic shunt is 10 mm. The magnetic shunt should be spread over 0.5 mm at both ends of windings in the 360 MVA/500 kV large power transformer. >

A boundary element method (BEM) in terms of magnetic vector potential and electric scalar potential is successfully applied to three-dimensional, multiply connected, and open boundary problems caused by the heavy current of leads in a three-phase combination transformer. The quasi-frontal method is employed for the solution of the set of equations with asymmetric and full coefficient matrix. At most a quarter of the whole matrix needs to be stored. The numerical results are compared with experimental values and show very good agreement. Corrective measures for reducing the losses and preventing the box from locally overheating are presented. >

The improved T- Omega method is employed to calculate the three-dimensional transient eddy current field in large transformers, as well as the short-circuit electromagnetic forces acting on the transformer windings. The effects of the magnetic shielding and initial angles of currents are studied, and the nonuniform distribution of ampere-turn along the height of the windings is taken into account. Thus, a common software package is developed. Two different transformers are analyzed, and the computed results show agreement with measured ones. >

A method for the computation of the transient electromagnetic torque in turbogenerators is given in which the iron saturation and the relative motion of the rotor and stator are taken into account. This method is based on the finite-element computation using a dynamic model coupled with the equations of external electric circuits. To avoid the airgap mesh distortion caused by the movement of the rotor, moving boundaries are introduced in the airgap. An example is given that involves the computation of the distribution of transient magnetic field and the electromagnetic torque of a 200-MW turbo-generator following different types of sudden short-circuits. The computed results agree with the measurements very well. >

A method using coupled fields and circuit equations is presented to calculate the end-region magnetic field, the current distribution in stator winding strands, and the circulating losses of the stator winding in large turbogenerators. In the calculation, the nonuniform distribution of current density in the stator winding and the influence on the end-region magnetic field of the copper shield are considered. Finally, the effect of the transposed angle on the circulating losses is analyzed. The method is used to calculate the end-region magnetic field and the current of the stator winding strands in a two-pole 300-MW generator. Numerical results are presented. >

A method combining the improved R- Psi method (in which the electrical vector potential R only exists in the conducting region) and the IEM (integral equation method) is described which synthesizes the advantages of the two methods and reduces the CPU time and the storage space required to solve 3-D eddy-current problems in open regions. The IEM is used to compute the magnetic field and losses due to heavy-current leads and the improved R- Psi method is used to calculate the magnetic field and losses due to windings. The coupled method is used to calculate the three-dimensional sinusoidal eddy-current field related to the open region in a 360 MVA/500 kV power transformer. The eddy current and loss computation and the estimation of the local hot spots makes it possible to prevent a large transmitter from local overheating. >