Weijie Xu

Dynamic Deformation Analysis of Power Transformer Windings in Short-Circuit Fault by FEM

This paper presents the investigations of short-circuit current, electromagnetic force, and transient dynamic response of windings deformation including mechanical stress, strain, and displacements for an oil-immersed-type 220-kV power transformer. The worst-case fault with three-phase short-circuit happening simultaneously is assumed. A considerable leakage magnetic field excited by short-circuit current can produce the dynamical electromagnetic force to act on copper disks in each winding. The two-dimensional finite element method (FEM) is employed to obtain the electromagnetic force and its dynamical characteristics in axial and radial directions. In addition, to calculate the windings deformation accurately, we measured the nonlinear elasticity characteristic of spacer and built three-dimensional FE kinetic model to analyze the axial dynamic deformation. The results of dynamic mechanical stress and strain induced by combining of short-circuit force and prestress are useful for transformer design and fault diagnosis.

Hysteresis Modeling of High-Temperature Superconductor Using Simplified Preisach Model

This paper presents a hysteresis model for the high-temperature superconductor using a simplified Preisach model, in which the Preisach distribution function is determined only based on the M-H limiting loop. The results of simulation and experiment of accuracy of the simplified Preisach model are reported for the case of silver-sheathed Bi2223 superconducting tape samples with an external magnetic field perpendicular to the tape surface.

Optimum Design of Rotor for High-Speed Switched Reluctance Motor Using Level Set Method

This paper presents a novel rotor structure optimized by using the level set method (LSM) to improve the static torque characteristics of a two-phase 4/2 high-speed switched reluctance motor. The magnetic equivalent circuit approach is used to analyze the influence factors on the static torque. The inner material boundary contained in the rotor saliency, which will be optimized, is implicitly represented by an embedded level set function. The evolution of the inner material boundaries is driven by the normal velocity derived through sensitivity analysis and adjoint variable computation. Optimal rotor configuration produces the nonuniform distribution of magnetic flux density in the air gap at aligned position, which can make the ratio of maximum and minimum inductances increased. High-permeability material is applied to enhance the mean torque. In order to achieve the optimal static torque, the strategy combining finite-element electromagnetic computation with LSM is conducted to optimize the rotor saliency at different rotor positions. The comparison of the optimized rotor configuration using LSM with that using density-based method suggests that the optimized rotor structure obtained using this presented method is easier to implement.

An Improved XFEM With Multiple High-Order Enrichment Functions and Low-Order Meshing Elements for Field Analysis of Electromagnetic Devices With Multiple Nearby Geometrical Interfaces

This paper proposes an improved extended finite element method for modeling electromagnetic devices with multiple nearby geometrical interfaces. In regions near these interfaces, the magnetic vector potential approximation is enriched by incorporating multiple derivative discontinuous fields based on the partition of unity method such that the interfaces can be represented independent of the mesh. The support of a node or an element can be cut by several interfaces. This method results in the high accuracy in the approximation field and derivative field. Numerical examples applied to the iron core in 1-D eddy current field involving level set-based parts, error analysis, and electromagnetic field computations are provided to demonstrate the utility of the proposed approach.

A new Preisach type hysteresis model of high temperature superconductors

This paper presents a new Preisach type hysteresis model for the high temperature superconductor. This model requires only the limiting hysteresis loop as the input data, and for this model, the limiting hysteresis loop is first separated into two limiting M−H loops based on the mechanisms, which can then be modeled by two separate modified Preisach algorithms. The area integrations of the Preisach distribution functions are determined only based on the limiting M−H loops. The validity and accuracy of this model is confirmed by comparing the simulation and experiment results of Bi-2223 and YBa2Cu3Ox superconducting tapes with external magnetic fields.

Analysis of Inter-Turn Insulation of High Voltage Electrical Machine by Using Multi-Conductor Transmission Line Model

In this paper, the inter-turn insulation of stator winding is comprehensively discussed. An equivalent circuit model and a multi-conductor transmission line (MTL) theory are established for the inter-turn voltage evaluation under the switching-impulse voltage, respectively. The distributed inductances are calculated by using small-signal analysis in finite element method (FEM) and the ac resistances of coils are analyzed considering the skin effect. The distributed capacitances of the coils are determined by using both electric field FEM and analytic equations. The electric potential distribution of inter-turn is calculated by the above two methods. The incident and reflect voltages are calculated by using the MTL model. The electric field of inter-turn calculated by using the FEM is presented. The FEM is used for the calculation of the inter-turn electric field distribution. The position where the inter-turn insulation may be more prone to breakdown is determined according to the result. Finally, the impact factors of inter-turn insulation including insulation materials and positions of failure in winding are analyzed in detail.

Extended Finite-Element Method for Electric Field Analysis of Insulating Plate With Crack

In this paper, the extended finite-element method (XFEM) is employed to analyze electric field of insulating plate with crack. To begin with, both the approximation of XFEM and features of the enrichment functions for crack tip are presented. Then, XFEM is used to solve electric potential of an insulating plate with a middle crack, which proves the method can save computational time by comparison with classic finite-element method (CFEM) under the same computational accuracy. In addition, XFEM is also applied to calculate electrical field at crack tip of an insulating plate with an edge crack, which reveals that the relative error between XFEM and CFEM is small when the meshing size of CFEM around crack tip is smaller than that of XFEM. Therefore, electric field analysis of discontinuous and singular solutions can demonstrate the superior performance of XFEM.

Modeling and Measurement of Magnetic Hysteresis of Soft Magnetic Composite Materials Under Different Magnetizations

The soft magnetic composite (SMC) materials and their application in electromagnetic devices have undergone significant development due to their unique advantages, such as low eddy-current loss, quasi-isotropy of mechanical and magnetic properties, low cost, and low material consumption during the production process. However, the magnetic properties of the SMC materials are very different from that of the laminated SiFe materials, which are particularly challenging for the design and application of electrical machines. This paper presents the modeling and measurement of magnetic properties of the SMC materials under both alternating and rotational magnetic excitations. Based on the underlying magnetization mechanisms, a vectorial elemental operator with biaxial anisotropy is introduced, and the concept of distribution function is utilized to describe the density of operators in the specimen. To verify this proposed model, the magnetic hysteresis of the SMC material is simulated and compared with the experimental results obtained by the three-dimensional magnetic property measurement system. The good agreement shows the validity and practicability of this vectorial elemental operator.

Extended finite element method for electromagnetic fields

This paper presents the fundamental principle of the extended finite element method (XFEM) for electromagnetic field analysis. This method provides an accurate approximation for locally non-smooth features within finite elements, such as singularities, discontinuities, and high derivatives. An alternative enrichment function is introduced to improve the approximation space of the conventional finite element method (CFEM) such that non-smooth solutions are modeled independent of the mesh. The level set method is employed to describe the interfaces among different materials. To demonstrate the advantages, the XFEM is compared with CFEM by solving a 1D electrical field problem.

Modeling and Insulation Performance Analysis of Composite Transmission Line Tower Under Lightning Overvoltage

In this paper, a new non-uniform multiconductor transmission line model combining with lumped circuit elements for a composite transmission line (TL) tower under lightning strokes is established. The metal ground-wire cross arms, the metal tower main body and its bracings are modeled as equivalent horizontal and vertical TLs, respectively. The two grounding ladders are regarded as two parallel-vertical TLs considering their mutual coupling. These distributed TL parameters are calculated by analytic equations. The composite arms are equivalent as lumped capacitances determined by employing 3-D finite-element method. The lightning overvoltage analysis for a 330-kV composite tower is performed by applying the hybrid model. The incident and reflected voltages are calculated. The insulation performance evaluation for the composite tower is discussed in detail.