Weimin Guan

Finite Element Modeling of Heat Transfer in a Nanofluid Filled Transformer

In order to investigate the heat transfer characteristics of transformer oil containing nanoparticles, an electric-thermal-fluidic analysis using the FEM is carried out in this paper. The temperature and velocity distribution of oil and density of nanoparticles in a nanofluid filled transformer under natural and forced convections are obtained. The results show that generally the heat transfer efficiency is better under the forced convection. In addition, the distribution of nanoparticles in the oil depends on the temperature and velocity of the oil. This indicates that the local density and electrophoresis of nanoparticles must be considered for nanofluid application in transformers because of their potential influence on the temperature and velocity of the oil. Therefore, improvement of the simulation model taking into account of the local density of nanoparticles using the effective thermal conductivity had been carried out. Furthermore, electrophoretic force term had been implemented in Navier-Stokes equation, whereas the effective dielectric permeability was obtained by a homogenization technique using FEM.

Analysis of Excess Loss in SiFe Laminations Considering Eddy-Current Dominated Domain Wall Motion

To develop efficient silicon steel plates and electric machines using silicon steel laminations, the evaluation of excess loss in silicon steel plates is important. In this paper, investigation on excess loss calculation using the finite element method is carried out. The excess loss, which is mainly due to two factors, lack of flux penetration and domain wall pinning, is considered by the proposed approach using the domain wall bowing degree (DWBD), which depends on the above two factors. The DWBD is obtained using nonlinear eddy-current analysis. The correlation between the excess loss and the DWBD can be expressed as an exponential equation. Using the equation, the excess loss can be predicted under harmonic frequency in the range of 50-2000 Hz. These results can be applied to iron loss estimation, especially, when there are harmonics in the power supply.

A Novel Hybrid Saturated Core Fault Current Limiter Topology Considering Permanent Magnet Stability and Performance

The saturated core fault current limiter (SFCL) is one of the most effective methods for limiting fault current. A permanent magnet (PM)-biased SFCL (PFCL) can reduce dc bias current and additional energy consumption. However, the thermal stability of NdFeB PMs in PFCL poses a major challenge. This paper proposes a novel hybrid-type SFCL (HSFCL) topology that can reduce this risk. In the HSFCL, a hybrid PM/dc coil is designed to saturate the core, and a fault-limiting reactor is used to guarantee fault-clipping performance. The new structure provides air-gap branches for the time-varying magnetic field produced by alternating current, which reduces the eddy current in the PMs and enhances their stability. Electromagnetic coupling is used to analyze the operating principles and characteristics of HSFCL. Finite-element analysis simulations were performed using Maxwell and compared with verification experiments, proving that HSFCL is efficient in fault clipping and PM reliability enhancement.

Numerical Modeling of Excess Loss in SiFe Sheet Considering Pinning Effect

Compared with grain-oriented SiFe sheets, the domain structure of nonoriented (NO) SiFe sheets is very different, and the magnetization processes of NO SiFe sheets are much less investigated. Thus, the excess loss of NO SiFe sheets is less well-known. In this paper, we propose a numerical method to estimate the excess loss in NO SiFe sheets, considering the pinning and skin effects. We apply an initial magnetic field distribution that reflects the pinning effect in our nonlinear eddy current finite element analysis model, which reflects the skin effect of the magnetic flux. Thus, the pinning effect can be coupled with the skin effect in the proposed numerical method. The hysteresis loss and eddy current loss calculated from the flux and eddy current distributions are directly obtained from the proposed analysis method. Thus, excess loss is included in both the hysteresis and the eddy current loss calculations. We first apply the proposed method to two different grades of NO SiFe sheets. The calculated iron losses in one sheet are compared at various frequencies with the measured losses from catalogue data, and less than 2% deviation can be achieved. Finally, we apply the proposed method to the loss calculation for the iron core of a reactor.

Research on electromagnetic performance affected by shielding enclosure of a coil launcher

Shielding enclosure is very important to designer in a coil launcher system, which directly determines the electromagnetic performance of the coil launcher. The formulation of eddy current field involving movement is deduced, and the time step transient finite element method (FEM) calculation theory is illustrated in the paper. The simulation model of a three stage coaxial induction coil launcher is built based on ANSOFT MAWELL software. In the transient simulation model involved in moving conductor eddy current, the power pulse generator and the power electronic component are modeled as circuit simulation in MAXWELL schematic module. The circuit model is well coupled with field model in transient solver. In every time step, the current values which are obtained in circuit simulation are loaded in field model as exciting source to the coils. Three typical shielding materials, such as, iron, steel and ferrite is set in 2D field model to study the influence of shielding enclosure on the electromagnetic performance of the coil launcher. Silicon steel (laminated sheets) is taken in 3D field model. Different geometric dimensions of shielding enclosure are also taken into consideration. The speed of the armature and the magnetic field are calculated during every calculation case. The current distribution in armature and shielding enclosing are displayed as well. The laws of shielding enclosure on the electromagnetic performance are concluded. A ferrite shielding enclosure will increase the speed of armature obviously. The conductive material will induce eddy currents and decrease the leakage magnetic field outside, that shows excellent shielding performance. A silicon steel shielding enclosure has the similar impact on the speed of the armature to the ferrite, but has better shielding effectiveness than the latter. Research results shows that the silicon steel is suitable for the coil launch shielding enclosure.

Proposal of Maxwell Stress Tensor for Local Force Calculation in Magnetic Body

To investigate the noise reduction of an electrical machine, the deformation and vibration of the cores are evaluated by using the coupled magnetic and mechanical analyses [1]. In these analyses, the local force calculated by using the flux distribution obtained from the magnetic field analysis is required in the mechanical analysis. In the local force calculation [2], the Maxwell stress tensor is usually used. The Maxwell stress tensors are often represented by the Minkowski and Chu models with linear and nonlinear energy expressions, respectively. In the nonlinear magnetic field analysis, the volume forces, which move the magnetic bodies, obtained from both models coincide with each other, whereas their local forces, which deform the magnetic bodies, are different. Therefore, more investigation is required to clarify the suitable expression of the Maxwell stress tensor. In this paper, to clarify the proper expression of the Maxwell stress tensor for the local force calculation of a nonlinear magnetic body, the Maxwell stress tensors are derived from the Fleming’s left hand rule because the volume current density can be used to determine the magnetization in the nonlinear magnetic body. As a result, a new Maxwell stress tensor is derived. The surface force obtained from the new Maxwell stress tensor is compared with those obtained from the ordinary Minkowski and Chu models, and the equivalent magnetizing current method [3] to show the effectiveness and validity of the proposed Maxwell stress tensor.

Noise reduction of saturable magnetically controlled reactor using magnetic-mechanical analyses

The saturable magnetically controlled reactor (MCR) applying dc control currents has been widely used in the voltage regulation and reactive power compensation in the electric power system [1].

The heat transfer characteristics of transformer vegetable oil-based nanofluid

To simulate the heat transfer characteristics of vegetable oil-based nanofluid under the condition of natural convection, we built up a 2-D axisymmetric thermal-fluid multiphysics coupled model in COMSOL Multiphysics. The effective thermal conductivity is calculated by the improved Maxwell model, the relationship between windings loss and temperature is also established. On this basis, the simulation of the movement and distribution of nanoparticles had completed. The results show that the heat transfer efficiency is better under the condition of vegetable oil-based silica nanofluid than that of vegetable oil. In the meanwhile, Nanoparticles easily stuck on the wall due to the small flow velocity near the boundary layer. Eddies at the upper part of the transformer are beneficial to the uniform distribution of nanoparticles.

Investigation on a modified hybrid compact saturated-core fault current limiter based on permanent magnets

To reduce the size and requirement of DC biasing capacity of saturated-core fault current limiter, a novel hybrid compact saturated-core fault current limiter (HCFCL) is proposed by this paper. The three-limb structure with Permanent magnets can reduce the size and DC biasing capacity. Firstly, the principle of HCFCL is introduced. To validate the performance, various FEA simulations with the 3D nonlinear field-circuit coupling transient analysis are carried out in Ansoft. And a 220V/10A prototype of HCFCL is designed and tested. The simulation and experimental results verify the effectiveness and excellent performance of proposed structure.

Transient characteristics analysis of a 380V/30kVar superconducting controlled reactor

This digest proposed a model of superconducting controllable reactor and calculates its reactance based on finite element method. In order to ensure the safe operation of superconducting reactor, the digest devises a control system for superconducting windings based on S7-200 PLC after analyzing the DC component and overvoltage of superconducting winding and puts forward an optimal control strategy. A 380V/30kVar SCR was fabricated for experiments and the experimental results show the correctness of this method and the feasibility of the control device.