Ruan Jiangjun

3-D FEM Simulation of Velocity Effects on Magnetic Flux Leakage Testing Signals

The development of computer-based defect characterization scheme for magnetic flux leakage (MFL) method require adequate mathematical models to describe the complicated interactions of sources, fields, velocity and defects in materials. In this paper, a 3-D transient finite element model based on T-Omega formulation was used to analyse MFL system at high velocity, with regard to eddy currents induced in a steel tube because of the relative movement between the exciters and specimen. The eddy currents change the profile of electromagnetic field, which increases difficulty in MFL signals interpretation and defect identification. Typical 3-D defects were modeled and MFL signals caused by the defects of different sizes were simulated respectively. Eddy currents induced by velocity and their characterization in high velocity MFL testing system were investigated.

Investigation of very fast transient overvoltage distribution in taper winding of tesla transformer

In order to study the very fast transient overvoltage (VFTO) distribution in the taper winding of a tesla transformer under high-frequency steep-fronted voltage surge, we built a distributed line model based on multiconductor transmission line (MTL) theory. We used a new hybrid algorithm combining finite-element-method (FEM) and interpolation formulas to quickly evaluate the induction coefficient matrix K by utilizing some characteristics of the taper structure. The turn-to-ground and interturn voltage distributions can be obtained by solving the telegraphists equations in the frequency domain. We measured the voltage distribution inside the taper winding to find some ways to weaken the voltage oscillations. Here, we compare the results with numerical values.

Improvement of Current Filament Method and Its Application in Performance Analysis of Induction Coil Gun

The performance analysis of induction coil guns is very useful in their experimental research and electromagnetic optimization design. This study improves the current filament method (CFM), which is commonly used in a coil guns circuit simulation. In the improved CFM, each excitation coil was divided into small subcoils to improve the calculation precision of inductance in the equivalent electric network. In order to check the effect of the improvement and analyze the electromagnetic transient in coil-gun launch, the field-circuit method for induction coil guns was introduced. The field model is built based on the 3-D composite grid method (CGM). CGM uses two sets of mutual independent grids to discretize the solution region, the coarse grids discretize the global region, and the fine grids discretize the moving part. In every time step, instead of remeshing, it is only necessary to change the coordinates of nodes in fine grids. A three-stage coil guns field-circuit model is built, and the validity of modification is proved by comparing the results of field-circuit model before and after the improvement of CFM.

A Composite Grid Method for Moving Conductor Eddy-Current Problem

We present fundamentals and procedures of a composite grid method (CGM) for determining eddy currents in moving conductors. Based on the finite-element method (FEM), CGM uses two separate mesh grids - one coarse and one fine - to calculate in the global region and local region separately. The results of the coarse mesh are interpolated onto the boundary of the fine mesh as its Dirichlets condition. Then two equations are solved in the fine mesh region in order to obtain the reaction force on the boundary, which is reacted on the coarse mesh to modify its right-hand-side load vector. And the equations in the coarse mesh are re-solved. The iteration continues until the results converge. The advantage of CGM is that it allows two overlapped grids differing greatly in size to be meshed independently. Also, the program is easy to modularize and thus has great flexibility and adaptability. Above all, it ensures good numerical accuracy in each grid set. As an example indicates, CGM is effective in handling 2-D moving conductor eddy-current problems that are tedious to solve by conventional methods such as re-meshing or using a Lagrange multiplier.

Capacitor-Driven Coil-Gun Scaling Relationships

Scaling method is an applied scientific method employed in experimental analysis. Also, in the analysis of an electromagnetic-launch system, the scaling method can be an aid in the conversion of test results from models to original. This paper focuses on the capacitor-driven coil-gun scaling relationships between the original and the model. The current-filament method is used to derive the scaling relationships of a capacitor-driven coil gun. If the input scale factors are chosen properly, output scale factors can be calculated by the scaling-relationship equations. Two single-stage coil guns were constructed and tested to verify the scaling method. The calculated scale factors agreed quite well with the test data. Further, three-stage coil guns were simulated based on Ansoft to verify the scaling method of the field results and changes in velocity with respect to the discharge position. It is shown that the scaling method is feasible. These laws allow, with proper application, the construction of experimental models in which phenomena similar to those occurring in the original are reproduced. The result can then be recalculated, using the scale factor for the physical quantity, into the original configuration. A detailed derivation and validation will be presented in this paper.

Application of a Composite Grid Method in the Analysis of 3-D Eddy Current Field Involving Movement

A composite grid method (CGM) was applied in the analysis of eddy current field involving movement. The region of current source and air including the background of moving conductor was discretized with a global grid. The region of moving conductor was discretized with a local grid. The trouble of remeshing in normal finite element technique based on one set of grid is overcome in CGM. At each time step, instead of mesh regeneration it is only necessary to change the coordinates of local grid nodes. The fields on the two sets of grids are solved independently by finite element method and related by an interpolation matrix. The dynamic characteristics of a 3-D coil gun model were calculated by CGM. As a comparison, a 2-D axisymmetric model was also simulated. Numerical results obtained were compared with experimental data.

Research on Differentiated Lightning Protection Comprehensive Management for the 500-kV Power Network in the Area Near the Three Gorges Project

For the serious problem on lightning disaster risk of the Three Gorges outgoing lines, new technology, which is based on risk assessment, has been proposed for the lightning disaster defense for transmission lines. In order to realize lightning disaster risk assessment and defense technology, the procedure consists of analysis on lightning activity characteristics, lightning failures replay analysis, lightning disaster risk assessment, establishment and implement of lightning protection improvement, and practical defense effect assessment of lightning protection improvement. Implied on the lightning disaster defense for the Three Gorges outgoing lines, this technology has been verified with positive results. The widespread use of this technology would provide technical support for the differentiation design and improvement of lightning protection for transmission lines and for realizing the meticulous lightning protection in the management of transmission lines.

Transient analysis of grounding system under lightning stroke using 3D FEM method

The transient behavior of a grounding electrode under lightning stroke is an interesting topic in EMC analysis of a substation environment. This paper uses a novel method, finite element time domain (FETD) method, to solve this problem. In the calculation, skin effect in the grounding electrode is considered, but neglecting the discharge around it. We get some calculation results of electromagnetic field distribution on a simplified model.

Shielding effectiveness of shielding cabinet in substation with 3D eddy current FEM calculation

Detail of the electromagnetic shielding effectiveness of shielding cabinets under serious electromagnetic environments in substation, in which control and protective equipment is installed, have not been very clear until now. The authors research is based upon a numerical simulation method-the 3D eddy current edge finite element method. With this novel analysis tool, considering the geometry, material and air window of the cabinet, they have obtained some interesting calculation results, which may be useful for the design of the cabinet.

Influence of CaSO4 content in contamination on the leakage current characteristics of porcelain insulator string

Leakage current was one of the important characterize of insulators insulation and has made a lot of studies. However, there are few studies about the influence on insulator leakage current characteristics considering the percentage of CaSO4 in contaminations. Seven pieces of suspension porcelain insulators XWP2-160 were used as test samples, and lots of artificial pollution tests were conducted in an artificial fog chamber. The results indicated that the peak value of leakage current will decrease with the increase of the CaSO4 percentage, in the way of negative exponential function, while the peak time of leakage current will increase in the way of positive exponential function. In addition, the value of flashover current will decrease slightly and the leakage current pulse decreases linearly with the increase of CaSO4 percentage. The critical flashover current will reduce in the way of negative exponential power function. The results can be an important reference for external insulation design, operation and maintenance of transmission lines.