Distribution Of Leakage Magnetic Field Research Articles

Detection and classification of invisible weld defects by magneto-optical imaging under alternating magnetic field excitation

Aiming at the difficult to detect invisible weld defects, a magneto-optical (MO) imaging non-destructive testing (NDT) system excited by an alternating magnetic field is proposed for feature extraction and detection classification of invisible weld defects. The relationship between the MO image features and the magnetic field intensity is analyzed based on the Faraday MO effect. A magnetic dipole model is proposed to study the magnetic field distribution on weld defect. A three-dimensional finite element model of invisible weld defects (with a width of 0.01 mm) is established, and the distribution of leakage magnetic field for different types of invisible defects is analyzed. In order to detect different invisible weld defects under the excitation of alternating magnetic field, MO imaging NDT experiments are carried out. The effectiveness of the finite element model is verified by the experiment. The gray value of the MO image can match the corresponding leakage magnetic field intensity. The Tamura method and gray-level co-occurrence matrix (GLCM) method are used to extract the texture features of natural invisible weld defect’s MO image, and these texture features of MO images are used as input data for the defect classification model established using back propagation (BP) neural network. Experimental results show that the classification accuracy of the GLCM + Tamura-BP model is higher than that of the GLCM-BP model, and its overall classification accuracy reaches 91.1%, indicating that this model can effectively and accurately classify natural invisible weld defects.

Influence of Magnetic Screen Material on Leakage Magnetic Field and Losses in the End Zone of Turbine Generator Based on Double Iterative Method

The overheating problem of the end components affects the safe and stable operation of the turbine generator. The magnetic screen layer can improve effectively the distribution of leakage magnetic field in the end zone. Taking the nonlinearity of the core material, rotor rotation, and the complex structure into consideration, mathematical model of nonlinear transient electromagnetic field is established in the end zone of 1407MVA turbine generator. The influence of different materials of magnetic screen layer on the end leakage magnetic field is studied based on double iterative method. Current density distribution of screen plate and finger plate are determined. The losses of the end components are obtained under the different materials of magnetic screen layer, which are used as heat sources in the three-dimensional fluid-solid coupled heat transfer model. Complex fluid flow and temperature distribution of the end components are obtained in the end zone of the turbine generator. The calculation results are verified by experiments.

Simulation of the space distribution characteristics of the leakage magnetic field in the transformer

The power transformer is the key equipment of the power system. Statistics show that two types of winding defects, winding deformation and inter-turn short circuit, are main causes of transformer fault. Therefore, timely and effective online monitoring of winding defects is the basis for ensuring the stable operation of transformers. At present, online monitoring methods of winding defects such as vibration method and dissolved gas analysis have low sensitivity and cannot accurately identify defects. Previous research shows that winding defects will change the space distribution of leakage magnetic field, so monitoring leakage magnetic field provides a new idea for online monitoring of winding defects. In this paper, the finite element simulation model of transformer is constructed, and the space distribution characteristics of leakage magnetic field under different winding defects are obtained. The results show that there are obvious differences in the space distribution of leakage magnetic field under different winding defects. On this basis, the optimal arrangement scheme of leakage magnetic field sensor is proposed, and the performance requirements of the sensor is given that the measurement range of the sensor should include 0.06 1.1 T and the resolution should be better than 1 mT.

Calculation and Analysis of Mechanical Characteristics of Transformer Windings Under Short-Circuit Condition

Accurate calculation of short-circuit (SC) dynamic characteristics of power transformer windings is of great significance to the safety of transformers. A 110 kV power transformer model is established, and SC electromagnetic characteristics and dynamic characteristics of the transformer windings are obtained through magnetic-structural coupling analysis by the finite-element method, considering the nonlinear mechanical characteristics of spacers. The distribution of leakage magnetic field and electromagnetic force (EF) and dynamic stress and displacement of windings are calculated due to SC fault. The SC EF of windings decreases exponentially with time. The axial EF of the disks at both ends of the winding is larger, while the radial EF is smaller, compared with that of central disks. The variation law of winding stress and displacement is similar to dynamic EF. The stress and displacement at both ends of the winding are larger while that in the middle part is smaller. The maximum stress and displacement of the end of windings are nearly 60% and 90% higher than that of the middle part, respectively. The axial displacement is the major part of the total displacement. The analysis method can provide a reference for checking the SC strength of the transformer.

Influence of Void Transposition Structure on the Leakage Magnetic Field and Circulating Current Loss of Stator Bars in Water-Cooled Turbo-Generators

In the design of stator bar structure, void transposition is an important way of reducing circulating current loss in water-cooled turbo-generators. The different void transposition structures have a great influence on circulating current loss. However, the influences of void transposition structure on the leakage magnetic field and circulating current loss are not definite. A 600-MW water-cooled turbo-generator is taken as an example. A three-dimensional (3-D) physical model of transposition bars in a single slot is established, and the field-circuit coupling method is used in 3-D numerical calculation of the leakage magnetic field and circulating current loss. The influences of segment number and length of the void transposition on the slot leakage magnetic field, current, and circulating current loss are obtained. The relationship between the slot leakage magnetic field distribution along the axial direction and circulating current loss based on different void transposition structures is clarified. The numerical calculation accuracy is validated with the current test experiment. New strategies and theoretical basis will be provided for the optimal design and technology of the transposition bars.

Winding Loss Mechanism Analysis of Magnetic Valve Controlled Reactor

With the increase of the magnetic valve controlled reactor capacity, winding loss brings more and more harm. Based on the physical model and mathematical model of magnetic valve controlled reactor, this paper proposes a valid method for calculating the winding loss .The proposed method calculates winding current harmonic content under different saturation, and obtains the leakage magnetic field distribution of magnetic distributed-valve reactor by using ANSYS. According to the leakage magnetic field distribution, winding loss can be calculated, and the loss curve can be deduced, which is only related to the saturation degree. The error between the ANSYS simulation results and the proposed method results is very small.

Numerical calculation of circulating current losses in stator transposition bar of large hydro‐generator

To reduce circulating current losses in stator strands of hydro-generator, strand transposition is widely used in stator bars. At present, the traditional analytical method for calculating circulating current losses cannot accurately simulate the distribution of leakage magnetic field, which will bring a large deviation to the calculation result and not benefit the winding design. The numerical calculation method for circulating current losses in stator bar is presented in this study, by which the leakage magnetic field distribution of stator transposition bar can be described in detail through establishing three-dimensional actual transposition model. The 0°/360°/0° transposition bar in a 180 MW hydro-generator is taken as an example and the circulating current losses in stator bar strands are calculated. The causes of errors in traditional analytical method are discussed through the comparison of the leakage magnetic field distribution obtained by numerical calculation method and traditional analytical method.

Study of Double-Sided Welding Defect Detection Technique Based on the Method of Magnetic Flux Leakage

This paper presents a new approach based on the method of magnetic flux leakage (MFL) for the double-sided butt weld (DSBW) of the welding equipment such as the pressure vessel in order to detect and identify the weld defect. In this approach, a new magnetization structure is adopted whose magnetization direction is perpendicular to weld line, also, a new continuous non-contact scanning method is used, what aims to solve the problems about complex leakage magnetic field (LMF) space distribution. Then, the LMF distribution laws with non-defect (ND) and laws with precrack from heat affected zone (PFHAZ) are obtained through using these methods such as theoretical analysis, numerical simulation, and experimental study. Afterwards, this paper reviews the recognition analysis of the above-mentioned two kinds of state (ND and PFHAZ) by binding the contrastive curves. The outcomes indicate that the suggested method of MFL has realized the recognition of ND and PFHAZ; The numerical simulation results and experimental results match each other well and their correlation coefficient R is 0.9729. Furthermore, the results verify the feasibility and the validity for the suggested method. This is a new way for exploration detection of the double-sided butt weld.

Current distribution in the low-voltage winding of the furnace transformer

The paper describes a mathematical model to be used in calculating the current distribution among the coils in the low-voltage winding of the furnace transformer. The method is derived from the leakage magnetic-field distribution in the transformer based on the minimum magnetic-field energy. The energy is calculated by using the principle of the minimum reactive power in the transformer window. The results are validated by using the finite-element model. The knowledge of the proper current distribution is very useful in designing the furnace-transformer cooling system to avoid local overheating.

The Analysis of Leakage Magnetic Field in the Large-Capacity Transformer

With the increase of transformer capacity, local overheating caused by leakage magnetic field affects the normal work and safe problems of transformer. How to reduce the additional loss and eliminate local overheating must be paid full attention to the design and manufacturing process of large transformer. In this paper, the leakage magnetic fields of clamps, clips and tank are analyzed. The distribution of leakage magnetic field in various parts of the transformer is obtained. Results of the analysis according with the engineering practice will be helpful for the optimal design of transformer.

Comparison of study on circulating current for different HTS transformer windings

In a HTS transformer a slight unbalance of the branch inductances of the windings might result in heavy circulating current because of zero resistance of HTS tapes. It is well-known that the circulating current worsens the distribution of leakage magnetic field and consequently does harm to the ac loss and critical current in the tapes. In this paper, the influence of the secondary winding configurations on the circulating current is studied by combining magnetic field calculation and circuit analysis, and some suggestions for improving the distribution of circulating current are also presented to make HTS transformers more efficient.

Sensitivity-enhanced electron holography and its application to magnetic recording investigations

The sensitivity for electron-phase measurement in electron holography has been improved to better than 2π/100 by the application of digital interferometry at the optical reconstruction stage. This enables quantitative measurement of magnetic flux as small as (1/100)(h/e)(=4.1×10−17 Wb), hitherto undetectable, with high spatial resolution. With this technique, we have observed the distribution of leakage magnetic field from a thin cross section of a perpendicularly magnetized recording film (cobalt-chromium) with a recording density as high as 300 kFCI (85 nm bit length), the highest density ever directly observed.