Electromagnetic pulse (EMP) de-icing technology refers to the form of pulsed current or pulsed magnetic field, which converts EMP energy into pulsed force, changes the shape of metal materials, destroys the adhesion between the ice layer and the surface of the material, and never achieves the purpose of ice layer falling off. This paper, elaborates on the working principle of EMP de-icing technology from three aspects: the research background of electromagnetic pulse technology, influencing factors of de-icing excitation, and industry applications. It reviews the progress of relevant fundamental theoretical research both domestically and internationally. Furthermore, it analyzes the factors affecting de-icing effectiveness from four perspectives: structural response, pulse current size, metal material thickness, and coil-to-metal material spacing. From the two dimensions of application and fundamental theory, the existing problems of EMP de-icing technology are identified respectively. The research direction of EMP de-icing technology is expected from three aspects: intelligent control system, optimization of metal materials and numerical calculation.
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With the continuous development of power grids, overhead transmission lines inevitably pass through regions with complex terrain and climatic conditions. In extremely cold environments, ice accumulation occurs on these lines. Under wind loads, the accumulated ice layers vibrate and detach, potentially causing accidents such as broken lines and tower collapses. The DC deicers serve as the primary equipment for power networks to defend against freezing disasters. They utilize Joule heating to raise line temperatures and melt ice and snow. However, existing DC deicers suffer from large size, heavy weight, and difficult transportation. As a core component of DC deicers, the rectifier transformer accounts for more than 70% of the total weight. To improve the mobility of these devices, it is necessary to optimize the structure of the rectifier transformer and reduce its weight.
Based on the transformer theory, a three-phase three-dimensional (3D) symmetric core and its lamination method for rectifier transformers are proposed. The electromagnetic field equations and the phasor relations of main and mutual fluxes were derived according to Ampere's law. A finite element model of a 4 MV·A rectifier transformer was established in Ansys Electronics Desktop. The time-varying law of the magnetic field and the characteristics of the loss density distribution were analyzed. The limitations of existing numerical calculation methods in dealing with the microscopic behavior of magnetic domains and the additional loss in the vertical symmetry plane were analyzed. To verify the analysis results, a 4 MV·A prototype was fabricated and tested under thermal cycling and no-load conditions. Finally, an additional loss shape function expression with the average flux density gradient as the independent variable was proposed. The undetermined coefficients and loss correction formula were obtained by collecting no-load test data of samples with different capacities.
The mutual fluxes of the three-phase 3D symmetric core were separated by the vertical symmetry plane of each phase. The core flux density was
By analyzing the electromagnetic characteristics of the proposed three-phase 3D symmetric core, its core loss characteristics and a correction formula were obtained. This study can provide theoretical support for the engineering application of 3D symmetric core rectifier transformers and thereby contribute to the lightweight design of DC deicers. This, in turn, can improve the transportation capability of DC deicers and ensure the winter safety of power grids.
Defects in operating outdoor overhead transmission lines with broken strands will cause local excess temperature rise. The maximum temperature occurs at the defect and decays rapidly to a defect-free area. Infrared thermography based on temperature distribution can identify the degree of broken strands defects. However, the wind speed will significantly reduce the surface temperature of the observed object, making infrared detection difficult. To study the axial temperature distribution at the broken strand area of overhead lines under low wind speed, the paper took the LGJ-240/30 type steel-cored aluminum strand as an example, and conducted thermal cycling tests in the State Key Laboratory of Power Transmission Equipment & System Security and New Technology of Chongqing University. The broken strand defect was produced by manual destruction, the homemade wind speed was regulated by the air collecting device, and the AC large current generator provided a stable Joule heat source. The influence of the number of broken strands on the maximum temperature rise at the back of the defect and the temperature difference in the axial defect-free area was obtained. And based on this, the infrared identification method of broken strand number when the wind speed is 1~3 m/s was proposed. Finally, the method was verified by natural experiments in the National Field Science Observation and Research Station. The results show that after the occurrence of strand breaks in overhead transmission lines, the axial temperature difference between the extreme value of the defective temperature and the normal temperature in the non-defective area decreases rapidly with the increase of the wind speed; the fitting coefficient b, which describes the heat transfer term in the fitting equation of the axial temperature difference θ and the wind speed u, increases with the increase of current carrying capacity and the number of strand breaks. The proposed method has a recognition rate of more than 90.1% for the number of broken strands and more than 94% for defects under the condition of low wind speed, the load current is 360, 480, and 600 A, and the number of broken strands is more than 3. It solves the problem of infrared thermal inspection project under low wind speed without missing the best maintenance time, greatly improves the maintenance efficiency of line maintenance, guarantees the safe and stable operation of power grid, and has guiding significance for the infrared thermal inspection project of overhead transmission lines.
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