Research On The Electromagnetic Force And Local Overheating In Large Power Transformers

In this dissertation, a research is made not only on the 3D steady and transient eddy current fields in large transformers,but also on stray losses, the local overheating as well as the mechanical strength of structures after the occurrence of sudden short circuit, using electromagnetic theory and modern numerical methods. Some new results which have both theoretical and practical significances, are gained.At first, the researches on the 3D steady and transient eddy current fields, stray losses, electromagnetic forces acting on the coils and the relevant numerical methods are summarized in this dissertation. The research is carried out around the relevant theory and key points in the numerical calculation. To lower the requirement for computer resources and satisfy the engineering applications, the decoupled T-Ω method is introduced, a fast by -and-by layer 3D automeshing algorithm is developed, and a fast method to solve the large finite element equations and its date structure and scheme are presented. Thus the computation of the short circuit electromagnetic forces and local overheating problem in engineering are solved properly. Based on the work mentioned above, a common and flexible software, which can be run on a VAX-4000 station to solve a complex engineering problem in which the number of nodes is up to 100,000 has been built.The space distribution and transient behavior of the electromagnetic forces acting on the coils are investigated on many practical large transformers. The space distribution of electromagnetic forces acting on the coils in the sudden short circuits process under different typical transformer structure, different arrangements of coils and turns, are illustrated. At the same time, the effects of the parameters of structure parts and materials, and different shieldings are also studied. All these studies focus on the dynamic forces acting on the coils and thedisplacement of the coil parts caused by the forces as well as the interaction of the dynamic forces and the displacements. Based on these works, some conclusions and suggestions are made so as to make a optimum design of the transformer in engineering.The distributions of the magnetic flux, eddy current, and stray losses, as well as local overheating under different projects are illustrated. The size and distribution of the leakage magnetic flux, the eddy current, and stray losses in oil tanks, shielding parts, pressing plates, pull plates etc. are depicted respectively. The effects of parameters of structure parts and materials, and different arrangements on the leakage fields and forces are analysed. The local overheating problem in the metal materials such as oil tanks is studied based on the criterion to test local overheating. Some efficient and concrete measures to solve these problems are presented. The aim of the studies is to research comprehensively the local overheating in the metal materials, and thus some new useful ideas for engineering are put forward.A program for testing the short circuit strength of the materials, using high speed camera, is presented. The mechanical strength and steadiness states of the coils in transformers can be exactly studied on basis of the transient displacements and deformation of coils tested and recorded by using the camera.