Wang Jianmin, Zhang Yuanlu, et al Baoding Tianwei Group co.,Ltd, Baoding 071056, china 1.Introduction Because lead current of low voltage winding is as large as 10 to 20 kA stray field in ce tie -plate of low voltage side is caused by both the lead current the winding current in large power transfmers , very uneven eddy current loss Local overheating are probably caused in ce tie plate. F this reason, 2D FEM model of lead stray field in L.V winding ends of large power transfmer is set up on the bases of both model experiment computing validation on simulating eddy current loss of ce tie-plate made of non-magnetic steel in large transfmers[1-2], analyses of both low voltage lead stray field Local overheating problem related to it are done in this paper. 2.Model Under the case of low frequency, the computation of lead stray field analysis of ce tie- plate local overheating are carried out with the following main assumption[3-4]: （1） conductivity of metal conduct is constant effect of displacing current is neglected. （2）The stray fields of both L.V lead windings are com-puted respectively, the stray fields in non-magnetic steel plate are linear. （3）The stray fields in transfmers are approximately com- puted in 2D eddy field with nonlinear materials. （4）The eddy current effects of windings conducts ce limb are igned. By the above assumptions, the simplified model of lead stray field in L.V winding ends of large power transfmer can be acquired, as shown in Fig.1. Using magnetic vect potential A（=AZ ） calculating magnetic field ,thus, its boundaries are all Dirichlet type condition（A=0）, boundary value problem of 2D nonlinear eddy field is:where, µ is permeability（H/m）; w is angle frequency （rad/s）; σ is conductivity of tie-plate; Js is current density of L.V lead（A/m2）.3.Analyses of both L.V lead stray field ce tie-plate overheating problem The lead stray field is computed in L.V winding ends of a large power transfmer the computed results are shown in Fig.2～Fig.3 Table 1. The practical overheating locations of ce tie-plate in large transfmers are usually taken place in the L.V side as shown in Fig.4. Ä、¡ represent directions of winding stray field xx、? represent directions of L.V lead stray field in the Fig.4 . Because relative locations of left to right from tie-plate to winding L.V lead are all symmetrical, it is considered by auths analyzing overheating phenomenon in Fig.4 that the tie -plate local overheating is caused by the stray fields of both L.V lead windings. It can be approximately thought that L.V lead stray field in a side of tie-plate ends is in phase with windings stray field they are positive, that the stray fields are negative in another side because of opposite phase of them. Thus, eddy current density eddy current loss density caused by them will be large enough to cause local overheating in ce tie-plate of large transfmers. Using positive principle linearly, analyzing fmula on maximum of both eddy current density eddy current loss density in ce tie-plate are given as follows: （2） （3）where, Bwmax is maximum of magnetic flux density vertical to tie-plate surface caused by winding currents Jwema isTable 1 Results of Tie-Plate Flux Density With Lead To Tie-Plate Distance ChangingScheme No. 1 2 3 4 5 6 7 8 9 10Distance of lead to tie-plate L （mm） 80 100 120 140 160 180 200 220 240 260Flux density （x10-4T） 138 127 115 108 102 92 84 77 70 64maximum of eddy current density in non-magnetic steel tie-plate by Bwmax; Blmax is maximum of magnetic flux density vertical to tie-plate surface caused by L.V lead current Pwema is maximum of eddy current loss density in non-magnetic steel tie-plate by Bwmax; JEMA PEMA are respectively maximum of eddy current density eddy current loss density caused by both winding stray field L.V lead stray field commonly. In this paper example, f example, when distance of lead to tie-plate is 140 mm, the Blmax is 0.0108（T） it is known from reference [2] that Bwmax is 0.103（T）, Jwema=1.51x106（A/m2）, Pwema=1.65x106（w/m3） .Using fmula （2）～（3）, thus, JEMA is 1.67x106（A/m2）, PEMA is 2.01x106（A/m3） eddy current density eddy loss density in a side of tie-plate increase 21.0% 44.0% than those of another side of tie-plate respectively. This is in consistence with practical overheating problem of tie-plate shown in Fig.4. Using analysis method given by this paper, overheating problems related to tie-plate in large power transfmers can be explained evaluated reasonable. 4.Conclusion The model of stray field of low voltage lead in large transfmers is set up its numerical computations are done by auths. Numerical analysis result is in consistence with practical overheating problem of tie- plate of L.V side in typical transfmers, thus, reliable method is given f protecting ce tie-plate from local overheating. Reference[1] Wang Jianmin, Cheng Zhiguang et al., “Experiment analysis on model simulating 3D eddy current losses in non-magnetic steel plate”, the Journal of Nth China Electric Power Univ. Vol.24 No 3,1997（in Chinese）.[2] Wang Jianmin, “Computation validation of winding stray field tie-plate loss in large transfmers”, ‘Master degree dissertation’, Nth China Electric Power Univ. , 1997（in Chinese）.[3] Lu Changbe, “Power transfmer computation”, China: Heilongjiang science technology publishing house, 1985（in Chinese）. [4] Zhou Keding et al, “Theetical methods applications of numerical computation in engineering electromagnetic field”, Higher education publishing house, 1994 （in Chinese）.
