Calculation Of Eddy Current Loss And Analysis Of Shielding Effect Of Power Transformer

With the continuously increase of the capacity and voltage level of power transformer, the leakage flux density of each structure of transformer is greatly increased, which will induce eddy current and form eddy current loss in the metal structure, not only waste energy and reduce the operating efficiency of the transformer, but also may form local overheating. Therefore, the corresponding shielding measures are needed to solve the magnetic leakage problem in the structure.For the problem of eddy current loss and shielding of the structure of large capacity power transformer, a 334MVA single phase auto transformer is taken as the research object to be studied. Firstly, the value and the distribution of the leakage magnetic field are calculated and analyzed by the two dimensional transformer model and the simulation results are compared with the theoretical calculation results which verifies the correctness of the Maxwell finite element software.The eddy current loss and shielding of the transformer tank are studied and calculated by using the three dimensional transient finite element method. Under fully considering the nonlinear magnetic properties of the tank and the anisotropic nonlinear magnetic properties of the magnetic shielding laminations, the shielding characteristics of different shielding mode and shielding structure of the tank are analyzed. The leakage magnetic field and eddy current loss of the tank under three conditions that installed the magnetic shield of front and back side of the tank, increased the upper and lower magnetic shield and added copper shield in the tank corner are calculated and the shielding structure of the first mode is optimized in five aspects:the width, the height and the thickness of the shielding plate, the spacing between the shielding plates and the distance from the shielding plate to the tank. Based on the calculation results, the shielding characteristics of different tank shielding mode and shielding structure are summarized.The loss and the shielding measures of the transformer clamp are studied. The equivalent permeability of the clamp material is obtained by comparing with the transient accurate calculation and the surface impedance approximate calculation of the eddy current loss. Then four kinds of shielding schemes which are optimized by using the surface impedance method are proposed according to the loss distribution of the clamp and the shielding scheme suitable for the clamp is obtained by comparing the advantages and disadvantages of shielding effectiveness of different shielding schemes. Further, the calculation accuracy of the surface impedance method after adding magnetic shielding is verified through the optimal process done by the transient method of the U type shielding scheme, which shows the calculation results obtained by the surface impedance method has certain precision when the equivalent permeability of the clamp is selected.Finally, the magnetic thermal coupling analysis is done to the tank and clamp of the transformer that the calculation results of the eddy current loss of the structure are used as the heat source load and the temperature distribution of the tank and clamp in the case of no magnetic shielding and with magnetic shielding are obtained. It is verified that when there is no shielding measure there will be local overheating problem in the transformer tank and clamp and the temperature rise of them can be reduced in different degree after taking different magnetic shielding measures.