Research On The Field-circuit Coupled Transient Eddy Current Field Of Magnetic-imbalance Transformer And Magnetic Field Analysis

It is very important to steadily operate for special structure large capacity generatortransformer and system-interconnection transformer which are used in large power stations,nuclear power plants or important transmission. Special structural large power transformersuch as transformer of disintegration structure and compression iron yoke structure emerge asincreasing capacity of transformer and restriction of transportation condition and siteinstallation. compared with general transformer, unbalanced magnetic field is established inthese transformers inner. It has great significance for improving the reliability of transformeroperation and ensuring the safety operation of power system to accurately calculate magneticfield of core, no-load loss and stray losses of a large power transformer and avoid the localoverheating. The local losses concentration is the direct cause of local overheating andoperational failure caused by the inconsequence of magnetic leakage field and stray lossdistribution with increasing of capacity and voltage. Therefore, to improve the calculationprecision of magnetic field and loss of large magnetic-imbalance power transformer and avoidlocal overheat, the performance and parameters of the special structure transformer must bestudied deeply to ensure the safe operation of special structure large capacity transformer andimprove the reliability of operation and determine the location of the heating source avoidinglocal heating of the structural parts.In order to analyze accurately the size and distribution of no-load loss and excitingcurrent and the magnetic field in the transformer core silicon steel laminations, a transientcalculation method coupling field-circuit is presented in this thesis considering nonlinearanisotropic magnetic properties of ferromagnetic materials. Transient calculation methodcoupling field-circuit has been improved compare with the traditional calculation method. Theexpressions of FE discretization are deduced in2D and3D transient eddy current fieldcoupling field-circuit considering the electromagnetic characteristics of different materialssuch as anisotropy and isotropy and nonlinearity respectively. Nonlinear algebraic equationsare solved by Newton-Raphson method which is common method to solve these equations,calculation accuracy of magnetic field and losses of large power transformers can be improvedgreatly using this method.A calculation program is developed considering2D and3D transient field-circuit coupling considering magnetic properties of laminated core such as nonlinear and anisotropic.The supplementary model of TEAM problem10and TEAM problem21(P21Bmodel,P21C-M1model and supplementary model of P21C-M1) are used as a example to verify thevalidity of proposed method and the computer program including nonlinear and anisotropyand isotropy, and the method is also applied to calculate the transient magnetic field in thelarge-capacity three-phase five-column transformer of disintegration structure and thetransformer of compression iron yoke structure, the transient magnetic field of transformercore, distribution of lossdensity of no-load and the difference and size of exciting currentchange with time are analyzed. Magnetic flux density and no-load loss of four different kindsof maximum sheet width model is calculated and analyzed aiming at compression iron yokestructure transformer. The validity and the correctness of the program code is verifiedcompared with the experimental results of two different kinds of power transformer with thespecial core structure.The calculation error of the coil current is larger using a conventional method when3Dleakage magnetic field and structural parts stray losses of large complex auto-powertransformer is calculated because the special structure is applied to the coil such as twocolumn parallel structure, side column excitation, voltage regulation and single-phase threecolumn parallel. A great calculation error is obtained when current of coil is calculated usingtraditional method (circuit method). To solve this problem, a3D field-circuit coupling methodis applied to calculate the coil current distribution in order to improve the calculation accuracyof structure losses and short-circuit impedance using accurate current. Coil current, impedanceand structural eddy current losses of transformers of large-capacity single-phase two-pillar ofexciting and a single three-column-pillar structure is simulated respectively.