Research Of Electromagnetic Characteristics For DC-biased Transformer Based On The Time-domain Magnetic Field-Electrical Circuit Coupled Model

It is very important to take research of DC-biased transformer and its impacts in monopole-earth system of high voltage direct current (HVDC) or magnetic storm in power network. Supported by the National and International Technology Cooperation Projects under Grant No.2010DFA64680, the electromagnetic characteristics of DC bias is studied in detail. A new method called time-domain magnetic field-electrical circuit coupled method is proposed, which is used to analyze the affection from DC disturbance to transformer. The main research achievements are as follows:Three-dimensional magnetic computation of different finite element methods based on magnetic vector potential function is studied. A new time-domain magnetic field-electrical circuit coupled model based on the edge finite element method is proposed. By this method, the time-varying electromagnetic computation of transformer is devided into nonlinear magnetic and time-domain circuit. And it is valid for the related analysis for electromagnetic devices such as transformer. Then the stable problem of computation in DC bias, which is based on the time-domain field-circuit coupled method, has been concerned. On the basis of stability analysis, an adaptive algorithm is proposed.Based on the time-domain magnetic field-electrical circuit coupled model, the electromagnetic characteristics of DC-biased transformer are analyzed. Different patterns of DC injection, as DC current flowing through the primary winding or the secondary winding, have been considered. Under DC-inject ways, the exciting current, magnetic filed and leak magnetic are learned and compared. The impact of DC current on AC excitation is analyzed. Then this method is used for computing the electromagnetic characteristics in DC biasing, and the varying regularity of coupling parameters is studied. With experiments carried out, the time-domain magnetic field-electrical circuit coupled method is proved.The time-domain magnetic field-electrical circuit coupled method is applied in eddy-current computation and DC-bias protection. The transient current is computed, which is treated as input in the three-dimensional nonlinear eddy field. The magnetic and eddy current are computed for transformer with different operating conditions. Analysis is carried out about the eddy losses of tie plate, clamp, shield and tank with DC current inrushing. Then the time-domain coupled computation is applied to compute the dynamic leakage inductances when DC bias happens. Variation of the dynamic leakage inductance is learned to conclude its regularity. Evaluation about the protection criterion based on leakage inductance is brought out, and reference is provided for the transformer protection in DC bias. The research work in this dissertation lays the foundation for further study of DC-biased transformer.