当电力变压器发生直流偏磁时，其励磁特性将受到一定程度的影响，从而可能对所在电力系统以及设备本身的正常运行造成干扰。本文首先推导建立了考虑漏磁支路的单相双柱、三柱和四柱双绕组心式变压器的电磁耦合模型。以单相双柱变压器为例，分析了直流偏磁状态下电力变压器励磁电流的特性，定量揭示了直流偏磁现象的产生机理和特征，并通过与解析方法的计算结果进行对比，验证了该模型的正确性。其次，基于Jiles-Atherton磁滞理论的能量平衡方程，推导实现了合理可逆磁化系数条件下，计及涡流损耗和异常损耗，以磁通密度作为输入量的变压器铁心动态磁滞损耗模型。在试验研究方面，开展了5 kVA/220 V单相三柱小型变压器的直流偏磁试验，测量分析了变压器励磁电压电流、磁滞回线、空载损耗和噪声的变化情况。通过与采用电磁耦合方法获得的仿真结果进行比较，验证了该模型的有效性。同时，利用遗传算法提取变压器铁心在正常工作状态下的动态磁滞损耗模型参数，用于分析直流偏磁状态下变压器的励磁电流和铁心动态磁滞损耗，取得了与试验数据较为吻合的结果。在小型变压器试验的基础上，还与生产厂家合作，开展了240 MVA/500 kV单相四柱大型电力变压器的直流偏磁试验。该变压器的电磁耦合模型仿真结果与试验数据的对比表明了该模型的有效性和工程应用前景。此外，利用场路间接耦合方法，还对该变压器内部漏磁场受直流偏磁电流的影响进行了简化分析，结果表明，直流偏磁状态下漏磁通增加较明显，可能导致金属结构件及油箱出现局部过热，值得引起重视。最后，本文利用铁心动态磁滞损耗模型分析了直流偏磁状态下三相组式变压器的励磁电流和铁心动态磁滞损耗，比较了直流偏磁和交流过励磁对变压器励磁特性的影响。结合试验研究结果，指出铁心损耗以及相应的温升不是限制变压器耐受直流偏磁电流能力的主要因素，而变压器在两种励磁状态下的差别也不容忽视。同时，通过扩展单相变压器的电磁耦合模型，对三相三柱、三相五柱变压器励磁电流受直流偏磁电流影响的情况进行了分析，重点讨论了漏磁支路磁阻对三相三柱变压器受直流偏磁电流影响的作用，指出随着漏磁支路磁阻的减小，三相三柱变压器同样会出现越来越明显的直流偏磁现象。

When power transformer is under the dc current biasing, its magnetization characteristics will be affected to some extent. As a result, the operation of power system and the transformer itself may be disturbed.Firstly, the electromagnetic coupling models including the leakage paths for single-phase two-limb, three-limb and four-limb two-winding core-form power transformers have been established. Taking the single-phase two-limb transformer as an example, the effects of dc biasing on the magnetization current of transformer are presented. The mechanism and characteristics of dc biasing are revealed quantitatively. By comparing the simulation results with these of analytic method, the correctness of this model is validated. Secondly, based on the energy balance equation of Jiles-Atherton hysteresis theory, the dynamic hysteresis loss model of the transformer core considering the eddy current loss and abnormal loss is developed, which is suitable for all the reasonable coefficient of reversible magnetization and adopting the flux density as entry variable.In the aspect of experimentation research, the dc biasing experiment of a 5 kVA/220 V single-phase three-limb transformer has been carried out. The magnetizing voltage, current, hysteresis loops, no-load losses and noise are measured and analyzed. Compared with the experimental results, the simulation ones of the electromagnetic coupling model show a good agreement and the effectiveness of this model is verified. By extracting the parameters of the dynamic hysteresis loss model from the experimental results of normal operation using genetic algorithm, the transformer magnetization characteristics under dc current biasing are simulated and the simulation results have a good agreement with the experimental ones. Based on the experience of small transformer test, the dc biasing experiment has been designed and executed on a 240 MVA/500 kV single-phase four-limb power transformer cooperating with the transformer manufacturer. The acceptable agreement of simulated and experimental results demonstrates the validity and engineering application perspective of the electromagnetic coupling model. Furthermore, the leakage flux inside the power transformer is evaluated with the indirectly coupled field-circuit method. The simulation results suggest that the leakage flux may increase obviously with the presence of dc biasing current, and it is possible to have local overheating spots in metal parts and oil tank, which more attention should be paid.At the end of this thesis, the magnetizing current and iron core dynamic hysteresis loss of three-phase power transformer bank is analyzed with the verified dynamic hysteresis loss model. The influences of dc biasing and ac overexcitation are compared as well. Along with the experimental results, the simulation ones propose that the iron core loss and corresponding temperature rising are not the limiting factor in deciding the capability of power transformer against the dc biasing current, and the differences between the ac overexcitation and dc biasing should not be ignored. Meanwhile, the validated single-phase electromagnetic coupling model has been extended for three-phase power transformers. The mathematical models of three-phase three-limb, three-phase five-limb core-form transformer are developed. The influences that the dc biasing current has on their magnetizing current are also studied. The magnetization characteristics of three-phase three-limb transformer with different magnitudes of leakage reluctances under dc current biasing are discussed in detail. The results demonstrate that the effects that dc biasing current on such type of transformer will become more and more obvious with the decrease of leakage reluctances.