Numerical Simulation On The DC Bias Phenomenon In Converter Transformer

As an obbligato equipment in E(U)HVDC transmission project, converter transformer can perform mode connection and coordination role between AC grids and DC lines. Because of the unique engineering design and running ways of DC grids, converter transformer is affected by DC bias phenomenon. The DC bias can cause distortion of the exciting current, increase of the low harmonics and the highly magnetic saturation of the iron core, which leads to the increasingly loss, local overheating and clear vibrating noise. All these may cause vicious accidents easily, such as grid schizolysis. So studying on the products’ internal electromagnetic characteristics is quite necessary under DC Bias.Based on the physical situation of single-phase transformer no-load operation and by using analytic calculation to analyze formative mechanism of DC bias, this paper explores the functional relationship between DC component and DC magnetic flux in iron core. In order to research the influence on the operating performance of converter transformer caused by DC bias, based on nonlinear magnetization curve of iron core and the transient "field-circuit coupled" method, two-dimensional solution model is established in ANSYS simulated environment by using the parameters of a405.2MVA/530kV single-phase duplex windings converter transformer. By comparing the analog value of short-circuit impedance and its analytical value, the rationalization of the models can be tested. After that, connect AC voltage to line side winding and put different levels of DC exciting on valve side winding. Based on it, the DC bias phenomenon can be simulated.With the condition of no-load and different levels of invasive DC, analysis of waveform curves of exciting current in converter transformer, the changing trend of positive and negative half-cycle peak value can be obtained. By utiliz ing MATLAB programming software, the data of exciting current can be changed into FFT and each harmonic content can be calculated. Drawing the changed curve of the ration between each harmonic and fundamental wave with the invading DC, in addition, total harmonic distortion is further computed. By loading DC component of different ratio, the contour plots of positive and negative half-cycle value of iron core magnetic flux density can be gotten separately, so that its position of saturation can be checked. Through choosing the specific paths of the magnetic distribution and calculating magnetic flux intensity in each node, its changing rule, peak value and the capability to bear DC bias in each position can be compared. The component and theoretical formula of no-load loss are got according to analytical method. Aiming at distributive feature of internal flux density, combine iron core limb with iron yoke to calculate together without oblique joint area, so that the changing curve of magnetic flux intensity with time and average peak of positive and negative half-cycle are computed. By making use of fitting relation of magnetic flux density and loss inside the silicon steel sheet, as well as conversion of no-load loss under fixed frequency, iron loss of unit mass is calculated in analytic domain. Finally, the no-load loss is fully explained with invasion of DC in accordance with half-wave average algorithm, which provides an effective means for the design and maintenance of product considering DC bias.