Research For Direct-current Magnetic Bias Of Large Converter Transformer

The direct current components of flux and exciting current arisen in a power transformer called DC bias magnetization which is an abnormal condition of transformer. In the DC transmission, because of the characteristics of converter station the DC bias magnetization phenomenon appears when DC component passes by the winding of converter transformer. This phenomenon is harmful to the characteristics of the transformers on line. It causes the increasing of magnetizing current, harmonics leakage magnetic flux, and thus the losses, which in turn results in the structures overheating and insulation ageing. So it is necessary to analysis the phenomenon from the angle of electrical magnetic.Because of the nonlinear of the ferromagnetic material, the method based on superstition principle can not be taken in the analysis of DC bias magnetization. Thus a new method is proposed in this paper which applies 2-D transient field-circuit direct coupling method of finite element. In this method, exciting current of non-load transformer is analyzed with the function of different level DC bias magnetization. The result is shown that DC bias magnetization causes the deviation of magnetic flux density in the core and serious asymmetry between the positive and negative semi-wave of exciting current waveform. Furthermore the more of DC bias magnetization, the more serious deformation of exciting Current.On the basis of relations between DC bias magnetization and peak value of exciting current, by a new method which is based on the data of non-load converter transformer losses, the condition of converter transformer losses is analyzed with the function of different level of DC bias magnetization. The result is shown that with the increasing DC bias magnetization, the core losses is also increased which leads to temperature-rising. In the serious condition this will cause the over-heat of local part of core.In the analysis of leakage field, the paper discusses the type and function of converter transformer leakage field and establishes the model of two dimension leakage field calculation by properly simplified. Based on 2-D transient field-circuit direct coupling method of finite element, the paper analyzes the transformer leakage field on the function with different DC bias magnetization current. The result is shown that with the increasing of DC bias magnetization, the variation regularity of DC leakage field at the different place is unchanged, while, the leakage is increased with different rate at different place.