Research On The DC Bias Phenomena Of Large Power Transformers

The direct current components of flux and exciting current arisen in a power transformeris called DC bias magnetization. It is harmful to the characteristics of the transformers online. It causes the increasing of magnetizing current, harmonics leakage magnetic flux, andthus the losses, which in turn results in the structures overheating and insulation ageing. DCbias phenomena might be resulted from the large transient fluctuations in the earth'smagnetic field triggered by solar flares or the interference of DC ground currents in theregion of the AC transmission system combined with DC system. The studies of thisphenomenon have been paid much attention by industrialized country, however, themechanism analysis and the analysis in quantities are still in initial stage.In this dissertation, according to the demands of the transformer manufactures andelectrical power design departments, a series of researches on the influence of DC bias onlarge power transformer have been completed. The theoretic baffling problems concernedhave been researched. The impact of DC bias magnetization is evaluated. The change oftransformer running behavior is analyzed in quantity. The DC bias endurance among thedifferent structural transformers is compared. Finally the reference value of permissible levelof DC bias in the conventionally designed transformer is given and the methods of enhancingthe ability of enduring DC bias are also suggested.Four parts and seven chapters are involved in this paper.The first part includes the first three chapters. In this part, a new method for analyzingtransformer DC biased affection which depending on the power system is presented. Thesystem simulation model and the 3D transient eddy current computing model for saturatediron core are set up. With the Multi-Step method presented in this dissertation for computingthe 3D transient eddy current problem, the oscillation occurred in Single-Step method can beavoided. The precision is higher with less computational efforts.The second part is Chapter 4. Some researches on the key problems of 3Delectromagnetic field analysis concerned with this subject have been finished. A method forcomputing reluctivities of iron core in the 3D transient analysis is given to reduce the CPUtime of nonlinear analysis. The conventional Surface Impedance method is improved toreduce the computational efforts of the problem which involves thin skin depth steel parts.The Surface Impedance Condition and surface impedance value are modified according tothe shape and saturated degree of the steel parts. The improved IC-CBCG and ICCG methodare suggested to reduce the storage and the computational time of the linear equations.The third part is Chapter 5. This part is for the verification of the programmed software,which involves the test of model transformer, the computation of TEAM WORKSHOPproblems and the engineering problem with known testing results.In the fourth part, which includes Chapter 6 and Chapter 7, the problems interested bythe transformer designing and manufacturing departments are studied in quantities. Thejudging regulation of the DC tolerance of a conventionally designed transformer and itsestimating method are presented. The methods of transformer structural design to promotethe DC bias endurance ability are suggested.A novel method for analyzing the transformer performance impacted by DC bias in thepower transmission system with neutral earth connection is presented in this dissertation. Theresearch results are very important to promote the development of the electromagnetic fieldnumerical analysis and application of electrical devices under DC bias. It also provides aneffective computational tool for enhancing the operating reliability of the large powertransformer.