Research On The DC Bias Excitation Current And Loss Of Transformers

Power transformers are the main electrical equipment in Grid Network. With the development of single-phase power transformers capacity and voltage level, in China, the maxiumum capacity of power transformer is 1000MVA, and voltage level up to 1000kV, which it is mainly used in southeastern power grid project. It is reported that the largest capacity of single-phase power transformer has reached 1300MVA, the highest voltage levels is above 1150kV, in foreign country. Nowadays, grid network is rising demand to Ultra High Voltage (UHV) large capacity power transformer, leads to the transformer manufacturing industry face up to fierce competition in the design, technology, manufacturing, test and so on. Furthermore, with the development of DC transmission technology, extra and ultra high voltage DC transmission lines are the rapid spread. DC transmission systerm equipment fault, routine preventive maintenance as well as monopole converter station pole operation, commissioning, which cause unipolar earth loop mode and bipolar operation inbalance mode operation phenonmenon have become more common during the DC transmission systerm. The non-normal operation of DC transmission line will not only arise the DC bias phenomenon in the neutral grounding transformer which cause a series of results, such as transformer core magnetic saturation in every half cycle, the distortion of magnetizing current, the increasing of leakage magnetic flux and corresponding losses, but also affect the related power transformer safe operation. From now on, along with extra and ultra high voltage transmission lines increase, these influence will become more serious. So, in the transformer industry, the transformer manufacturers have aroused great attention on how to effectively control to the influence of DC bias and strengthen ability to withstand DC current of transformer, how to make measures to curb impact of transformer DC bias in design.Research and analysis on the power transformer DC bias effect. The two tests here are helpful for research on transformer DC bias phenomenon:one researches the different lap ways of electrical steel sheet have an effect on transformer magnetic circuit by using the double Epstein square devices; another one designs a unique AC and DC hybrid supply voltage source, according to the model transformer characterictics, successfully simulates the site condition on transformer DC bias. By using the field-circuit coulped method, the transformer DC bias phenomenon is simulated and analyzed, excitation current waveformers and the field distributions with different DC bias condition are obtained, the judgement method on transformer withstand DC capacity is given, and then comparison with the simulation results and experimental results to verify the field-circuit coupled method has a guidance significance on practical engineer. By using the equivalent magnetic circuit network method, the 2D core leakage field distribution is researched and calculated.