Evaluation Of DC Biased Power Transformer

DC bias has got more and more attention in China with the developing of HVDC power grid. Ability to withstand DC bias has clearly become of main performance of large power transformers. However related open literature is very less, and also has problems of un-clear theory, less quantitative analysis and un-defined testing process. On the other hand, passive measures like lower flux density are applied in transformers. Practical application is lag behind the study of theory on this topic.This document is trying to make thorough and complete study on DC bias at following sections.1) DC bias will be studied from root cause. Related characteristics of the transformer are also discussed and analysis.2) Some comment misunderstandings are clarified. Microscopic features (material) and the macro characteristics (Transformer) under DC bias are described relatively complete.3) Corresponding characteristics of each type of dc bias will be proposed in this document. And also list most kinds of measures at present.4) Three methods are proposed to evaluate ability to withstand DC bias for transformers. Complete parameters and processed is presented for each methods. A reference transformer is calculated and evaluated according to the calculation methods. Main part of this calculation is show from request of customer to the detailed physical quantity. The calculation method is deeply discussed with the calculation results.5) A test schemed is proposed as the DC bias withstanding testing. Parameters calculation of the equipment, specific testing process and testing attention, etc are discussed in this part. Actual transformers are tested with this test schemed. The result is analysis at the end of the document.