Research On Improving Technology Of Anti-short Circuit Ability Of Oil-immersed Distribution Transformer

With the improvement of China's economic development and the progress of urbanization,there is a demand for continuous expansion of urban distribution networks.The rural power distribution network covers a large area,the reliability of power supply is low,and old equipment also needs to be upgraded.As a hub connecting the distribution network and users,distribution transformers are in huge demand.Judging from the quality statistical results of power grid material sampling inspection,the qualification rate of ordinary oil-immersed distribution transformers withstand short circuit is only about 70%.Not only does it leave a big hidden danger for the safe operation of the distribution network,but also causes a large number of safety accidents and economic losses.This paper first analyzes the reasons for the low qualification rate of common oil-immersed distribution transformers to withstand short circuits from a macro perspective,and illustrates the importance of improving the research on the ability of oil-immersed distribution transformers to withstand short circuits.The history of domestic and foreign research on short-circuit faults of power transformers is introduced,and the limitations of current research on short-circuit faults of distribution transformers are discussed.Secondly,the short-circuit steady-state currents of different short-circuit types are calculated and compared.The transient expression of short-circuit current is derived,and the maximum multiple of short-circuit current is estimated.Based on the analysis of the leakage magnetic field of the distribution transformer,the common failure modes of the transformer short circuit are analyzed in terms of the radial force and axial force of the electric power.A complete model for simulating the harmonic electromagnetic field of the transformer under rated operating conditions and a 1/4 model for simulating the transient electromagnetic field of the transformer under short-circuit conditions are established.It reveals the distribution characteristics of the magnetic flux leakage density of the transformer during short circuit,the distribution characteristics of the stress inside the winding,and their time variation.Statistical analysis of the short-circuit withstand test results of more than 300 oil-immersed distribution transformers sampled by China Southern Power Grid from 2012 to 2018 by a certain testing center,and the range of qualified short-circuit tests for oil-immersed distribution transformers in the industry are speculated.Explore the correlation between the short circuit qualification rate of oil-immersed distribution transformers and the selection of materials for each component and the manufacturing process adopted.Two types of material selection / process combinations for large-capacity distribution transformers to improve the ability to withstandshort circuits are given.Finally,according to the three aspects of design,material selection,and process,the relevant factors of the transformer's short circuit withstand are reorganized.The design is divided into two aspects of electromagnetic design and structural design,mainly considering factors such as reducing short-circuit electromotive force and verifying whether the strength design of the component can meet the accounting requirements.Material selection focuses on the material and type selection of each component.Summarize the measures to improve the short-circuit withstand capacity of oil-immersed distribution transformers,and provide theoretical guidance for the practice of improving their short-circuit withstand capacity.By assisting an enterprise that fails to pass the short circuit test of the S13-M-200 /10oil-immersed transformer,the company adopts measures for improvement on the basis of the original product and submits the test to the short circuit test again.The test passed and verified the effectiveness of the summary measures to improve the short-circuit withstand capacity of oil-immersed distribution transformers.