Disassembly analysis is an effective way to verify the causes of defects of electric equipment. Recently

such problems as disqualified capacitance and dielectric loss as well as excessive dissolved gas in the oil are found successively on more than ten sets current transformers

which are produced in a same batch

in GM switch station. For exploring the cause of the defects

the characteristics of defects in this batch of the transformers at GM station are sorted firstly. The current transformer in phase A of bay No.159 is selected for disassembly and analysis. However

no abnormality is found in all parts of the transformer during the disassembly process. In view of this

such two defective models as"improper drying"and"oil immersion without evacuation"are prepared by the use of two sets type LB6-110 W instrument transformers.And an experimental platform for applying voltage and current is set up so to simulate the development process of the defect of the current transformer at rated operating condition. It is shown by the simulation test that in case of applying voltage and current

the defects develop rapidly with short time

showing such defect features as remarkable increase of capacitance and dielectric loss

contents of H2 in excess of 10 000 μL/L

extremely fast gas production rate with CH4 in second place and with a small amount of C2 H2. It is thought by comparing the feature of the defect development and analysis that the main reason for the defect of the current transformer is no filling oil at evacuation condition in the oil filling process or the vacuum cannot meet the requirement during the production period. On this basis

such suggestions as shortening detection period of the dissolved gas in the oil and use of constant voltage and current test for sampling test in the delivery is proposed in this paper.