The Defective Causes Analysis for a Batch of Current Transformers Based on Simulation Tests

Recently, quality problems were found successively in more than ten current transformers (CTs), which were produced in a same batch, in Guangming switch station of Inner Mongolia province. Both the capacitance and dielectric loss did not meet the requirement of criteria. And the contents of dissolved gas in oil in those CTs exceeded the warming value. In order to explore the formative reasons of defects, the characteristics of defects in this batch of the transformers were sorted firstly. A typical CT which was used to be installed at an A-phase outgoing of the No.159 interval was selected for disassembly analysis. However, no abnormality was found in all parts of the transformer during the disassembling process. In view of this, two defective models were prepared by using the LB6-IIOW type of transformers separately. One of them was improper dried whereas the other was simply oil-immersed without using vacuum. The development process of defects in current transformer was studied. The simulation test showed that under the external applied voltage and current stresses, the capacitance and dielectric loss of models increased rapidly. And the dissolved gases were produced very fast in oil. H2 is the dominating dissolved gas followed by CH4. A small amount of C2H2 could be also found in oil. It was believed that the unsatisfactory vacuum immersion oil is the main cause for the failure of this batch of CTs. Based on this research, corresponding technical suggestions were put forward.