Abstract:The vacuum on load tap changer is one of most important components of UHV converter transformers. During an annual maintenance for converter stations, the abnormal increase of acetylene in the oil chamber of vacuum on load tap changers of UHV converter transformer is reported for the first time. Based on the oil chromatographic analysis and hanging core inspec tion, it is found that the discharge ablation caused by the loose fit between the bolt and the screw hole directly leads to the abnormal acetylene. According to the mechanical analysis of abnormal phenomenon, the suggestions for maintenance standards and daily operation of on load tap changers are put forward, which provides precious experiences for the follow up similar works.