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In this paper, four vacuum tap changer failures in power and converter transformers are investigated. It is shown that oil contamination and arcing of auxiliary contacts are the main reasons for the failures. The protection and recorded fault current waves are analyzed. The three-phase imbalance current rate is proposed for the early warning of tap changer failure due to the delay initiation and activation of differential protection. A magnetic field FEM simulation model is also established to study the effect of tap changer failure on convert transformer, and the failure progress process has been reproduced successfully. Finally, the historical DGA data sets of thousands of vacuum tap changers in a certain region are collected and analyzed. Caution limits of acetylene of vacuum tap changers are obtained. According to the failure analysis and inspections, maintenance suggestions are proposed for the reference of power grid and manufacturers.