This paper analyzes the process of oil-paper insulation breakdown

introduces the insulation structure characteristics of U-model oil-immersed current transformers

and designs the unbalance current protect device.This device obtains unbalance signal by way of making use of the current changing due to partial insulation trouble during the breakdown

and transmits this current signal to the on-line monitoring equipment

thus achieving the on-line insulation monitoring of the current transformer

providing a reliable basis for the early prevention of the insulation breakdown of current transformer and avoid the fault of current transformer due to insufficient period maintenance.