The discharge defects in oil-paper bushing can not be timely discovered due to lack of effective online monitoring method

thus explosion on bushing occasionally occurs. In order to provide an effective online monitoring device to find the discharge defects in oil-paper bushings

we researched the influences of characteristic gases produced in bushing on the inner pressure of bushing

and developed an online monitoring device based on inner pressure response. Moreover

we analyzed the sensitivity of the online device in finding discharge defect

and verified the validity of the online monitoring device by partial discharge（PD） tests performed on real bushing. The results show that characteristic gases such as hydrogen

methane

and carbon monoxide produced by discharge defects of oil-paper insulation can cause significant increase in the internal pressure

and the internal pressure will increase with the development of defects. The method to monitor the pressure of bushing is available to discover discharge defect in busing

and the online monitoring device developed in this paper has sufficient sensitivity to discover initial discharge defect in bushing. Therefore

the on-line monitoring device developed in this paper can be used to realize the defect warning of oil-paper insulating bushing

so as to avoid explosion accident of bushing.