The operation of circuit breakers and disconnectors in substations can produce transient earth voltage rise（TEVR） and severe space electromagnetic radiation

causing serious electromagnetic disturbance to the on-site electronic devices in substations. The power supply of on-site electronic devices is a major coupling path of electromagnetic disturbance. Therefore

this paper studies the coupling characteristics and suppression of electromagnetic disturbance caused by on-site power supply. The research results show that most of the existing power supply circuits of on-site electronic devices contain power lines connected between the inside and outside of the shielding

which can significantly reduce the efficiency of the shielding and further introduce external electromagnetic disturbance into the shielding. In this paper

an isolation transformer structure is proposed. Its primary and secondary windings are located inside and outside the shielding box respectively

averting the power lines connected across the shielding. Further experiments on this isolation transformer show that the electromagnetic disturbance introduced by the power supply can be effectively suppressed.