Broadband couplers for UHF detection of partial discharge in gas-insulated substations

The paper describes investigations of the UHF performance of a disc coupler designed for continuous monitoring of partial discharge in gas-insulated substations and indicates how the UHF sensitivity can be improved. Resonant frequencies of the disc are calculated, and the coupler is shown to be most sensitive at these frequencies. The bandwidth of the coupler can be extended by coupling the UHF signal from the edge of the disc, allowing detection of a greater number of the possible resonant modes of the disc, including the dominant, lowest frequency mode. Sensitivity to the UHF signals generated by partial discharge can be further improved by various means. The disc can be earthed at certain points while still retaining the coupling function. A novel nonresonant experimental coupler design is also presented, based on the design techniques used for broadband log-spiral antennas. The frequency response of the spiral coupler is much flatter than that of a comparable disc coupler.