Study on partial discharge characteristics of C4F7N/CO2 gas mixture GIL based on the integrated optical-UHF detection

With the application of gas-insulated metal-enclosed transmission line (GIL), the gas insulation of environmentally friendly gas named "g3" which is the gas mixture of C4F7N/CO2 has been considered as an alternative to SF6 to reduce the greenhouse effect. Partial discharge (PD) is one of the most influential harm in the insulation system. In order to confirm reliability and safety in the operation of insulation, the characteristics of C4F7N/CO2 gas mixture GIL is required to be detected and analyzed. In this paper, artificial models of three typical defects in the GIL are established and researched for PD characteristics using the integrated optical-UHF method that can improve the range, the accuracy and the sensitivity of detection. The ratio of 4%C4F7N/96%CO2 is selected as the gas insulation for the PD experiment of GIL at 0.2 MPa, applying the voltage of 16kV. The result shows that each defect has their own PD characteristics which differ from phases, pulse number and amplitude, according to the phase-resolved partial discharge (PRPD) patterns. The PRPD patterns indicate that the needle-plate defect and the floating electrodes defect have strong regularity of phase distribution, while that of free particle defect is weak relatively. Furthermore, the optical sensor and UHF sensor have different effective response range from different defects. Thus, based on the results we can monitor the insulation defect and help to recognize the type of defects in GIL.