Diagnosis of partial discharge by employing inner grading electrode of insulating spacer in gas insulated switchgear

In order to further develop the detection and diagnosis measures of partial discharges (PDs) in gas insulated switchgear(GIS), we proposed to adopt the inner grading electrode inside insulting spacer as an ultra-high frequency(UHF) sensor for PDs. Using the finite difference time domain (FDTD) method, we studied the main waveguide modes of GIS and the frequency response characteristics of the inner grading electrode. Then, we used the proposed UHF sensor to test the PD characteristics of three typical defects, and compared the sensor with commercial UHF sensors in aspects of detection characteristics. The simulation results show that the TEM mode component of the GIS model is the largest and the TE11 follows when there is PD in the GIS. The cutoff frequencies of high-order waveguide modes distribute around 250 MHz~1 GHz, and there are a number of resonance frequencies below 3 GHz for the grading electrode UHF sensor, while the largest resonance frequency appears at about 140 MHz. The experimental results show that the PD detection sensitivity of the grading electrode UHF sensor is much higher than that of external sensors, however, it is slightly lower than that of interior sensors. The main frequency spectra of both metal protrusion on high voltage conductor and floating potential defects distribute below 1.5 GHz, which is consistent with the simulation of the cutoff frequencies of high-order waveguide modes. According to the experiments, the largest resonance frequency of grading electrode UHF sensor is about 115 MHz, close to the simulation value, 140 MHz.