| Internal ultra-high-frequency (UHF) couplers have not been installed on most of gas insulated switchgears (GISs) currently running. Insulators were surrounded by metal flange. In this condition, UHF signal couldn’t be detected by the conventional methods. Detecting UHF signal passing through the rein sprue in metal flange is the only way to measure partial discharges (PDs) in GIS. Due to the influence of the tiny dimension and special configuration of the resin sprue, the UHF Signals propagating from it may get weaker. Furthermore, the electric field structure and frequency spectrum distribution have already been varied significantly. For the reasons above, if the conventional external UHF sensors were still used to detect the UHF signals leaking from the resin sprue, the sensitivity of detection may drop substantially, and it wound not satisfy the requirements of sensitivity of GIS PD detection. The topic of this paper is to develop an external UHF coupler which is specially detecting the UHF signal passing through the rein sprue.We analyzed the propagation of UHF signals passing through the sprue based on the theory of electromagnetic shielding of slots. Comparing with the electromagnetic wave whose direction of electric field is vertical to the short side of sprue, the electromagnetic wave of parallel direction is more easily passing through the sprue. With the increasing of the frequency, SE (shielding effectiveness) falls down gradually. SE of the sprue of 45×20mm is 20dB when the frequency is 0.85GHz, and it will fall to OdB when the frequency is 1.71 GHz. The longer the long side of the sprue is, the wider the available frequency is. Such as available frequency of sprue of 45x20mm exceeds 0.85GHz, and that sprue of 55×20mm falls to 0.68GHz. The propagation of UHF signal was simulated by using finite-difference-time-domain (FDTD) method. The electric field structure and frequency spectrum of UHF signals in the surface of sprue and exposed insulator were analyzed. According to the research, the main direction of electric field is parallel to the short side of sprue, which is more 30dB than the other two directions, so the UHF electromagnetic wave passing through the sprue should be intensively linear polarization and the energy is mostly on the frequency band of 0.8-2 GHz; UHF electromagnetic wave passing through the exposed insulator is weakly linear polarization, its direction distribution of electric field is relatively symmetrical. The result of test of 252kV GIS PD detection in laboratory was well consistent with the theory and simulation.By means of fractal transformation, resistively loaded and cavity backed technology, the bow-tie antenna was designed. Its working frequency band is mainly in 0.8-2GHz, whose effective height is 2.6 times of the conventional PD coupler. For the sake of broaden the frequency band, the double ridge was added to the rectangular waveguide, so the double ridge waveguide antenna was designed. Its effective height is 1.7 times of the conventional PD coupler.An experimental platform was established for the 252kV GIS PD detection. Some kinds of typical PD defect were embedded in GIS, such as protrusion on the bus bar, surface contamination on the insulator and air void in spacer. In the experiments, the power spectrums of the UHF signals passing through the rein sprue were analyzed. The feasibility of detecting the PD UHF signals leaking from the sprue was verified. The sensitivity of the bow-tie antenna detecting the PD signals leaking from the sprue was tested, which indicates the sensitivity is below 15pC. |
PDF Full Text Request