The ultra-high frequency(UHF)method has been widely used in a gas-insulated system(GIS)for partial-discharge detection,and many achievements have been realised.In addition,many studies based on artificial defects have been made to confirm its validity.Therefore,the UHF method is generally believed to be sufficiently effective for GIS monitoring.However,in practical application,the authors find that for some micro-crack discharge in GIS insulator,the UHF method has low sensitivity.To fully study the characteristics of the micro-crack discharge in the GIS insulator,an experiment is conducted in this study using an actual post insulator with a micro-crack defect.The current signal based on IEC 60270 standard and the radio-frequency electromagnetic signal is simultaneously measured for thorough analysis.The results show that some submillimetre crack defects may occur in the GIS insulator.Its discharge is mainly presented as glow discharge,and the discharge signal frequency usually cannot reach the UHF band;thus,it cannot be effectively detected by the UHF method.This study provides complementary information to the applicability of the UHF method and inspires further study of the GIS insulator and its monitoring technology.
With the increase in the voltage and capacitance of gas-insulated transmission lines(GILs),the insulation failure of GIL has attracted more and more attention.Further understanding of the partial discharge(PD)and flashover characteristics of metal particles on the insulator surface,as well as the flashover mechanism,is necessary to reduce the failure rate and improve the reliability of the equipment.In this study,an ultra-high voltage(UHV)AC insulator PD test and measurement system based on the cross-reference pulse current(PC)and ultra-high frequency(UHF)methods were established.The PD development and flashover characteristics of 5 mm-long metal particles at different positions on the surface of a UHV AC insulator,as well as the surface charge accumulation,were studied.The results show that the discharge of millimetre-scale metal particles on the insulator surface under PD test conditions is relatively low(generally lower than 2PC),and it is difficult to be detected by the conventional PC and UHF methods due to their insufficient sensitivity.Moreover,it is found that the weak PDs of the millimetrescale metal particles will result in a charge accumulation on the insulator surface under AC voltage,which eventually will lead to the insulator flashover.The PDs of the UHV AC insulator before flashover are generally small,which will make it difficult for online monitoring systems to give an effective alarm before flashover.
Based on the Estuarine, Coastal and Ocean Modeling System with Sediments （ECOMSED） model, a 3-D hydrodynamic-transport numerical model was established for the offshore area near the Yangtze Estuary in the East China Sea .The hydrodynamic module was driven by tide and wind. Sediment module included sediment resuspension, transport and deposition of cohesive and non-cohesive sediment. The settling of cohesive sediment in the water column was modeled as a fimction of aggregation （flocculation） and deposition. The numerical results were compared with observation data for August, 2006. It shows that the sediment concentration reduces gradually from the seashore to the offshore area. Numerical results of concentration time series in the observation stations show two peaks and two valleys, according with the observation data. It is mainly affected by tidal current. The suspended sediment concentration is related to the tidal current during a tidal cycle, and the maximum concentration appears 1 h-4 h after the current maximum velocity has reached.
