Zhong Lipeng

Ultraviolet Images of 50-Hz AC Discharges in SF 6 Atmosphere

Detection of ultraviolet images is proved to be both effective and convenient in discharge monitoring nowadays. In a self-designed gas chamber with observation windows installed, characteristics of 50-Hz ac discharges in SF6 atmosphere were observed in the needle-to-plane model and the floating potential model. Ultraviolet images of the discharges in different periods of the voltage process are presented, and correspondence is found with the variation of the discharge capacity.

The mechanism of SF6 decomposition characteristics under partial discharge at different gas pressures and voltage

SF6 is widely applied in gas insulated switchgear (GIS) due to its excellent insulating and arcing performance. However, the partial discharge often is caused by some insulation defects because of the production, installation and operation of the GIS. Under partial discharge, The decomposition products including SO2F2, SOF2, SO2, CF4, CS2, will be formed. The types, content and change trend of products will encounter serious disagreements due to the difference of discharge type. The partial discharge can be detected by the analysis of the decomposition products of SF6. In recent years, because the method to detect the partial discharge is of a greater reduction of interferences and a higher sensitivity, the researchers pay more attention to the decomposition characteristics of SF6. In this paper, the decomposition characteristics of SF6 under partial discharge at different gas pressures and voltage level were studied. In the experiments, gas chromatography was adopted to quantitatively determine the characteristic components of decomposition. The results show that SF6 decomposition products include CO2, SOF2, SO2F2, SO2 under the needle-plate defects. The total concentration of (SOF2+SO2) and SO2F2 vary with the pressure increasing according to the rule of negative exponent, and the concentration of CO2, SOF2, SO2F2 increase linearly and gets saturated at last as the voltage increases. Then, characteristic parameters of SF6 dissociation rate is put forward to explain the SF6 gas decomposition mechanism under different pressures and voltage level. Under partial discharge, first, SF6 will decompose into SF5, SF4, SF2, etc, by electron impact-induced dissociation. The low fluorine sulfides will then react chemically with insulating material, and oxygen in SF6 to form the decomposition products. And dissociation rate affected by gas pressures and voltage controls to form stable decomposition product rate. Finally, by calculating dissociation rate, the conclusions that dissociation rate at different gas pressures and voltage level influenced the decomposition product concentration were drawn.

The application of sweep frequency impedance on the diagnosis of short circuit faults within transformer winding

As a central device of a power system, the safe running of a power transformer is considered to be significance. Through the statistics of onsite faults, the amount of the transformer winding deformation caused by short-circuit faults within transformer winding is maximum, therefore the detecting on the short circuit faults is important to improve the reliability of power transformers in service. In this paper, a data acquisition system has been developed based on a signal source and a power amplifier, and the short circuit faults are simulated by using a three-phase transformer in lab, as well as the sweep frequency impedance (SFI) curves are extracted and analyzed to find out the inner relevancy. The result indicates that the resonance frequencies and their amplitudes of the sweep frequency response curves vary as the degree and type of short circuit faults differ, and by analyzing the relationship between each curve, some preliminary conclusions can be drawn here. This investigation has some degree of guiding significance in predicting structural faults of windings in the transformers and is the basis for further experimental studies.