Zhu Lingyu

The effect of thermal aging on surface structure and mechanical strength of oil-impregnated pressboard

Oil-impregnated paper is generally used as insulation material in power transformer, which consists of transformer oil and insulating paper (pressboard). And the pressboard made of cellulose is one of important components in transformer insulating system. Oil-impregnated pressboard stays in the environment with high temperature during normal operation of transformer. It is generally recognized that the hot-spot temperature may be the chief factor determining the lifespan of transformer. Furthermore, the inner temperature of transformer will also influence both electrical and mechanical performances of oil- impregnated pressboard. In this paper, the micro surface appearance and mechanical strength of insulating pressboard under different aging time are studied. The sample preparation includes the following steps: vacuum drying for pressboard and transformer oil, impregnating pressboard with transformer oil, and taking oil-impregnated pressboard into vacuum drying oven for thermal aging. When the aging time of oil-immersed pressboard reaches 100h, 200h, 400h and 800h, the pressboard will be taken out of the oven as specimens respectively. After all the specimens under different aging time are obtained, the micro surface appearance of them was observed by scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM). Then the tensile strength of them was tested by tensile strength tester. The results indicate that long cellulose chain in the surface of pressboard is broken into the shorter one or shed directly with the aging time becoming longer, meanwhile the mechanical strength of pressboard becomes weaker.

Noise and vibration survey of ac filter capacitors in Feng Xian HVdc converter station

The audible noise radiated from the ac filter capacitors in HVdc converter stations has become a serious environment problem. The purpose of this study is to apply portable digital vibrometer for in-situ surface vibration measurement of capacitors, to survey the vibration isolation and noise reduction effect of double-bottom capacitors which are installed in Feng Xian Converter Station for the first time, and to analyze the vibration and noise characteristics of ac filter capacitors. Portable digital vibrometer is a new technology using eye-safe visible laser and digital signal processing to conduct non-contact on-line vibration velocity measurement. The vibration of capacitor surfaces and steel frames are measured and amended according to the measuring elevation angle. On the other hand, the noise at the points arranged around the rail of filter field is measured by NA28 sound-level meter and 1/3 octave band realtime analyzer. The data and analyses show that the main frequency component in vibration spectrums of capacitor surfaces and steel frames are both 100Hz. And as the doublebottom design being adopted, the vibration at bottom surface decreases obviously. Besides, the noise at the side near the top surfaces of capacitors is much higher than that at other places.

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.

Application of Excitation Function to the Prediction of RI Level Caused by Corona Discharge

Radio interference (RI), as an aftereffect of corona discharge, is an important research topic in the field of electromagnetic compatibility, where excitation function is applied broadly to the prediction of RI level. This paper presents the theory of excitation function method used in the RI level prediction. Then, some practical problems related to this method are discussed. The propagation procedure of corona current is solved by the phase-modal transformation, and the impedance matrix of multi transmission lines is calculated by a double logarithmic approximate model of Carsons Ground-Return impedance. At the same time, in order to calculate the RI level when total line corona is assumed, an analytical formula is deduced for integral operation. Based on the above solutions, an algorithm is presented and applied to the prediction of RI level of a practical overhead transmission line. Comparison of prediction and measurement results indicates that the algorithm proposed in this paper is effective and feasible.