Yuchang Qiu

Application of fractals to PD signal recognition

A fractal random model namely the fractional Brownian motion (FBM) is established to describe a single PD pulse. The fractal dimension ruler variable (FDRV) and fractal dimension (FD), as two features derived through the least-square fitting, are introduced to extract characteristics from the single PD pulse. Analysis of experimental results indicates that the fractal dimension ruler variable has a higher sensitivity compared with fractal dimension, and therefore can he used as an important practical feature to recognize PD signals.

Investigation of SF/sub 6/-N/sub 2/, SF/sub 6/-CO/sub 2/ and SF/sub 6/-air as substitutes for SF/sub 6/ insulation

The dielectric strength of SF/sub 6/-N/sub 2/ is slightly higher than that of SF/sub 6/-CO/sub 2/ in slightly nonuniform fields encountered in gas-insulated switchgear (GIS) and gas-insulated cables, but the opposite is true in the cases of highly nonuniform fields and gas/film insulation existing in gas-insulated transformers. Experiments also show that the SF/sub 6/-air mixture with the SF/sub 6/ concentration being 0.8 can be used in cubicle type GIS, where the impulse dielectric strength of the gas mixture is apparently higher than that of SF/sub 6/ gas.

The influence of O/sub 2/ and N/sub 2/ additives on impulse breakdown of SF/sub 6/ in a non-uniform field

The influence of O/sub 2/ and N/sub 2/ additives on positive impulse breakdown of SF/sub 6/ in a non-uniform field is reported. N/sub 2/ and O/sub 2/ were mixed in the ratio of 80/20 as a substitute for natural air, and the ratio was varied afterwards. Different impulse waveshapes were employed with wavefront being in the range from 0.6 to 80 /spl mu/s and wavetail duration about 500 /spl mu/s in all cases. Experiments showed that the optimal air content in the SF/sub 6//artificial air mixture was around 20% when the total gas pressure was 0.1, 0.2 or 0.3 MPa. By varying the ratio of N/sub 2/ to O/sub 2/ in the mixture containing 90% SF/sub 6/, the highest breakdown voltage under 2/500 /spl mu/s impulse was observed in the case of 5% N/sub 2/ plus 5% O/sub 2/. The relationship between the breakdown voltage and mixing ratio indicated that synergistic effects of SF/sub 6//O/sub 2/, SF/sub 6//N/sub 2/ and O/sub 2//N/sub 2/ were apparently different. The maximum breakdown strength is ascribed to the best synergism of O/sub 2/ and N/sub 2/ acting together with SF/sub 6/, and the synergistic effect of O/sub 2/ and N/sub 2/ in SF/sub 6/ mixtures is related to the wavefront duration of the applied impulse voltage.<<ETX>>

Propagation characteristics of UHF electromagnetic waves induced by partial discharge in GIS

The principle of the UHF method to detect partial discharge (PD) in GIS is to measure the UHF electromagnetic waves (EMWs) excited by PD pulses. Such EMWs propagate along the GIS busbar in not only TEM modes, but also of TE and TM modes. The coaxial busbar of the GIS acts as a waveguide in which the UHF EMWs travel. The PD source can be located accurately by this method. Experimental results show that single PD pulse can result in resonances of EMWs inside the GIS which are of much longer duration compared with the pulse width itself. In addition, the attenuation of the EMWs along the GIS busbar are investigated on a GIS test setup in laboratory.

Voltage-time characteristics of SF/sub 6/ and SF/sub 6/-N/sub 2/ under lightning and steep-fronted impulses

Voltage-time characteristics of SF/sub 6/ gas and an SF/sub 6/-N/sub 2/ gas mixture with a mixing ratio of 50/50 were measured in a 50/20-mm-diameter coaxial electrode system with and without a spacer under both lightning impulse and steep-fronted impulse voltages. A nonoscillating steep-fronted impulse with a risetime as short as 50 ns was derived from a 1.2/60- mu s lightning impulse by means of a wave-front-steepening gap, and was measured using a compact resistive type voltage divider, with the measuring system response time being less than 5 ns. Experiments showed that the breakdown voltage under the steep-fronted impulse was lower than that under the lightning impulse for the same time to breakdown in both SF/sub 6/ and the SF/sub 6/-N/sub 2/ gas mixture, but it was still higher than the 50% lightning impulse breakdown voltage used for insulation co-ordination. For the case with a spacer, breakdowns in both SF/sub 6/ and SF/sub 6/-N/sub 2/ were greatly scattered compared with gas insulation without any spacer.<<ETX>>

Effect of surface charges on impulse flashover voltages of spacer in SF/sub 6/

The positive and negative charges were injected onto the spacer surface by DC corona discharges, and the flashover characteristics were investigated by a sudden change from DC to lightning or steep-fronted impulse in the presence of a conducting-particle attached on the spacer in SF/sub 6/ gas. Experimental results showed that the polarity of corona charges has a great influence on impulse flashover voltages. For the opposite polarity impulse to corona charges, there is a significant shift of the time domain where the minimum flashover voltage appears in the U-shaped curves of relationships between flashover voltage and wavefront duration (U/sub f/-t/sub f/). Also, the influence of DC polarity reversal on the corona discharge current has been studied.

A method for calculating low probability impulse breakdown voltages in SF/sub 6/-filled gaps

From the analysis of the abnormal discharge in SF/sub 6/-filled gaps, a method for calculating low probability impulse breakdown voltage is put forward, which consists of three limiting lines: the streamer breakdown line, the low probability impulse breakdown level and the streamer onset line in different gas pressure ranges. The calculated value gives a safety margin compared with the measured value.

An expression of critical pressure in SF/sub 6/-filled point/plane gaps

A new method for calculating low probability impulse breakdown voltages is put forward, which consists of three limiting lines: the streamer breakdown line, the low probability impulse breakdown level and the streamer corona onset line in different gas pressure ranges. The streamer breakdown line and the streamer corona onset line can be calculated using the basic knowledge of SF/sub 6/ gas insulation. If the critical pressure P/sub c/ is known, then the low probability impulse breakdown level can be predicted, because the streamer corona onset voltage and the low probability impulse breakdown voltage are of almost the same value at P/sub c/. So the expression of P/sub c/ is very useful for insulation design. In this paper, different point/plane gaps are tested, and an expression of P/sub c/ is derived based on the experimental results. Compared with the measured values for given gaps, the calculated values have a relative error no more than 2%.

Leader Step Time and Low Probability Impulse Breakdown Voltage Measured in SF6 Gas Mixtures

SF6 is used worldwide today owing to its high dielectric strength and good heat transfer properties. But SF6 is extremely sensitive to non-uniformity of the electrical field. Therefore in some electrical apparatus, such as cubicle type GIS, SF6 gas mixtures are used1. Experiments show that when SF6 is mixed with low electrical strength gases, such as air or CO2, the low probability impulse breakdown voltage U i and dc breakdown voltage U b of the mixtures are higher than that of SF6 2,3. the view of corona stabilization enhancement4 is usually used to explain the increase of U b: when low electrical strength gas is added to SF6, the corona onset voltage of mixtures decreases, so more ions are produced leading to the enhancement of corona stabilization. But the increase of U i can not be explained by this viewpoint because there is hardly any corona stabilization under impulse conditions. Moreover, it is reported5 that when Freon 113 (C2C13F3), whose electrical strength is higher than that of SF6, is mixed with SF6, not only will U b and U i increase, but the corona onset voltage will also increase. Obviously, this phenomenon conflicts with the view of corona stabilization enhancement.

Dielectric characteristics of SF/sub 6/ under the steep-fronted impulses

The influence of the wavefront duration on impulse breakdown voltage of the nonuniform field gap in compressed SF/sub 6/ gas has been studied using a point-plane electrode arrangement. Voltage-time curves are also given in this paper. For comparison purposes, dielectric characteristics of SF/sub 6/ in a sphere-plane gap have been measured. The wavefronts used in this work are 0.05, 0.3, 0.6, 0.9, 1.2 and 1.5 /spl mu/s. The experimental results show that the wavefront duration has significant influence on the dielectric behaviour of SF/sub 6/ in the nonuniform field. It is found that the breakdown voltage-wavefront relationships and voltage-time curves of nonuniform field tend to be U-shaped.