Li Dajian

Study of time delay of UHF signal arrival in location partial discharge

In the study of location partial discharge in transformer based on the UHF method, in order to accurately locate partial discharge source, precisely achieving time delay of different signals which are received by different UHF sensors and radiated from the same partial discharge source is essential. Three methods of time delay estimation are introduced and compared, including the initial peak, the inflection point of cumulative energy curve, cross correlation, and the measures of signal de-noising based on the wavelet transform and cubic splines interpolation are introduced to optimize the methods mentioned above. Then the pin-to-flat electrode model and the wedge model are adopted respectively to experiment these methods. Results demonstrate that absolute error between the time delay estimation through optimized cross correlation and the time delay in theory, which is less than 30 ps, is the least among the errors through other methods mentioned above, and the optimized cross correlation analysis can achieve the highest precision among other methods and can be used to accurately locate partial discharge source.

The discovery and analysis of suspended discharge defects in TV

Reducing the influences of suspended discharge on insulating property can improve power supply reliability. In this Paper, through analyzing an explosion accident of 220kV SF6 gas-insulated electromagnetic voltage transformer (TV), it shown that the primary cause of the accident is attributed to the suspended discharge in the internal equipment, resulting in a reducing performance of SF6 gas-insulation. Under the low lightning overvoltage, short circuit of TVs primary lead over the ground is caused. Meanwhile, the experiment verifies that the suspended discharge causes instable electrical connection in internal equipment, resulting in fluctuation of TVs secondary terminal voltage and misjudge of equipment insulation online monitoring. At last, the paper proposes the countermeasures and recommendations on preventing from suspended discharge in internal electrical equipment.