Brief Introduction to Waveform Simulation Analysis of Lightning Wave Impulse Test for Bushing with Voltage Divider Screen

Author： GOZ Electric Time：2024-06-09 09:23:18 Read：16

    Aiming at the problem that the wiring of the voltage divider, stray impedance and capacitive reactance will cause the response waveform of the bushing lightning wave type test to be distorted, the bushing capacitance value is measured and the spectrum information of the voltage waveform is analyzed. The spectrum information of a specific time period is analyzed by short-time Fourier transform to determine whether the bushing has interlayer breakdown during the test. The results show that stray impedance will cause the voltage waveform at both ends of the voltage divider to be distorted in the lightning wave type test. The spectrum of the voltage of the voltage divider in different time periods can be processed by short-time Fourier analysis to determine whether the bushing has breakdown. The number of breakdown layers has a quantitative relationship with the extreme value of the spectrum and the change in amplitude. When the number of breakdown layers is small, this method is more accurate than the capacitance value measurement.

    With the rapid development of my country's power system, especially the construction and commissioning of ultra-high voltage transmission projects, higher requirements are put forward for the insulation reliability and safety of a large number of power equipment on the transmission and distribution lines. As a capacitive device widely used in the power system, the reliability and safety of dry bushings have always been valued by people. In recent years, transformer failures have occurred frequently, and bushing failures account for about 30% of the total number of transformer failures. The online monitoring device of the traditional dry bushing is connected to the end screen of the bushing through the grounding wire. Once the online monitoring device fails, serious consequences may occur. Based on the reliability and safety requirements of the dry bushing, a new layer of plate is added outside the outermost plate of the dry bushing as a voltage divider. From a design perspective, this bushing with a voltage divider can effectively increase the safety of the equipment and the feasibility of online monitoring. However, when the actual product was tested for lightning wave type before leaving the factory, it was found that the waveform of the lightning wave response at both ends of the voltage divider was severely distorted, which brought certain difficulties to the engineers in judging whether the bushing was broken down under the lightning wave. Relevant personnel studied an overvoltage online monitoring system based on the transformer bushing voltage divider. By simulating the system with lightning and operating impulse voltage waves, the output waveform response was obtained. The research of Hu Wei et al. [10] showed that the high-frequency impedance of the grounding wire is much greater than the power frequency impedance, which is one of the important reasons for the increase of the voltage divider potential of the bushing during the lightning impulse voltage test.

    This study uses NI's circuit simulation software Multisim to simulate the distortion of the lightning wave response waveform at both ends of the voltage divider, and verifies that stray inductive reactance and capacitive reactance are the causes of the response waveform distortion. Further research is conducted on the response waveform, and the response waveform is analyzed using short-time Fourier transform, and an effective method for detecting interlayer breakdown under a small number of breakdown layers is proposed.

Conclusion

(1) Stray inductive reactance and capacitive reactance will cause the waveform of the voltage u2 at both ends of the voltage divider to be distorted in the lightning wave type test.

(2) Even if the u2 waveform is distorted, the waveform mutation caused by the breakdown phenomenon can still be observed from the oscilloscope waveform, and the capacitance value of the bushing main capacitor C1 before and after the test can be measured to determine whether the bushing has broken down, but this method is difficult to implement when the number of breakdown layers is small.

(3) Using short-time Fourier analysis to process the spectrum in different time periods of u2, it is possible to determine whether the casing has been broken down. The percentage of amplitude change increases approximately linearly with the increase in the number of breakdown layers. When wavefront breakdown occurs, the spectrum extreme value increases with the increase in the number of breakdown layers. When waveback breakdown occurs, the spectrum extreme value decreases with the increase in the number of breakdown layers. This method still has high accuracy when the number of breakdown layers is small.