DC superimposed lightning impulse voltage test results and analysis

Author： GOZ Electric Time：2024-08-20 09:20:24 Read：19

    When the GIS busbar basin insulator is in a clean state, ±1675kV or ±2400kV lightning impulse voltage is applied to the GIS busbar separately for 10 times, and no flashover occurs in the test sections of the 1100 kV and 550 kV GIS busbars. It is analyzed that 2400kV is the lightning impulse withstand voltage of the 1100kV GIS busbar, and the busbar is not prone to flashover under this voltage. Although the lightning withstand impulse voltage of the 550kV GIS busbar is 1675kV, it has a large insulation margin and no flashover occurs.

    The GIS busbar was pre-applied with DC superimposed lightning impulse voltage test, and the test results of flashover under various working conditions were statistically analyzed, see the table. The above test results are plotted in Figures 5 and 6. The horizontal axis of Figure 5 is DC voltage, and the vertical axis is the superimposed voltage of DC and lightning impulse voltage. The horizontal axis of Figure 6 is self-sinking e, and the vertical axis is lightning impulse voltage. The gray dots in the two figures indicate the voltage corresponding to the flashover, and the black dots indicate the voltage corresponding to the flashover when no flashover occurs.

    As shown in Table 1, when the amplitude of the lightning impulse voltage is 2400 kV, the busbar flashes 11 times when the negative polarity DC is superimposed on the negative polarity lightning impulse voltage, including 3 flashovers when the pre-applied -225 kV DC voltage is applied, and 4 flashovers when the pre-applied -360 kV DC voltage is applied. Only 1 flashover occurs when the positive polarity DC is superimposed on the positive polarity lightning impulse voltage. In the heteropolarity superposition test, no flashover occurs on the busbar.

    According to the analysis of the test results of single lightning impulse voltage, when the single -2400 kV lightning impulse voltage was applied, the GIS bus did not flash over, but when the DC voltage was superimposed over -225 kV, multiple flashovers occurred, indicating that when the negative polarity DC voltage was superimposed, the bus was more likely to flash over than the single lightning impulse voltage, and the bus's lightning impulse voltage tolerance level decreased. When the -450 kV DC was superimposed on the -2400 kV lightning impulse voltage, the superimposed lightning impulse flashover voltage amplitude decreased by about 28.5% compared with the single -2400 kV lightning impulse voltage. Combined with Figures 5 and 6, it can be seen that with the increase of the negative polarity DC voltage, the superimposed lightning impulse voltage amplitude during flashover and the superimposed value of the two have a decreasing trend. The analysis shows that the GIS busbar is a slightly non-uniform field. When the negative polarity DC voltage is pre-applied, compared with the lightning impulse voltage, the high-voltage center conductor has more time to accumulate positive charges near the high-voltage center conductor, especially the local field strength at the junction of the high-voltage center conductor, SF6 gas, and pot-type insulator is greater. When the lightning impulse voltage is superimposed, the local field strength at the three junctions is greater than the local field strength when the lightning impulse voltage is applied alone, and it is easier to flash over. Therefore, the flashover voltage of negative-negative superposition is lower than that of single lightning impulse voltage. With the increase of pre-applied DC voltage, this effect is more obvious.

    Comparing the test results of different polarity combinations in Figures 5 and 6, the flashover voltage of negative-negative superposition is lower than that of positive-positive superposition, and it is easier to flash over. The analysis shows that, as a slightly non-uniform field, when the GIS busbar is pre-applied with negative polarity DC voltage, positive charges are easily accumulated near the high-voltage center conductor, especially near the junction with SF6 gas and pot-type insulator 3. The electric field distortion is more serious than that of applying positive polarity voltage. When the impulse voltage of the same polarity is superimposed, this effect is further enhanced. Therefore, it is easier to flash over when the negative polarity is superimposed.

    The path of busbar discharge is mainly along the surface of the pot-type insulator, or first from the busbar conductor through the gas gap to the pot-type insulator and then along the surface of the pot-type insulator. One typical flashover trace of pot-type insulator discharge is shown in Figure 7. The DC voltage during flashover is -226.01 kV, the lightning impulse voltage is -1862.09 kV, and the superimposed voltage is -2088.10 kV. As can be seen from Figure 7, the flashover occurs at the 7 o'clock direction of the concave surface of the pot-type insulator, starting directly from the high-voltage center conductor, SF6 gas, and the junction of the pot-type insulator 3, and developing along the surface of the pot-type insulator to the busbar barrel.

    Considering the influence of pre-applied DC voltage time on the surface charge accumulation of the pot-type insulator, the superimposed lightning impulse voltage test with pre-applied DC voltage time of 0.5, 2, and 4 h was also carried out. There was no obvious effect on the flashover voltage at different pre-applied times. It was analyzed that the pre-applied DC voltage did not cause a significant change in the surface charge of the pot-type insulator due to the large insulation margin of the pot-type insulator or the insufficient pressure time.

1. Results and analysis of DC superimposed operating impulse voltage test

    When the GIS busbar pot-type insulator was in a clean state, ±1300 kV or ±1800 kV operating impulse voltage was applied to the GIS busbar separately for 10 times, and no flashover occurred on the pot-type insulator.

    The GIS busbar was subjected to a pre-applied DC superimposed operating impulse voltage test, and the test results are shown in Figures 8.9. In Figure 8, the horizontal axis is the DC voltage, and the vertical axis is the superimposed voltage of the DC and operating impulse voltages; in Figure 9, the horizontal axis is the DC voltage, and the vertical axis is the operating impulse voltage. In the two figures, the gray dots represent the voltage corresponding to the occurrence of flashover, and the black dots represent the voltage corresponding to the absence of flashover.

     It can be seen from Figures 8 and 9 that: ① When the operating impulse voltage amplitude is 1300 kV, no flashover occurs on the pot-type insulator under various combinations of superimposed voltages; ② When the operating impulse voltage amplitude is 1800 kV, only two flashovers occur on the body insulator when the negative polarity DC is superimposed on the negative polarity operating voltage, and both flashover discharges start from the gas gap, and the gas gap breaks down and develops to the surface of the pot-type insulator. However, no flashover occurs on the pot-type insulator under superimposed voltages of other polarities. The discharge law in the DC superimposed operating impulse voltage test is similar to the above-mentioned lightning impulse. Superimposed operating impulse voltage tests with pre-applied DC voltage time of 0.5, 2, and 4 h were carried out. Different pre-applied times had no obvious effect on the flashover voltage, and the reason was the same as the superimposed lightning impulse voltage.

2. Conclusion

    1) When the GIS busbar is in a clean state, no flashover occurs on the GIS busbar when a single ±2400 kV lightning impulse voltage or a single ±1800 kV operating impulse voltage acts.

    2) When negative polarity DC is superimposed on negative polarity impulse voltage, flashover occurs multiple times in the 550kV GIS bus section. The flashover voltage of negative-negative superposition is lower than that of single impulse voltage. The flashover voltage of negative-negative superposition is lower than that of other polarity combinations. When -450kV DC is superimposed on -2400 kV lightning impulse voltage, the amplitude of the superimposed lightning impulse flashover voltage decreases by about 28.5% compared with the single -2400 kV lightning impulse voltage.