Analysis of factors affecting the residual voltage waveform of lightning impulse

Author： GOZ Electric Time：2024-07-25 09:26:31 Read：14

    The lightning impulse residual voltage test (hereinafter referred to as the residual voltage test) is an important test to verify the lightning impulse protection characteristics of the arrester. The accuracy of the measurement is the basis of the test. The influence of factors such as oscilloscope channel interference, voltage divider ratio and low-voltage arm impedance on the residual voltage waveform in the test was analyzed, and different methods were used to improve the residual voltage measurement waveform according to the influencing factors, providing a reference scheme for improving the accuracy of lightning impulse residual voltage measurement. Keywords: lightning impulse residual voltage; oscilloscope channel interference; voltage divider; current sensor; attenuator.

    Lightning overvoltage protection measures are an important part of power system research, and different scholars have carried out various studies from different aspects. Different lightning protection measures for overhead lines, transformers and towers are analyzed respectively, and suggestions for lightning overvoltage protection measures for power systems are given based on the influencing factors such as grounding resistance, shielding line installation method, use of gapless metal oxide arresters, and use of out-of-band series metal oxide arresters. Among them, arresters, as a common means of overvoltage protection measures for power systems, are discussed in detail. The electrical properties of the arrester, such as the protection performance, high current tolerance performance, and square wave current performance, are important performances for evaluating the quality of the arrester. A large number of studies have been conducted on the above performances of the arrester, including various factors affecting the current capacity of the resistor, and the current tolerance performance of the resistor or the arrester has been analyzed by factors such as the test waveform, the size of the resistor, and the formula of the resistor. The residual voltage characteristics of the entire arrester were specifically analyzed through the test. Since the lightning residual voltage test of the arrester determines its lightning protection characteristics, it is of great significance to determine the insulation coordination of the power system, so the accuracy of the residual voltage test measurement needs to be paid attention to. In order to more accurately measure the current in the residual voltage, a current sensor was developed. Based on the problem of measuring voltage in the residual voltage test, the influence of the voltage divider resistance value on the residual voltage measurement result was analyzed, and the reasons why the residual voltage waveform in the residual voltage test may be affected were analyzed from the aspects of attenuation probe, cable impedance matching, and triggering mode. In addition to the existing research, the author discovered and analyzed the typical factors causing abnormal residual voltage waveform through a large number of experiments.

Conclusion Analysis

1) When using an oscilloscope to measure residual voltage, if the two channels of the oscilloscope share a grounding point, it is necessary to pay attention to the possibility that the residual voltage signal and the current signal may interfere with each other. The smaller the signal amplitude of the current and residual voltage input to the oscilloscope, the more obvious the interference.

2) The method of independent measurement of current and residual voltage signals and optical fiber transmission is an effective way to solve the mutual interference between current and residual voltage signals in residual voltage measurement.

3) In the residual voltage measurement circuit, the introduction of factors such as attenuators and probes brings difficulties to the impedance matching of the measurement circuit, and may distort the measured residual voltage waveform, affecting the accuracy of the measurement. Therefore, in the case of uncertainty about the degree of influence of attenuators and probes on the measurement accuracy, the impedance of the high and low voltage arms of the voltage divider is controlled so that the voltage divider signal can be directly input into the signal analysis unit (such as an oscilloscope) to ensure that the measured waveform is not affected by the attenuator and probe to the greatest extent. At this time, the loop impedance matching state can be achieved by adjusting the parameters of the low voltage arm of the voltage divider.

4) The adjustment of the low-voltage arm parameters of the voltage divider not only affects the response characteristics of the voltage divider itself, but also affects the impedance matching in the measurement circuit, thereby affecting the accuracy of the residual voltage measurement.

5) Without adding matching resistance in the voltage divider measurement circuit, the overshoot phenomenon in the lightning impulse residual voltage waveform can be effectively suppressed by adjusting the low-voltage arm parameters of the voltage divider. However, for the impulse current waveform with a steeper apparent wavefront time, the impact of this method on the waveform response time needs to be considered.