| The grounding device provides a common reference ground for all kinds of electrical equipment during normal operation.When the power system fails or suffers from lightning stroke,the fault current or lightning current can diffuse to the earth through the grounding device,so as to limit the ground potential rise and ensure the safety of electrical equipment and personnel.The transient performance of the grounding device is closely related to the flashover probability.At present,the current flowing into the grounding electrode ranges from low-frequency DC and power frequency to impulse current containing high-frequency components.Only analyzing the safety performance of grounding device from a single frequency can not meet the requirements of lightning protection and grounding.It is necessary to establish a new measurement method of grounding device to detect and evaluate the grounding performance from multiple frequency dimensions.Therefore,this paper mainly studies the evaluation method of grounding body based on frequency domain analysis and the law of grounding electrode impact dispersion.Firstly,this paper establishes the simulation models of three typical tower grounding devices in CDEGS software,analyzes the spectrum curve of grounding body,discusses the possible application direction of frequency domain analysis in grounding detection,and analyzes the reasonable arrangement method of auxiliary electrode in actual measurement.It is found that the grounding impedance increases with the increase of frequency and is inductive as a whole.The frequency of the influence of soil resistivity on the grounding response is wide,and the influence of the length of the inner conductor of the grounding device on the spectrum response gradually decreases with the increase of frequency.In addition,when the distribution of voltage pole and current,i.e.the lead wire is 90 °,the measured spectrum is closest to the ideal result.According to the time-frequency transformation,the principle of frequency domain decomposition method for calculating the transient response of grounding electrode under arbitrary impulse current waveform is studied.The spectrum characteristics of different impulse current are analyzed,and the frequency points needed for frequency domain decomposition method are found.The simulation and experimental results show that the error of measuring the impulse response of grounding electrode by frequency domain decomposition method is less than 10%.Then,an experimental platform is built to study the effective length discrimination method according to the spectrum impedance characteristics of grounding body,that is,when the high-frequency impedance is much larger than the low-frequency impedance and the amplitude frequency curve is inductive as a whole,the effective length is achieved.The frequency domain decomposition method is used to calculate the impulse voltage response and judge whether it reaches the effective length.Finally,the distribution characteristics of impulse voltage and impulse current and the time-varying law of transient impedance are analyzed through impulse tests of horizontal grounding electrode with different lengths.It is proposed that the impulse dispersal process of grounding electrode can be divided into two stages: the electromagnetic transient process of wave head and the scattering current of wave tail passing through the whole grounding electrode.The transient impedance transits from the injection impedance to the wave tail impedance,and the wave head impedance is affected by the impulse current waveform The tail impedance is close to the DC steady-state impedance,and is greatly affected by the length and soil resistivity.A time-domain method to judge the effective length of impulse is proposed,that is,when the impulse grounding resistance of grounding electrode is greater than the wave tail impedance,the grounding electrode exceeds the effective length. |
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