Measurement Method And Experimental Study Of Tower Impulse Grounding Resistance Based On Discrete Different Frequency

The grounding device of transmission line tower is an important condition for the stable operation of power system and power equipment,and its impact characteristics directly affect the lightning protection performance of power system.The impulse grounding resistance is an important parameter to reflect the grounding performance of grounding device.Excessive impulse grounding resistance will lead to the instantaneous rise of transmission tower potential and cause lightning trip accident.At present,the measurement methods of tower impulse grounding resistance mainly include on-site measurement method and impact coefficient method.The on-site measurement method has the problems of too large impact equipment and high construction difficulty.The impact coefficient method multiplies the measured power frequency grounding resistance by the impact coefficient to get the impulse grounding resistance,which has the problems of large measurement error.In order to solve the problems of existing measurement methods,this paper first studies the periodic continuation process of lightning current with different waveform parameters,analyzes the discrete spectrum characteristics and energy characteristics of lightning current,and determines the frequency point and frequency range of grounding electrode frequency domain impedance response calculation.The calculation method of frequency dependent parameters of soil is studied,and the calculation method of mutual resistance of grounding electrode considering soil medium is put forward.At the same time,the mutual inductance and self inductance between grounding conductors are considered to form an accurate calculation method of frequency domain impedance response of grounding electrode.The time domain response of impulse voltage of grounding electrode under lightning current is studied,and the expression of impulse voltage time domain response function with four unknown characteristic parameters is proposed innovatively.By writing the impedance response equation of grounding electrode to solve the unknown parameters,the calculation method of impulse grounding resistance is obtained.Combined with short distance electrode arrangement method and four pole method,the impulse grounding resistance measurement method of grounding electrode is proposed Based on this method,the accurate measurement of tower impulse grounding resistance is realized,which lays a theoretical foundation for the accurate evaluation of transmission line lightning protection performance.The main research work is as follows:(1)Based on the basic principle of Fourier transform,the periodic continuation process of lightning current with different waveform parameters is studied.The discrete spectrum characteristics and energy characteristics of lightning current after periodic continuation are analyzed.The harmonic frequency after lightning current decomposition and the energy proportion in different frequency ranges are obtained.Finally,the grounding fault is determined according to the harmonic frequency after lightning current decomposition and the frequency range of energy concentration The calculated frequency point and frequency range of impedance response in polar frequency domain.(2)Combined with the change of soil frequency related parameters,the calculation method of grounding electrode mutual resistance considering soil medium is studied.Combined with the Galerkin method of moment,the calculation formulas of soil coefficient of horizontal grounding electrode and square grounding electrode are derived.At the same time,considering the self inductance and mutual inductance of grounding conductor,the accurate calculation method of frequency domain impedance response of typical grounding electrode is proposed.The error analysis of calculation results and CDEGS calculation results shows that the calculation results are basically consistent,which verifies the accuracy of frequency domain impedance response calculation method of grounding electrode.(3)The transformation characteristics of impulse voltage time domain response function of grounding electrode under lightning current are studied.The expression of impulse voltage time domain response function of grounding electrode with four unknown characteristic parameters is proposed.Combined with the calculation method of frequency domain impedance response of grounding electrode,the unknown characteristic parameters are solved,and the calculation method of impulse grounding resistance of grounding electrode suitable for different structure sizes is obtained.At the same time,combined with the short distance electrode arrangement method for measuring the power frequency grounding resistance of grounding electrode and the four pole method for measuring the soil resistivity,a method for measuring the impulse grounding resistance of grounding electrode is proposed.The accuracy of the proposed method is further verified by error analysis based on the proposed method and CDEGS calculation results.(4)10 horizontal grounding electrodes with different lengths and 10 transmission line towers are selected for relevant tests,and the horizontal grounding electrode is simulated.According to the measured soil resistivity and power frequency grounding resistance,the length of horizontal grounding electrode is calculated,and then the impulse grounding resistance of 10 horizontal grounding electrodes with different lengths is obtained.The measured results in this paper are compared with those calculated by CDEGS.The accuracy of the method is verified by the difference analysis.Similarly,the field test of transmission line tower grounding electrode is carried out,and the measurement accuracy of this method is determined by comparing the calculation results of this method and CDEGS.