| The grounding system is responsible for dispersing failure current, lightning current or any other transient currents quickly and effectively, and protecting the power system equipment and workers. With the increment of lightning activity and applaction of extra-high voltage and smart substation technology, it is significant for reaserching the transient characteristics of grounding system to improve safty and stable operation of electric power system.Firstly, this paper introduces the research situation of transient characteristics of grounding system, analyses the application range and open question of existing transient computation model and test method of grounding system. In order to overcome the above shortcomings in existing transmission line models, the electromagnetic radiation theory is introduced into the calculation of segment conductor size depending on the electromagnetic characteristics of soil and the frequency of applied current, furthermore, a simple and accurate recursive method for calculating electromagnetic mutual coupling parameters is deduced based on electromagnetic theory and the characteristics of mutual coupling parameters matrix. Comparing with the existing transmission line models, the optimized one keeps the advantage that flexible to model and convenient to take consideration of soil ionization, at the same time, its calculation precision is greatly enhanced to be close to the electromagnetic field model, so the optimized transmission line model can meet the requirement of transient calculation(less than10MHz) of grounding system.Secondly, the impulse behavior of transmission line twoer grounding device is calculated by transient calculation model, and discuss the factors that influence the precision in computation of grounding electrodes’ impulse impedance. The impulse characteristics of grounding electrode are analyzed in time domain and frequency domain respectively, and the empirical formula of impulse effective length of horizontal grounding electrode is presented. The grounding grid is modeled and its transient grounding potential rise is calculated when it is excited by impulse current. Moreover, the influence of grounding grid potential rise on substation secondary equipment is analyzed, and the Anti-interference measures of secondary equipment are proposed.Furthermore, the impulse characteristic parameters are defined based on the classic fall of potential method. Acrooding to the current distribution in earth at the condition that grounding test circuit is injected by impulse current, an impulse characteristic parameters test method is presented, which includes the selection of testing power source, arragement of test circuit, anti-interference measures, and impulse impedance frequency spectrum calculation method. The impulse characteristics of transmission line tower grounding devices and grounding grid are measured in field by the presented impulse characteristic parameters test method, and the impulse impedance of tower footing and impulse potential rise of grounding grid are obtained.At last, based on the measurement theory of impulse characteristic of grounding system, an impulse impedance measurement apparatus is developed. The apparatus has built-in impulse current generator, which adopt adjustable inductance and resistance to control the output waveform of impulse current, so the lightning current can be simulated. When injecting the impulse current into grounding system, the data acquisition device collects the voltage and current signals of grounding systems respectively, which are then converted by A/D device and transformed by fast fourier transform(FFT) into frequency domain. Through the spectrum calculation method, the impulse grounding impedance can be obtained and divided into resistance and inductance component. Laboratory and field tests are performed and compared with the test results tested by megger method, verified the correctness and accuracy of the developed apparatus. |
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