Study On The Impulse Characteristics Of The Grounding Devices And The Morphology Features Of The Soil Sparkover

As the transmission lines and substation are struck by the lightning, the grounding devices provide the main dissipating parth of the lightning current to the earth. The trasient electromagnetic field generated by such large current may cause great damage to the equipments of the power system and personal safety of the operators. As a result, study of the impulse characteristics of the grounding devices under the lightning current is of great significance to the lightning protection of the power system. Since the lightning current is of large amplitude and high frequency, the impulse charateristics of the grounding devices differs greatly from the power frequency characteristic due to the soil sparkover and inductive effect. Moreover, the impulse characteristics of the grounding devices is also affected by the soil parameters, the amplitude and waveform of the injected current, the structure and size the grounding devices. All those factors are interacted by each other. Many domestic and foreign scholars have conducted plenty of theoretical and experimental research on this subject. By summarizing the limitations of existing research and the need of practical project, the further research in the paper are carried out as follows:Since the high-frequency components of the current and response voltage are ignored during the FFT and IFFT, the trasient electrical network model of the grounding devices is established in the time domain. This model contains the self-resistance and self-/mutual-inductance of the conductors, the equivalent self-/mutual-conductance and self-/mutual-capacitance between the dissipating conductors. Combined with the soil ionization model, the proposed model could sonsider the soil sparkover effectively. To overcome the shortage of the frequency domain algorithm that the high-frequency component are ignored during the FFT and IFFT, the time domain difference method is proposed. Accordingly, the difference solution of p-order accuracy is derived. The proposed algorithm is of high-accuracy and high-effciency.The electromagnetic field simulation theory spends a large number of memory as meshing the medium space around the thin wire. Therefore, the transient characteristics of the grounding devices based on FDTD considering soil ionization is established. The thin wire model in FDTD does not need to mesh the conductors, which could save lots of computational resources. This method could be used to solve the transient electromagnetic distribution of large-sized grounding grid. To use the CPML as the boundary, the proposed method could model the grounding devices located in the semi-infinite space. This method could sovle the problems that the trasient electrical network model could not deal, such as:blocky soil, the wave process on the current line and so on.As the portable impulse current generator used to measure the impulse resistance of the tower grounding devices, the waveform of injected current is not the typical waveform generally due the long current line. The convolution calculation principle is proposed to convert the measured response to the standard response under the lightning current. Moreover, the steep peak of the response voltage is discoveried during the field experiments, which would cause great errors. The condition as such problem occurs is studied by simulating the measurement loop using Matlab. The corresponding solutions are also presented in the paper. In addition, the field experiments of grounding grid is carried out. Combined with the transient electromagnetic field model based on FDTD, the step rise of the current at the end of the generator and oscillation of the response voltage at the injected point is analysed. Finally, the impulse characteristics of the large-sized grounding devices is studied.The impulse experimental plarform is built up to conduct the impulse experiments and study the impulse characteristics of the grounding devices. Combined with the test results and the trasient electrical network model, the structure of the grounding devices is optimized. Besides, the optimal angle of the extended conductors and needle electrode is studied. Finally, the optimum structure of the tower grounding devices is presented. The resistance-reducing measure of uneven electric field is proposed. This method adds the thin conductors with spines into the soil to simulate the tree creeping discharge in the soil, which could reduce the soil critical breakdown electric field and resistivity and intensify the soil sparkover. Then, the simulation experiments of the resistance-reducing measure. Furthermore, the resistance-reducing ratio of the propose method is studied by FDTD.The transient GPR of the grounding grid whether the concentrated grounding devices and grounding grid is connected or unconnected is analyzed by the TDDM. Accordingly, the risk assessment of back striking of the substation transformer is tudied. Finnally, the safety distance between the concentrated grounding devices and grounding grid is calculated. Besides, the danger threshold of the impulse resistance of the concentrated grounding devices is given.The sensitive films are adopted to observe the soil sparkover between the plate electrodes. Combined with the impulse experiments of the electrodes, the soil ionization can be divided into four stages:partial ionization stage, ionization delay stage, ionization stage and deionization stage. The characteristics of the electric parameters in four stages is studied. Accordingly, the change regulation of the instantaneous resistance can be explaned by the soil ionization stages.The sparkover of the single horizontal electrode and grid is observed by the same method. The types and features of the discharge, sparkover development and discharge point distribution around the grounding devices are analyzed. The sparkover observation experiments reveal that the sparkover in the soil can be divided into partial discharge and tree creeping discharge. The partial discharge has many discharge points and spreads over the surface of the electrode densely. When the electric field exceeds the critical breakdown field, several partial discharge points will develop into tree creeping discharge.