Experimental Investigation On Impulse Characteristic Of Grounding Grid In Substation

The grounding grid of substation has the function of providing the discharge channel of the fault current or lightning current,stabilizing the grounding potential rise and providing the potential reference point.Its reliability and safety are an important conditions to ensure the normal running.At present,it is usual to evaluate the safety performance of the grounding grid from the power frequency characteristics.However,since the current amplitude is larger and equivalent frequency is higher,the lightning current and the power frequency current have big difference.Only evaluated the power frequency characteristics of grounding grid has some limitations.Presently,the researches on the impulse characteristics of grounding grid are focused on the simulating calculation and simulation experiment,the impulse test of actual grounding grid under high current is lacking.Therefore,it is necessary to carry out field test,so that the result of test can verify the simulation calculation and provide reference for the evaluation of grounding grid and the lightning protection grounding design of the substation.Based on the analysis of the lightning accident of the substations,this paper summarized the research achievements of the impulse characteristics of the grounding device and put forward the single side re-flowed test arrangement based on the three electrodes arranged method as the test method.According to the site environment,the impulse current was injected into the grounding grid from the center or the edge,the current was returned from the vertical electrode on the grass,the potential reference point was set 160m far from the edge of grounding grid.The impulse characteristics included the impulse grounding resistance,potential distribution and current distribution under different current injection points(center or edge),different wave-forms(8/20?s or 2.6/50?s),different current amplitudes(200A-4200A)are measured.Based on the field test results,combined with the simulation calculation,the difference between impulse characteristics and power frequency characteristics were contrast analysis,the difference between test and simulation were studied.The experimental and simulation results show that the impulse grounding resistance is larger than the power frequency grounding resistance because of the inductance effect.With the increase of the injection current,the soil lionization around the ground electrode happens,the impulse grounding resistance is gradually decreased,the simulation value of ground conductor potential rises are larger than the test value.The waveform of ground potential rise is affected by the position of the ground conductor and the impulse current waveform.When the position is closer to the injection point or the front time of impulse current is shorter,the inductance effect will be more serious,the time to half value of ground potential rise waveform will be shorter and the vibration of waveform will be serious.Under the impact current,the high potential area is concentrated near the current injection point.When the current amplitude is larger or the front time of waveform is shorter,the distribution of ground conductor potential will be more uneven.When the current amplitude is larger,the distribution of current will be more uneven.The distribution of current under different wave-forms are similar.