Research On Defect Diagnosis Of Power System Grounding Grids Using Transient Electromagnetic Imaging

In China,grounding grids are mostly made of galvanized steel,which has poor corrosion resistance.After years of burying underground,corrosion and break point faults occur frequently due to soil erosion and stray current or short-circuit current damages.These kinds of fault result in significant economic loss and bring hidden danger to power system and personnel safety.This thesis focus on characteristics analysis of electromagnetic field response when grounding grids in different defect situations,and develops new defect diagnosis method of power system grounding grids using transient electromagnetic imaging(GG-TEM).This thesis develops transient electromagnetic 3D time domain finite difference forward modeling algorithm matching for grounding grids model.This algorithm is specific in many aspects due to the difference between grounding grids model and regular transient electromagnetic detecting objects.Defect of grounding grids is diagnosed using transient electromagnetic 3D FDTD algorithm based on forward modeling results.Maxwell is employed to obtain the 3D finite element forward modeling results.Fast inversion equivalent resistivity and imaging to diagnose the defect existing in grounding grids.Non-uniform grid scheme is proposed for grounding grids model.Use uniform small grid near grounding grid conductor and source,and non-uniform grid in the region away from grounding grid conductor and source.Non-uniform grid scheme is also used inner the grounding grid mesh.Simulate the air layer and set the conductivity a small value.The air layer is discretized and taken as a part of calculation model.Use the same iterative equation for calculation in air layer and lossy earth layer.On the other hand,the precision of numerical calculation is guaranteed.The accelerated time stepping scheme adapting to the shape of transmitter current is proposed.According to the shape of transmitter current,the accelerated time stepping is timely used during different period of transmitter current.Adjust the time parameter in empirical formula for time discretization during different transmitter current period.Increase parameter ? in empirical formula for time discretization when calculating grounding grids model,which improves computational efficiency of the developed algorithm.An initial field calculation method is developed to displace source discretization for the ungrounded loop TEM grounding grid model.The stable magnetic field created by source in modeling region is calculated and taken as the initial value for finite difference iteration.It avoids iterative calculation during the rising and lasting period of transmitter current,and improves the computational efficiency of developed algorithm.The defect existing in grounding grid is diagnosed using transient electromagnetic 3D time domain finite difference forward modeling.Distribution characteristics of surface magnetic intensity present distinct changes when corrosion or break point defect occurs to grounding grid.The severer the corrosion is,the more dramatic drop of magnetic intensity is observed.From the imaging of surface magnetic intensity,the fault in the grounding mesh right under the transmitting loop is diagnosed.Propose the method of defect diagnosis of grounding grid using transient electromagnetic apparent resistivity fast imaging.Maxwell is employed to obtain 3D forward modeling results.Fast imaging based on smoke-ring theory,the fault existing in grounding grid is diagnosed directly from contour map of apparent resistivity.Whether the measuring line is arranged parallel to grounding grid conductor or arbitrarily,the fault of grounding grid can be diagnosed accurately.The influence of fault in different degrees is analyzed.The result shows that the severer the defect of grounding conductor is,the more dramatic drop of surface magnetic intensity is observed.The influence of grounding conductor material,and soil resistivity on the diagnosis results of grounding grid is analyzed in detail.Results show that different material of grounding conductor and soil resistivity make no impact on defect diagnosis of grounding grid.The effect of underground pipeline and current-carrying cable on defect diagnosis of grounding grid is investigated.Results show that the pipeline with high conductivity lying above grounding grid and in grounding mesh under measuring line,even though its size is as small as outer diameter 0.1m and wall thickness 0.01 m,will make impact on defect diagnosis.The pipeline lying beneath grounding grid or made of low conductivity material make no impact on diagnosis result.Current-carrying cable has serious influence on diagnosis result.The method of configuration detection of grounding grid is proposed.From the measuring results of surface magnetic intensity,the grounding conductor is accurately located.After fast imaging of recorded magnetic field,the configuration of grounding grid will be clearly and accurately shown in the contour map and 3D surface map of apparent resistivity.Field tests of break point diagnosis and configuration detection on a constructed grounding grid are performed.Another field test is carried out in the Chongqing Liangting 220 kV substation.The test result is verified by excavation.This thesis demonstrates the accuracy and efficiency of the proposed transient electromagnetic imaging method for defect diagnosis of power system grounding grid.