Research On Non-destructive Testing Of Grounding Grids In Substations Using Ultrasonic Guided Waves

The fault diagnose of grounding grids used in substations is an importantmeasure to maintain the safety operation of power system. With the increasing ofoperation time, the resistance of grounding grids decreases gradually due to theelectrochemical corrosion of soil. Therefore, it’s of great significance to monitor anddiagnose the degradation of grounding grids to maintain the safe operation of powersystem. Up to now the researches on the evaluation of the corrosion of groundinggrids are limited, and the diagnosis methods can be categorized into three groups:circuit network method, electromagnetic measurement method and electrochemicalmeasurement method. The performance of these methods is not good enough forintegrity evaluation of grounding grids. Therefore it is necessary to develop moreeffective methods, like non-destructive testing method, to diagnose the corrosion ofgrounding grids efficiently. In this paper, a kind of guided waves in plate, SH0wave,is introduced to the health monitoring of flat steel used in industry of power system.The main research work includes:(1) The propagation characteristics of Lamb waves propagating in infinite plateis investigated, and Low order shear horizontal wave (SH0) is theoretically consideredas a appropriate ultrasonic Lamb wave for nondestructive detection of flat steel;(2) An Electromagnetic Acoustic Transducer (EMAT) is designed fornondestructive testing of flat steel. The sound field spatial distribution model of SH0wave electromagnetic acoustic transducer (EMAT) is developed based on theprinciple of Huygens. The influence of structural parameters on the sound fielddistribution of EMAT is researched, aiming to optimize the parameters ofthe transducers. The results show that the designed SH0wave EMAT can excite thepurity SH0wave in flat steel;(3) The influence of width of plates on the different mode lamb wavepropagating on the flat steel is researched. Experimentaland numerical simulationresults show that it is difficult to excite the purity Low order symmetry andantisymmetry Lamb wave in rectangle section bar by the piezoelectric angle probes.The designed EMAT can excite the Low order shear horizontal wave (SH0). Thevariation in width of plates has little influence on the propagation of SH0wave;(4) Experiments are carried out on the flat steels in different conditions in the laboratory. The influence of width, thickness, the length, the position of the defect,grounding and the buried environment of flat steels on the performance of testing areresearched. The results show that the developed SH0wave EMAT is effective for thenon-destructive testing of the flat steels. And the method has strong adaptability andhigh sensitivity. Finally, the proposed technique is applied to the NDT of groundinggrid in field.