Research On The Diagnosis Method Of The Electrical Connection Faults In Grounding Grid

Grounding is the electrical connection between the power apparatus and the ground. It provides assurance for the power equipments connecting with one another maintaining the same electrical potential through a low-resistance tunnel. The grounding grid is an important facility which fulfills the grounding standards and provides the security operation for power plants and substations. Its grounding performance has been put great attention to both at home and abroad.In recent years, with the booming of electrical industry, the security affairs of the grounding grid is on the rise in the construction of UHV and EHV transmission system as well as the urban distributed power grid, which promotes the requirements for the diagnosis for grounding grid. The thesis analyzes in detail the current research for electrical connection monitoring on grounding grid, and conducts further research on the fault diagnosis for the grounding grid on the basis of relevant techniques. It provides a thorough discuss on the corresponding theory and its engineering application. The main work involving is as follows:①It provides a diagnosis equation basing on the relative variation of branch resistance of the grounding grid as well as its solution through improved Newton iteration method. The resistance between the ports of the down lead conductor of the grounding grid is small, thus its measurement inaccuracy may have great influence on the diagnosis result. Basing on the structure chart and the measured value of the resistance between the down lead conductor ports, the thesis constructs the relative variation of branch resistance dependant erosion diagnosis equation and solves this equation through the improved Newton iteration method by the introduction of the regularization factor. By comparing the relative difference between the branch resistance before and after the fault, this method therefore is characterized with a better precision. Meanwhile, the improved Newton iteration method reduces the morbidity of the Hessian Matrix, which leads to the disconvergence, and thus improves the solution for the diagnosis equation, promotes its calculating speed and accuracy.②In the fault diagnosis for grounding grid, the variation of the branch resistance is a basic approach to judge whether the grounding grid is in good performance. But the calculation of the critical value for branch resistance in fault condition exists unsettled. By the joint application of finite element method and charge simulation method, the thesis puts forward a calculation model for the temperature field of the grounding conductor and the earth nearby. Based on the electrical and thermal model of the grounding conductor, the connection fault criteria are analyzed. And the relation between the relative variation of the branch resistance and the fault level is concluded.③The structure topology of the grounding grid in power plants and substations is huge, containing hundreds of nodes and branches. Only tens of them are connected with the down lead conductor. Therefore, the ports selection is of great importance to the construction of the diagnosis equation, the optimal solution and finally the diagnosis accuracy. By introducing the topological transformation theory, the ATP-EMTP simulation platform is constructed according to the network hierarchy, which is grounding grid, quasi-tiny plate, tiny plat, tiny grid, Accessibility grounding grid and Intrinsic grounding grid. By the comparison between measurement schemes, the selection of the measurement ports which is applicable for the grounding grid fault diagnosis.④The grounding material is usually good electrical conductor. The port resistance of the down lead conductor is usually in the range of mΩ, which is hard to measure in a strong electrical environment. The DC double bridge and four-terminal measurement is introduced to measure the port resistance in the thesis and the diagnosis courses are decided by simulated experiments. The testing results show that this method can precisely measure the port resistance and suppress the DC interference in the substation.⑤The existing connection fault diagnosis methods for grounding grid require the design chart in detail, which is hard to acquire when the fault happens after several years'operation. The thesis introduces the geophysical exploration technique, and discusses the locating method using the ground penetrating radar (GPR) in the grounding grid fault diagnosis. The selection of center frequency of antenna, Time window, sampling rate, distance of eachother measuring point, distance of antenna and measuret grid are discussed. The grounding grid topological reconfiguration method based on the GPR results is provided, and successfully conquers the difficulty led by the absence of original design material and the subsequent low reliability of the diagnosis result.