# Advances and Challenges of Corrosion and Topology Detection of Grounding Grid
## 2 Corrosion Detection Method of Substation Grounding Grid
### Surface Electromagnetic Field Method  
The surface electromagnetic field method carries out data acquisition from the surface magnetic field, analyzes surface magnetic field characteristics and determines the distribution and fault of grounding grid conductors. Two kinds of grounding grid methods are present: injected and non-injected. The non-injected type uses the magnetic field induced by the ground current in the ground network to directly measure and analyze its characteristics for fault diagnosis. [PERSON] et al. [27] designed the surface electromagnetic field detection system and developed a fault diagnosis program on the basis of the finite difference method. This method can test the surface potential, test the distribution of grounding grid, and evaluate the unbalanced current of transformer and the corrosion degree of grounding grid. The non-injected surface electromagnetic field method requires the presence of grid current from power station, which is simple and convenient to set up. However, this method is subject to the change of in situ current and therefore is less used for research and applications. The injected surface electromagnetic field method was proposed by [PERSON] [28] in 1986 to measure the electromagnetic field distribution on the ground surface which is induced by current injected into the down-line of grounding grids. By comparing data with the theoretical model, local electromagnetic field changes were obtained to determine the locations of grounding grid faults. [PERSON] analyzed the impact of injection mode (location and number of injection points), frequency, and network structure on the diagnosis. Since then, many scholars have studied various forms of injected current, namely DC, alternating current (AC), square wave, and HF. [PERSON] et al. [29] applied high-frequency AC current injection (up to 1 MHz) to diagnose the grounding grid breakpoint on the basis of the leakage current. The higher the injection current frequency, the better the diagnostic resolution. However, developing a 1 MHz high-frequency current source is difficult. The influence of displacement current should also be considered. [PERSON] and [PERSON] adopted a 379 Hz AC injection current to determine the corrosion degree of ground grid by measuring the drop in magnetic field amplitude [30]. A low-frequency current source reduces the difficulty of power supply design. He [31] injected 50 Hz and 10 A current into high-speed railway grounding systems and realized the corrosion diagnosis of the grounding conductor through the sur face magnetic field. On the basis of the geophysical frequency-domain detection and dual-frequency induced polarization method, the dispersion characteristics of soil and proposed injecting double-frequency square wave current could be realized [32]. The square wave current has broadband characteristics, and some frequency points may amplify the corrosion information of the grounding grid to improve the detection effect of the surface magnetic field method. [PERSON] [33; 34] used the differential operation to amplify the characteristic signal (the third-order differential of the surface magnetic field), locate the grounding grid conductor with peak value, realize grounding network topology detection, and determine the flat steel fracture on the basis of the missing third-order differential peak of the surface magnetic field. However, the differential operation will amplify the noise, which is not practical under the presence of strong electromagnetic interference in the substation. In order to reduce measurement workload, [PERSON] [35] measured the magnetic induction intensity of some measuring points above the grounding grid of substation, established the inverse problem of the grounding grid magnetic field, solved the inverse problem using the reconstruction method based on magnetic detection electrical impedance tomography (MDEIT) method, reconstructed the magnetic field model above the grounding grid, and realized the corrosion fault diagnosis of the grounding grid.