# Advances and Challenges of Corrosion and Topology Detection of Grounding Grid
## 2 Corrosion Detection Method of Substation Grounding Grid
### Electromagnetic Imaging  
In recent years, the research achievements in geophysical exploration, biomedical imaging, and other fields have brought vitality to the diagnosis of grounding grid corrosion, forming a branch of the electromagnetic imaging method. TEM is a geophysical method. It establishes a pulsed magnetic field underground by sending current through a loop. During a field intermission, the secondary eddy current field is observed by a receiving coil, and the longitudinal resistivity profile is formed by inversion. TEM has the characteristics of strong penetration, high resolution, small volume effect, and non-contact measurement. It is mainly used in the field of geophysical exploration. Therefore, the author proposed the transient electromagnetic method for grounding grids (GG-TEM method) [44; 45; 46]. The basic principle of GG-TEM is shown in Figure 2. The transmitter outputs bipolar pulse current and excites the magnetic field through the coil. At the moment of a magnetic field turn-off, the secondary eddy current field is excited at the ground grid, and the eddy current is confined within the grounded grid and attenuated. The receiving coil measures the secondary eddy current magnetic field in the ground network intermittently in one pulse. The magnitude and attenuation rate of the secondary eddy current field are determined by the thickness of grounding grid conductor, electromagnetic characteristics of the material, and corrosion degree. The apparent resistivity imaging method is used to obtain the sectional map of the grounding grid. The corrosion degree of the grounding body can be analyzed by the apparent resistivity section characteristics. In 2016, we conducted experiments in the Wuhan Nannui Grounding Network Experimental Field and achieved fairly ideal detection results (Figures 3-5). The grounding grid, grid spacing, trench width, and depth were24 m \(\times\) 24 m, 4 m, 0.4 m, and 0.6 m, respectively. The degree of corrosion was simulated by the thickness of the flat steel. The thinner the flat steel, the higher the corrosion degree. Four zones (60 \(\times\) 6 mm (A), 40 \(\times\) 4 mm (B), 20 \(\times\) 3 mm (C), and 40 \(\times\) 4 mm flat steel (D)) were set, with four fracture ports (numbered 1, 2, 3, and 4). Figure 3: Wuhan Nannui grounding grid experimental site. Figure 2: A Schematic field setup of transient electromagnetic method [46]. Two lines were arranged to detect the imaging characteristics of the thickness and breakpoint of flat steel. In Figure 4, the measuring line passes along the flat steel through areas of different thickness (\(60\times 6\) mm and \(20\times 3\) mm). As shown in the figure, the high-resistivity layer is thin, and the resistivity is low in the thick conductor section. In the thin conductor section, the high-resistivity layer is thicker, and the resistivity is higher. The electrical resistivity imaging characteristics coincide with the thickness of flat steel. Breakpoint detection: The line passes through the BD area, which has the same flat steel thickness (\(40\times 4\) mm), and the line passes through the breakpoint grids of 4, 1, and 2. Figure 5 shows that the apparent grid resistivity of the three grids where the breakpoints are located is significantly higher than that of the intact grid. Moreover, the thickness of the high-resistivity layer is obvious, and the resistivity of the intact grid is low. Compared with the surface electromagnetic field method, GG-TEM can solve the problem of current injection point of electromagnetic field method, normalization of injection point, primary field influence problem of electromagnetic induction method, and contact resistance problem and realize contactless measurement. [PERSON] et al. [47] proposed the grounding net imaging method of endogenous EIT. The 16-channel device mode and imaging principle of biomedical electrical impedance tomography were adopted. The amount of information was greatly increased, and the construction efficiency and diagnostic accuracy were improved.