*2.5. 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 e ffect, 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) [44–46]. 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-o ff, 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.

**Figure 2.** A Schematic field setup of transient electromagnetic method [46].

In 2016, we conducted experiments in the Wuhan Nanrui Grounding Network Experimental Field and achieved fairly ideal detection results (Figures 3–5). The grounding grid, grid spacing, trench width, and depth were24 m × 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 × 6 mm (A), 40 × 4 mm (B), 20 × 3 mm (C), and 40 × 4 mm flat steel (D)) were set, with four fracture ports (numbered 1, 2, 3, and 4).

**Figure 3.** Wuhan Nanrui grounding grid experimental site.

**Figure 4.** Apparent resistivity section of flat steel.

**Figure 5.** Apparent resistivity section with breakpoints.

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 × 6 mm and 20 × 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 × 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.

Yang 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. Wu [48] and Wang [49] proposed the radar imaging method, where the ultrawideband emission signal propagates through the soil medium to reach the grounding body and the echo signal is used to obtain the physical state of the grounding body.

In recent years, the electromagnetic imaging method has become a hotspot in the research of grounding grid corrosion diagnosis. However, this method is limited by problems such as reinforced bar shielding of concrete pavement, cable trench, power equipment impact, and large calculation workload.

#### **3. Grounding Network Topology Detection Method**

The current injection method and magnetic field excitation method are the main detection methods of grounding grid topology structure.

#### *3.1. Current Injection Method*

The current injection method injects a sinusoidal current by selecting a suitable injection node, measures the magnetic field on the surface, and analyzes the magnetic field distribution characteristics to realize the grounding conductor positioning.

Yang [30] designed and developed a different frequency current supply system, mobile weak magnetic induction intensity measurement system, and corresponding software system. Qamar [33] proposed a grounding network topology detection method on the basis of the differential method and established a magnetic field shape function for describing the direction of the ground surface parallel or perpendicular to the grounding grid. This method uses the principal peak characteristic of the even derivative of horizontal component shape function and odd derivative of vertical component shape function to filter and differentiate the measured data of a square magnetic field in the grounding grid after injecting a current, so as to determine the topological structure of the grounding grid. However, the differential method is very sensitive to noise and measurement error. The detection path has a good effect perpendicular to the conductor direction, and the positioning result may have a large error when a large deviation from the vertical direction exists [30,33]. Fu et al. [51] proposed a grounding network topology detection method on the basis of wavelet edge detection technology. Wavelet edge detection effectively strips the magnetic gradient characteristics of each conductor segmen<sup>t</sup> from the background field, which greatly reduces the influence of magnetic field superposition on positioning accuracy. At the same time, the strong anti-interference ability of the wavelet transform suppresses the error introduced in the measurement and magnetic field fitting process and ensures the method's stability.

#### *3.2. Magnetic Field Excitation Method*

The magnetic field excitation method excites the grounded flat steel through the coil to generate the eddy current field (the secondary field), and the topology of the grounding network is obtained by detecting the eddy current field. The transient electromagnetic method [46] measures the net secondary field, and it has high detection accuracy. The topography of the grounding grid can be visually obtained using the apparent resistivity imaging technology, and the image is intuitive, but it has a large amount of calculation. In addition, the topology of the grounding grid can be obtained by analyzing the amplitude characteristic of the magnetic field which generated by the sinusoidal current. The method is simple in measurement and small in calculation, but the background field is included in the measurement result of the method, so the detection accuracy is relatively low.

#### **4. Challenges and Opportunities**

Although a variety of grounding grid corrosion and topological detection methods have been proposed, these methods are still immature, and commercial equipment and technical regulations are lacking. The following are challenges from a technical perspective:

#### 1. Strong electromagnetic interference

The electromagnetic environment of the substation is very complicated, and the power frequency interference magnetic field of the substation operating state can reach more than 10 μT. The high-frequency transient interference caused by high-voltage switching action; large power frequency current from ground power equipment, such as transformers, knife gates, transformers, and gate brackets; power grounding current; harmonic interference; and various secondary and communication equipment will a ffect the normal operation of the detection instrument by means of spatial and conduction coupling.

The strong electromagnetic interference poses a serious threat to the electrical network method, surface electromagnetic field method, electrochemical method, and electromagnetic imaging method. By using an active source, the excitation source power is increased, and the signal-to-noise ratio is improved. Changing the frequency of the excitation current signal and using a frequency-selective amplification technique, such as notch, amplification, and filtering, can suppress the power frequency and avoid the main interference frequency.

#### 2. Strong metal interference

The substation has a large number of metal facilities, and the connection of several important facilities to the grounding network brings grea<sup>t</sup> di fficulties to the detection of the grounding grid. For example, a large number of underground metal pipes, cables, leads and cement roads with steel structures will have a grea<sup>t</sup> impact on the distribution of surface magnetic fields. This makes it di fficult to distinguish the grounding grid information.

#### 3. Corrosion detection of down-line

The grounding grid down conductor is a communication channel between the power equipment and the grounding network. The stray current density is large near the surface layer of the soil, and because of the direct contact of the air, it is susceptible to changes in air and soil humidity, so that the corrosion rate of the part of the grounding conductor is higher than that of the horizontal conductor of the grounding grid. Therefore, the down-line underground part is easily corroded. Moreover, diagnosing down-line corrosion is very di fficult, and studies on the corrosion diagnosis of the down-line are lacking.

#### 4. Small target body of flat steel in grounding grid

The common cross-sectional area of the grounding grid conductor is very small compared to other geophysical targets. The cross-sectional area of the grounding grid is much smaller than the length of the conductor, which brings grea<sup>t</sup> di fficulty to the software simulation. For the electromagnetic induction method, because the conductor cross section is small, the eddy current response intensity and the influence range are small; for the ground penetrating radar, the reflection surface is too small, which brings grea<sup>t</sup> di fficulty for detection.
