# Advances and Challenges of Corrosion and Topology Detection of Grounding Grid
## 2 Corrosion Detection Method of Substation Grounding Grid
### Electric Network Method
#### 2.1.3 Improved Sensitivity Analysis Method  
Because the sensitivity analysis method only makes use of a very limited number of equations and does not take non-linearity into consideration, the resistance of each conductor segment deviates greatly from the actual value. [PERSON] [10] and [PERSON] [11] have improved the sensitivity analysis method. They rotated the excitation position of the current source in the accessible node and measured the voltage of nodes at many locations when each excitation occurs. They made full use of the limited accessible nodes and greatly expanded the number of equations.  
The diagnostic equation of the electric network method is a set of undetermined or overdetermined non-linear equations, and the optimization algorithm is a research hotspot. The simplex method [7; 8], TABU search algorithm [12], support vector regression [13], differential evolution algorithm [14], constrained total least squares algorithm [15], genetic algorithm [16], and dynamic chaotic PSO [17] have all been applied. [PERSON] [18] proposed Tikhonov regularization on the basis of the least square method in order to reduce the ill-conditioning of non-linear equations, and used the L-curve method to identify grounding grid faults.  
Fault diagnosis for large-scale networks requires not only large size matrix operations but measurement errors and mutual-interference. Tearing and block testing methods [19; 20; 21] have been developed. In the partial fault diagnosis of grounding grid, only the relevant parts can be tested, which can reduce the testing workload and effectively improve the fault diagnosis efficiency.  
[PERSON] [22] and [PERSON] [23] analyzed the branch corrosion measurability of the grounding grid. They eliminated inaccessible nodes by network transformation and carried out inversions to determine the measurability of each branch gradually. The testability of fault diagnosis of the grounding grid is predictable, which has guiding significance for optimizing the test scheme and making scientific use of the diagnosis results.  
The substation site is complex, and the practical application of the network method involves many obstacles. The influence of down-line offset [24], portal frame [25], and cable trench [26] is a difficult problem to be solved urgently in the power network method; thus, further research is necessary.  
The electric network method has been put forward for nearly 30 years, nevertheless, the testing equipment and implementation methods are not sufficiently practical, as the result, their popularization are obstructed. The network method needs to be developed in more practical manners to determine the topological structure of the grounding network. Establishing an accurate grounding network model is difficult because of irregular construction, lack of drawings, inconsistency of construction and drawings, incomplete grounding network, and connection between the new grounding network and the old one. Moreover, the selection principle of down-line is difficult to determine, and the grounding conductor resistance is very small. In order to suppress strong electromagnetic interference, high power supply is required. There are factors such as the deviation of the positions of the accessible nodes, measuring the resistance of the wire, contact resistance, cable trench and surrounding equipment, which will affect the measurement results. In addition, it has large measurement workload, difficult data management, and low detection efficiency. Hence, operators should have a high technical threshold and limited practicability.