# Advances and Challenges of Corrosion and Topology Detection of Grounding Grid
## 1 Introduction  
The grounding engineering of power systems mainly includes substation grounding, tower grounding, and a high-voltage direct current grounding rod. A major purpose of grounding engineering is to ensure surface point stability and to reinforce the safety of personnel and equipment in the event of a lightning striking or massive amount of current discharged into the ground due to the failure of a power system. However, due to the fact that corrosion will inevitably occur in power systems at the subsurface level, this has become a potential safety hazard of grounding systems. Stray current corrosion, bacterial corrosion, and electrochemical corrosion are the main causes of grounding body corrosion [1; 2]. Therefore, the diagnose of grounding body corrosion is one of the major challenges in the evaluation of grounding system conditions [3]. In addition, the detection of subsurface power system involves grounding network topology and grounding impedance detection. Grounding impedance detection technology and equipment are fairly developed [4]. Due to the high difficulty of the grounding body corrosion and topology detection, more advanced technology and equipment are required for such operations. Research concerning grounding body corrosion detection technology has been conducted for decades, yet it remains difficult to put theories into practice at a large scale. Copper and flat steel are commonly used as grounding conductors. Although copper has stronger corrosion resistance than flat steel, they both have been buried underground for a long time which can be up to more than 30 or 40 years. Thus, their possibility of being corroded cannot be ignored. In China, flat steel is mainly used as a grounding material, but it can be easily corroded. The research of corrosion detection is extensive. Corrosion detection technology of a substation grounding grid mainly includes the network method [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26], electromagnetic field method [27; 28; 29; 30; 31; 32; 33; 34; 35], and electrochemical method [3; 36; 37; 38; 39; 40]. In recent years, emerging technologies, such as ultrasonic testing [41; 42], electromagnetic pulse time difference location [43] and electromagnetic imaging [44; 45; 46; 47; 48; 49], have been considered for corrosion detection. Ultrahigh-voltage DC transmission has developed intensively in recent decades, and DC grounding corrosion is a severe challenge. When single pole earth return is used, the carbon steel corrodes rapidly. Thus, grounding corrosion has become the most difficult problem to solve in DC transmission projects. However, the depth, structure, and operation of grounding rods are quite different from those of grounding grids of substation. To our knowledge, no study has been conducted on the diagnosis of grounding rod corrosion. With the rapid development of new energy power generation, the corrosion of grounding bodies caused by a DC stray current in photovoltaic power plants is a new problem that is rarely discussed at present. [PERSON] et al. [50] discussed a method of carrying out evaluations for grounding body corrosion caused by DC stray current in photovoltaic power plants. Because the grounding body is buried underground, it is concealed, and the degree of corrosion is difficult to detect directly. At present, evaluating grounding body corrosion mainly depends on site excavation, which is extremely low cost-efficient. It is also restricted by operating conditions. Therefore, the trenchless diagnosis and evaluation technology of grounding body corrosion is demanded. Recently, grounding grid topology diagnosis technology has come to play an increasingly significant role. In grounding network fault detection and state assessment, accurate drawings of the grounding network are needed. However, in practical projects, old drawings of grounding networks with long a service life are missing or defective due to reconstruction and expansion.