Abstract: This paper introduces the relevant concepts of neutral point grounding methods in power systems, discusses the application and advantages of neutral point resonant grounding methods both domestically and internationally, and finally briefly introduces some development and application of automatic tracking compensation devices in China, and puts forward constructive suggestions on neutral point grounding methods. Keywords: Resonant grounding; Arc suppression coil; Automatic compensation 1 Introduction The neutral point grounding method in a power system is a comprehensive technical issue, closely related to the system's power supply reliability, personal safety, equipment safety, insulation level, overvoltage protection, relay protection, communication interference (electromagnetic environment), and grounding devices. Power systems have various voltage levels, and the neutral point grounding methods of different rated voltage grids also have their own characteristics. In the medium voltage range below 110kV, the power grids of China and many other countries generally adopt the low-current grounding method, whose prominent feature is that the grounding arc of a single-phase fault can be extinguished by itself. The low-current grounding method is an ineffective grounding method, among which the neutral point resonant (through arc suppression coil) grounding method has received the most attention. Neutral point resonant grounding involves many technical issues, and its development and changes have been rapid in recent years, and its operating characteristics have been optimized. 2. Concepts Related to Neutral Point Grounding Before discussing the neutral point resonant grounding method, it is necessary to clarify several concepts related to neutral point grounding. 2.1 Zero-Sequence Impedance Zero-sequence impedance is a fundamental physical concept in neutral point grounding. According to circuit theory, the voltage and current of the power grid can be decomposed into three symmetrical systems: positive-sequence, negative-sequence, and zero-sequence. The impedance of the loop through which the zero-sequence current flows is called the zero-sequence impedance. The essence of various neutral point grounding methods is actually the difference in the magnitude of the zero-sequence impedance, or the difference in the ratio of the zero-sequence impedance to the positive-sequence impedance of the system. 2.2 Neutral Point Resonant Grounding Generally, grounding the neutral point through an arc suppression coil is called neutral point resonant grounding. Although the tuning inductance only varies within a small range, the zero-sequence impedance of the system is close to infinite. Under normal circumstances, the arc suppression coil and automatic tracking compensation device in operation often adopt an overcompensated operation mode that is slightly deviated from the resonant point. Since the technical term "resonant grounding" is more in line with the actual situation of the neutral point grounded through an arc suppression coil system, the power system with the neutral point grounded through an arc suppression coil is usually called a resonant grounding system. 2.3 The category of "ineffective neutral grounding" includes systems where the neutral point is ungrounded, resonant grounding, and high-resistance grounding, provided the single-phase grounding arc can extinguish itself. It also includes systems where the neutral point is grounded through medium or low resistance and medium or low reactance, where the grounding arc cannot extinguish itself. 2.4 Small-current and large-current grounding methods in power systems mainly fall into two categories: Small-current grounding methods are those where the single-phase grounding arc can extinguish instantaneously. These mainly include neutral point resonant (through an arc-suppression coil) grounding, ungrounded neutral point, and neutral point grounding through high resistance. Large-current grounding methods are those requiring a circuit breaker to interrupt a single-phase grounding fault. These mainly include direct neutral point grounding, and neutral point grounding through low reactance, medium resistance, and low resistance. 3. Application of Neutral Point Resonant Grounding Various grounding methods are used in domestic and international power systems, but generally speaking, neutral point resonant grounding is widely used in medium and high voltage power grids and generator grounding systems both domestically and internationally. 3.1 Both low-current and high-current grounding methods are used in the neutral point grounding of medium-voltage power grids, with the most representative being neutral point resonant grounding and low-resistance grounding. In the United States, medium-voltage power grids generally use neutral point grounding via low or medium resistance or direct grounding, rarely employing resonant grounding. This is mainly due to historical reasons and its current operating system; however, resonant grounding is increasingly used in American power grids. Japan's power system previously primarily used resonant grounding for its neutral point, but after World War II, due to US influence, it switched to high-current grounding. However, resonant grounding and ungrounded neutral point grounding have since seen significant development. Furthermore, economically developed European medium-voltage power grids primarily use low-current grounding. For example, the medium-voltage power grids of Germany, France, Sweden, Finland, Italy, and Austria commonly use resonant grounding. In the CIS and several Eastern European countries, resonant grounding holds a considerable or absolute advantage in medium-voltage power grids. EDF (Électricité de France) comprehensively considered its national power grid technology policy and began upgrading the neutral grounding method of its medium-voltage power grid around 1990, converting all high-current grounding systems that had been in operation for 30 years to resonant grounding. In the Scandinavian countries of Northern Europe, resonant grounding is also widely used for the neutral point of the power system. Finland's entire 10-20kV medium-voltage power grid uses low-current grounding, with 80% operating with ungrounded neutrals and 20% with resonant grounding. Furthermore, the 30kV medium-voltage cable networks in Berlin, Germany; Moscow, Russia; Vienna, Austria; and Geneva, Switzerland; and others also use resonant grounding for their neutral points, and their operation is satisfactory. For decades, my country's medium-voltage power grid has used low-current grounding, with the majority operating with ungrounded neutrals. In recent years, the national and local governments have invested heavily in the upgrading of urban and rural power grids, leading to an expansion of the power grid and a continuous increase in cable lines. The original ungrounded neutral point method for 6-35kV medium-voltage distribution networks is no longer suitable, and the old-style manually adjustable arc suppression coil grounding method also has many problems in operation. Therefore, it should be replaced by an automatic arc suppression coil grounding method. The capacitive current in the power grid is the main basis for selecting the parameters of the arc suppression coil. my country and the former Soviet Union stipulated the following limits for the capacitive current of the power grid: for 3-6kV power grids composed of overhead lines with non-reinforced concrete or non-metallic towers, the limit for the capacitive current IC is 30A; for 10kV power grids, the limit for IC is 20A; for 3-10kV power grids composed of overhead lines with reinforced concrete or metal towers, and all 35kV and 66kV power grids, the limit for IC is 10A. The limit for IC in medium-voltage power grids composed of 3-10kV cable lines is 30A. When the power grid IC exceeds the above limits and operation is required under ground fault conditions, the neutral point should adopt an arc suppression coil (resonant) grounding method. 3.2 High-Voltage Power Systems High-voltage power systems generally refer to 110–220 kV systems. Some countries, such as Germany, Sweden, Norway, Finland, and Austria, previously used resonant grounding in their 220 kV systems. During their development, the neutral point in these systems initially used resonant grounding. However, due to the large scale of the power grid and cross-border interconnections, the neutral point was subsequently switched to effective grounding. Currently, there is a general consensus on the use of effective grounding in 220 kV systems, although exceptions exist. There have always been differing opinions regarding the neutral grounding method for 110 kV systems. my country has a vast territory with significant differences in meteorological, geographical, and geological conditions across regions. However, the original 110 kV system was uniformly designed and manufactured using effective grounding, resulting in numerous problems for safe operation. In my country's "First Ten-Year Science and Technology Plan," the neutral point grounding method of 110kV power systems was listed as a major research topic, and pilot projects were carried out. The pilot results proved that, under certain special conditions, neutral point grounding via an arc-suppression coil is an effective method in 110kV systems. Furthermore, the successful operation of the original 154kV system in Northeast my country, which used an arc-suppression coil grounding method for nearly 30 years, also illustrates this point. In ultra-high voltage and extra-high voltage power systems, the technology of using the arc-suppression coil principle to extinguish the arc of the surging current and configuring single-phase reclosing is also an important measure for stable grid operation. 3.3 Generators Generators are the driving force of power systems and must have the ability to respond to sudden faults during operation. The neutral point grounding method of generators is closely related to this. According to their development history, generator neutral point grounding methods can be divided into: direct neutral point grounding, neutral point grounding via low impedance, ungrounded neutral point, neutral point grounding via high resistance, and neutral point grounding via an arc-suppression coil (resonance), etc. Currently, large generator sets worldwide mostly employ either high-resistance grounding or arc-suppression coil grounding for their neutral points, each with its own advantages and disadvantages. High-resistance grounding of the neutral point in large generators is a common practice in countries like the US and Japan, with a permissible ground fault current of 5–15A. In recent years, many large generator sets imported into my country have also adopted this grounding method. However, due to the increasing capacity of individual generators, the grounding capacitance current easily exceeds this limit. With advancements in generator structure, materials, and manufacturing processes, the advantages of high-resistance grounding for the neutral point are becoming less pronounced, especially for large hydroelectric generator sets, where it is increasingly difficult to apply. In my country and the former Soviet Union, except for generators with smaller capacitance currents operating without grounding the neutral point, large-capacity hydroelectric and steam turbines all use arc-suppression coil grounding for their neutral points. Furthermore, large generators in the New England power system in the US and some generators in Europe also use arc-suppression coil grounding for their neutral points. Using resonant grounding for the generator neutral point can easily and conveniently meet the safety grounding current limits for various types of generators, including large hydroelectric generators. Furthermore, according to the consultation report of the International Conference on Large Electric Systems (CIGRE), 99% of users advocate keeping the grounding current at a very low level, and the neutral point resonant grounding method can easily meet this requirement. 4. Advantages of Neutral Point Grounding via Arc Suppression Coil Summarizing the application of arc suppression coils at home and abroad over the past few decades, it can be seen that neutral point grounding via arc suppression coil has the following advantages: Instantaneous single-phase grounding faults can be eliminated by the operation of the arc suppression coil, ensuring uninterrupted power supply to the system; in the event of a permanent single-phase grounding fault, the operation of the arc suppression coil can maintain system operation for a certain period of time, giving the operating department sufficient time to start backup power or transfer loads, preventing passive situations; in the event of a single-phase grounding fault, the operation of the arc suppression coil can effectively prevent arc grounding overvoltage, protecting all power equipment in the network; due to the shortened duration of the grounding arc, its hazards are limited, thus reducing maintenance workload; since instantaneous grounding faults can be automatically eliminated by the arc suppression coil, the probability of erroneous protection operation is reduced; system neutral point grounding via arc suppression coil can effectively suppress single-phase grounding current, thus reducing the requirements for substation and line grounding devices, reducing personnel casualties, and also benefiting electromagnetic compatibility. It is evident that neutral point resonant grounding is a relatively ideal neutral point grounding method for medium-voltage power grids (including cable networks) and even high-voltage systems. 5. Development and Application of Neutral Point Resonant Grounding Devices in China In the past, China mainly used manually adjustable arc-suppression coils. This method is very inconvenient, generally requiring the arc-suppression coil to be disconnected from the power grid before adjustment; furthermore, the manual method has poor adaptability to line variations. In recent years, some universities and research institutions in China have developed various automatic power grid capacitance current compensation devices using various principles, some of which have achieved good results. Currently, in terms of the automatic capacitance current measurement principle, there are frequency conversion signal method, node equation method, tuning measurement method, state comparison method, phase angle control method, current injection method, etc. These methods each have their advantages and disadvantages in practical applications, and can be selected according to actual needs. Regarding the coil body of the arc-suppression coil, various types have emerged, from the most primitive on-load tap changer type, to the air gap type, DC-assisted magnetizing type, magnetic valve type, and now the high-impedance transformer type and capacity-adjusting type utilizing power electronic devices. Because arc suppression coils utilize power electronic devices, the original adjustment mechanism has shifted from mechanical movement to electronic switching. Therefore, the response speed of current arc suppression coils is increasingly faster, reaching tens of milliseconds or even a few milliseconds. This makes it possible to adjust arc suppression coils only when a single-phase ground fault is detected, while the coil operates in maximum overcompensation mode during normal system operation. Furthermore, the use of microcomputers in automatic arc suppression coil control makes device protection more reliable than before. For example, ensuring the accuracy of grounding protection has always been a difficult problem; when using microcomputer control, algorithms such as residual increments can easily solve this issue. 6. Conclusion In summary, it can be seen that the neutral point grounding method via arc suppression coil (resonance) is widely applicable to generators and medium-voltage power grids below 110kV. In 110–220kV high-voltage systems, the neutral point grounding method via arc suppression coil (resonance) is also suitable for use under some special conditions. With the application of computer technology and power electronics technology to automatic arc suppression coil devices, resonant grounding technology has been continuously optimized. The optimized resonant grounding method has the characteristics of high safety performance, good electromagnetic environment and high comprehensive technical and economic indicators. Therefore, the neutral point grounding method through the arc suppression coil has a wide range of application prospects. References: [1] Yao Huannian, Cao Meiyue. Resonant grounding of power system [M]. China Electric Power Press, 2000. [2] Liu Yancun, Li Xiang. A fast-response automatic tuning arc suppression complete set of device [J]. Journal of Electrical Engineering, 2001, (8). [3] Zeng Xiangjun, Yu Yongyuan, Yin Xianggen, et al. 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