[Abstract] With the continuous expansion of medium-voltage power grid systems with ungrounded neutral points and the increase in cable feeder circuits, the single-phase grounding capacitive current is also constantly increasing. Modifying the neutral grounding method and rationally selecting the neutral grounding method have become key technical issues related to the reliability of power grid operation. This paper analyzes and discusses the neutral grounding methods of power grids. [Keywords] Power supply system , neutral grounding reliability 1. Overview Medium-voltage power grids are commonly used at voltages of 35kV, 10kV, and 6kV, all of which are ungrounded neutral systems. However, with the development of power supply networks, especially the increasing number of users using cable lines, the single-phase grounding capacitive current in the system is constantly increasing, leading to the expansion of single-phase grounding faults into accidents within the power grid. China's electrical equipment design standards stipulate that for 35kV power grids, if the single-phase grounding capacitive current exceeds 10A, and for 3kV-10kV power grids, if the grounding capacitive current exceeds 30A, a neutral point grounding method via an arc suppression coil is required. However, Article 59 of the "Urban Power Grid Planning and Design Guidelines" (implementation) states that "for 35kV and 10kV urban power grids, when cable lines are long and the system capacitive current is large, a resistance method can also be used." Because different countries have different viewpoints and operational experiences regarding the neutral point grounding method for medium-voltage power grids, this is a hot topic of discussion in both theoretical and engineering circles in China. In the transformation of medium-voltage power grids, the issue of the neutral point grounding method has attracted much attention and faces a decision-making issue regarding its development direction. 2. Different Neutral Point Grounding Methods and Power Supply Reliability In China's medium-voltage power grid power supply system, most are low-current grounding systems (i.e., neutral point ungrounded or grounded via an arc suppression coil or resistor). my country has been using the arc suppression coil grounding method for many years, but in recent years, some areas have adopted the neutral point grounding method with small resistance. Therefore, this paper analyzes these two grounding methods. For ungrounded neutral systems, this is a transitional form, and with the development of the power grid, it will eventually evolve into the two methods mentioned above. 2.1) Neutral point grounding with small resistance: Some countries, mainly the United States, use this method because the US historically overestimated the dangers of arc grounding overvoltages. This method is used to discharge excess charge on the line and limit such overvoltages. In the neutral point grounding method with small resistance, the resistance value is generally chosen to be relatively small. When a single phase of the system is grounded, the current flowing through the grounding point is controlled at around 500A, and sometimes around 100A. The current flowing through the grounding point is used to activate the zero-sequence protection and disconnect the faulty line. Its advantages and disadvantages are: 2.1.1. When a single phase of the system is grounded, the voltage of the healthy phase does not rise or the rise is small, the insulation level requirements for equipment are lower, and its withstand voltage level can be selected according to the phase voltage. 2.1.2. During grounding, due to the large current flowing through the faulty line, the zero-sequence overcurrent protection has good sensitivity and can easily detect the grounded line. 2.1.3. Due to the large current at the grounding point, if the zero-sequence protection fails to operate in time or fails to operate, the insulation at and near the grounding point will be more severely damaged, leading to phase-to-phase faults. 2.1.4. When a single-phase grounding fault occurs, whether permanent or non-permanent, it will trigger a trip, greatly increasing the number of line trips and seriously affecting the normal power supply to users, thus reducing the reliability of the power supply. 2.2 Neutral point grounding via arc suppression coil: The arc suppression coil was invented in 1916, and the first unit was put into operation at the Pleidelshein power plant in Germany in 1917. With 84 years of history, operational experience shows that it is widely applicable to medium-voltage power grids. Medium-voltage power grids in countries such as Germany, China, the former Soviet Union, and Sweden have long used this method, significantly improving the safe and economical operation of medium-voltage power grids. Using a neutral point grounding method via an arc suppression coil, the current flowing through the grounding point is relatively small when a single-phase ground fault occurs in the system. Its advantage is that the line does not trip immediately upon a single-phase ground fault, and according to regulations, the power grid can operate with a single-phase ground fault for 2 hours. Practical experience and data show that when the grounding current is less than 10A, the arc can self-extinguish because the inductive current of the arc suppression coil can offset the capacitive current flowing through the grounding point. If properly adjusted, the arc can self-extinguish. For the increasing number of cable feeder circuits in medium-voltage power grids, although the probability of grounding faults is rising, single-phase grounding faults do not develop into phase-to-phase faults because the grounding current is compensated. Therefore, the power supply reliability of the neutral point grounding method via arc suppression coil is significantly higher than that of the neutral point grounding method via small resistance. However, the neutral point grounding method via arc suppression coil also has the following problems: 2.2.1. When a ground fault occurs in the system, because the residual current at the grounding point is very small, and according to the regulations, the arc suppression coil must be in an overcompensated state, the zero-sequence current flowing through the grounded and ungrounded lines is in the same direction. Therefore, zero-sequence overcurrent and zero-sequence directional protection cannot detect the grounded fault line. 2.2.2. Since most of the arc suppression coils currently operating in medium-voltage power grids are manually adjustable, they can only be adjusted after being taken out of operation. There is also no equipment for online real-time detection of the single-phase grounding capacitance current of the power grid. Therefore, during operation, they cannot be adjusted in a timely manner according to the changes in the capacitance current of the power grid, so they cannot play a good compensation role, and problems such as arc failure and overvoltage still occur. The neutral point grounding method via arc suppression coil has two major drawbacks, which are also two major technical challenges. For many years, power engineers have been dedicated to solving these technical problems. With the development and application of microelectronics and detection technologies, China has developed and produced automatic tracking arc suppression coils and single-phase grounding line selection devices, which have been put into practical operation with good results and are now in the stage of promotion and application. 3. Single-phase grounding capacitive current Because the neutral point ungrounded method is only a short-term transitional method in medium-voltage power grids, the ultimate goal is to transition to grounding via arc suppression coils or low resistance. Before the transformation, the capacitive current in the power grid must be calculated and measured to provide technical data for the transformation. The single-phase grounding capacitive current in a medium-voltage power grid consists of the following components: 3.1. The capacitive current of all lines (cable lines, overhead lines) with all electrical connections in the system. 3.2 The capacitive current generated by the capacitors bridging the phase and ground in the system. 3.3 The increase in the power grid capacitive current caused by power distribution equipment. The capacitive current in the system can be calculated using the following formula: ΣIc = (Σic1 + Σic2) (1 + k%) Where: Σic is the sum of single-phase grounding capacitive currents on the power grid; ΣIc1 is the sum of single-phase grounding capacitive currents of lines and cables; Σic2 is the sum of capacitive currents generated by capacitors bridging phases and ground in the system; k% is the increase in power grid capacitive current caused by distribution equipment. For 10KV, take 16%; for 35KV, take 13%. Based on the calculation of single-phase capacitive current on the power grid, in order to accurately select and rationally configure the capacity of the arc suppression coil, it is essential to measure the single-phase capacitive current during system operation. A microcomputer-based online real-time detection device provides a fast and accurate means for measuring the single-phase capacitive current on the power grid. Its principle is to detect the unbalanced voltage E0 of the system, and detect the line voltage UAB, neutral point displacement voltage U0, and neutral point displacement current I0 at a certain sampling period, and calculate the single-phase grounding capacitive current according to the following formula. E0 = U0 + I0 × Xc Where: Xc is the system's capacitive reactance to ground; Since Xc = (E0 - U0) / I0, then Ic = U-phase / Xc = U-phaseI0 / E0 - U0 Where Ic is the single-phase grounding capacitance current. The detection of single-phase capacitance current can also be achieved using the bias capacitor method and the neutral point external capacitor method. In the test, several different capacities of Cf (the added bias capacitor) can be selected to measure several sets of data. The moving average value is used to obtain the single-phase grounding capacitance current to reduce the error in the test. 4. Microcomputer-controlled arc suppression device Artificially tuned arc suppression coils cannot adjust the compensation amount in real time with the operation of the power grid. This cannot guarantee that the power grid is always in an overcompensated state, and may even lead to system resonance. It is also difficult to minimize the ground current when a fault occurs. China began developing microcomputer-controlled automatic tracking arc suppression devices in the 1980s. These devices have been continuously improved and developed into a series of products, and are equipped with automatic grounding line selection links, effectively solving the long-standing technical problems of power grids with neutral point grounded via arc suppression coils. The Z-type grounding transformer in this device features low zero-sequence impedance, low loss, and the ability to carry secondary loads. Its adjustable reactor is a steplessly continuously adjustable iron core with a full air gap structure, exhibiting excellent regulation characteristics, high linearity, and low noise. The device employs a series resistor grounding method with an arc-suppression coil to suppress resonance caused by the arc-suppression coil. Its microcomputer control unit is the core for automatic tracking, detection, adjustment, and line selection. The system response time is less than 20 seconds, and it operates in three modes: over-compensation, under-compensation, and minimum residual current. During operation, the computer periodically samples data to obtain real-time parameters of the power grid. The computer calculates the system's capacitive current and residual current, and automatically adjusts the reactor's inductance based on the deviation between the set value and the calculated value, thereby ensuring the arc-suppression coil operates at the set value. The line selection device uses computer sampling of the line's zero-sequence current. The computer determines the grounded line based on the amplitude and direction of the sampled current, achieving accurate and timely detection of lines with grounding faults. Conclusion There are differing opinions on the neutral point grounding method for medium-voltage power grids in China, and it has become a hot topic in power grid transformation. Based on years of operational experience and advancements in science and technology, the long-standing technical challenges of neutral point grounding via arc suppression coils in medium-voltage power grids have been resolved. The continuous improvement and widespread application of automatic tracking arc suppression coils and grounding line selection devices have provided technical support for neutral point grounding via arc suppression coils in medium-voltage power grids. Therefore, adopting neutral point grounding via arc suppression coils is the development direction for China's medium-voltage power grid.