Discussion on several issues regarding the neutral grounding method in distribution networks

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Discussion on Several Issues Concerning the Neutral Point Grounding Methods in Distribution Networks Abstract: This paper compares the advantages and disadvantages of different neutral point grounding methods in distribution networks. The impact of resistive grounding on the distribution network on power supply reliability, communication, personal safety, and switch maintenance was analyzed. It is pointed out that for cable-based distribution networks, grounding the neutral point through a resistor is the preferred method. The article also explains the principles for selecting the resistance value of the neutral point resistor, as well as the practice of grounding the neutral point through a resistor in China. 1 Neutral point grounding method In the early days in China, it was stipulated that the neutral point grounding methods in power systems be divided into two categories: systems with high grounding short-circuit currents and systems with low grounding short-circuit currents. Since the magnitude of current cannot be clearly defined using the classification of neutral point grounding methods in power systems, it is instead divided into effectively grounded neutral point systems and inadequately grounded neutral point systems. 　　The neutral point of a power system is effectively grounded, either by direct grounding or through a low-value resistor or reactor. It is required that the ratio of the zero-sequence reactance (X0) to the positive-sequence reactance (X1), namely (X0/X1), be positive and less than 3, while the ratio of the zero-sequence resistance (R0) to the positive-sequence reactance (X1) should also be positive and less than 1. Conversely, it is a system with a neutral point that is not effectively grounded. 　　The neutral point of a power system is not effectively grounded, including resonant (arc-suppression coil) grounding and ungrounding. 2 Advantages and disadvantages of different grounding methods for the neutral point in distribution networks The electrical connection between the neutral point of a distribution network and the reference ground can vary depending on operational requirements; these methods include leaving the neutral point ungrounded, grounding it through arc-suppression coils, grounding it through resistors of high, medium, or low value, grounding it through reactors of low value, and direct grounding. Each of these neutral-point grounding methods has its own unique advantages and disadvantages. 2.1 Advantages and disadvantages of an ungrounded distribution network neutral point An ungrounded distribution network neutral point means that the neutral point is not artificially connected to the ground. In fact, such a distribution network is grounded through the capacitance of the power grid to ground. 　　Main advantages of a neutral-ungrounded system: A small steady-state power frequency current occurs in the event of a single-phase ground fault in the power grid. In this way, instantaneous faults such as lightning-induced insulation flashovers can be automatically cleared without the need for tripping. 　· In the case of metallic grounding faults, it can operate in single-phase grounding mode, which improves the uninterrupted power supply in the grid and enhances power supply reliability. 　· The grounding current is low, which reduces the rise in ground potential. It reduces step voltage and contact voltage. It reduces interference with information systems. It reduces backflow to the low-voltage network, etc. 　　Economically: Grounding equipment is saved, resulting in lower investment in the grounding system. 　　Disadvantages of ungrounded neutral system: a. Compared with systems grounded through a neutral resistor, it generates higher overvoltages (such as arc flash overvoltages and ferroresonance overvoltages), increasing the likelihood of breakdown in weak insulations. 　　b The high-frequency oscillation current generated during intermittent arc ground faults is large, reaching several hundred amperes, and may cause inter-phase short circuits. 　　c To date, fault location is difficult, preventing the grounding fault circuit from being properly and quickly isolated. 2.2 Advantages and disadvantages of resonant grounding (arc-suppression coil) of the distribution network neutral point Resonant grounding of the distribution network refers to the connection of one or more neutral points of the distribution network to the ground through arc-suppression coils. The steady-state power-frequency inductive current generated by these arc-suppression coils tunes the steady-state power-frequency capacitive current in the grid; hence this type of grounding is called resonant grounding. The purpose is to reduce the residual current during a ground fault, thereby enabling such faults to be resolved on their own. Therefore, all the advantages of a neutral-point-ungrounded system are present in, and even better than, a neutral-point arc-suppression coil grounded system. Similarly, all the disadvantages of the ungrounded neutral system are also present in the system with a neutral arc-suppression coil grounding; it is only less likely that arc overvoltages of maximum amplitude will occur. This is because the arc-suppression coil reduces the arcing rate during single-phase grounding. 　　The success of using the arc-suppression coil grounding method largely depends on the reliability of the arc-suppression coil itself, the tracking system, and the line selection device. 2.3 Advantages and disadvantages of direct grounding of the distribution network neutral point Direct grounding of the distribution network neutral point refers to the situation where all or some of the transformer neutral points in the distribution network are directly and fully connected to the ground (ground grid) without any artificial impedance added. Make the power grid satisfy R 0 ≤ X 1 and X 0 / X 1 ≤ 3. The advantages of a system with the neutral point directly grounded include: a lower level of internal overvoltage, which allows for the use of lower insulation standards and thus saves on capital investment. 　　b High ground current facilitates fault location, allowing the grounded fault circuit to be disconnected accurately and quickly. The disadvantages of a system with the neutral point directly grounded are: a. The faulty circuit is disconnected quickly, resulting in intermittent power supply. 　　b The ground current is high, resulting in a significant rise in ground potential. In this way: l increases damage to electrical equipment. It increases contact voltage and step voltage. l Increases interference with information systems. l Increase the backflow against the low-voltage network. 2.4 Advantages and disadvantages of grounding the neutral point resistor in distribution networks At least one neutral point in a distribution network is connected to a resistor, with the aim of limiting the current during ground faults. The neutral point is grounded through a resistor (with zero resistance per phase, R0 ≤ Xc0 being the capacitive reactance per phase to ground), which eliminates the disadvantages of systems with an ungrounded neutral point and systems using arc-suppression coils for grounding; this reduces the amplitude of transient overvoltages and enables ground protection that is sensitive and selective in fault location. Since the grounding current of this system is smaller than that of a directly grounded system, the rise in ground potential, as well as interference with information systems and backfeed into low-voltage power grids, are all reduced. Therefore, the neutral resistor grounding system possesses some of the advantages of the ungrounded neutral system as well as the arc-suppression coil grounding system or directly grounded system, while also having to some extent some of the disadvantages of these two grounding methods. 　　Depending on the requirements for limiting the magnitude of ground fault current, grounding systems using high, medium, or low-value resistors exist, each with its own advantages and disadvantages. 2.4.1 Advantages and disadvantages of the neutral-point high-resistance resistor grounding system The neutral-point high-resistance resistor grounding system is used to limit the level of ground fault current to 10 A or less. The design of such a high-resistance grounding system must comply with the criterion that the per-phase zero-sequence resistance R0 ≤ Xc0 (the per-phase capacitive reactance), in order to limit the transient overvoltages that occur during intermittent arc ground faults. 　　Advantages: a It can prevent and dampen resonant overvoltages and intermittent arc grounding overvoltages, up to 2.5P · U. 　　b The ground current level is 10A or less, reducing the rise in potential. 　　c A ground fault does not need to be cleared immediately, allowing the system to operate with a single-phase ground fault. 　　Disadvantage: Its application range is limited; it is suitable for certain small 6–10KV distribution networks and plant auxiliary power systems. 2.4.2 Advantages and disadvantages of the neutral point low-resistance resistor grounding system To enable the rapid and proper isolation of the circuit with a ground fault, it is necessary to reduce the resistance value of the resistor. Advantages: a Low level of internal overvoltage (including arc overvoltage, resonance overvoltage, etc.), which enhances the reliability of the network and equipment. 　　b High ground current (100 – 1000A); fault location is easy, allowing for the rapid and accurate isolation of the circuit with a ground fault. Disadvantage: a Due to ground fault, the current flowing into the ground is If = 100 – 1000 A, resulting in a higher rise in ground potential compared to systems with an ungrounded neutral point, arc-suppression coil grounding, or high-value resistor grounding. b The grounded fault circuit is quickly disconnected, resulting in intermittent power supply. 2.4.3 Advantages and disadvantages of the neutral-point resistive grounding system A resistive value at the midpoint is employed in order to overcome the drawbacks of high-value and low-value grounding systems while retaining their advantages. By keeping the ground fault current within the range of 50 to 100A, it retains advantages such as low internal overvoltage levels, minimal rise in ground potential, and the ability to quickly disconnect the circuit affected by the ground fault. However, it also has disadvantages such as the interruption of power supply to the circuit affected by the ground fault. 3 Problems with the grounding of the neutral point in China’s urban distribution networks using arc-suppression coils In recent years, with the rapid development of China’s power industry, there have been significant changes in the structure of urban distribution networks, with cables playing an increasingly important role in the feeders. Various problems associated with the grounding of the neutral point using arc-suppression coils have become more apparent over time. 　　As the capacitive current in the distribution network increases rapidly, it becomes difficult to ensure that the arc-suppression coil operates in an over-compensated state at a certain detuning degree. The main reasons are: (1) The adjustment range of the arc-suppression coil is limited, usually at a ratio of 1:2, which is not suitable for the requirements at the beginning and end of the project. (2) The nominal and actual current values at each tap of the arc-suppression coil differ significantly; in some cases, this difference can reach 15%. Resonance has occurred during operation due to the discrepancy between the actual current values and those indicated on the nameplate. (3) There is a significant error between the calculated capacitive current and the actual capacitive current. In most substations, the power supply network consists of both cables and overhead lines; it is difficult to determine the length of the distribution lines accurately and in a timely manner. Moreover, there are many different types of cables, and the capacitive current per unit length varies as well. (4) Some distribution networks contain a certain proportion of 5th harmonic current in their total grounding capacitance current, with this proportion ranging from 5% to 15%. Even if the power-frequency grounding current is calculated with high precision, it is still not possible to compensate for the harmonic current component present in 5% to 15% of the grounding capacitance current. In summary, in a cable-based distribution network, when a single-phase ground fault occurs, the residual grounding current is high, and the conditions for overcompensation often cannot be met. 　　In cable-based distribution networks, single-phase ground faults are often caused by the breakdown of system equipment due to its own insulation defects under certain conditions. The residual current resulting from grounding is relatively large; especially when the grounding point is located in the cable, the grounding arc becomes a closed arc, making it even more difficult for such an arc to extinguish itself. (The value at which the arc light generated by the single-phase ground capacitance current can extinguish itself is much lower than the value specified in the regulations – only 5A for cross-linked polyethylene cables.) Therefore, single-phase ground faults in cable-based distribution networks are usually permanent faults. Since a system in which the neutral point is grounded through an arc-suppression coil is a low-current grounding system, it is difficult to locate the point of a single-phase grounding fault once such a fault occurs, and it is not possible to quickly identify the circuit in which the fault lies. In this way, on the one hand, it exposes the system equipment to overvoltage for extended periods, posing a threat to the equipment’s insulation; on the other hand, the advantage of not requiring the user to cut off power is no longer available. 　　In a system grounded at the neutral point through an arc-suppression coil, the overvoltage levels are high, posing a threat to the insulation of the equipment. (1) The detection of the line at the single-phase ground fault point generally involves attempting to disconnect it. During the testing of circuits with circuit breakers, operational overvoltages of high amplitude can sometimes occur. (2) Compared to systems with an ungrounded neutral point, systems with a grounded neutral point through arc-suppression coils can only reduce the probability of arc-grounding overvoltages occurring; they cannot reduce the amplitude of such overvoltages. (3) In certain conditions, systems with a grounded neutral point through arc-suppression coils can experience resonant overvoltages. For the above reasons, and also because cables are devices with weak insulation – for example, the 1-minute power-frequency withstanding voltage of 10kV cross-linked polyethylene cables is 28kV, which is more than 20% lower than that of ordinary devices – during the detection of a single-phase ground fault, prolonged exposure to power-frequency or transient overvoltages often leads to inter-phase faults, resulting in the tripping of one or multiple wires. 　　In the case of single-phase grounding, the voltage of the non-faulty phases rises to the line voltage or even higher. When it is not possible to locate the fault quickly, gapless metal oxide arresters (MOAs) operate under high line voltages for extended periods, which can lead to damage or even explosion. Such incidents were not uncommon in previous years. By increasing the rated voltage of the MOA, such accidents can be significantly reduced. However, if the characteristics of the MOA valve itself do not improve, it will inevitably lead to an increase in the residual voltage of the MOA under lightning impulse currents, thereby reducing its protective performance. Furthermore, in a system where the neutral point is grounded through an arc-suppression coil, arc grounding overvoltage and resonance overvoltage can occur, and the duration of such overvoltages may be relatively long. Due to issues related to the operating load, MOAs are generally not required to suppress these types of overvoltages. This reduces the voltage-limiting effect of MOAs and weakens their advantages, which is not conducive to the widespread use of MOAs in distribution networks. 4 Several Issues of Concern Regarding the Grounding of the Neutral Point in Distribution Networks through Low-Value Resistors 4.1 Reliability 4.1.1 Requirements for power supply reliability and factors affecting it: According to the relevant regulations on power supply reliability management in China, there are three main indicators used to assess the level of power supply reliability: the frequency of outages, the duration of outages, and the amount of power not supplied. These indicators are related to many factors, including those caused by planned outages and those resulting from failures. The main factors affecting the reliability of power supply in 10kV distribution networks can be primarily attributed to user-related issues, climate conditions, municipal construction activities, and equipment aging. It can be said that the different neutral grounding methods in 10kV distribution networks have a comprehensive impact on the reliability of power supply in such networks. After a change in the neutral grounding method, it may increase the likelihood of failures for certain types of faults, while for other types of faults it may reduce the likelihood of failures or have no effect at all. To improve power supply reliability, certain measures should be taken based on the impact of grounding methods on faults. 4.1.2 Impact of neutral point grounding methods on power supply reliability: As is well known, compared to the methods of grounding the distribution network’s neutral point either without any grounding or through an arc-suppression coil, the greatest advantage of grounding the neutral point through a small resistor is that, in the event of a single-phase ground fault, if it is an instantaneous fault and the system’s capacitive current or the residual current after compensation by the arc-suppression coil is low enough to cause the fault to extinguish itself, then the fault can be resolved on its own. In the case of a permanent fault, the system can continue to operate with this single-phase ground fault for 2 hours, providing sufficient time to resolve the fault and ensure uninterrupted power supply to users. However, this advantage is not prominent in urban distribution networks that rely primarily on cables. Based on statistical data, the causes of cable failures are mainly insulation aging, cable quality, and external damage; these are generally permanent faults. When a grounding fault occurs, the cable should not be operated with that fault present. Based on actual operation experience, in distribution networks dominated by cables, with the neutral point either ungrounded or grounded through arc-suppression coils, phase-to-phase short-circuit faults resulting from single-phase ground faults are relatively common. Some actual accidents have shown that a single-phase ground fault developing into an inter-phase fault actually expands the scope of power outages. This is especially true when it progresses to a busbar short-circuit fault, which is equivalent to a short circuit at the transformer outlet. Since some transformers currently have weak resistance to short-circuit impacts, this can lead to damage to those transformers. 　　Considering the actual conditions of power supply in urban distribution networks, factors such as the dual-power supply system, the use of overhead insulated wires, ring network configurations, open-circuit operation modes, and the proportion of cable lines result in the advantages of a neutral point that is either ungrounded or grounded through arc-suppression coils not being particularly prominent. Analysis based on the actual operation of substations that have currently switched to reduced-resistance grounding ； Proper protection configuration can maintain power supply reliability. 　　Based on the above analysis, substations that rely primarily on cable power supply and use a small-resistance grounding for the neutral point do not have a significant impact on power supply reliability; in fact, they can be beneficial in improving it in certain aspects. 4.2 Impact on communications The ground fault current and the zero-sequence current during operation induce longitudinal electromotive forces through inductive coupling with adjacent communication lines. The asymmetric voltage generated by the three phases induces electrostatic voltages through capacitive coupling with adjacent communication lines. 　　The rise in ground potential caused by the current flowing into the ground due to grounding faults in the distribution network generates voltage on the grounded telecommunication lines through resistive coupling between the grounding electrodes; this is known as resistive coupling or direct transmission. 　　The voltages and currents generated in the communication system as mentioned above are harmful to the communication system, and are referred to as hazardous effects. Those that result in a decrease in communication quality, noise in telephone calls, and distortion of telegraph signals and data transmissions are referred to as interference effects. 　　Due to the direct grounding of the power grid neutral point, the neutral point resistor (or reactor) is also grounded, resulting in a higher ground fault current compared to scenarios where the neutral point is not grounded (insulated) or where an arc-suppression coil is used for grounding. Consequently, the impact on communication systems is greater in the former case than in the latter. This concept arises from single-power supply feeding, where a single-phase ground fault occurs at the end of the line (point F). The fault current induces a high voltage in the communication lines that run parallel to the power lines; if one end of the communication line is grounded, this voltage can be measured at the other end, and it increases as the fault current grows. 　　The judgment regarding electromagnetic induction in the communication line, derived from this simple basic concept, is clearly excessive. In actual urban distribution networks, it is rare for only one end of the neutral point to be grounded while the other end remains open-circuited. The actual power distribution network is much more complex than this. When a single-phase ground fault occurs at point F along the line, the ground fault current flows toward the fault point F from both ends in opposite directions. The induced voltage along the entire length of the communication line is proportional to the absolute value of (i1l1 – i2l2). Therefore, in systems with a directly grounded neutral point or systems where the neutral point is connected to a low-value resistor (or reactor), the induced voltage on the communication lines is not necessarily higher than that in systems with an arc-suppression coil at the neutral point or in systems with an ungrounded (insulated) neutral point. Specific calculations and measurements are required. If the most severe extreme conditions are considered, then in systems with an arc-suppression coil at the neutral point and in systems with an ungrounded (insulated) neutral point, induced voltages on the communication lines can be even more severe in the case of two-phase ground faults (such faults often occur due to fog flashover on the overhead lines in these systems). 　　In actual large cities, both the power distribution networks and communication networks are made of cables, and the ground fault current flows along the outer surface of these cables, so generally there is no impact. In short, the specific situation needs to be calculated and analyzed on a case-by-case basis. It should also be noted that there are many protective measures that can be employed when the induced voltage exceeds the specified value. 4.3 Regarding personal safety 　　Based on actual examples provided by power supply companies, there have been cases of electric shock injuries and deaths as well as incidents of escaping from electric shocks, whether in systems with ungrounded or arc-suppression coil grounding, or in systems with low-resistance grounding. Therefore, for such accidents involving direct contact with high voltage, the key factor determining whether injury or death will occur is not the type of neutral-point grounding used, but rather the way in which the person comes into contact with the live conductors and the time it takes for them to get away from it after being shocked. Therefore, from the perspective of protecting personal safety, in systems where the neutral point is ungrounded or grounded through an arc-suppression coil, no immediate tripping occurs when a single-phase ground fault happens; as a result, this can pose a significant risk to those who accidentally come into contact with live wires and are unable to disconnect from the power source promptly. In contrast, in systems where the neutral point is grounded through a small resistor, when a metallic single-phase ground fault occurs, the short duration of such faults allows the protective devices to act correctly and in a timely manner, enabling the person who has been electrocuted to disconnect from the power source immediately. Thus, despite the high value of the short-circuit current, the harm caused to humans is relatively minor. However, in systems with small-resistor grounding, when a single-phase ground fault occurs through a transitional resistor (as in the case of the Zhuhai Airport substation), the protective devices cannot act accurately and in a timely manner, and this can still result in harm to human beings. Therefore, factors such as the manner of electric shock and the operation of protective devices after an electric shock should be taken into consideration. In many cities, overhead lines have been replaced with insulated ones; as a result, accidents caused by external forces leading to single-phase grounding of overhead lines have decreased significantly. When a cable experiences single-phase grounding, the current that flows into the ground is limited due to the current-diverging effect of the cable’s insulation, so the resulting voltage rise is also small. From this perspective, a low-resistance grounding system for 10kV distribution systems offers better protection for human safety compared to ungrounded or arc-suppression coil grounding systems. 4.4 Regarding circuit breakers Theoretically, in systems with an originally ungrounded (insulated) neutral point and arc-suppression coil grounding, the line circuit breakers do not trip in the event of a single-phase ground fault. By switching to a grounding system with a low-value resistor (or low-value reactor) at the neutral point, the line circuit breaker will trip in the event of a single-phase ground fault; this leads to the problems of \"frequent tripping and equipment damage\" as well as an increased amount of maintenance work. Long-term operational experience in cities such as Shanghai shows that this will not happen. The maintenance workload for circuit breakers in the 23KV neutral point low-resistance resistor grounding system at the Shanghai West Suburb Substation is not greater than that for circuit breakers in the 35KV neutral point arc-suppression coil grounding system at the same substation. The reason for this is that the fault current is not high; the current flowing into the ground in the case of a single-phase ground fault is limited to between 1 and 2 KA, which is slightly higher than the load current but less than one-eighth of the circuit breaker’s breaking current. As a result, it does not cause severe damage to the circuit breaker. The conditions under which a circuit breaker can break a single-phase short circuit are much better than those for breaking an inter-phase short circuit. In neutral-ungrounded (insulated) and arc-suppression coil grounded systems, the probability of inter-phase short-circuit faults occurring as a result of single-phase arc grounding faults is quite high. 5 Considerations for relay protection when the resistor is used for grounding 　When the neutral point is grounded through a small resistor, in the case of a single-phase fault, the fault current increases and zero-sequence current is generated; therefore, zero-sequence protection should be added to the protection setup. Based on experience, protection configurations should adopt zero-sequence current protection with different time delays, or zero-sequence direction protection. The protection configuration should also take into account: (1) For distribution lines, zero-sequence current transformers and zero-sequence current grounding protection that responds to the power-frequency current value are used as the primary protection against single-phase grounding, triggering tripping. (2) The protection setting value is selected to bypass the capacitance current in this section, and a reliability factor of 2.0 can be adopted. (3) Sensitivity is verified based on the capacitive current flowing through the faulty line. Sensitive systems engineering >1.25. (4) The open delta 3U0 of the busbar voltage transformer in this section is used as a signal. (5) It is preferable to use a single CT wrapped around the three-phase cable for the zero-sequence CT, in order to avoid unbalanced currents caused by errors and saturation differences among the three CTs. (6) The protection configuration can be coordinated over time to minimize the scope of the fault. 6 Proper selection of resistance value When using neutral point resistance grounding, the choice of resistance value must be based on the specific conditions of the power grid. Factors such as the level of overvoltage that can be limited, the sensitivity of relay protection systems, the impact on communication, and human safety must all be taken into consideration. (1) For high-resistance grounding, when a single-phase ground fault occurs in the system, operation with the fault is permitted, and the current at the fault point should be limited to below 10A. Therefore, the value of the ground resistance R0 is chosen such that Xc ≧ R0, and R0 ≧ Uφ/10A. Xc is the per-phase reactance of the system, and Uφ is the phase voltage of the system. (2) For low-resistance grounding: a From the perspective of reducing internal overvoltages, based on TNA simulations and computer calculations, when I₀ ≥ Ic (where I₀ is the current flowing through the neutral point resistance and Ic is the system capacitance current), the overvoltage magnitude on the healthy phases can be limited to less than 2.8 times; when I₀ ≥ 1.5Ic, it can be limited to less than 2 times. After I 0 ≥1.5Ic, the effect of limiting overvoltage changes little. Therefore, the resistance value can be selected according to 1.5Ic ≥ I₀ ≥ Ic. R 0 = U φ / I 0. 　　b From the perspective of ensuring the sensitivity of relay protection, the smaller the resistance value, the better. Modern microcomputer-based protection systems generally come equipped with zero-sequence protection functions, and the starting current value for these functions is quite low. The current resulting from a single-phase ground fault is much greater than the capacitive current between each line and the ground, so the sensitivity requirements of zero-sequence protection are usually met. The problem is that when the grounding transition resistance is high, the sensitivity of relay protection is affected. According to the resistance value selected in a), when the transition resistance is not greater than 100Ω, the protection sensitivity is generally satisfactory; in distribution circuits that primarily use cables, the transition resistance is usually less than 100Ω. 　　c For the purpose of reducing interference with communication, the resistance should not be chosen to be too low. The four agreements in our country stipulate that, if no arrester is installed between the communication cable and the ground, the hazardous influence voltage shall not exceed 430 V; for high-reliability lines, it shall not exceed 630 V. Currently, the neutral point resistance for the Shenzhen power grid is set at 15Ω, 10Ω for the Beijing power grid, and 5.7Ω for the Shanghai power grid. The corresponding currents are 400A, 600A, and 1000A respectively. None of them caused any impact on the communication lines. 　　d For reasons of personal safety, the higher the resistance value of the neutral point grounding, the better. Because the neutral point is connected through a low resistance, during a single-phase ground fault, the grounding short-circuit current flowing through the fault point is relatively large, which causes the ground potential at the fault point to rise. This can result in step voltages and contact potentials exceeding the allowable values. Therefore, when selecting the resistance value, it is necessary to consider the ground resistance of the grounding grid, the protection operation time, the grounding short-circuit current, and whether the step voltage and contact potential exceed the specified limits. Based on the practical experience from Shenzhen, Guangzhou, Shanghai, and Beijing, the adoption of resistive grounding has not led to any personal injuries resulting from excessively high step voltages or contact potentials. 7 Conclusion The selection of the neutral point grounding method in distribution networks is a comprehensive technical issue. Ungrounded neutral point, resonant grounding, and resistive grounding each have their own advantages and disadvantages; the choice should be determined through technical and economic comparisons based on the specific conditions of the power grid. In other words, systems with each type of neutral point grounding method have their own unique advantages, which is why they have been developed. In the same city and at the same nominal voltage level, systems with various neutral-point grounding methods coexist. It is incorrect to determine the neutral grounding method in a one-size-fits-all manner based on voltage levels. This is because each system with a different neutral grounding method has its own drawbacks. Therefore, when making a choice, one must proceed from specific realities, weigh the pros and cons, and choose the option that offers more advantages than disadvantages. 　　For example, in small power grids with overhead lines, where the network capacitive current is low, a grounding system using high-value resistors at the neutral point can be employed. 　　For large power grids with overhead lines, where the network capacitive current is high, a neutral point resonance grounding system can be adopted. 　　In urban cable distribution networks, which have a good network structure, an earthing system using neutral point resistors with medium or low values can be adopted. If it is required to use a compensation network with capacitive current to limit the ground fault current, a grounding scheme in which the neutral point is connected in parallel to an arc-suppression coil through a neutral resistor can be adopted. 　　The grounding method using a resistor with a medium or low resistance at the neutral point, as well as the method of grounding the medium-resistance resistor in parallel with the arc-suppression coil, can overcome the two main drawbacks of the ungrounded and resonant grounding methods: (1) limiting the transient overvoltages and transient currents that occur during single-phase intermittent arc grounding. (2) Overcome the difficulty in selecting the appropriate line, enabling quick and accurate identification of the line with a single-phase ground fault for disconnection. 　　Grounding of the neutral point through a resistor has been in use abroad since the 1940s. In 1995, Hualete Electric Company was the first to introduce neutral point grounding resistance from the American company PGR. It has since been used by power supply agencies in regions such as Shenzhen, Shanghai, Beijing, Tianjin, Jiangsu, and Fujian, as well as in industries including petrochemicals, steel manufacturing, subways, and power generation plants. The use of over 2,000 annual resistance cabinets has shown that they possess advanced performance and reliability. References: 1. Technical analysis of the neutral point grounding methods in the distribution networks in Shanghai’s urban area. Shanghai Electric Power, Issue 6, 1993. 2. Dong Zhenya. Development and improvement of neutral point grounding methods in urban distribution networks. China Electric Power, Issue 8, 1998. 3. Xu Ying. Several issues related to neutral point grounding methods in 3–66 kV power grids. 4. Feng Baoyi. The impact of neutral point grounding methods in cable-based distribution networks on power supply reliability. Last edited by laoyier on 2009-3-11 at 10:55