my country's sustained and rapid economic development has placed higher demands on power supply reliability, and the rise of smart grids has greatly boosted the development of power distribution technology. State Grid Corporation of China and China Southern Power Grid have successively carried out power distribution automation construction in numerous cities. As a mainstream equipment supplier, Shandong Kehui Power Automation Co., Ltd. has consistently focused on power distribution automation research and technology application, achieving rich research results and engineering experience.
Since pioneering the first fully functional distribution network automation terminal in China in 1996, our company has achieved excellent results in distribution automation projects in Zhejiang, Fujian, Shandong, Sichuan, Guangdong, Shanghai, and Shaanxi provinces. Our product for selecting fault locations using transient fault information and low-current grounding faults has been applied in over a thousand substations, achieving a success rate exceeding 95%, making a significant contribution to improving power supply reliability. In recent years, with the deepening of distribution automation construction and the integration of distributed power sources, a number of new technologies have emerged in areas such as low-current grounding fault detection and location, short-circuit fault detection and handling, relay protection, and the application of IEC 61850 in distribution networks. Kehui Company presents this series of lectures, combining our latest understanding, research results, and practical applications, to stimulate further discussion and encourage collaboration with distribution engineering technicians to jointly promote the development of distribution automation technology in my country.
This lecture series consists of five parts: ① Discussion on neutral grounding methods in distribution networks; ② Low-current grounding fault selection and location technology; ③ Fault handling technology of distributed intelligent control; ④ Wide-area protection technology for distribution networks; ⑤ Plug and play technology.
The neutral point grounding method of a distribution network, that is, the electrical connection between the neutral point and the earth, is a fundamental issue in distribution networks. It involves many technical and economic issues such as power supply reliability, overvoltage, and relay protection. Currently, there is much discussion in the industry, but understandings vary, and the grounding methods of distribution networks differ from country to country. This article mainly introduces the commonly used neutral point grounding methods in distribution networks and their characteristics, and provides suggestions for selecting grounding methods based on this.
Grounding method classification
The neutral grounding methods of power distribution networks can be divided into two main categories: effective grounding and ineffective grounding.
Effective grounding methods include three specific methods: direct grounding (the neutral point is directly connected to the earth), grounding through a small resistor (connected to the earth through a resistor with a small resistance value), and grounding through a reactance (connected to the earth through an inductor with a small reactance value). Because the fault current is relatively large when a single-phase grounding occurs, it is conventionally called high-current grounding.
Ineffective grounding includes three specific methods: ungrounded neutral point (neutral point is suspended or absent), resonant grounding (connected to the ground via an arc suppression coil, i.e., an inductor with a large inductive reactance), and high-resistance grounding (connected to the ground via a resistor with a large resistance). Because the current flowing through the fault point during a single-phase grounding event is very small, it is also called low-current grounding. Active grounding, which uses power electronic devices to compensate for all electrical components of the fault current, including reactive, active, and harmonic components, can also be classified as ineffective grounding. Since its active current generating device is a flexible distribution system (DFACTS) device, it can also be called flexible grounding.
Characteristics of different grounding methods
In directly grounded systems, a single-phase ground fault current will exceed 50% of the three-phase short-circuit current. This enormous short-circuit current can damage electrical equipment and interfere with nearby communication lines. It can also easily generate touch voltage and step voltage, posing a risk to personal safety. Therefore, relay protection devices need to operate immediately to disconnect the faulty line. Since single-phase grounding is the most common fault type in distribution networks, it will frequently cause power outages, affecting power supply reliability. Its advantages include not generating overvoltage and relatively easy implementation of relay protection.
In a low-resistance grounding system, the single-phase ground fault current is significantly reduced compared to a directly grounded system due to the current-limiting effect of the resistor, which lessens the damage to the distribution network and equipment. However, it is still necessary to immediately disconnect the faulty line, resulting in power outages and affecting power supply reliability. Simultaneously, the overvoltage is slightly increased, but it will not damage the distribution equipment.
In an ungrounded system, the line voltage between the three phases remains essentially unchanged during a single-phase ground fault, without affecting the power supply to the load. Furthermore, since the fault current is the system's distributed capacitance current to ground, its value is relatively small, posing minimal harm to equipment, communication, and personnel. Therefore, operation can continue for a period of time under grounding conditions, allowing operators to take appropriate measures. In fact, if the grounding current is small, the arc will extinguish itself, creating a "transient" fault, and the system will return to normal operation, achieving a "self-healing" effect. The main advantages of ungrounded systems are that single-phase grounding does not cause power outages and offers high power supply reliability. However, its disadvantages include: when the grounding current is large, it can create a stable or intermittent arc grounding, generating an arc grounding overvoltage of up to 3.2 times the phase voltage, which can endanger the insulation safety of lines and equipment and potentially cause phase-to-phase short-circuit faults, leading to line tripping and power outages; the small fault current in a single-phase grounding system makes relay protection (fault selection, location, etc.) difficult.
In a resonant grounding system, during a single-phase ground fault, the fault current is the sum of the system-to-ground capacitance current and the arc suppression coil inductance current. Adjusting the arc suppression coil can minimize the fault current, making it easier to extinguish the arc. After arc extinguishing, it can limit the recovery speed of the fault phase voltage, reduce the probability of arc reignition, and promote fault self-elimination and system recovery to normal operation. Based on the compensation inductance current being equal to, less than, and greater than the system-to-ground capacitance current, the arc suppression coil can be classified into three states: fully compensated, undercompensated, and overcompensated. From the perspective of compensation effect alone, full compensation is the best, but the arc suppression coil and the system-to-ground capacitance are prone to series resonance; undercompensated methods are prone to full compensation after disconnecting part of the line; therefore, a moderately overcompensated state is generally adopted.
Early arc suppression coil adjustments were done manually, which made it difficult to track changes in the system's capacitance current in a timely and accurate manner. Now, automatic tracking compensation technology is generally used, which greatly improves the compensation effect of the arc suppression coil.
Active grounding can minimize the fault current (approaching zero), making the fault arc easier to self-extinguish, minimizing the risk of arc reignition, and promoting more self-recovery of grounding faults. By avoiding intermittent grounding, it also reduces the hazards of arc overvoltage.
Selection of neutral grounding method in distribution network
This is both a technical and an economic issue. It requires consideration of the operation of the distribution network, the reliability requirements of power supply, and factors such as overvoltage, personal safety, communication interference, relay protection, and equipment investment during faults. It is a systematic project.
The overvoltage hazards, difficulties in relay protection, and complex operation and management associated with ineffective grounding are all disadvantages for power supply companies. From a purely self-interested perspective, there is reason to choose effective grounding. However, from the perspective of improving power supply reliability and reducing personal injury risks, ineffective grounding offers greater advantages. When choosing between ungrounded and resonant grounding for ineffective grounding, the arc extinction rate of the fault arc should be a primary consideration. According to relevant Chinese standards, resonant grounding should be used when the grounding current exceeds 10 amperes in a mixed cable and overhead line network and when the grounding current exceeds 20 amperes in a pure cable network.
Currently, the neutral points of distribution networks in the United States, the United Kingdom, Singapore, and Hong Kong generally adopt an effective grounding method, while the neutral points of distribution networks in European countries such as Germany and France, as well as countries such as Japan and Russia, generally adopt an ineffective grounding method.
The vast majority of my country's power distribution networks use non-effective grounding methods, while some power distribution cable networks in coastal cities and central areas of megacities use low-resistance grounding methods.
The use of low-resistance grounding in power distribution cable networks is typically based on two considerations: ① large capacitive current, resulting in poor compensation from arc suppression coils; ② faults are permanent and difficult to self-recover. In fact, current automatic arc suppression coil compensation technology fully meets the needs of cable networks. In Germany, cable networks with capacitive currents reaching hundreds of amperes still use resonant grounding. Statistics show that a significant portion of faults in cable networks are transient faults occurring outside the cable joint or the cable itself (such as user transformers); actual faults indicate that even grounding of the cable itself can self-extinguish the arc. Therefore, using resonant grounding in cable networks can also avoid unnecessary power outages, or in other words, maintain normal power supply for a period of time.
In recent years, resonant grounding has gained attention from power supply companies worldwide. Socioeconomic development has placed higher demands on power supply reliability, and resonant grounding can minimize power outages caused by single-phase grounding. The maturity of technologies such as automatic tuning of arc suppression coils and low-current grounding fault protection (e.g., line selection using transient signals) has also played a significant role in this shift. In the UK, which traditionally used effective grounding, some distribution networks have already switched to resonant grounding.
Active grounding technology is currently in the research and limited operation stage. Because it can greatly improve the self-healing level of single-phase grounding, it is an important fault self-healing technology in smart distribution networks and represents the development direction of neutral point grounding (especially non-effective grounding) in distribution networks.
For smart distribution networks, when the main grid adopts an effective grounding method, the grounding method of distributed generation (connected to transformers) faces a dilemma. If it also adopts direct grounding or low-resistance grounding, the single-phase ground fault current in grid-connected mode will increase significantly; if it adopts an ungrounded method, the system is effectively grounded when connected to the grid, but ineffectively grounded in the islanded grid after disconnection. The system needs to meet the requirements of two grounding methods with completely different characteristics. When the main grid adopts an ineffective grounding method, the problem is relatively simpler; distributed generation can adopt an ungrounded method, and the grounding method will not fundamentally change whether in grid-connected or disconnected mode.
Therefore, from a technical perspective, the neutral point of a distribution network containing distributed power sources should adopt an ineffective grounding method, especially an active grounding technology. This not only allows the main grid and the isolated grid to maintain the same grounding method, but also fully realizes the self-healing of grounding faults and maximizes the reliability of power supply.
