I. The Importance of Single-Phase Grounding Protection Line Selection? In substations, switching stations, and power plants, 66kV, 35kV, 10kV, 6kV, and 3kV distribution lines are the main components of the power system. In these voltage levels, the neutral points of transformers and generators are either ungrounded or grounded via arc suppression coils or resistors for power transmission and distribution. Furthermore, multiple output or input distribution lines are connected to the busbar at the same voltage level, mostly using aluminum (or copper) overhead lines or high-voltage power cables; the number of lines typically ranges from five or six to a dozen or even twenty or thirty; each distribution line has many branches, arranged in a radial pattern, and then connected to various distribution transformers, which step down the voltage to a lower level before supplying it to a wide range of users. In these types of distribution lines, faults such as phase-to-phase short circuits, overcurrent (overload), and single-phase grounding frequently occur. Among these, single-phase grounding occurs most frequently, accounting for over 70% of the total system failure rate; short-circuit faults also often evolve from single-phase grounding into multi-phase grounding. "Single-phase grounding" refers to the phenomenon where any one of the three phases (A, B, and C) of a distribution line breaks off and falls to the ground, forming a conductive loop; or where atmospheric lightning or other causes lead to overvoltage, damaging the insulation material of the distribution equipment and resulting in significantly low insulation resistance to ground. Because the neutral point of the main transformer in the system is either ungrounded or grounded through an arc-suppression coil or a high-resistance ground, when multiple distribution lines are on the same busbar, a single-phase grounding in any of them cannot directly form a loop with the winding coils of the main transformer, and there will be no large current phenomena such as short circuits or overloads in the line. Only the capacitive current formed between the line and the ground changes, manifesting as a weak zero-sequence current in each line. This current is extremely small, ranging from a few milliamps to hundreds of milliamps or even several amperes, and is directly proportional to the length of the line; under normal conditions, it is approximately 15 milliamps per kilometer of overhead line. In the power industry, this type of power supply system is called a "small grounding current system" or "small current grounding system." In this system, because the primary winding of the voltage transformer (PT) is connected to the system busbar using a Y0 connection, when a single-phase ground fault occurs on any line, a zero-sequence voltage is generated at the delta opening of the secondary winding. A zero-sequence overvoltage alarm can be set; however, it cannot be set for a specific line. During a ground fault, the voltage to ground of the ungrounded phase can rise to √3 times the phase voltage. When the system is accompanied by ferroresonance, the phase voltage will rise by 1-5 times, or even higher, forming an overvoltage. This accelerates the aging of the insulation materials of the power equipment, shortens its service life, and leads to insulation breakdown. This can result in two or more phases simultaneously grounding, causing a short circuit and increasing the damage to the power equipment. Therefore, accidents such as voltage transformer and circuit breaker explosions, distribution transformer burnouts, and power cable and insulator punctures frequently occur in power systems. Existing relay protection or integrated automation protection devices' "short circuit protection," "overcurrent protection," and "zero-sequence current" protection are all high-current-starting protection devices; the small current during a single-phase ground fault cannot drive these protection devices to operate. Therefore, they cannot trip the high-voltage switch (circuit breaker), and the faulty and non-faulty lines cannot be isolated. To prevent the escalation of accidents, it is necessary to promptly distinguish between faulty and non-faulty lines. In substations, switching stations, or power plants, if a reliable "single-phase ground fault protection line selection device" is not installed, manual power outages must be performed sequentially to select the faulty line. Sometimes, all distribution lines connected to the busbar must be disconnected to find the faulty line. This causes power outages of faultless lines, leading to large-scale blackouts; at the same time, it increases the number of high-voltage switch (circuit breaker) trips, shortens their service life, and reduces the reliability and quantity of power supply. Locating the grounding point on the power line requires disconnecting numerous branch lines from the main line one by one, measuring the insulation resistance of each section or branch to ground using an insulation resistance meter, and manually determining the location of the fault. This process is extremely complex and labor-intensive; for personal safety, multiple safety measures are required, consuming significant manpower, resources, and time, increasing the workload of power workers, and posing safety hazards. In summary, while systems with ungrounded neutral points or those using arc suppression coils or resistor grounding can extend the tripping time of a faulty line during a single-phase ground fault, they also lead to the interruption of power supply to multiple non-faulty lines, causing a wider power outage and posing a greater risk to people's lives and property. The State Electricity Regulatory Commission and power grid companies are imposing increasingly stringent requirements on power supply reliability, demanding that rural power grids achieve at least 99.8% reliability and urban power grids at least 99.9% to be considered合格 (qualified). There are also limits on the number and duration of power outages caused by faults on each line per year. In some areas, exceeding the limit results in a 200 yuan fine for each outage. The application of "integrated automation protection devices for substations" is widespread in power systems, leading to many substations operating unmanned. However, because the solutions for single-phase grounding protection in low-current systems using "integrated automation" are not perfect, grounding fault information cannot be accurately reported to the dispatch and monitoring center in a timely manner when a single-phase grounding occurs, delaying the location and repair of the fault by line maintenance personnel. Several serious accidents have occurred in some areas where the failure to promptly disconnect power after a single-phase grounding on a 10kV high-voltage distribution line resulted in the electrocution deaths of people near the grounding point. This has caused significant trouble, economic losses, and negative social impact for power supply departments. Therefore, according to the requirements of the national standard GB50062-92 "Design Code for Relay Protection and Automatic Devices of Power Installations", the use of accurate and reliable single-phase grounding protection line selection devices in "small grounding current systems" is one of the important technical measures to improve the automation level of power supply systems. II. Current Technological Status in this Field at Home and Abroad Single-phase grounding protection line selection in small grounding current systems is a global challenge; it has not been completely solved in power production for over a century. In the early 20th century, many power engineers and universities abroad conducted extensive research on this project, resulting in a deeper understanding, more and more technical solutions, and a gradual improvement in accuracy. Representative theories include the "first half-wave" theory of German power engineer Bach and the "reactive power direction" theory of Russia. Devices developed based on these theories have been used in power systems, achieving a line selection accuracy of approximately 50%. China began developing automatic single-phase grounding line selection devices for small current grounding systems in the 1980s. Although starting late, the development speed has been rapid, and it has now reached an advanced level globally. In China, based on the principle of zero-sequence current magnitude, sensitive relays and transistor electronic protection technologies have been used to select faulty lines by setting zero-sequence current action values. Various models of these devices have been developed. After years of use, their line selection accuracy has approached 50%. Later, by combining the "first half-wave" theory with transistor electronic technology, several different specifications of line selection devices were produced and widely used in my country's power system, further improving the accuracy to around 60%. In the 1990s, with the widespread application of microcontrollers in my country, many technology companies began applying this high-tech microcomputer technology to this field. Simultaneously, based on the theory proposed by Professor Xu Yuanheng of North China Electric Power University that "in a power supply system with an ungrounded neutral point (or grounded via an arc suppression coil), the direction of the zero-sequence harmonic current of the faulty line is opposite to the direction of the zero-sequence harmonic current of the non-faulty line," several research institutes and enterprises have developed and manufactured small-current grounding line selection devices. Subsequently, Professor Sang Zaizhong of Shandong University of Technology proposed the "S-injection method" theory through research. The method involves injecting an AC current signal of approximately 250Hz from the low-voltage side into the high-voltage busbar by reverse power transmission from the secondary winding of a voltage transformer to the primary winding; then detecting and comparing the magnitude of this current signal in each line to ultimately identify the faulty line. The "wavelet analysis method" is a new theoretical approach for single-phase grounding protection line selection in systems with low grounding currents that has emerged in recent years. Its principle is based on the fundamental principle of the "fault recorder" already used in power systems. By collecting and recording the waveforms of high-order harmonic currents in each line during a single-phase grounding event, and then analyzing and comparing them using computer software, the line with a significantly different current waveform from other lines is identified as the faulty line. Devices developed based on this method have been tested in some substations. However, the results are still unsatisfactory, with a line selection accuracy of less than 50%. Further testing and improvement are needed. In substation integrated automation microcomputer protection devices, the principle used for single-phase grounding protection is still largely the early zero-sequence current overcurrent setting principle. That is, zero-sequence current overcurrent protection is added to the unit of the line protection device; the principle is the same as short-circuit protection and overcurrent protection, relying on the setting current starting value for protection line selection. This method has long been proven to be highly unreliable. In some integrated automation microcomputer protection devices, although independent small-current single-phase ground fault location devices are equipped, most have not been put into normal use due to poor line selection reliability and incompatible components. Some integrated automation manufacturers, fearing user doubts about the quality of their complete sets of equipment due to inaccurate ground fault location in their devices, have opted to use products from other specialized manufacturers of single-phase ground fault location protection devices. However, due to various technical reasons, even specialized manufacturers' line selection devices are unreliable and frequently misselect during use. III. Analysis of the Reasons for Poor Reliability of Existing Technology Because the neutral point of the three-phase windings on the secondary side of the main transformer is not grounded (or grounded through an arc suppression coil or resistor), when any phase in the line is grounded, it does not form a circuit with the transformer windings (or the resistance is very high), so no current can be formed (or it is very small), and no large current will flow through the faulty line and the grounding point. The only factor affecting the accuracy of protection line selection is the capacitive current formed between the line and the ground, which is proportional to the line length, approximately 15mA per kilometer for a 10kV line. The key reason for this is its small size. The ground fault current is very small, ranging from a few milliamps to tens or hundreds of milliamps, with a maximum of only a few amperes. This is thousands or tens of thousands of times smaller than the hundreds or thousands of amperes of load current in the power supply line. Furthermore, the current during a fault is not significantly different from the current during a non-fault fault. Moreover, the unbalanced current generated by the measurement errors of many current transformers is much larger than the zero-sequence current value during a ground fault. Existing relay protection and integrated automation protection equipment cannot distinguish between the fault ground current and load current fluctuations. In addition, when a ground fault occurs, not only does the faulty line have a capacitive current to ground, but the non-faulty line also has a capacitive current to ground. This necessitates measuring and distinguishing the capacitive currents of both lines, making the process even more challenging. In some large-scale substations, although there are many long distribution lines, grounding can generate a large capacitive current. However, a large grounding current can also easily cause arcing at the fault point, leading to short circuits in two-phase or three-phase lines. Therefore, relevant national departments have clearly stipulated that arc suppression coils or resistors must be installed in systems with grounding currents greater than 10A. This is to reduce excessive current at the grounding point and avoid arcing; thus, the current at the grounding point of the faulty line is smaller, and the fault characteristics are less obvious. Furthermore, because power systems are environments with strong electric and magnetic fields, interference signals are significant, often overwhelming the grounding signal. In addition, during grounding, the system frequently experiences ferroresonance, changing the direction of the zero-sequence current between the faulty and non-faulty lines, thus negating the theory of using the "direction of zero-sequence harmonic current" to determine the faulty line. According to feedback from on-duty personnel, the main reasons for inaccurate line selection using the "S-injection method" for protection are: 1. The signal energy that the device can "inject" into the high-voltage system is limited, and the current is too small, negligible compared to the large current of the power system. In reality, the harmonic interference signal in the substation is much larger; 2. It is constrained by various conditions such as the size of the power supply system, the number and length of lines, and changes in the operating mode of the bus section; 3. It is affected by the grounding resistance of the fault point; when the fault point is grounded with high resistance, the signal current flowing through the faulty line is very weak, while the current signal of other non-faulty lines, if long, is many times stronger than that of the faulty line; 4. The power loss is significant, causing the voltage transformer to overheat during use, leading to metering errors in the energy meter and easily causing other protection devices to malfunction; this frequency of current is harmful in the power system, affecting the quality of power. Therefore, many who have used this type of device have now abandoned it and are seeking new methods. Due to various reasons, using only one or two "theories" and methods as the working principle and judgment method of the device cannot guarantee the accuracy of protection line selection. Currently, although many enterprises and research institutes both domestically and internationally have developed and produced various similar protection line selection devices, their reliability is not high. Devices with an accuracy rate exceeding 90% are still few and far between. More efforts from enterprises and scientific and technological personnel are needed to explore this area. IV. Successful Cases of Reliable and Accurate Line Selection With the strong support of the Hubei Provincial Power Bureau and the Xiangfan Power Supply Bureau, Xiangfan Keneng Company cooperated with our unit, gaining not only technical information and testing equipment but also research funding. From the late 1980s, a group of engineering and technical personnel with long-term experience in power generation and distribution conducted nearly 20 years of research, deeply analyzing the reasons for the unreliability of such devices both domestically and internationally. They conducted on-site investigations of many substations and switching stations, acquiring a large amount of operational data and discovering various problems in the application of these theories. They summarized the changing patterns of current, voltage, and various interference signals under grounding conditions, and based on the principles of "fuzzy theory," comprehensively adopted multiple principles and methods such as "first half-wave," "reactive power direction," "harmonic current direction and magnitude," and "wavelet analysis." By combining various data collected during grounding status with operational experience, and utilizing fuzzy theory and computer technology, the system employs logical relationships, fuzzy weighted calculation methods, and self-developed software. On the hardware side, it utilizes large-scale integrated circuits and high-speed industrial control computers imported from the United States. Structurally, it features a distributed network structure with multiple processors working simultaneously, enabling real-time tracking, synchronous acquisition and processing of various data, and rapid determination of judgment results. Simultaneously, the company has successfully developed the LXMZ-10 busbar-type zero-sequence current transformer and cable-type zero-sequence current transformer, which are highly accurate and sensitive and are compatible with single-phase grounding protection devices. These advancements ensure the reliability of protection line selection from multiple perspectives. Since the successful development of the first-generation line selection device in 1989 (originally named: Single-phase grounding detector, Single-phase grounding fault online monitoring instrument), it passed the inspection and testing of the Low-voltage Electrical Appliance Quality Inspection and Testing Center of the Ministry of Water Resources and Electric Power in 1993. Furthermore, a technical achievement appraisal meeting was held under the joint organization of the Central China Power Grid Bureau and the Hubei Provincial Electric Power Industry Bureau, receiving high praise from more than thirty experts and engineers present. The project's key technical personnel have now comprehensively upgraded the device, making it more powerful and further improving the reliability of line selection. The accuracy rate has increased from 70-80% in the early stages to 100% now. It solves the needs of line selection for protection in systems with a completely ungrounded neutral point, as well as for systems with a neutral point grounded via an arc suppression coil or resistance. It can also meet the requirements for line selection when multiple lines experience simultaneous grounding in various combinations or segmented operation modes of multiple busbars. Even when the transition resistance during grounding is high or the system experiences resonance, it still achieves accurate line selection. This device has now been installed and used in over two hundred substations. User feedback indicates excellent performance, meeting the reliability requirements of the power system and reaching a world-leading level. Therefore, it fundamentally solves the global problem of poor reliability in single-phase grounding protection line selection in low grounding current systems, making an innovative contribution to improving the automation level of power systems. (The author of this article is an installation and maintenance engineer at Yicheng Power Supply Company.)