[b]1 Introduction[/b] In modern power grids, inductive loads such as electric motors account for a considerable proportion. While consuming active power, they also absorb a large amount of reactive power. The presence of reactive power not only leads to a decrease in generator output and reduces the efficiency of transmission and distribution equipment, but also increases network losses, seriously affecting power supply quality. Currently, developed countries such as Japan and the United States have compensation ratios of over 0.5 and trunk power factors close to 1.0, while my country's is only 0.45. In low-voltage system compensation, centralized compensation at substations is mainly adopted, while decentralized compensation is adopted for large electrical equipment. With the improvement of people's living standards, the electricity consumption of low-voltage users, especially residential users, has increased significantly. The recommended power capacity for residential design has reached over 40VA/m2. However, due to the relatively slow pace of upgrading and renovation of power distribution lines in factories, mines, residential areas, and military camps, the voltage at the end of the lines is far below the allowable range, making it difficult for non-lighting loads such as washing machines and air conditioners to work normally, and causing great damage to electrical equipment. Meanwhile, due to the large number of new electrical loads using rotating equipment such as motors and compressors and power electronic devices, the demand for reactive power is very high, resulting in a significant increase in line losses within the community. The main solution to this problem is capacity expansion, i.e., increasing the capacity of transformers and distribution lines to improve power supply capacity. However, capacity expansion involves large investments, extensive construction work, and long cycles. Furthermore, since reactive power at the end still needs to be provided by the centralized compensation system on the low-voltage side, the utilization efficiency of transmission lines remains low. Therefore, effectively reducing the reactive current in the lines not only increases the active power transmission capacity but also helps reduce line losses between the low-voltage side of the transformer and the end load, improving the voltage quality at the end. Researching and developing reactive power compensation devices for line terminals has clear economic significance and social benefits. [b]2 Basic Analysis[/b] 2.1 Low-Voltage Terminal Reactive Power Compensation In existing community power supply designs, low-voltage reactive power compensation cabinets are typically used for centralized compensation, located at the beginning of the low-voltage distribution lines, as shown in Figure 1. Compared to centralized compensation, terminal reactive power compensation is located at the load end of the low-voltage distribution line, directly providing the reactive power required by the load, thereby reducing the reactive power flow of the low-voltage distribution network and reducing line loss and voltage drop. The "Design Code for Power Supply Systems" (GB50052-95) states that "reactive power loads of electrical equipment with large capacity, stable load and frequent use should be compensated separately on-site." Data shows that under certain conditions, reactive power compensation for a 11 kW asynchronous motor is economical and reasonable. Based on a typical 8-story, 2-unit residential unit, the equipment capacity is approximately 200 kW, the calculated capacity reaches over 40 kW, and the typical power factor is 0.7 [3]. Therefore, setting up a separate reactive power compensation device not only meets the design specifications but also has a high input-output ratio. [img=455,156]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtjqzdhxb/2002-2/47-2.jpg[/img] 2.2 Advantages of Low-Voltage Terminal Reactive Power Compensation Compared with centralized compensation, low-voltage terminal reactive power compensation has many similarities with decentralized compensation in terms of operation effect and function. It can be summarized as follows [4~6]: (1) Line current can be reduced by 10%~15%, and line loss rate can be reduced by 20%; (2) Voltage loss is reduced, the voltage quality of electricity sales is improved, and the starting and operating conditions of electrical equipment are improved; (3) System capacity is released and the power supply capacity of the line is improved. Under the same power supply capacity, line investment can be saved. In addition, it also helps to reduce the load of upstream switches and contactors, and even reduce their capacity specifications. 2.3 Analysis of the Special Characteristics of Low-Voltage Terminal Reactive Power Compensation Studies have found that compared with the low-voltage centralized compensation method, terminal reactive power compensation has obvious special characteristics. First, the load fluctuation at the end of the line is large, and the proportion of the base load is small. The load amplitude varies greatly in different seasons, weekdays and holidays, and different times of day; second, the load capacity is small and the location is scattered, so the economic power factor of compensation is different from that of centralized compensation; third, terminal compensation generally does not have a reserved installation location, no dedicated personnel to manage it, and usually requires phase control. Terminal compensation is also significantly different from local compensation and decentralized compensation. Local compensation and decentralized compensation are mostly used for large-capacity single-machine loads. The compensation equipment is generally put into operation with the equipment and cut off when the equipment is shut down, and its detection, analysis and control are relatively simple. Therefore, traditional centralized compensation, local compensation and decentralized compensation equipment cannot meet the needs of terminal compensation, and the development of new terminal compensation devices is of great significance. Based on the above analysis, the terminal reactive power compensation device should have the following characteristics: (1) complete and perfect control and protection functions, high degree of intelligence, maintenance-free or low maintenance; (2) small size, light weight, suitable for wall embedding or wall hanging; (3) low cost and multi-functional. The device should have a wide range of functions, such as power quality detection including reliability, voltage quality, and frequency deviation, and should have a high performance-price ratio. [b]3 Main Research Content[/b] The research content of reactive power compensation for terminal lines can be divided into three main parts: theoretical research, applied research, and system implementation. Among them, the theoretical part mainly determines the specific location of terminal compensation, the optimal compensation capacity, and the system feasibility analysis, focusing on the first two parts; the applied research designs and proposes implementation schemes, scheme optimization, and safety and reliability analysis, focusing on completing system structure optimization and compensation control method determination; while the system implementation mainly completes line design, manufacturing, software programming, and comprehensive debugging. These are discussed below. 3.1 Determination of Compensation Location Determination of compensation location is the first step in reactive power compensation and an important part of reactive power optimization. In general residential areas or military camps, the installation location of low-voltage line terminal compensation is usually only three: at the main distribution box inlet of the residential building (N1～Nn), at the distribution box in the stairwell unit (M1～Mm), or at the meter box in the household (K1～Kk), as shown in Figure 2. Since most individual household loads are below 6kW, and reactive power demand fluctuates greatly, with short commissioning times and frequent switching, the terminal compensation locations are generally selected from the first two compensation points based on the total capacity of the line terminals. When a user's reactive power load is particularly prominent, a separate compensation control box can be installed. The specific location can be preliminarily determined through input/output analysis. [img=352,191]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtjqzdhxb/2002-2/47-1.jpg[/img] 3.2 Optimal Compensation Capacity Optimal compensation capacity is one of the main research contents of terminal line reactive power compensation. Before compensation, the power factor of the load is usually lagging and the value is relatively low. The "National Electricity Supply and Consumption Rules" stipulates the minimum power factor for different users during peak loads and adjusts electricity charges according to the power factor. Another part of the cost savings is the direct benefit from reduced line losses and the indirect benefit from reduced capacity expansion pressure. On the other hand, reactive power compensation requires increased equipment investment, operating costs, and additional losses. Therefore, determining the optimal compensation capacity is to find a balance between the two. The economic power factor varies depending on the installation location and load type, significantly impacting the reactive power compensation capacity and the minimum compensation group capacity. Generally, the higher the average reactive power of the user, the farther the compensation point is from the power source, the higher the cost of line losses, and the greater the compensation degree should be. 3.3 System Structure Optimization The terminal compensation device consists of the following parts: power detection, processing and storage, protection control, capacitor bank, etc., as shown in Figure 3. Since this device not only has reactive power compensation functions but also needs to consider voltage monitoring, frequency offset, reliability calculation, data security storage, and other new functions, and has comprehensive self-monitoring and protection characteristics, the complexity of the circuit and the amount of software calculation are greatly increased. Optimizing the system structure, especially the core part (shown by the dotted line in the figure), will help reduce system complexity, coordinate hardware and software modules, and improve the system's own safety and reliability. On the other hand, high-precision reactive current and voltage detection is a key aspect of reactive power compensation. Due to the need for phase-by-phase control, the three-phase current and voltage must be detected separately, which not only greatly increases the investment in hardware equipment, but also increases the size and weight of the compensation device, making this part account for a large proportion of the overall cost of the compensation system. Improving the detection link is one of the main contents of system structure optimization. [img=400,182]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtjqzdhxb/2002-2/48-1.jpg[/img] 3.4 Compensation Control Methods Currently, conventional compensation control mainly includes two types: AC contactor control and thyristor control. According to the type of compensation device, there are self-saturating reactors (SR), controllable saturating reactors (CSR), thyristor switched capacitors (TSC), thyristor switched reactors (TCT), thyristor controlled reactors (TCR), thyristor controlled capacitors (TCC), etc. [7,8]. By employing techniques such as equipotential bonding and zero-crossing current cutoff, the impact on thyristors and capacitor banks can be significantly reduced. Simultaneously, the switching control should consider the relationship between the required capacity and the capacity of the capacitor banks to be switched on/off, ensuring a one-step switching process to avoid repeated trial switching impacts on the power grid and extend capacitor life. It should be noted that when using TSC compensation equipment, since its output cannot be continuously adjusted, capacitor grouping significantly affects the compensation effect. To extend capacitor life, the switching frequency of each capacitor group should be minimized, and the number of switching operations for each group should be roughly equal. To achieve a high compensation degree while avoiding overcompensation, the group capacity should be as small as possible. There is a certain contradiction between these two aspects. Proposing a reasonable grouping strategy is also a research topic. On the other hand, the influence of different load curve types should be fully considered during grouping. [b]4 Conclusion[/b] Terminal line reactive power compensation is of great significance for reducing line losses within factories, mines, schools, residential areas, and military camps, and improving the power supply capacity of existing power distribution systems. Due to the special nature of terminal reactive power compensation, it is necessary to develop and manufacture dedicated compensation devices. This paper focuses on several aspects that need attention in the development of terminal reactive power compensation devices. [b]5 References[/b] 1 Liu Bo, Zhao Hongwei, Feng Puqiao. Discussion on some issues in urban power grid transformation. Journal of Chongqing Communication Institute, 2000, (1) 2 Liu Bo, Zhao Hongwei, et al. Research on intelligent reactive power compensation of low-voltage distribution line terminals based on neurofuzzy theory. Journal of Chongqing Communication Institute, 2000, (3) 3 Dai Yuxing (ed.). Electrical Design Manual for Civil Buildings. Beijing: China Building Industry Press, 1999 4 Cao Guangzu. Distributed and terminal reactive power compensation should be systematically emphasized. Low Voltage Electrical Appliances, 1999, (5): 27-30 5 Zhang Hongwu, et al. Analysis of several reactive power measurement methods. Electrical Measurement & Instrumentation, 1997, 34 (382): 1-3 6 Xu Baiyu. Re-understanding of reactive power and capacitor compensation. Guangdong Electric Power, 1995, (3): 43-45 7 Gong Chenglong, Xu Qiwen. Microcomputer-based detection method for reactive components in reactive power compensation devices. Electrical Engineering Technology, 1996, (7): 37-40 8 Xi Ziqiang et al. Research on thyristor-controlled static active reactive power compensator. Power Electronics Technology, 1998, (1): 43-45