Abstract: This paper introduces the composition and functional characteristics of an information-based intelligent management system for low-voltage power distribution. The effectiveness of the system's operation on the Beiliu City Power Supply Company's grid was analyzed, showing positive results. The system's application has improved the company's management level of the low-voltage power distribution network, enhanced power quality, created significant economic and social benefits, and contributed to the company's basic management standards. Keywords: Low-voltage power distribution, line loss analysis, power quality, GPRS network Introduction With the gradual improvement of urban and rural power grid transformation, power production has placed new demands on line loss management, improving power quality, and ensuring the reliability and safety of power supply. The scientific nature of power marketing management and the intelligent nature of power demand-side management (DSM) require accurate and scientific intelligent monitoring of the low-voltage side of distribution transformers, understanding their real-time operating status, and rationally managing them according to demand to improve power quality. To achieve real-time monitoring of equipment in the distribution network, including public distribution transformers, plant-specific transformers, distribution cabinets, substation outgoing lines, and other power collection and equipment monitoring points; to understand the operating status and harmonic levels of electrical equipment; and to provide a scientific basis for line loss analysis, load forecasting, voltage qualification rate statistics, power supply reliability, power supply scheme optimization, distribution network planning, user connection, and power quality management; and to improve the economic, efficient, and safe operation of the distribution network, Beiliu Power Supply Company and Wuhan Huajun Technology Co., Ltd., following relevant standards, regulations, and requirements, and based on extensive collection of user needs and suggestions, combined with the actual situation of urban and rural power grids in my country and drawing on the advantages of similar products at home and abroad, have successfully developed the "Information-based Low-voltage Distribution Intelligent Management System" (hereinafter referred to as the System). It is a multi-functional system integrating distribution transformer electrical parameter monitoring, power metering, harmonic monitoring, reactive power compensation, line loss analysis, wireless communication, and RTU (remote control, remote sensing, and remote operation) functions. 1. System Composition The system consists of a management system main station software, low-voltage distribution intelligent terminals, low-voltage distribution intelligent terminal cabinets, communication front-end units, database servers, WEB application servers, GPRS networks, etc., as shown in Figure 1. Figure 1 System Network Diagram 1.1 The management system main station software consists of an operating system, front-end communication software, large-scale commercial database software, WEB application service software, maintenance and development software, and report publishing software. It mainly performs functions such as data acquisition, data processing, data statistics, alarm handling, screen display, curve display, clock synchronization, access control, and WEB browsing. 1.2 The low-voltage distribution intelligent terminal is installed at the distribution transformer site. It integrates functions such as real-time transformer monitoring, dynamic reactive power compensation, harmonic monitoring, voltage qualification rate statistics, power supply reliability statistics, over-line alarms, and energy metering. Its data can be stored cyclically for two months and has multiple communication interfaces (RS232, RS485, infrared, GPRS, etc.). Data can be automatically uploaded to the communication front-end unit through the communication interface, and parameters can be remotely modified and software upgraded. This terminal can simultaneously perform the functions of a real-time monitoring instrument, reactive power compensation device, harmonic monitoring instrument, voltage monitoring instrument, and electricity meter. 1.3 Low-voltage distribution intelligent terminal cabinet: The low-voltage distribution intelligent terminal cabinet mainly consists of a low-voltage distribution intelligent terminal, reactive power compensation capacitors, composite switches, and protection devices. It can also be equipped with electricity meters and load switches according to the user's needs. The low-voltage distribution intelligent terminal can be used independently or as part of a terminal cabinet. 1.4 Communication front-end unit: Establishes a communication connection with the terminal, completes the acquisition and processing of terminal data, and stores it in the data server. It remotely enables software upgrades, parameter distribution, and clock synchronization for the terminal. 1.5 Database server: Runs a commercial database to store all data uploaded by the terminal, including measured data, harmonic analysis data, capacitor switching status, various statistical data, historical alarm data, alarm information, transformer parameters, reactive power compensation parameters, etc. 1.6 The WEB application server runs WEB application service software and system application software, providing the MIS network with information such as browsing graphs, reports, historical curves, and various alarm operation records through the WEB server. 1.7 The GPRS network is the public wireless network of the mobile communication company. This network enables network connection between terminals and communication front-end units, thereby achieving data transmission. The GPRS network has advantages such as short construction period, low investment, low communication costs, security, and stability. 1.8 The browsing workstation can be accessed from any computer connected to the local area network using an IE browser to log in to the system and perform operations such as equipment management, data query, line loss statistics, harmonic analysis, voltage qualification rate statistics, power supply reliability statistics, system maintenance, and alarm information query. 2 System Functions, Features, and Advantages 2.1 Overview The system's main functions include data acquisition, data processing, data transmission from the distribution transformer; capacitor switching; curve display; parameter setting; event alarms; report query and statistics; clock synchronization; access control; data retrieval; and WEB browsing functionality. 2.2 Data Acquisition The following data are acquired: ◇ Load data: active power, reactive power; ◇ Energy data: active power, reactive power; ◇ Power quality data: voltage, current, power factor, harmonics; ◇ Meter reading data: electricity meter data; ◇ Operating condition data: abnormal alarm status. 2.3 Data Processing ◇ Event alarms: voltage and current over-limit alarms, phase loss, power outage, three-phase imbalance, harmonic distortion, etc.; ◇ Calculation and statistical analysis: Statistical analysis can be performed on daily, monthly, and yearly maximum and minimum values ​​and occurrence times of distribution operation data, as well as line loss analysis, voltage qualification rate, and power supply reliability; ◇ Curves: Provides real-time and historical curve functions, displaying real-time trend curves of load and energy consumption. 2.4 Data Transmission ◇ Communicates with terminals and enables remote upgrades and parameter settings; ◇ Exchanges information with the power marketing management system; ◇ Exchanges information with other systems. 2.5 Application Functions ◇ Demand-side management and service support functions: load analysis and forecasting, distribution transformer information query service, execution of orderly power consumption plans, and online power quality monitoring; ◇ Power marketing management technical support functions: remote meter reading and online monitoring of electricity meter operation status; ◇ Marketing analysis and decision analysis: classification and statistical analysis of load and energy data, and line loss analysis; ◇ Report generation: able to automatically generate various daily, monthly, and annual reports. 2.6 Features and Functions of the Intelligent Terminal ◇ 32-bit embedded microprocessor (ARM) and large-screen LCD; ◇ Data that can be collected and displayed includes: voltage, current, active power, reactive power, apparent power, power factor, active energy, reactive energy, total harmonic distortion rate and 2nd to 19th harmonic content, harmonic histogram, and various alarm information; ◇ Automatically determines capacitor bank switching based on reactive power through intelligent calculations to achieve local reactive power balance and improve power factor; ◇ Large-capacity data storage, capable of storing 2 months of data; ◇ Embedded GPRS/GSM communication module and low-voltage power line carrier module; ◇ Reads electricity meters. 2.7 Communication Network Features ◇ Uses China Mobile's dedicated network, with data transmitted to the management platform via GPRS communication; ◇ High data security, unaffected by external interference; ◇ Short construction period, low investment, low operating costs, low resource consumption, and rapid and stable data transmission. 2.8 System Advantages ◇Comprehensive data collection, including voltage, current, active power, reactive power, power factor, energy consumption, harmonics, various alarm information, power supply reliability time, voltage qualification time, power outage time, and meter reading data. ◇Comprehensive functions, equivalent to establishing an intelligent management system integrating multiple functions such as a real-time distribution transformer monitoring system, a voltage, current, and harmonic monitoring network, a local reactive power compensation device, electronic energy meters, and meter reading terminals. ◇Threshold control is implemented for parameters such as voltage, current, harmonics, power, and frequency. When these parameters exceed the limits, alarm information is generated and actively reported to the management center, ensuring timely reporting of alarm information for management personnel to handle promptly and prevent accidents. 3. Effect Analysis The system began operation on the Beiliu power grid in July 2004 and passed the scientific and technological achievement appraisal organized by the Guangxi Science and Technology Department in November of the same year. In 2005, it won the second prize of Guangxi Power Grid Company Science and Technology Progress Award and the second prize of Yulin City Science and Technology Progress Award. In 2006, it won the second prize of Guangxi Computer Promotion and Application Achievement Award. We have successively installed low-voltage distribution intelligent terminals on 165 public transformers and 67 user-dedicated transformers. Among them, 100 public transformers have been equipped with reactive power compensation devices, with a total compensation capacity of 7270 kvar. Additionally, checkpoint terminals have been installed at the 10kV western urban area line switch in the substation. During three years of operation, the system has proven safe and reliable, achieving real-time monitoring and data acquisition of transformer parameters and 10kV line checkpoint parameters. The installed reactive power compensation devices dynamically and automatically switch capacitors, and the switching effect and device self-testing can be monitored in real time. The data provided by the system has provided decision support for our scientific management of the distribution network and the transformation of the county power grid, achieving good results: 3.1 Accurate data acquisition improves work efficiency. The system can comprehensively, accurately, and in real-time collect various operating data of the distribution transformers, such as three-phase voltage and current, neutral current, active power, reactive power, power factor, harmonics, etc. It can also collect active and reactive power at any time period with high accuracy. During operation, we conducted tests at three typical sites and compared the field data with the system monitoring data. The results showed that the error between the system's measurement results and the actual measured results was within acceptable limits. This solved the problems of heavy workload, poor real-time performance, and human error in manual field data collection, effectively improving work efficiency. 3.2 The system is powerful, providing a basis for scientific management. The system's management platform has powerful statistical analysis functions, capable of statistically analyzing operating data such as voltage qualification rate, power supply reliability, line loss rate, harmonic content rate, three-phase imbalance rate, and load rate at any time period, point, or line. It automatically generates various reports and graphs, allowing users to intuitively and quickly understand the operating status of the distribution transformers. Various analytical reports can also be designed according to user needs. During operation, based on the data provided by the system, our company readjusted 25 overloaded distribution transformers and 13 distribution transformers with low load rates, adjusted the tap positions of 77 distribution transformers with voltage exceeding limits, and adjusted the phase sequence of the incoming lines for 263 users, bringing the three-phase load of the distribution transformers to a basic balance. Simultaneously, based on the load conditions of the distribution transformers and the current status of the urban distribution network, the capacity expansion of distribution transformers and the upgrading of lines were scientifically planned and designed to make the load configuration in the urban area more reasonable. 3.3 Communication method: low investment, safe and reliable operation. The system adopts China Mobile's dedicated network, and data is transmitted in real time through the most secure and stable GPRS wireless communication method. Data transmission is fast and stable, and the operation is safe and reliable, unaffected by external interference. We make full use of China Mobile's existing wireless network and fiber optic channels, making reasonable use of their resources, which greatly saves the cost of building and maintaining channels. This communication method overcomes the shortcomings of power line carrier and spread spectrum communication methods, such as poor anti-interference, unstable data, and susceptibility to lightning strikes. In the two years since the system started operating, it has withstood the test of high temperature, high humidity and lightning weather conditions in the south, and there have been basically no data interruptions, loss or errors (except for China Mobile network failures), and the stability is good. 3.4 Strengthen harmonic monitoring and rationally select equipment. In the past, we only focused on harmonic monitoring of the 35kV and above power grid, and neglected harmonic monitoring of the low-voltage distribution network. However, after installing the low-voltage distribution intelligent terminal, real-time monitoring of the harmonic conditions on the 10kV lines and the low-voltage side of distribution transformers revealed that the total harmonic distortion rate of the low-voltage grid voltage was far greater than the harmonic voltage limit in the public power grid harmonics standard (GB/T14549-1993). Monitoring of some distribution transformers with pre-installed reactive power compensation switching devices showed that these devices amplified harmonics during capacitor switching, increasing the harmonic current injected into the grid and polluting it. These data provide a scientific basis for our distribution network management, power grid planning and design, and power equipment selection. Simultaneously, we added a harmonic blocking function to the setting of the capacitor switching control in the low-voltage distribution intelligent terminal. This improved the system's stability and reliability and prevented harmonics from being amplified by reactive power compensation capacitors and causing damage to the power grid. 3.5 Dynamic Reactive Power Compensation for Significant Energy Saving and Loss Reduction: The system can monitor reactive power compensation devices installed on the low-voltage side of distribution transformers in real time and can automatically or remotely control the switching of capacitors from the control center. The management platform allows users to query the reactive power compensation status of each distribution transformer, the operating time and switching status of each capacitor, and perform statistical analysis on the operating status for any time period. This solves the previous problems of not being able to monitor whether the reactive power compensation devices were switching on and off under normal conditions, whether the switching effect was ideal, and whether the switching devices were intact. For public transformers of 200kVA and above, we initially configured the reactive power compensation capacitor capacity at 30% of the transformer capacity. After a period of operation, analysis of the operating conditions revealed that some transformers had high reactive power demand and insufficient compensation capacity, while others had excessive compensation capacity, with some capacitors rarely in operation. Therefore, we adjusted the capacity of 11 sets of compensation devices, significantly saving costs. The 10kV West Urban District line in the city has 29 public transformers and 19 user-dedicated transformers. In April 2006, we installed low-voltage distribution intelligent terminals on all 48 transformers, and installed reactive power compensation devices on 26 of the public transformers, with a total installed capacity of 2145kvar. In 2005, the active power of this line was 27.504 million kWh, the reactive power was 12.957 million kWh, and the annual average power factor was 0.905. After the installation of the distribution intelligent terminals from April to June 2006, the active power supplied by the line was 5.3 million kWh, the reactive power was 2.025 million kWh, and the three-month average power factor was 0.934. Furthermore, from June 4th to 11th, we conducted an effectiveness test on the West Urban District line. On the evening of June 4th, all reactive power compensation capacitors on the line were remotely disconnected; on the morning of June 8th, all reactive power compensation devices on the line were restored to normal operation. Data analysis shows that the line loss rate was 8.32% from 00:00 on June 5th to 24:00 on June 7th, and 4.22% from 00:00 on June 9th to 24:00 on June 11th. The above experiments demonstrate that the addition and removal of reactive power compensation capacitors resulted in a 4.1 percentage point difference in the line loss rate. Without calculating the losses in the 110kV and 35kV transmission lines, and only considering the reduction in losses on the 10kV lines and distribution transformers, based on the line's active power of 27.504 million kWh in 2005, the reactive power compensation device can reduce losses by 1.1277 million kWh annually. At a purchase price of 0.4 yuan per unit of electricity, this translates to savings of 450,000 yuan. The entire investment can be recovered in approximately 3 years. 4. Conclusion The system improved the company's management level of the low-voltage distribution network, improved power quality, created significant economic and social benefits, and contributed to the company's basic management standards. These data provide a scientific basis for our management of distribution networks, power grid planning and design, and selection of power equipment. At the same time, we added harmonic blocking function to the setting value of the switching capacitor in the low-voltage distribution intelligent terminal control. On the one hand, it improves the stability and reliability of the system, and on the other hand, it avoids the harm caused to the power grid by the amplification of harmonics through reactive power compensation capacitor. References [1] Zhang Wenfei. Constructing a new generation of power information management system, Journal of Guangdong University of Technology, 2001.4 [2] Lü Runyu. High-order harmonics of power system. Beijing: China Electric Power Press, 1998.5 [3] Dong Yunlong, Wu Jie, Review of reactive power compensation technology, Energy Saving, 2003.9 [4] Lei Haibin, Yi Benshun. Design and implementation of power transformer load monitoring system based on GPRS. Instrumentation Technology and Sensors, 2005.4 [5] Zhu Jiong. Functions and characteristics of distribution transformer real-time monitoring system. Zhejiang Electric Power, 2001(5), pp58～60