I. Introduction
With the development of science and technology, realizing the automation of medium and low voltage distribution networks has become a trend in the development of power systems. The low-voltage distribution network is the last link of the transmission and distribution system, and its degree of automation is closely related to the quality and reliability of the power supply. For this reason, this article discusses the necessity of the automation of the low-voltage distribution network and its implementation.
Distribution transformers are power devices that distribute electrical energy directly to low-voltage users. Their operational data are an important part of the basic data of the entire distribution network. Therefore, it is necessary to monitor its operating parameters and status. This article discusses the use of GPRS as a means of communication to establish distribution transformer monitoring systems.
In the communication of the distribution transformer monitoring system, digital radio stations, GSM short messages, and optical fiber access can be used. The advantage of digital radio stations is that in addition to the annual frequency fees, there is no extra cost for normal operation; the disadvantage is that it is affected by the terrain and climate, resulting in poor system reliability and real-time performance, and it cannot be reported actively. GSM short message way can realize initiative reporting; The disadvantage is to charge according to the bar, the operation cost is high, and the time delay is very long while the holiday short message center server is busy. Fiber-optic communication is stable and reliable, but it is inconvenient for construction costs, poor scalability, and maintenance of equipment. The GPRS communication avoids the above problems.
General Packet Radio Service GPRS is a new wireless data transmission service developed on the existing GSM system. The purpose is to provide mobile users with high-speed wireless IP or GPRS 1X services. The theoretical bandwidth of GPRS can reach 170Kbit/s. TCP/IP connection can be provided on this channel. It can be used for applications such as INTERNET connection and data transmission. GPRS uses packet switching technology. Each user can occupy multiple wireless channels at the same time. The same wireless channel can be shared by multiple users, and resources can be used effectively. GPRS allows users to send and receive data in the end-to-end packet transfer mode without the need to utilize circuit switched mode network resources. GPRS is always online, according to traffic accounting, thus providing an efficient, low-cost wireless packet data service. Especially suitable for intermittent, sudden and frequent, multi-disperse, small and medium-sized data transmission, but also for occasional large data transmission.
Second, the system structure
The development goal of the distribution transformer integrated management system is to establish a safe and reliable, capable of providing various advanced services, and an open platform that provides strong functions for the execution and implementation of applications and multiple independent operations on the platform. The system composed of application subsystems becomes a distributed, open, modular, and expandable distribution transformer integrated management system that meets the modernization management requirements of distribution systems. In addition to providing basic hardware technologies and operating system kernels, basic platforms also provide basic services such as database systems, information transfer, human-machine interfaces, and real-time environment applications. Each application subsystem based on the basic platform includes: distribution transformer management, line loss management, automatic meter reading, VQC function, harmonic data, purchasing power management, data query, automatic reporting, charts and interfaces with other systems, etc. . These subsystems realize organic horizontal and vertical integration on the basis of open system architecture (OSA) to form a complete distribution transformer integrated management information system.
The network structure of the GPRS distribution transformer monitoring system is shown in the figure below. The system is divided into four parts: the monitoring center master station, network proxy server, GPRS data transmission terminal, and monitoring terminal equipment. The main station mainly completes the human-computer interaction; the network proxy server is mainly responsible for the establishment of the network data link and the transparent transfer of data transmission and receiving; the GPRS data transmission terminal completes the sending and receiving of the information; the monitoring terminal equipment mainly completes the digital and analog quantities. collection.
In this system, the network proxy server can be connected with INTELNET public network such as ADSL, LAN, and the public IP or DNS dynamic IP+DNS resolution service. For security reasons, the monitoring center master station is not directly connected to the LAN. INTERNET online, through the network proxy server to be responsible for the establishment of the network data link and transparent data transfer.
After the GPRS data transmission terminal is powered on, it will actively access the network proxy server according to the IP address or domain name that is pre-configured in it, and establish a TCP/IP link through the proxy server and the monitoring center. The monitoring center master station maintains the IP address and ID number of each terminal that it accesses. When the master station requests data from a monitoring terminal, it will find the corresponding terminal based on the IP address and ID number. Send to the terminal, the terminal responds to the data through the GPRS data transmission terminal to the network proxy server port, through the port mapping forwarded to the monitoring center master station, which completes a responsive communication process.
The above method is implemented through the Internet (CMNET), so that the user's implementation cost is relatively low. If you need high reliability data transmission. China Mobile provides direct access to GPRS networks for this application. The monitoring center master station directly accesses the GPRS mobile station through a dedicated line and obtains a fixed IP address. The monitoring point data does not need to be routed to the Internet. The monitoring point DTU also uses a SIM card with a fixed IP address. The point-to-multipoint network constructed in this way has the characteristics of good real-time performance and high security, but the access cost is relatively high.
First, monitoring terminal
The power distribution comprehensive monitor adopted by the monitoring system is a high-tech product designed and manufactured according to the technical requirements of the power demand side management and the relevant standards of the distribution load monitoring. It adopts high-precision digital sampling processing technology, integrates data acquisition, display, storage, communication and other functions into one, and can use background software to display, print, and analyze the stored data in the form of charts or reports. Its technology is leading, powerful and reliable, and can be widely used in the monitoring of distribution automation systems. Its main functions are:
Basic measurement parameters:
There are nine types and more than thirty parameters for three-phase voltage, current, frequency, active power, reactive power, power factor, four-quadrant electric energy, and ambient temperature.
Technical Parameters
• Rated technical parameters
Rated voltage Un: 100V
Rated current In: 5A
Rated frequency Fn: 50Hz
• Measurement accuracy
Voltage / current / frequency: 0.2
Power factor / active power / reactive power: 0.5
Active/reactive power: 1.0
• Built-in flash memory capacity
Storage capacity: 1024k bytes, can achieve continuous 84 days electricity data recording (recorded every 15 minutes)
Accuracy
Voltage / current / frequency: 0.2
Power factor / active power / reactive power: 0.5
Active/reactive power: 1.0
System parameter setting
CT, PT ratio of the device
Data storage interval
System clock
LCD contrast
Overpressure parameters
Undervoltage parameters
Overcurrent parameters
Fourth, GPRS data transmission terminal
The GPRS data transmission terminal realizes the communication between the monitoring center master station and the monitoring terminal. KB3000 is used in this system. Features and characteristics are as follows:
A, function:
1. Standard AT command interface to facilitate program design;
2. TCP/IP Internet embedded protocol stack, easy interconnection;
3. RS232 interface mode, more convenient communication method;
4. You can upgrade the program through the serial port.
B, performance:
1. Use low-power high-performance CPU as processor, 16-bit internal data bus, 128KB SRAM, 512KB FLASH, high-speed processing protocol and large amounts of data;
2. Support full transparency and frame format data transmission;
3. Support TCP, IP, PPP, ICMP, UDP, TELNET, HTTP protocol stack;
4. A SIM card holder conforming to the ISO7816-3IC card standard, supporting GSM11.11 and GSM11.10 compliance, and specification of the SIM card;
5. Simplified interface design, using a common 232 interface and simple AT command interactive interface;
6. For a more convenient power connector, you can use an external power supply. Can also take power directly from the serial port;
7. The overall power consumption is less than 2W;
8. Perfected fake dialing in GPRS network, automatic restart after disconnection, remote wakeup and other functions;
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