The distribution transformer is the basis for contacting the power grid and users, and its operating status is directly related to whether the users can reliably use electrical energy, so it is necessary to perform data collection and real-time monitoring on it. At present, there are mainly two methods of data acquisition and communication for distribution transformers in China: wired and wireless. Wired communication methods include telephone lines, power line carriers, and optical fibers. They have shortcomings such as communication being susceptible to interference and low reliability, which is difficult to popularize. However, the radio station communication method requires higher costs due to the need for master station construction. With the rapid development of wireless communication technology and communication network, especially the maturity of GPRS technology based on GSM digital mobile communication system, it provides a new communication method for distribution transformer monitoring system.
This paper presents a design scheme of remote monitoring system for distribution transformer based on GPRS network, which solves the problem of remote transmission of distribution transformer parameters and realizes the functions of real-time data collection and real-time load monitoring. And based on the ARM controller, an advanced distribution transformer monitor is designed to provide real-time data for professional analysis of the power sector (such as load forecasting, line loss analysis, fault judgment, power quality, etc.), thereby improving the operation of the distribution network The economy and safety.
2. GPRS communication technology
General Packet Radio Service (GPRS) is a mobile packet service developed on the basis of the existing GSM digital mobile communication system. It uses packet switching technology to provide end-to-end, wide area wireless IP connection Compared with the GSM circuit-switched data transmission method, GPRS is particularly suitable for intermittent, sudden or frequent, small amounts of data transmission, and is also suitable for occasional large data transmission. Its main technical features are:
(1) High transmission rate. GPRS uses 4 different encoding methods: CS-l ~ CS-4, each user can use up to 8 time slots at the same time, the highest theoretical speed is l71.2kbps transmission rate.
(2) Real-time online. Even if there is no data to be transmitted, it remains connected to the network and is in the "always online" state.
(3) The core network layer of GPRS adopts IP technology to provide seamless connection with the existing data network.
(4) Low cost. In terms of network construction, it is only necessary to follow the existing GSM network for development. Since the data is sent and received in packets, the system usually charges by flow rate, users can always be online, and the service cost is low.
(5) The coverage of the network is very wide.
The use of GPRS is very extensive, including sending and receiving e-mails through mobile phones, browsing data on the Internet, etc. Applying GPRS to the distribution transformer monitoring system can not only meet the real-time requirements of data transmission of the distribution transformer monitoring system, but also save costs and be flexible and convenient to implement.
3. System structure
The system consists of monitoring terminal, GPRS communication network and monitoring center station, as shown in Figure 1. The transformer monitoring terminal is installed at the transformer site, collects the electrical parameters and some switching values ​​of the secondary terminal of the transformer, and periodically uploads the collected information to the monitoring central station located in the substation through a GPRS modem. When the three-phase power parameter of the transformer becomes abnormal, it is immediately uploaded to the central station Alarm information; at the same time, when the central station sends control commands to the monitoring terminal, the monitoring terminal receives commands through the GPRS modem and can control some switching values ​​of the transformer. The GPRS network is the communication bridge between the monitoring terminal and the central station. The collected information and alarm information of the monitoring terminal are uploaded. The central station control commands are issued through the GPRS network for wireless communication. The GPRS network and the Internet network are seamlessly connected. The central station accesses the Internet through the ADSL dedicated line to enter the GPRS network. On the one hand, it communicates with the monitoring terminal in two directions. On the other hand, it provides a visual interface to let users understand the remote transformer without leaving the house. 'S health status. When an alarm message appears in the transformer, the central station can promptly inform and notify the nearest duty officer to eliminate the fault, greatly shorten the troubleshooting time, and improve the reliability and safety of the distribution network operation.
Figure 1: System structure diagram
4 Distribution transformer monitoring terminal hardware composition and principle
The monitoring terminal is the core component of the system, and its main functions are:
(1) Over-current protection: Over-current protection of the distribution transformer is realized to ensure the reliability of power supply;
(2) Remote communication: realize the remote transmission of data from the terminal;
(3) Collect analog quantity: Realize to collect three-phase voltage and three-phase current;
(4) Meter reading: copy the electric value indication of the electronic meter.
The power distribution monitoring terminal is composed of ARM processor, ferroelectric memory, tripping and closing relay, meter reading RS485 module, button, display module, power AD chip ATT7022B, GPRS module and meter reading RS485 module. The block diagram is shown in Figure 2.
Figure 2: Principle box of distribution transformer terminal
The ARM processor reads and processes the internal data of ATT7022B through the SPI interface, and stores the obtained data and the electric energy reading value of the electric meter read through the RS485 module into the ferroelectric memory, and the GPRS module Send the data to the master server. The ARM processor judges whether the overcurrent protection generates a trip signal to protect the distribution transformer through the data of AD, and sends the alarm to the master station through the GPRS module.
4.1 CPU module
The CPU is the core part of the terminal. The ST company's STR710 processor has an ARM7 core. The ARM7 processor has a low-power 32-bit RISC processor, supports 16-bit compressed instruction set Thumb, and has embedded ICE-RT logic. Easy debugging and development. The STR710 processor has a wealth of peripherals and I / O interfaces. It has a large amount of program memory FLASH and RAM inside. The powerful ARM processor can handle many complex control applications due to its rich peripherals and numerous I.O. The / O port can simplify many peripherals, reduce costs, and has the advantages of low cost, low power consumption, and high performance. Therefore, STR710 can handle data collection and analysis, as well as control and coordination of communication and other work.
4.2 AD module
The AD module adopts the sampling module based on the electric energy AD chip ATT7022B. ATT7022B is a three-phase fundamental / harmonic energy measurement chip with high reliability, high accuracy and high stability. The power measurement accuracy is better than 0.1% in the dynamic range of 1000: 1, and the effective value measurement of current and voltage The accuracy is better than 0.5%. ATT7022B can separately measure the fundamental wave energy, eliminate the negative impact of harmonics on energy measurement, and provide a reference basis for the fair billing of the power sector. It integrates a 7-channel 16-bit high-precision ADC and a 24-bit high-speed DSP internally. The seventh ADC can be used to prevent electricity theft. A temperature sensor is integrated on the chip to output various electrical energy parameters required by the three-phase multi-function electric meter through the SPI communication interface. . ATT7022B can simultaneously give the total active / reactive energy, fundamental active / reactive energy, harmonic active / reactive energy, apparent energy and other parameters. The voltage acquisition method is a voltage sensor, and the current acquisition method is a current sensor.
4.3 Meter reading 485 module
The serial port of the STR710 processor's own peripheral device is connected to MAX3485 to 485 communication. Through the 485 communication and the watt-hour meter, the watt-hour meter reading value in accordance with the DL / T645 multi-function energy meter communication protocol is read out.
4.4 Data storage
Use ferroelectric memory FM3164 to save all kinds of data and prevent these data from being lost after power-off. These data include the data collected by the terminal, the IP address of the master station, the calibration parameters of the meter and other parameter settings. Ferroelectric memory can also ensure that the terminal clock runs when power is off. The STR710 processor uses the IIC bus to connect to the FM3164.
4.5 GPRS module
The GPRS module is based on the data transmission of the distribution transformer monitoring system. The stable transmission of the terminal plays a vital role in the normal operation of the system. Its working time is greater than 99% of the system running time, and its operating temperature is in the range of -15 degrees to 60 degrees.
The 232 interface of the ARM processor peripheral of the distribution transformer monitoring terminal serves as the communication interface between the processor and the GPRS module. The GPRS wireless communication module uses the ITM-100 enhanced version of SIMCOM. The module has a built-in TCP / IP protocol stack and supports CMNET access. The CPU uses the AT command set to control the GPRS module. The AT command set is used to send data to the master station in the form of TCP protocol. In actual operation, in order to ensure the reliability of the data, a supplementary test and retransmission mechanism is adopted.
5. Master station software and terminal software of distribution management system
The distribution master station software mainly provides the following functions:
(1) Operating curve: curve of voltage, current, active power, reactive power, power factor, electric energy, etc. at 96 points of three phases per day;
(2) Daily report: maximum and minimum voltage and current occurrence time and value, transformer running time, power supply quality analysis, power factor limit time, power outage record, daily meter reading, voltage qualification rate, etc .;
(3) Management functions such as load curve and electric energy measurement;
(4) Abnormal alarm function. The software flowchart 3 of the power distribution terminal is shown as follows:
Figure 3: Software flow chart of the power distribution terminal
6 Conclusion
GPRS communication has the advantages of fast access, low price, stable and reliable, and strong real-time performance. It has irreplaceable advantages over other communication methods. The distribution transformer management system based on GPRS realizes the functions of automatic meter reading, line loss analysis, distribution transformer monitoring and abnormal alarm for power companies. The system is well used in practice and the work is stable.
The author's innovations in this article: 1) The wireless remote monitoring of the distribution information is realized through the wireless data transmission function of the GPRS module. 2) A remote monitoring platform is built through reasonable software and hardware design, which can realize the safety, reliability, compatibility and economy of the power safety monitoring system.
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