Design and implementation of a distribution transformer monitoring system based on GPRS

1. Introduction

The distribution transformer is the basis for connecting the power grid and users. Its operating status is directly related to whether users can use electricity reliably. Therefore, it is necessary to collect data and monitor it in real time. At present, there are two main types of distribution transformer data collection communication in China: wired and wireless. Wired communication methods include telephone lines, power line carriers, and optical fibers. They have the disadvantages of being easily interfered with and having low reliability, making them difficult to popularize. The radio station communication method is expensive because it requires the construction of a master station. With the rapid development of wireless communication technology and communication networks, especially the maturity of GPRS technology based on the GSM digital mobile communication system, a new communication method has been provided for the distribution transformer monitoring system.

This paper proposes a design scheme for a remote monitoring system for distribution transformers based on GPRS network, which solves the problem of remote transmission of distribution transformer parameters and realizes the functions of real-time data acquisition and real-time load monitoring. An advanced distribution transformer monitor is designed based on ARM controller to provide real-time data for professional analysis of the power department (such as load forecasting, line loss analysis, fault judgment, power quality, etc.), thereby improving the economy and safety of distribution network operation.

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 circuit switching data transmission method of GSM, GPRS is particularly suitable for intermittent, sudden or frequent, small amount of data transmission, and also for occasional large data transmission. Its main technical features are:

(1) High transmission rate. GPRS uses four different coding methods: CS-1 to CS-4. Each user can use up to 8 time slots at the same time, and the highest theoretical transmission rate is 171.2 kbps.

(2) Real-time online. Even if there is no data to be transmitted, it remains connected to the network and is in an "always online" state.

(3) The core network layer of GPRS uses IP technology, providing seamless connection with existing data networks.

(4) Low cost. In terms of network construction, it is only necessary to continue to use the existing GSM network. Since data is sent and received in packets, the system usually charges according to traffic, users can always be online, and the service cost is low.

(5) The network has a wide coverage area.

GPRS has a wide range of uses, including sending and receiving emails 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 the distribution transformer monitoring system data transmission, but also save costs and achieve flexibility and convenience. [page]

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 on the transformer site, collects the electrical parameters and some switching quantities of the transformer secondary end, and uploads the collected information to the monitoring center station located in the substation through GPRS modem at regular intervals. When the three-phase power parameters of the transformer are abnormal, the alarm information is immediately uploaded to the center station; at the same time, when the center station sends a control command to the monitoring terminal, the monitoring terminal receives the command through GPRS modem and can control some switching quantities of the transformer. The GPRS network is a bridge for the communication between the monitoring terminal and the center station. The collection information and alarm information of the monitoring terminal are uploaded, and the control command of the center station is issued through wireless communication through the GPRS network. The GPRS network is seamlessly connected to the Internet. The center station uses ADSL dedicated line to access the Internet and enter the GPRS network. On the one hand, it communicates with the monitoring terminal in a two-way manner, and on the other hand, it provides a visual interface, allowing users to understand the operating status of the remote transformer without leaving home. When the transformer has an alarm message, the center station can be informed in time and notify the nearby duty officer to troubleshoot the fault, which greatly shortens the troubleshooting time and improves the reliability and safety of the distribution network operation.

Figure 1: System structure diagram[page]

4. Hardware composition and principle of distribution transformer monitoring terminal

The monitoring terminal is the core component of the system, and its main functions are:

(1) Overcurrent protection: Overcurrent protection for distribution transformers is realized, ensuring the reliability of power supply;

(2) Remote communication: realizing remote data transmission of terminals;

(3) Collecting analog quantities: collecting three-phase voltage and three-phase current;

(4) Meter reading: Read the electricity reading from the electronic meter.

The power distribution monitoring terminal is composed of an ARM processor, a ferroelectric memory, a tripping and closing relay, a meter reading RS485 module, a button, a display module, an electric energy AD chip ATT7022B, a GPRS module, and a meter reading RS485 module. Its block diagram is shown in Figure 2.

Figure 2: Schematic diagram of distribution transformer terminal

The ARM processor reads the data inside the ATT7022B through the SPI interface and processes it, and stores the acquired data and the watt-hour indication of the watt-hour meter read through the RS485 module into the ferroelectric memory, and sends the data to the main station server through the GPRS module at regular intervals. The ARM processor determines whether the overcurrent protection generates a tripping signal to protect the distribution transformer based on the AD data, and sends the alarm to the main station through the GPRS module.

4.1 CPU Module

The CPU is the core of the terminal. It uses ST's STR710 processor with an ARM7 core. The ARM7 processor is a low-power 32-bit RISC processor that supports 16-bit compressed instruction set Thumb and has embedded ICE-RT logic, making debugging and development easy. The STR710 processor has a wealth of peripherals and I/O interfaces, and has a large-capacity program memory FLASH and RAM. The powerful ARM processor can handle many complex control applications. Due to its rich peripherals and numerous I/O ports, it can simplify many peripherals and reduce costs. It has the advantages of low cost, low power consumption and high performance. Therefore, STR710 can handle data collection and data analysis, as well as communication and control and coordination of other tasks.

4.2 AD Module

The AD module uses a sampling module based on the energy AD chip ATT7022B. ATT7022B is a three-phase fundamental/harmonic energy metering chip with high reliability, high precision and high stability. The power measurement accuracy is better than 0.1% within the dynamic range of 1000:1, and the effective value measurement accuracy of current and voltage is better than 0.5%. ATT7022B can measure fundamental wave energy alone, eliminate the negative impact of harmonics on energy measurement, and provide a reference for fair billing in the power sector. It integrates 7 channels of 16-bit high-precision ADC and 24-bit high-speed DSP. The seventh ADC can be used to prevent electricity theft. The chip integrates a temperature sensor and outputs various energy parameters required by the three-phase multi-function meter through the SPI communication interface. ATT7022B can simultaneously give total active/reactive energy, fundamental active/reactive energy, harmonic active/reactive energy, apparent energy and other parameters. The voltage acquisition method uses a voltage sensor, and the current acquisition method uses a current sensor. [page]

4.3 Meter reading 485 module

The serial port of the STR710 processor's peripherals is connected to the MAX3485 and converted to 485 communication. Through 485 communication, it is connected to the watt-hour meter to read out the watt-hour indication of the watt-hour meter that complies with the DL/T645 multi-function watt-hour meter communication protocol.

4.4 Data Storage

Ferroelectric memory FM3164 is used to save various data to prevent these data from being lost when power is off. These data include the data collected by the terminal, the IP address of the master station, the calibration accuracy parameters 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 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 is crucial to the normal operation of the system. Its working time should be greater than 99% of the system operation time, and the working temperature should be in the range of -15 degrees to 60 degrees. The 232 interface in the ARM processor peripherals of the distribution transformer monitoring terminal is used as the communication interface between the processor and the GPRS module. The GPRS wireless communication module adopts the enhanced ITM-100 of SIMCOM. The module has a built-in TCP/IP protocol stack and supports CMNET access. The CPU uses the AT instruction set to control the GPRS module, and uses the AT instruction set to send data to the master station in the form of TCP protocol. In actual operation, in order to ensure the reliability of data, a retest and retransmission mechanism is adopted.

5. Power distribution management system master station software and terminal software

The power distribution master station software mainly provides the following functions:

(1) Operation curve: curves of voltage, current, active power, reactive power, power factor, electricity consumption, etc. at 96 points of three-phase power supply every day;

(2) Daily report: time and value of maximum and minimum voltage and current, transformer operation time, power supply quality analysis, power factor over-limit time, power outage record, daily meter reading, voltage qualification rate, etc.;

(3) Management functions such as load curve and electric energy metering;

(4) Abnormal alarm function. The software flow chart of the power distribution terminal is shown in Figure 3:

Figure 3: Software flow chart of power distribution terminal

6 Conclusion

GPRS communication has the advantages of fast access, low price, stable and reliable, and strong real-time performance, which is irreplaceable compared with other communication methods. Based on GPRS, the distribution transformer management system realizes automatic meter reading, line loss analysis, distribution transformer monitoring, abnormal alarm and other functions for the power company. The system is well used in actual application and works stably.

The author's innovations in this paper are as follows: 1) Wireless remote monitoring of distribution transformer 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.