Remote monitoring of high voltage frequency conversion system through GPRS

1 Introduction

With the rapid development of modern power electronics technology and microelectronics technology, the continuous improvement of automation and intelligence, the research and application of medium-voltage and high- power variable frequency speed regulation devices have gradually matured.

The guarantee of reliability is the key to the recognition and promotion of high-voltage inverters . In order to meet this requirement, some inverter manufacturers have designed a way to use communication to transmit the real-time data, operating status and fault status of the site to the remote terminal to achieve remote monitoring and maintenance of the site equipment. In the past, this remote monitoring system was generally implemented by using the public telephone network, dialing through a modem, and cooperating with corresponding software to achieve monitoring. Its obvious disadvantage is that it can only communicate after dialing, and it cannot communicate "point to multipoint" at the same time. Other ways to achieve remote communication are to use satellites, microwaves, optical fibers or radio stations, and these methods not only cost a lot of money in equipment investment, but also come with high procurement costs and operation and maintenance costs.

With the advancement and popularization of wireless communication technology, GPRS networks have become ubiquitous, and the application cost has been reduced to a very low level. Using GPRS embedded in high-voltage inverters to achieve remote monitoring communication not only has the system's easy maintainability, scalability and security, but also has all the advantages of GPRS communication.

GPRS-based remote monitoring has the following characteristics:

(1) Quick login, always online, and wide coverage.

(2) High security: the system incorporates an encryption mechanism during data transmission, allowing data to be transmitted securely over the public Internet.

(3) High reliability, the system has error correction and retransmission mechanisms to ensure the integrity and correctness of the data. Secondly, the system has an automatic recovery function to ensure stable operation of the system without manual intervention when the GPRS network status is unstable.

(4) Multiple-user service multiplexing: different network users share the same set of GPRS channels, but only when a certain user needs to send or receive data will the channel resources be occupied. The system relies on the corresponding software to flexibly realize point-to-point and point-to-multipoint data transmission at the same time.

(5) Power saving function: the device can be in standby mode when not sending data, and log in to the network by timing or data trigger.

The comparison between gprs access and telephone dial-up access is shown in the attached table.

At present, the GPRS service launched by China Mobile Communications is becoming more and more mature, and the application of GPRS has gradually become a research hotspot in the field of monitoring. Moreover, users can further reduce the cost of GPRS communication in various forms such as monthly rental and monthly package according to their needs. Therefore, as a networking method that is currently and in the future developing on a large scale, GPRS has the characteristics of distributed, centralized, networked, and open, and will surely be more and more widely used in industries such as industrial control , environmental protection, water conservancy and hydrological monitoring, and alarm.

2 Implementation of GPRS remote monitoring

The following describes the implementation of remote monitoring through GPRS, and explains how to use AT commands to communicate with the GPRS module, and proposes the use of

The idea of ​​monitoring based on the (browser/server) architecture is presented, and the experimental steps and application examples are given.

2.1 GPRS remote monitoring system

The GPRS remote monitoring system consists of an intelligent controller , a GPRS communication module, China Mobile's communication network (GPRS network), the Internet public network, a data server, an enterprise's local area network , and its computer equipment. The high-voltage inverter GPRS remote monitoring system is shown in Figure 1.

First, the high-voltage frequency conversion system collects the field parameters, encrypts and compresses them, and then connects them to the GPRS communication module in the form of a data stream through a serial mode, and communicates with the China Mobile base station. The base station SGSN then communicates with the gateway support node GGSN, and GGSN processes the packet data accordingly. The GPRS module sends the data to China Mobile's internal network (CMNET) through the GPRS network in the form of a GPRS data packet, and then China Mobile sends the data to the Internet through the GPRS service node (GSN), and searches for a server with a specified IP address on the Internet. Since the GPRS network works based on IP address addressing, the Internet data server on the public network only needs to be simply connected to the Internet and have an IP address assigned by the public network. The data server uses Win Sock programming to realize the sending and receiving of network data and forward it to the database of the internal network. The database facilitates data maintenance and webserver calls. The system can also realize the reverse transmission of data and instructions to achieve the purpose of remote control. The workstation accesses the web server through the Internet to browse the detailed information of each monitoring point.

The remote monitoring system based on GPRS relies on the mobile communication network. After the on-site parameter collection is completed, the GPRS terminal communication program and related software can be written according to actual needs.

2.2 Implementation of GPRS Remote Terminal

The data collection of the gprs remote terminal is shown in figure 2.

The GPRS remote terminal is mainly composed of a data collector and a GPRS module. First, the control unit of the high-voltage inverter will display and control the collected industrial field parameters on the spot, and send them to the GPRS module to control the operation of the GPRS module itself, and send the encrypted and fault-tolerant data to the data server in the data mode.

The communication protocol between the high-voltage frequency converter and the GPRS module is a serial port protocol. The high-voltage frequency converter sets the GPRS module through the serial port. The setting instructions before establishing the data channel include:

at+cgdcont=1,"ip","cmnet"

Description: Initialize the network session, create a profile account on the module, and access the China Mobile GPRS network.

Return: OK

at×e2ipa=1,1 Description: IP connection on the current session

Return: OK

at×e2ipo=1, "218.79.163.201", 5000

Description: Initialize a TCP connection, 218.79.163.201 is the server IP address.

Return: connect indicates that the connection is established successfully.

After the connection is established, the GPRS module enters the data state and the high-voltage frequency conversion device can send and receive information data.

2.3 The structure of remote monitoring system

2.3.1 Server

In the remote monitoring system based on GPRS, the data server sends and receives data with the GPRS terminal through the socket based on the TCP/IP protocol. The data is processed by the software developed and run on the server, and then updated to the SQL database, so that legal users can view the data through the browser at any time and anywhere. The browser views the information just updated to the database by the remote device through the Internet, achieving the functions of on-site data analysis and display. Its workflow is shown in Figure 3:

2.3.2 Browser

No special software is required on the browser side, the system's built-in Internet software can access the web interface on the server .

2.4 Communication test of gprs monitoring system

(1) First, insert the SIM card (disconnect the power supply when inserting the card ), connect the antenna, and connect the GPRS module to the computer serial port using an RS-232 serial cable. Turn on the GPRS module power supply, and when the signal indicator starts flashing, it means that the module has been turned on and has successfully registered the GSM network.

(2) The GPRS device is powered on. After the device performs a self-check, it enters the command mode and the indicator light shows that it is working normally.

(3) The gprs module can be set up through third-party software to configure parameters or through the hyperterminal . Send the at command at+cgdcont=1, "ip", "cmnet". If there is data, press Enter and it will return OK. This means that the hardware connection is correct. You can also test it with your own software. Figure 4 is the screen of the hyperterminal connecting to gprsdtu.

(4) Start the application server socket, assuming that the server IP address is 218.79.163.201 and the port is 5000.

(5) Use at×e2ipo=1, "218.79.163.201", 5000 to set the data server IP address and port number. Press Enter and return to connect to indicate that the connection is successfully established.

(6) The server side uses software written by winsock to send and receive data, and the gprs side uses the hyperterminal to simulate the remote device to send and receive data.

(7) The server runs the upper-level software program to store the data transmitted by GPRS into the Mic ROSoft Access database and display it.

(8) The client launches the browser to view and modify the contents of the database on the server.

3 Server-side software

The upper software of this system is configured by using the general version of Beijing Kunlun Tongtai's mcgs configuration software. Since the configuration software integrates the gprsdtu driver , it is relatively easy for users to use. As long as the various parameters are set during configuration and the project file is run, the real-time connection between the server and the remote monitoring unit can be realized through gprs.

The remote high-voltage inverter and the gprs module have been connected with the serial port, and the corresponding settings have been made. The local server sends a call command to the gprs module every 5 seconds. After receiving the call command and verifying it, the gprs module transmits data to the server, including the "actual frequency", "stator voltage", "stator current", "pressure" and some state quantities and alarm information of the system operation, etc., which are saved by the server through the real-time database and the historical database. Users can view all the data on the site through the browser and can learn about the current operation of the inverter. They can also adjust some current parameters of the inverter according to the actual situation, and pass the modified parameter values ​​to the high-voltage inverter through the gprs module, thereby realizing remote monitoring and maintenance of the high-voltage inverter equipment, greatly improving the reliability of the system operation, making the operation more flexible, and reducing maintenance costs. Users can also compile software for historical data query, reporting, statistics, trend analysis, printing, etc. to achieve personalized operations.

3.1 Main Screen

After opening and running the configuration software, a screen as shown in Figure 5 will appear:

The buttons in the main screen include refresh, alarm view, historical data, other indications, curve, parameter view, and serial port close. The refresh button is used to open the serial port and call gprsdtu; when the serial port close button is pressed, the call to gprsdtu is closed, that is, no new data will be uploaded; when other buttons are pressed, the corresponding screen is entered. There is an alarm description status bar in the main screen, which is used for alarm information. The indicator lights in the main screen include permission, external control, closing, ready, connection, operation, bypass, alarm, remote control box, DCS , closed loop, and the display bar in the main screen is used to display the set frequency, actual frequency, stator voltage, stator current, and pressure. All these displayed data are uploaded through gprsdtu.

It should be noted that the displayed values ​​of date and time in the main screen are the time of the last GPRS DTU upload of the lower computer, not the running time of the upper computer.

3.2 Other instructions

When the other indication button is pressed, the other indication screen is entered, as shown in FIG6 .

There are three areas in the other indication interface: in the setting area, set the calling cycle of each call to the lower computer and the time for the scheduled call of the lower computer three times a day; in the time area, display the current time of the upper computer system and the running time of the lower computer uploaded by the lower computer for the last time; in the indication area, display the high-voltage closing status of the inverter of the lower computer and the communication status of the upper computer. If the upper communication termination indicator light is on, it means that the upper computer has stopped calling the lower computer and the lower computer will no longer upload the latest data.

3.3 Curve

When the curve button is pressed, the curve screen is entered, as shown in Figure 7. At this time, the real-time curves of voltage and current are displayed on the screen. Press the return button to exit the curve screen.

3.4 Parameter View

When the parameter view button is pressed, the parameter display screen is entered, as shown in Figure 8. This interface is mainly used to display the parameter settings of the lower computer and the frequency step settings. Enter the lower computer parameter number to be searched in the "p parameter number" input box, and the parameter value corresponding to the parameter number will be displayed in the "p parameter current value".

3.5 Alarm View

When the alarm view button is pressed, the alarm information browsing table is entered, as shown in Figure 9. This table is mainly used to store the fault alarm conditions of the lower computer.

3.6 Historical Data

When the historical data button is pressed, the saved data browsing table is entered, as shown in Figure 10. This table is mainly used to store real-time data uploaded by the lower computer.

3.7 Excel report output

In the menu bar of the configuration software, there is an Excel report output button. Pressing this button will open an Excel spreadsheet that has stored data, as shown in Figure 11. Users can process and print data on the spreadsheet.

4 Conclusion

This article introduces a solution to achieve real-time connection monitoring between high-voltage inverters at remote sites and local data servers through GPRS technology. The data at the remote inverter site can be transmitted to the local server for display, processing, and storage via GPRS, and remote fault diagnosis can also be performed. When necessary, the local operator can also control the remote inverter. The system has been put into actual operation and has achieved good results through remote customer contract energy management .