"1 Characteristics of SCADA system
SCADA system generally adopts decentralized measurement and control and centralized management. The whole system is composed of monitoring center, several decentralized remote measurement and control terminal RTU (remote terminal unit) and communication medium. The monitoring center, also known as the master station, is the core of SCADA system and is responsible for controlling and managing the operation of the whole system; RTU, also known as peripheral station, is an independent intelligent measurement and control module using microprocessor or DSP to complete the collection and processing of various remote field data, the control of field actuator and the communication with remote control center. It has the characteristics of easy expansibility and easy maintenance; Communication media can be implemented in different ways according to actual requirements and application objects. The following is a brief analysis.
2 selection of communication media
Data transmission media are divided into wired and wireless. Wired transmission modes, such as power line carrier, RS-485 fieldbus and PSTN public telephone network. Wireless transmission modes, such as VHF / UHF radio station, ISM spread spectrum radio station, GSM mobile phone network, satellite communication network, etc. Each method has its own characteristics.
The power line carrier uses the existing power supply line without laying another special communication line, but the inherent interference of the power line is large, and the carrier signal can only be transmitted in one distribution transformer area, and the communication tracking is short; RS-485 fieldbus has the advantages of high communication efficiency and good reliability, but when it is used in large capacity system, the project cost of laying special line is too high.
The initial investment cost of PSTN public telephone network and GSM mobile telephone network is small and there is no blind area coverage, but the accumulated operation cost is very high; Spread spectrum radio (2.4GHz) and microwave (above 4GHz) in ISM (industrial, scientific and medical) band can be used for long-distance and high-performance transmission, but the price is expensive.
VHF / UHF radio station: it has a long transmission distance, only needs maintenance cost without operation cost, and the system use and site expansion are very convenient. It is especially suitable for large-capacity distributed SCADA system with high performance price ratio, but it can be used only after applying for corresponding frequency points from the local radio management department.
It can be seen that wired and wireless methods have their own advantages and disadvantages. The advantage of wired communication lies in the reliability of data transmission, but when RTU is scattered, the cost of auxiliary dedicated lines in measurement and control system is too high; Wireless communication has obvious advantages in high-capacity SCADA system. The urban street lamp monitoring system adopts VHF / UHF radio communication mode, which has been running in Nanjing, Fuzhou, Changchun and other provincial capital cities for several years, and the customer has responded well. Practice has proved that as long as the key problem of data transmission reliability is solved, the normal and reliable operation of the whole wireless communication SCADA system can be guaranteed.
3 design of SCADA system for wireless communication of urban street lamps
The overall structure of the system is mainly composed of three parts: monitoring center, several peripheral stations RTU and VHF / UHF radio station for data communication. Tait 855 / 856 base station is selected for the monitoring center, and Motorola GM300 vehicle radio is selected for peripheral stations. The networking structure of urban street lamp wireless communication SCADA system is shown in Figure 1.
3.1 composition and function of monitoring center
The control center of urban street lamp wireless communication SCADA system is mainly composed of main control computer, backup computer, server, large screen multimedia projector, radio, tower and antenna.
The main control computer and the server are networked according to the client / server structure. As the client of the system, the main control computer is composed of an Intel industrial control computer and programmed with VB language. Its main function is to realize the man-machine interface, connect with the radio through the wireless modem, send control instructions to RTU and receive the returned detection data and status parameters. In order to improve the reliability of the system, another Intel industrial computer is used as the backup computer to back up the main control computer.
An IBM server is used as the database server of wireless communication SCADA system. The data collected by RTU is processed by the main control computer and stored in the data server. Other management systems are authorized to directly access the server, which can realize data sharing between systems.
The monitoring center mainly performs the following functions:
(1) The large screen multimedia projector dynamically displays the location of each RTU in the city map and the working status and real-time parameters of the station in the SCADA system (including AC voltage, current, power, watt hour meter reading, lighting rate, etc. of each line).
(2) According to the correction degree and sunrise and sunset time of the city, formulate the daily on / off time curve corresponding to the whole year, so as to control the on / off of street lamps in real time; In addition, the controller can modify the on / off time according to the specific situation and transmit it to RTU for control.
(3) Regular patrol inspection / fixed-point detection of RTU, and obtain the switch status, data parameters and alarm code returned by RTU through wireless modem.
(4) In the process of patrol inspection, in case of fault or parameter out of limit, audible and visual alarm can be carried out, and the enlarged map with the station location as the main center, corresponding working conditions, measured data and fault form can be dynamically displayed on the large screen.
(5) It has the function of saving energy and implements two working modes of all night light / Midnight light. The controller decides whether to adopt the midnight light mode and arbitrarily adjust the start and end time of midnight light for each line. At the same time, it also has the anti stealing monitoring function: when it is detected that the current value of a branch is greater than the normal operation value, if the current value of other branches in the same RTU is normal (indicating that the A / D chip works normally), the branch has the phenomenon of power loss.
(6) The virtual instrument method is used to display the shape of voltmeter, ammeter, watt hour meter and other instruments, and the VB control meter is used to visually display the dynamic change of analog data on the instrument panel.
(7) Timing all RTUs regularly to ensure the time accuracy of peripheral stations.
(8) Data processing, storage and report printing functions.
3.2 wireless communication
3.2.1 communication media
VHF / UHF radio station is used for data transmission between the monitoring center of wireless communication SCADA system and RTU. The working frequency is 203 ~ 450hz (corresponding frequency point must be applied to the local radio management department). The modulation mode of data communication is FSK and the transmission rate is 300 ~ 1200BPS. RTU stations are scattered and far away from the monitoring center, so the communication system adopts large area system. The monitoring center adopts Tait 855 / 856 base station with transmission power of 45W. It works in full duplex mode. The coverage is related to the height of high gain omnidirectional antenna tower, and the radius can reach tens of kilometers. The RTU adopts Motorola GM300 vehicle radio, equipped with directional antenna, with transmission power of 25W and working in half duplex mode.
3.2.2 communication mode
The monitoring center sends commands to RTU in broadcast mode or point-to-point mode. When broadcast mode is adopted, RTU only receives and executes commands, such as proofreading time, modifying on / off time and so on; When point-to-point mode is adopted, such as telemetry, remote control and other commands, RTU shall confirm the station number first. If yes, it shall perform corresponding operations according to the command format, such as on / off control circuit, return current, voltage, switch status, fault code data frame, etc. to the monitoring center.
3.3 RTU structure module design of peripheral station
The RTU structure diagram of peripheral station of wireless communication SCADA system is shown in Figure 2.
RTU is mainly composed of MCU measurement and control system, including data acquisition and processing and a / D conversion circuit, keyboard display circuit, clock circuit, street lamp all night / Midnight lamp control circuit and wireless communication circuit. All peripheral stations can operate independently. Even if the monitoring center cannot be contacted due to communication failure, the street lamp system can be monitored independently.
The MCU measurement and control system uses 80C31 as CPU, 27c256 as EPROM and 62256 as RAM. In the address decoding circuit, 74LS138 and p2.7 are respectively connected with 62256 chip selection signal (zero selection) and 74LS138 pin G1 (1 selection), P2.4, p2.5 and p2.6 are respectively connected with 74LS138 pins a, B and C, and the output chip selection signal controls multi-channel switch, ICL7109, DS12887, 74hc245 and LCD.
(1) Data acquisition and a / D conversion circuit
RTU real-time measurement parameters include line voltage, current, watt hour meter reading, etc. The analog input signal of the module has three-phase AC voltage UA ~ UC. The total current of the line is ia ~ IC, the current of each branch (the system can detect 8 branches at most) is i1a ~ i1c,..., i8a ~ i8c, and the signal passes through V / I transmitter and multiplexer (4051 & MES); 4) the signal processing circuit (including the conversion of AC signal rectification and filtering) is sent to the double integral A / D converter ICL7109 and converted into twelve bit binary numbers, in which the lower eight bits D1 ~ D8 are connected with p0.0 ~ p0.7, and the upper four bits D9 ~ D12 are connected with p0.0 ~ p0.3. The CPU reads the upper four bits and lower eight bits from the data bus by controlling the high / low byte enabling terminals hben and LBEN respectively.
(2) Keyboard display circuit
Liquid crystal display is adopted. The key functions mainly include: setting station number, time, current change value of each branch, current / voltage zero adjustment correction, all night light / Midnight light mode, patrol inspection / fixed-point detection of parameters of each branch, etc.
(3) Clock circuit
The clock chip DS12C887 is used to provide accurate clock signals, including year, month, day, hour and minute. The time can be modified manually or uniformly calibrated by the monitoring center.
(4) Street lamp on / off circuit
The on / off of the street lamp line of the station can be automatically executed through the daily on / off time solidified in EPROM, or the line on / off can be realized through manual setting or remote control by the monitoring center.
(5) Wireless communication circuit
The modulation / demodulation chip tcm3105 demodulates the received analog carrier signal into digital signal and transmits it to the CPU, modulates the digital signal to be transmitted into analog carrier signal and transmits the data through GM300 vehicle radio, directional antenna and monitoring center.
4 precautions for realization of street lamp wireless communication SCADA system
The urban street lamp wireless communication SCADA system using VFH / UFH radio communication has been in normal operation for several consecutive years. Experience shows that as long as the reliability of wireless data transmission is solved, this method is obviously superior to wired transmission in urban large capacity monitoring system, with high performance price ratio, and RTU can be easily expanded at will. The author believes that the following aspects should be paid attention to in the process of design, commissioning and installation:
(1) Pay special attention to the field strength test of wireless signal. Before the installation of peripheral stations, the signal shall be sent by the base station of the monitoring center. The field strength test shall be carried out at each RTU installation, there shall be no obvious co frequency interference and different frequency intermodulation interference, and the signal level shall be more than 20dB. If a RTU cannot meet the requirements, its address must be changed; When most RTUs cannot meet the requirements, the operating frequency of the radio station must be changed.
(2) Do not select too high communication baud rate, otherwise it is easy to cause code loss.
(3) During the debugging of a / D conversion circuit, it is found that the pins 17 (test) and 27 (send) of ICL7109 chip cannot be suspended, and should be connected to + 5V with 26 (run / hold). Otherwise, the A / D conversion data will often be unstable.
(4) The V / I transmitter used to measure three-phase voltage cannot be directly installed on the main board, otherwise it is very easy to burn the main board by lightning.
(5) In mountainous areas, if the wireless signal is affected, an interrupt station can be set up.
Reprinted from - viku electronic market network
wireless communication
A / D converter
ICL7109"
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