A case of wireless spread spectrum communication technology applied to airport power supply and distribution system
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As the power supply and distribution management unit can save operating personnel, reduce operating costs, improve operational reliability, improve service quality, and improve work efficiency, power supply and distribution automation control technology has become an important technical means to ensure the safe operation of power grids today. The power supply and distribution automation monitoring system uses automatic control technology, computer technology, electric power technology, communication technology and other means to carry out real-time intelligent monitoring and control of the power grid, so that the power grid is always in a safe, reliable, high-quality, and efficient operating state. The ultimate goal is to improve the reliability and quality of power supply, shorten the handling time of accidents, reduce the scope of power outages, improve the traditional passive management, and improve the management level and work efficiency of the entire power supply and distribution system. Communication is an important part of the power supply and distribution monitoring system, and it occupies a very important position in it. It is responsible for data transmission and command release between the monitoring center and each substation RTU (Remote Teminal Unit). Task.
The reliability of communication technology, the strength of anti-interference, and the ease of operation and maintenance have also become one of the keys to the success of the power supply and distribution monitoring system, which is particularly prominent in the airport power supply and distribution monitoring system.
1. The characteristics of the airport power supply system and the communication technology problems arising from it
1. The characteristics of the airport power supply system
Most of our civil airports have a relatively complete power transformation and distribution system. The system is not only large in scale, diverse in equipment, but also complex in operation and requires high reliability. Taking Beijing Capital International Airport as an example, it has one 110/10kV total step-down station and one 35/10kV total step-down station, and more than 60 10/0.4kV substations. The number of substations is large, the distribution is wide, and the lines are long. Some of the communication navigation stations are located outside the airport area. The distance between the substations and the monitoring center for their power supply is more than ten kilometers, which brings a great deal to the operation and management of the power supply management department. difficulty. In order to discuss and study the application of automation technology in the airport power supply and distribution system to achieve monitoring, the Civil Aviation Administration of China listed the development of the Capital Airport power supply automation control system as a major civil aviation science and technology project in 1996 (No. MD.SE.1996.11).
2. Communication problems of airport power supply and distribution monitoring system
Since the airport’s power supply and distribution network has already taken shape, and in history, when the airport was constructed, some remote substations were not laid with control cables or communication cables. Substations (such as remote stations, etc.) still have problems with temporary land occupation. Therefore, the particularity of the airport environment determines that some substations cannot use wired communication. Carrier communication cannot be regarded as a good communication method because of its high cost, uneconomical, and poor anti-interference. Therefore, only wireless communication is the preferred method. The electromagnetic environment of the airport is complex. There are a variety of wireless communication equipment in the airport, including long wave, medium wave, short wave and other frequency bands. Many of them are used for air command, communication and navigation, and work intercom communication. How to solve this problem? The wireless communication problem of the system has become a key issue for the success of the entire power supply and distribution monitoring system.
2. Basic principles and characteristics of spread spectrum communication
1. The concept of spread spectrum communication
The frequency spectrum is the description of an electrical signal in the frequency domain. Spread Spectrum Communication, that is, spread spectrum communication, is a new development in communication. Together with optical fiber communication and satellite communication, it is known as the three high-tech communication transmission methods in the information age. It refers to the communication system that expands the frequency spectrum of the transmission information with a specific spreading function to become a wideband signal, sends it to the channel for transmission, and then compresses it by corresponding means to obtain the communication system of the transmission information.
The working principle of spread spectrum communication is shown in Figure 1.
2. Theoretical basis of spread spectrum technology
According to the Shannon (cEShannon) channel capacity formula in information theory, C=Wlog2 (1+5/, v), where C is the communication capacity (limit transmission rate), is the channel bandwidth, v is the noise power, and S is the signal power. This formula shows that when the signal-to-noise power ratio S/N of the transmitted signal drops in the Gaussian channel, the method of increasing the system transmission bandwidth can be used to keep the communication capacity C unchanged. For any given signal-to-noise power ratio, the transmission bandwidth can be increased to obtain a lower information error rate. If a signal with a bandwidth of one hundred times or even thousands of times the signal bandwidth is used to transmit the signal, the communication can be carried out safely under strong interference conditions. This is the basic idea of spread spectrum communication.
3. Features of spread spectrum communication
Spread-spectrum communication technology was originally invented for the military’s confidential communication. Because of its many unique advantages, it has been widely used in computer local area network interconnection, data communication, image transmission, power systems, microwave communication networks and other fields. Its characteristics are as follows :
① High reliability and low bit error rate (usually below 10.).
②The communication capacity is large, and the spread spectrum communication uses code division multiple access, and its frequency utilization rate is higher than that of the single-line carrier.
③Good security and confidentiality, the spread spectrum system can submerge the signal in noise, so it is suitable for working in areas with severe noise such as substations.
④ It can resist multiple interference and utilize the correlation characteristics of spreading codes to effectively suppress multiple interference.
⑤ Spread spectrum equipment is generally small in size, low in power consumption, low in transmission power, and relatively cheap.
⑥ The network construction period is short, and the networking is fast and flexible. It can form a point-to-point and point-to-multipoint system according to the needs.
3. Application of wireless spread spectrum communication technology in the power supply and distribution system of Capital Airport
1. Selection of remote demonstration station and determination of communication method
After discussion and research, we decided to select 5 remote substations with various characteristics for real-time monitoring in the first phase of the R&D project. The 5 remote stations are the Capital Airport 35kV main step-down station, 6# East Runway South Light Substation, 20# South Road Xili Dormitory Substation, 25# Faxintai Substation and 78# Airport Catering Building Substation. Except for station 6# using the existing control cable to use wired communication, the rest have decided to use wireless spread-spectrum communication. Install wireless spread-spectrum radio and omnidirectional antennas in the monitoring center, and install wireless spread-spectrum radio and directional antennas at remote stations. The antenna adopts point-to-multipoint full-duplex communication. Among them, station 25# is located outside the airport area, about 8km away from the monitoring center, and is surrounded by wireless short-wave antenna arrays for communication and navigation. The anti-interference of wireless spread spectrum equipment is undoubtedly a test.
2. The function of wireless spread spectrum radio
The wireless spread spectrum radio (SSWirelessModem) uses spread spectrum technology to work. It can realize point-to-point and point-to-multipoint data, voice, and image transmission, and can operate in full duplex. The transmitting station modulates the signal from the controller through the data reception controller, modulates it into a spread-spectrum signal and transmits it through the antenna. The receiving station demodulates the spread-spectrum signal received from the antenna, and then sends the signal to the controller through the data interface.
3. Selection and parameter setting of wireless spread spectrum radio station
After investigation and careful consideration, we selected the AirlinkS-band 64SMP radio station of Cylink, a world-renowned supplier of confidential products and wireless spread spectrum communication products. The Airlink series of radio stations have been approved and approved by the National Radio Regulatory Commission and are in line with the National Radio Management and technical standards are the first certified product among wireless spread spectrum equipment. The radio can work in a full-duplex Modem point-to-point and point-to-multipoint environment. The operating frequency ranges from 2400MHz to 2483.5MHz. It uses spread spectrum technology to provide data and voice channels. It supports 1.2, 2.4, 4.8, 9.6, 19.2 , 56, 64kbps full-duplex synchronous data and 1.2 to 19.2kbps asynchronous data, the communication distance is 33km to 48km, and it has the characteristics of error correction, transmission frequency and output power can be selected.
The system adopts a point-to-multipoint working mode. The monitoring center uses a Cylink64SMPModem and a pair of omnidirectional antennas, and each collection site uses a Cylink64SMPModem and a pair of directional antennas. The Modem of the monitoring center is set as the master unit (MasterUnit). The Modem of the acquisition site is set as the slave unit (SlaveUnit), and the master unit and the slave unit use the same radio frequency (RF) channel.
The schematic diagram of the wireless spread spectrum communication network of the power supply control system of the Capital Airport is shown in Figure 2.
The system parameters of the master and slave unit stations are:
(1) Monitoring center station radio
RF channel: 15
Transmit output power: 650 (28dbm)
Burst synchronization timing: internal string timing
Transmit clock source: DCE (ST)
Mode selection: asynchronous 10-bit
Pseudo-random code: 5
Data rate: 2.4kbps
RTS: ONRTS/CTS
Delay: 0 milliseconds
Transceiver mode: normal
Master/Slave station: Master station
(2) Collect live radio stations
RF channel: 15 launches
Output power: 650 (28dbm)
Burst synchronization timing: internal string timing
Transmit clock source: DCE (ST)
Mode selection: asynchronous 10-bit
Pseudo-random code: 5
Data rate: 2.4kbps
RTS: OFFRTS/CTS
Delay: 0 milliseconds
Transceiver mode: normal
Master/Slave station: Slave station
Fourth, the debugging of the communication system
At the beginning of the commissioning, we wrote a special radio test program, using the radio to communicate between computers in order to fully test the characteristics of the radio. First of all, we conduct a comprehensive test on the radio indoors, and all communication radio stations use small indoor antennas. During the point-to-point test, the data sent and received were completely correct, which far exceeded the accuracy parameters mentioned in the radio manual. However, during the point-to-multipoint testing process, it was found that the data was occasionally abnormal, but the bit error rate exceeded the accuracy requirements in the radio’s manual. After consulting relevant information and repeated exploration, it was later found that the slave radio sent the data after sending ” Before the “Clear to Send (CTS)” command, the delay must be at least 17ms, otherwise the radio transmission may be wrong. This discovery greatly improved the point-to-multipoint transmission accuracy and achieved the expected effect.
Then the test is transferred to the on-site environment for further testing. The communication stations all use the outdoor antenna of the collection site, but the communication is still carried out between the computers. At the beginning, the bit error rate far exceeded the indicators specified by the system. After analyzing and repeatedly adjusting the directional antenna, shortening the connection cable between the radio and the antenna, and improving the wiring plan, the communication quality has been further improved, and the communication quality has completely reached Prescribed requirements.
Through the test, I have basically mastered the use methods and characteristics of the radio. In summary, I have the following feelings: ①When adjusting the direction of the directional antenna, the lobes emitted by the directional antenna should be aligned with the receiving end of the omnidirectional antenna. Pay attention to the antenna At the same time of orientation, attention should be paid to the adjustment of the elevation angle. ②The connecting cable between the antenna and the radio should be shortened as much as possible. The connecting cable should not have any bends when it encounters a bend during the wiring, and the transition should be smooth to prevent signal radiation in the connecting cable, thereby causing signal superimposition. ③When debugging, test different combinations of transmitting frequency and PN code, and finally determine the combination of transmitting frequency and PN code that can achieve the best communication effect in the whole network. ④In point-to-multipoint communication, the master station is in the constant receiving and sending status, while the slave station is only in the constant receiving status. Data can be sent only when sending is requested. After sending the data, the “clear to send (CTS) )” before the command, the delay must be at least 17ms or more.
Five, concluding remarks
The introduction of wireless spread spectrum communication technology overcomes the unique communication difficulties of the airport power supply monitoring system, enabling the project to progress smoothly. The project has passed the acceptance and put into trial operation, and the system has been operating normally. Facts have proved that wireless spread spectrum communication technology is suitable for the airport power supply system. It can avoid the interference of the airport environment on wireless communication without affecting the existing communication system in the airport. The communication error rate is low, the installation is simple, and investment is saved. , Maintenance-free. Wireless spread spectrum communication indeed surpasses traditional communication methods in many aspects such as reliability, availability and survivability. Especially in some special geographical environments such as airports, it shows its unique advantages.
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