The principle and application of medium voltage power line communication technology
Overview: This paper summarizes the research achievements in the related fields of medium-voltage power line communication at home and abroad in recent years, analyzes the channel characteristics of medium-voltage power line channels, and introduces the physical layer, MAC layer key technologies involved in medium-voltage power line communication and related The development of communication system.
Medium voltage power line communication (MV-PLC) technology refers to the use of medium voltage power lines (usually 10KV voltage levels) in the power transmission network as a signal transmission medium for the transmission of voice and data information. This technology was first applied to the automatic data transmission platform of the medium-voltage distribution network; in recent years, the medium-voltage power line broadband network access has attracted more and more attention due to its complete infrastructure, wide distribution, and low cost, especially It is in a remote rural area or a sparsely populated area and has strong practical value.
Application area of ​​medium voltage PLC
The medium voltage transmission network covers a large area and has many application fields. Medium voltage distribution automation is of great significance to the development of the national economy. Related applications include electricity load control, power grid operation monitoring, and centralized meter reading. Distribution network automation often has a large number of distributed nodes that need to be controlled and data collected, so there are higher requirements for the economics of data channels. The medium-voltage PLC technology transforms the traditional medium-voltage power grid into a data communication network. It has natural advantages in terms of construction cost, operation and maintenance costs. At present, it has been well applied in South Korea, the United States, Spain and other countries. Research and practice.
With the rapid development of the Internet, the Internet plays an increasingly important role in production and life, and PLC technology also occupies an important position in broadband network access methods. The access scheme from outdoor medium-voltage PLC to indoor low-voltage PLC is considered to be an ideal solution to the "last mile" problem of broadband access. This type of research has emerged in North America and has developed rapidly in recent years.
For the medium-voltage power line network, due to its already widespread distribution, it has become an ideal medium for high-speed network access in remote areas to shorten and eliminate the "digital divide" between urban and rural areas, and between developed and undeveloped areas. ". In remote areas or areas with low population density, it will be more difficult to achieve larger bandwidth digital communication services through other technical means than PLC in the short term; xDSL or communication optical cables widely used in urban areas are generally difficult to lay. These areas; satellite communication can be realized in some areas, but the low communication rate and the high cost of channel leasing and terminals have greatly restricted their large-scale applications; cellular represented by GSM, IS-95, WCDMA, etc. The communication technology itself is designed for application scenarios with high user density. If it is used in areas with low user density, it will cause serious waste of communication capabilities and high operating costs, making it difficult to be promoted and popularized. In developing countries, This contradiction is particularly prominent. As far as the domestic situation is concerned, China has a vast territory and a considerable number of rural areas are located in remote and sparsely populated areas. The development of communications in these areas is relatively lagging behind. According to statistics, the fixed telephone coverage rate of administrative villages in China is 94% ~ 97%, and the coverage rate of data communication is even lower. Due to the constraints of natural and economic conditions, if the existing communication methods are adopted, the improvement of information services in these areas will face more and more difficulties. The power supply rate of the administrative village is close to 100%, and the coverage of the power grid is the basic condition for regional development. Therefore, using the power transmission network to achieve broadband network access is a feasible, economical solution suitable for the national conditions. Schematic diagram of the structure of the access network system.
Figure 1 PLC access network system in the countryside
Research on Medium Voltage Power Line Channel
The original intention of power line design is to transmit power signals with a frequency of 50 to 60 Hz, and to extend its functions to voice or data signal transmission will face a complex electromagnetic environment. Therefore, for PLC technology, the research on power line channel characteristics is very basic and critical. In China, due to the more complicated distribution and load conditions of medium-voltage lines, relatively short lines between substations and relatively strong interference, the physical layer environment faced by medium-voltage power line networking communications is relatively complex and harsh. In recent years, relevant scientific research institutions have conducted fruitful research on the characteristics of medium-voltage power line channels. The author has also conducted several field measurements and analysis of the characteristics of medium-voltage power line channels in typical rural areas in northern China. These research results provide practical engineering applications. For reference.
In general, the negative impact of power line channels on digital communications mainly comes from three aspects: varying impedance characteristics; frequency-selective channel attenuation characteristics; colored background noise and multiple impulse noises.
The purpose of studying the impedance characteristics of the channel is mainly to achieve impedance matching between the channel and the transmitter and receiver. When the impedance is mismatched, it will cause a waste of signal energy and even a dumb signal point. Due to the many branches of the medium voltage distribution network, the load situation is complex, and the line impedance will vary with distance, frequency, and time, and the range of change is large, generally between tens of ohms and hundreds of ohms. Figure 2 is the medium voltage power line The curve of impedance characteristic with frequency. Therefore, when implementing broadband network access, impedance matching is more difficult; at present, the method of adapting the impedance in the wide range of the line side by sacrificing matching performance is usually used in the coupling technology, and some systems use impedance adaptive power amplification at the transmitter end. Equipment, etc., to achieve a better impedance matching effect.
Figure 2 Medium voltage line impedance characteristic curve
Channel attenuation characteristics have an important impact on the effectiveness of digital communications. The attenuation of the medium-voltage line is more serious than that of the low-voltage line, and experiments have shown that the average attenuation per 100m amplitude can reach 8dB ~ 11dB. At the same time, the attenuation of the medium-voltage line also shows obvious frequency selectivity, and at some frequency points or frequency bands, there will be deep transmission attenuation. The multipath effect caused by a large number of branch points is considered to be a main reason. Experiments prove that it is difficult to achieve a successful communication connection in these deeply attenuated frequency bands, and it must be avoided in the actual communication system.
The average noise power of the medium voltage power line is around -60 ~ -70dBm / Hz. Among them, the colored background noise power formed by the superposition of a variety of low-power noise is generally between -60 ~ -70dBm / Hz, in some frequency bands can be lower than -80dBm / Hz, its overall attenuation with increasing frequency, and the power The spectrum changes slowly, generally on the order of minutes or even hours; the narrowband noise mainly caused by other wireless communication signals such as broadcasting stations within the communication bandwidth, occupies an average bandwidth of 2k ~ 4kHz, and the power is higher, which is about 30% higher than the background noise ~ 50dBm / Hz, this kind of interference generally exists stably for a long time; the most impact on the communication effect is the impulse noise, this type of noise is randomly generated, the duration is very short, generally tens or hundreds of milliseconds, most of the power is high When the background noise is 10 ~ 30dBm / Hz, when impulse noise occurs, the data transmission in the noise band may have serious burst errors. Figure 3 shows the typical noise spectrum of the rural medium-voltage power lines in China in the 40k ~ 560kHz frequency band.
Fig. 3 Noise characteristic curve of domestic rural medium voltage line (40k-560kHz)
Key technology and application system of medium voltage PLC
The medium voltage power line channel is a very unstable, high noise, strong attenuation transmission channel. Efficient and reliable modulation and coding technology is very important for power line communication. At present, at home and abroad, a large number of researches and tests have been conducted on the application of various modulation techniques in medium voltage PLCs in accordance with the requirements of different communication scenarios for speed and reliability.
According to the research on the characteristics of the medium voltage channel, the noise power generally decreases with increasing frequency, but at the same time, the deep fading caused by the multipath effect is also more serious at the high frequency end. Therefore, when selecting the PLC carrier frequency, it is necessary to The line situation is a compromise between the two. A lot of practice at home and abroad has proved that the carrier frequency of 5k ~ 50kHz is more suitable for most medium voltage distribution automation systems. In the application of distribution automation, it is mostly one-way data transmission, and the main requirement is high reliability, and the real-time requirements are not very high. Therefore, generally lower transmission rates are selected, generally between 10bps ~ 1000bps between. In terms of modulation technology, the narrowband modulation method currently used the most, such as ASK, FSK or CPSK, has been widely used. In recent years, in order to realize data transmission under the environment of strong noise interference, spread spectrum communication technologies such as frequency hopping, direct sequence spread spectrum, and Chirp frequency hopping have also been introduced into the medium voltage PLC system.
In the scenario of using medium-voltage PLC to achieve data network access, due to the high communication rate, the channel utilization rate of the modulation and coding technology used, and the ability to avoid or counteract burst noise and impulse noise are all higher. Claim. At present, in the research of medium and low rate access networks, modulation methods such as BPSK and QPSK have been applied. In order to combat the channel characteristics of frequency selective fading, high-order error control coding is generally applied at the same time, which is the same as this modulation method. The combination of not-so-high frequency band utilization makes the system's communication rate more restricted. CDMA technology can effectively combat narrow-band noise and other interference in the transmission channel, but the higher processing gain required in the CDMA system is difficult to achieve on power line channels with severe frequency selective fading, so the advantages of the CDMA system are in PLC can't get full play. Generally, it is no longer applicable when the rate exceeds 1 Mbps. For higher transmission rate access networks, multi-carrier orthogonal frequency division multiplexing (OFDM) technology is considered to be the most suitable technical solution. OFDM modulates data with multiple mutually orthogonal carriers, transforming the serial data stream into parallel processing. It has a high channel utilization rate close to the Shannon limit; it can effectively resist the multipath effect, solve the problem of crosstalk between codes, and has a strong ability to resist burst interference; in addition, OFDM also provides flexibility in channel allocation The possibility of operation can avoid the frequency band of deep fading in the communication bandwidth; OFDM technology is widely used in high-speed PLC.
In the MAC layer protocol, the current research shows that the competitive CSMA / CA protocol with conflict avoidance, the TDMA-based non-competitive reservation protocol, and the hybrid protocol combining the two are more suitable for medium-voltage broadband access Network, and has been applied in the actual system.
In general, the PLC application system has gone through the development process of analog-single chip integrated circuit-modern digital signal processing technology. The starting point of the medium voltage PLC system is relatively high, and the solutions of DSP devices and special chips have been widely adopted. After the mid-1990s, the high-speed PLC chip industry has developed rapidly, and many foreign companies have researched and developed corresponding products.
Driven by the market, the medium-voltage PLC broadband access application system in North America has developed rapidly. Amperion has studied the high-speed data transmission technology on the medium-voltage power line between the power generation substation and the transformer, and the interface technology of MV-PLC and LV-PLC for one end, and optical fiber and wireless network for the other end, thus providing transmission from medium voltage to Low-voltage transmission, end-to-end PLC solution from outdoor access to indoor networking. At present, the main technology of the system has passed laboratory tests, and Amperion and its partners are actively promoting the business process of such access networks.
In recent years, the application of MV-PLC access, which has received widespread attention in rural areas, is still mainly at the stage of experimental systems. The physical layer of the access system introduced in [1] uses the BPSK modulation method and uses BCH coding, interleaving and other technologies to combat channel attenuation and burst noise. The MAC layer uses a hybrid protocol of CSMA / CD and TDMA. In the low-density area, the medium-voltage power line network transmission experiment achieved Internet access, and the transmission distance reached 4 km, but failed to meet the QoS guarantee required by VoIP. In [2], a full-duplex digital broadband communication scheme based on QPSK modulation was adopted on the 13.8 kV power grid in rural North America, and a TCP / IP connection of 2 Mbps was realized on the 17 MHz and 83 MHz frequency bands.
Conclusion
Medium-voltage power line communication technology has unique advantages and great development potential in medium-voltage distribution automation, broadband network access in urban and rural areas, etc. If it is widely used, it will have a positive role in promoting the development of the national economy. In recent years, relevant research and applications at home and abroad have been rapidly developed. It is reasonable to believe that with the progress and maturity of related technologies, medium-voltage power line communications will play an increasingly important role in the information society.
references:
1. J Anatory, NH Mvungi, MM Kissaka, Modeling IP Based Powerline Network for communicaTIon and interconnecTIons with Wireless networks for CommunicaTIon in non-urban and Low-density areas for developing Countries, Southern African TelecommunicaTIon Network & Application Conference, September 2003, George , South Africa
2. Sanderson W, Broadband communications over a rural power distribution circuit, Proceedings of the IEEE, 7-9 April 2000, Page: 497 ~ 504
3. Xing Zhimin, Hou Sizu, Li Jing, etc., "Measurement and Research on Channel Characteristics of Medium Voltage Power Lines," North China Electric Power Technology, 2005, 10
4. Amirshahi P, Kavehrad M, Transmission channel model and capacity of overhead multi-conductor medium-voltage power-lines for broadband communications, Consumer Communications and Networking Conference, 2005, CCNC, 2005 Second IEEE, 3-6 Jan. 2005, Page , 354 ~ 358
5. JJ Lee, Measurement of the communications environment in medium voltage power distribution lines for wide-band power line communications, ISPLC 2004, Zaragoza, Spain, Mar / Apr. 2004, pp: 69-73.
6. Cheng Xiaorong, Yuan Lusha, Hou Sizu, etc., "Test and Analysis of Medium Voltage Broadband Power Line Communication Access and Channel Characteristics," Power System Automation, 2005, 14.
7. Yin Xiaogong, Yin Boyun, Wang Jing, "Research on Medium Voltage Distribution Network Communication System and OFDM Terminal Design", Electrician
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