Abstract: This paper summarizes the research results in the field 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 key technologies of the physical layer and MAC layer involved in medium-voltage power line communication and the development of related communication systems.
Keywords: medium voltage; power line communication; distribution automation; broadband network access
Introduction
Medium voltage power line communication (MV-PLC) technology refers to the use of medium voltage power lines (usually 10KV voltage level) in the power transmission network as a signal transmission medium to transmit voice and data information. This technology was first applied to the automated data transmission platform of the medium voltage distribution network; in recent years, medium voltage power line broadband network access has attracted more and more attention due to its complete infrastructure, wide distribution and low cost, especially in remote rural areas or sparsely populated areas, and has great practical value.
Medium voltage PLC application field
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 power load control, power grid operation monitoring, centralized meter reading, etc. Distribution network automation often has a huge number of nodes that need to be controlled and data collected, so it has high requirements for the economy of data channels. Medium voltage PLC technology transforms the traditional medium voltage power grid into a data communication network, and has natural advantages in construction cost, operation and maintenance costs. It has been well applied in South Korea, the United States, Spain and other countries, and a lot of research and practice has been carried out in China.
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 means. The access solution 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 emerged in North America and has developed rapidly in recent years.
For medium-voltage power line networks, due to their existing wide distribution, they have become an ideal medium for achieving high-speed network access in remote areas, so as to shorten and eliminate the "digital divide" between urban and rural areas, and between developed and underdeveloped areas. In remote or low-density areas, it will be difficult to achieve digital communication services with larger bandwidths through other technical means other than PLC in the short term; xDSL or communication optical cables widely used in urban areas are generally difficult to lay in these areas; satellite communication can be achieved in some areas, but the low communication rate and high costs of channel leasing and terminals have greatly limited its large-scale application; cellular communication technologies represented by GSM, IS-95, WCDMA, etc. are designed for application scenarios with high user density. If used in areas with low user density, it will bring serious waste of communication capabilities and high operating costs, making it difficult to promote and popularize. This contradiction is particularly prominent in developing countries. As far as the domestic situation is concerned, my country has a vast territory, and a considerable number of rural areas are in remote and sparsely populated areas, and the communication development in these areas is relatively backward. According to statistics, the fixed-line telephone coverage rate in administrative villages in my country is 94%~97%, and the data communication coverage rate is even lower. Due to the constraints of natural and economic conditions, if the existing communication methods are used, it will face more and more difficulties to improve the information services in these areas. The electrification rate of administrative villages is close to 100%, and grid coverage is a basic condition for regional development. Therefore, using the power transmission network to achieve broadband network access is a feasible and economical solution that suits national conditions. Figure 1 is a structural diagram of the rural medium-voltage PLC access network system.
Figure 1 Study on medium-voltage power line channels in rural PLC access network systems
The original intention of power line design is to transmit power signals with a frequency of 50~60Hz. When its function is extended to voice or data signal transmission, it will face a complex electromagnetic environment. Therefore, for PLC technology, the study of power line channel characteristics is very basic and critical. In China, due to the more complex distribution and load of medium-voltage lines, relatively short lines between substations and relatively strong interference, the physical layer environment faced by medium-voltage power line networking communication 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 multiple field measurements and analyses on the characteristics of medium-voltage power line channels in typical rural areas in northern China. These research results provide references for practical engineering applications.
Generally speaking, the negative impact of power line channels on digital communications mainly comes from three aspects: changing impedance characteristics; frequency-selective channel attenuation characteristics; colored background noise and various impulse noises.
The purpose of studying channel impedance characteristics is mainly to achieve impedance matching between the channel and the transmitter and receiver. When the impedance is mismatched, it will cause waste of signal energy and even the appearance of dumb signal points. Due to the large number of branches and complex load conditions of the medium-voltage distribution network, the line impedance will change 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 curve of the impedance characteristics of the medium-voltage power line changing with frequency. Therefore, impedance matching is difficult when achieving broadband network access; currently, the method of sacrificing matching performance in coupling technology to adapt to the impedance in a wide range on the line side is usually adopted. There are also systems that use impedance-adaptive power amplifier equipment at the transmitter end to achieve better impedance matching effects.
Figure 2 Impedance characteristic curve of medium voltage line
The channel attenuation characteristics have an important influence on the effect of digital communication. The attenuation of medium voltage line is more serious than that of low voltage line. Experiments have shown that the average amplitude attenuation can reach 8dB~11dB per 100m. At the same time, the attenuation of medium voltage line also shows obvious frequency selectivity. 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 major reason. Experiments have shown that it is difficult to achieve successful communication connection in these deeply attenuated frequency bands, and it must be avoided in actual communication systems.
The average noise power of medium voltage power lines is around -60~-70dBm/Hz. Among them, the power of colored background noise, which is formed by the superposition of multiple low-power noises, is generally between -60 and -70 dBm/Hz, and can be lower than -80 dBm/Hz in some frequency bands. It weakens as the frequency increases, and the power spectrum changes slowly, generally in the order of minutes or even hours. The narrowband noise caused by other wireless communication signals such as radio stations within the communication bandwidth occupies an average bandwidth of 2k~4kHz, has a high power, and is about 30~50 dBm/Hz higher than the background noise. This type of interference generally exists stably for a long time. The impulse noise has the greatest impact on the communication effect. This type of noise is randomly generated and lasts for a very short time, generally tens or hundreds of milliseconds. Most of the power is 10~30 dBm/Hz higher than the background noise. When impulse noise occurs, data transmission in the noise frequency band may have serious sudden bit errors. Figure 3 shows the typical noise spectrum of my country's rural medium-voltage power lines in the 40k~560kHz frequency band.
Figure 3 Noise characteristic curve of domestic rural medium-voltage line (40k-560kHz)
Medium-voltage PLC key technology and application system
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, domestic and foreign countries have conducted a lot of research and testing on the application of various modulation technologies in medium-voltage PLC according to the requirements of different communication scenarios for rate and reliability.
According to the research on the characteristics of medium-voltage channels, the noise power generally decreases with the increase of 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 compromise between the two according to the actual line conditions. A large number of practices at home and abroad have proved that the carrier frequency of 5k~50kHz is more suitable for most medium-voltage distribution automation systems. In the application of distribution automation, most of them are one-way data transmission, and the main requirement is high reliability, and the requirement for real-time performance is not very high. Therefore, a lower transmission rate is generally selected, generally between 10bps~1000bps. In terms of modulation technology, the most commonly used is narrowband modulation method, such as ASK, FSK or CPSK, which have been widely used. In recent years, in order to achieve data transmission in an environment with strong noise interference, spread spectrum communication technologies such as frequency hopping, direct sequence spread spectrum, and Chirp frequency hopping have also been introduced into medium-voltage PLC systems.
In the scenario of using medium-voltage PLC to achieve data network access, due to the high communication rate, high requirements are placed on the channel utilization rate of the modulation and coding technology used, and the ability to avoid or resist burst noise and pulse noise. 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. This, combined with the low frequency band utilization of the modulation method itself, greatly limits the communication rate of the system. CDMA technology can effectively combat interference such as narrowband noise in the transmission channel, but the higher processing gain required in the CDMA system is difficult to achieve on the power line channel with severe frequency selective fading, so the advantages of the CDMA system cannot be fully utilized in PLC. It is generally believed that it is no longer applicable when the rate exceeds 1 Mbps. For access networks with higher transmission rates, 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 to convert serial data streams into parallel processing. It has a high channel utilization rate close to the Shannon limit; it can effectively combat multipath effects, solve the problem of inter-code interference, and has a strong ability to resist burst interference; in addition, in terms of channel allocation, OFDM also provides the possibility of flexible operation to avoid deep fading frequency bands within the communication bandwidth; OFDM technology is widely used in high-speed PLC.
In the MAC layer protocol, current research shows that the competitive CSMA/CA protocol with conflict avoidance, the non-competition reservation protocol based on TDMA, and the hybrid protocol combining the two are more suitable for medium-voltage broadband access networks and have been applied in actual systems.
In general, the PLC application system has gone through the development process of analog-single-chip integrated circuit-modern digital signal processing technology. The medium-voltage PLC system has a high starting point, and DSP devices and special chip solutions have been widely used. Since 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 substation and the transformer, as well as the interface technology with MV-PLC and LV-PLC at one end and optical fiber and wireless network at the other end, thus providing an end-to-end PLC solution from medium-voltage transmission to low-voltage transmission, from outdoor access to indoor networking. At present, the main technology of the system has passed laboratory tests, and Amperion and its partners are vigorously promoting the commercial process of this type of access network.
The application of MV-PLC access in rural areas, which has received extensive attention in recent years, is still mainly at the stage of experimental systems. The access system introduced in reference [1] uses BPSK modulation method in the physical layer, 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. Internet access was achieved in the medium-voltage power line network transmission experiment in suburban and low-density areas of South Africa. The transmission distance reached 4 km, but the QoS guarantee required by VoIP was not achieved. In reference [2], a full-duplex digital broadband communication solution based on QPSK modulation was used on a 13.8 kV power grid in rural North America, and a 2 Mbps TCP/IP connection was achieved on the 17 MHz and 83 MHz frequency bands. Conclusion Medium
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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 developed rapidly. There is reason to believe that with the advancement and maturity of related technologies, medium-voltage power line communication will play an increasingly important role in the information society.
References:
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