1. Introduction Distribution network automation is an automated system for remote real-time monitoring, control, and coordination of equipment on a distribution network. The key to information technology-based distribution network automation lies in three aspects: a large number of intelligent terminals, communication technology, and rich backend software. Communication technology is one of the three key technologies. Distribution network automation systems require effective and reliable communication links to transmit commands from the backend control center to numerous intelligent terminals, while simultaneously sending real-time data from the intelligent terminals back to the backend control center. Specifically, the communication system in distribution network automation is divided into two main levels: The first level is the backbone network communication between the master station and the substations. In this level, communication methods such as fiber optics, microwave, and spread spectrum can be used to transmit distribution information through SDH fiber optic ring networks, dedicated data networks, etc. If a communication network already exists between the master station and the substations, data transmission between the master station and the substations can be completed using various dedicated communication access devices. The second level is the branch network communication between the substations and FTUs/DTUs, which is the main focus of this paper. At this level, the main communication media available are twisted-pair cables, fiber optics, and carrier waves. Twisted-pair cable offers lower data transmission rates but guarantees a certain level of transmission quality (meeting error rate requirements). However, it requires cable laying, resulting in significant construction work. Fiber optic cable, on the other hand, boasts high data rates, low error rates, and superior performance, making it a high-quality transmission medium. Fiber optic cable has largely replaced twisted-pair cable as the preferred transmission medium between substations and FTUs/DTUs, a consensus widely held. However, fiber optic transmission systems require cable laying in practical applications, leading to high engineering requirements and substantial construction work. Regarding carrier transmission, power transmission lines are a natural network for transmitting distribution automation data. Utilizing carrier modulation technology to establish communication between FTUs/DTUs and distribution substations solves the transmission channel problem. The probability of 10kV distribution lines being vandalized is lower than that of twisted-pair and fiber optic cables. From this perspective, carrier transmission offers higher reliability and security than twisted-pair and fiber optic cables. Furthermore, carrier communication has been widely applied in power systems above 35kV, accumulating substantial technical experience that can serve as valuable lessons for distribution network carrier communication. Ultimately, distribution network automation will utilize a hybrid approach combining multiple communication methods, primarily fiber optic and carrier communication. At the same time, we should also see that the carrier communication of the distribution system is very different from the previous carrier communication. The following first explains the technical problems existing in the carrier communication of the distribution system, and then introduces the corresponding solutions. 2 Problems to be solved by the carrier communication of the distribution network Carrier communication has a long history of development in the communication of the power system and has very mature application experience in the power system above 35kV. However, the carrier communication of the distribution network is very different from the previous carrier communication, which is reflected in: (1) Different communication methods. The existing carrier communication of the power system above 35kV implements the point-to-point communication method. Wave traps are installed on both sides of the channel, and the carrier channel has relative independence. However, the carrier communication in the distribution network needs to complete the data transmission between the distribution substation, FTU, and DTU. Due to the consideration of communication cost, wave traps will not be installed in the distribution network. Only the point-to-multipoint communication method can be adopted. If the SSB (single sideband suppressed carrier) and FSK (frequency shift keying) modulation technology in the traditional carrier communication is still used, its output power will increase a lot, and the communication cost and equipment volume will also increase a lot. This is difficult to accept in engineering implementation. (2) Different channel attenuation characteristics. Practice has shown that channel attenuation is related to the load. The power load of power systems above 35kV is relatively stable, so the change in channel attenuation is not too drastic and can basically be regarded as not changing with time. However, the power load in the distribution network varies greatly throughout the day, which causes the channel attenuation value to change more drastically. This requires the communication equipment to have a wider automatic gain control capability. (3) Different line conditions. Power systems above 35kV mostly use overhead lines and less often use underground cables. In the distribution network system, there are both overhead lines and underground cables, and there are also cases where overhead lines and underground cables are laid together. At the connection between overhead lines and underground cables, due to the inconsistency of characteristic impedance, obvious echo reflection is formed. This reduces the transmission efficiency and causes greater signal loss. In addition, the echo reflection forms standing waves in the channel, which makes it very difficult for the receiver to receive the signal correctly. (4) Different coupling methods. As shown in Figure 1, carrier communication in 35kV systems and above usually adopts single-end coupling, i.e., phase-to-ground coupling. In the distribution network carrier communication shown in Figure 2, if the tie switch S3 or the sectionalizing switches S1 and S2 are disconnected, the data transmission between each FTU and the distribution substation should still be maintained. Therefore, the double-end coupling method shown in Figure 2 should be adopted. In order to ensure normal communication between FTU and distribution substation under the condition of line grounding fault, the phase-to-phase coupling method should be adopted. When a fault occurs, the phase-to-phase coupling is converted to phase-to-ground coupling. Although the transmission loss increases, the coupling channel can still be kept open. Alternatively, the network technology mentioned later can be used to solve the problem. [img=366,187]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/26-1.jpg[/img] (5) The noise level and interference are different. The background noise in the distribution network carrier channel is measured to be -50 to -40dBm. In carrier channels above 35kV, the background noise is recommended according to the noise level suggested in the "Design Guidelines for Single-Sideband Power Line Carrier Systems" (GB/T 14430-93) [3]: [img=433,160]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/26-2.jpg[/img] It can be seen that the noise in the distribution network carrier channel is not much different from that in the 35kV system. However, the pulse interference in the distribution network carrier channel is more frequent and higher than that in the 35kV system. This is because the distribution network system is connected to high-voltage motors, medium-voltage frequency converters, and other various electrical equipment. When high-voltage motors and medium-voltage frequency converters are working, they will generate very high interference pulses, which must be carefully considered when designing distribution carrier equipment. In summary, the characteristics of distribution network carrier channels are: a) the background noise level is similar to that of 35kV systems, but the frequency and amplitude of pulse interference are higher than those of 35kV and above power systems; b) the transmission distance is not far, generally not exceeding 10km, but there are many channel branches. Compared with systems above 35kV, the transmission attenuation is much higher for the same transmission distance, and the characteristics of time-varying behavior are more obvious, with significant channel attenuation; c) the information transmission problem in the event of channel failure must be considered. Below, we will introduce the relevant technologies and solutions using Siemens' newly developed distribution network carrier communication unit DCS-3000. [b]3 Distribution Network Carrier Communication Technology[/b] 3.1 Modulation Technology As mentioned earlier, there are many spike interferences in distribution network carrier channels, which is one of the main factors affecting the correct reception of data. Existing carrier communication technologies use SSB (Single Sideband Suppressed Carrier) modulation technology and FSK (Frequency Shift Keying) modulation technology, which have limited ability to suppress spikes. Therefore, in recent years, many manufacturers both domestically and internationally have been researching and adopting new carrier communication technologies to suppress spike interference. Spread spectrum modulation and OFDM (Orthogonal Frequency Division Multiplexing) are representative technologies. Spread spectrum modulation modulates lower-frequency data onto a wider frequency band, thus effectively suppressing spikes. Although spike amplitudes are large, their relative proportion is smaller after being modulated onto a wider frequency band, making them easy to filter out at the receiver. Spread spectrum modulation has good spike suppression capabilities. However, its drawback is that the high-frequency characteristics of the distribution network carrier channel are poor, limiting the usable effective frequency band to below 500kHz. This limits the data transmission rate, otherwise the spike suppression capability would be correspondingly weakened. Siemens' newly developed DCS-3000 distribution network communication unit uses OFDM modulation technology. It can suppress spike interference while also achieving a high data transmission rate. The basic principle of an OFDM system is shown in Figure 3. High-speed serial data (fs) is converted into N parallel low-speed sub-data streams. These low-speed sub-data streams are modulated by N subcarriers (f0, f1...fN-1), where f0, f1...fN-1 are mutually orthogonal. Partial overlap of the subcarrier spectra is allowed, thus effectively utilizing the bandwidth. At the receiver, because these subcarriers are mutually orthogonal, their mutual influence is minimized, resulting in minimal bit errors caused by crosstalk between subchannels. OFDM converts serial symbols into parallel symbols, modulates them orthogonally, and then transmits them in parallel within the channels. Compared to single-carrier modulation systems such as SSB and FSK, at the same transmission rate f_s, the transmission rate on each subchannel is only fs/N, increasing the time occupied by each symbol by a factor of N. This effectively suppresses spike interference. Although the spike pulse has a large amplitude, it occupies a wide frequency spectrum and has less energy per unit bandwidth. Relative to each subchannel f0, f1...fN-1, its energy is reduced, thus reducing its impact on the demodulation output. For narrowband pulse interference, it can only affect a few subchannels of f0, f1...fN-1, not all of them. The system can reduce the transmission rate on these affected subchannels or temporarily shut down these subchannels to overcome the impact of narrowband pulse interference. [img=291,206]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/27-1.jpg[/img] [img=291,158]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/27-2.jpg[/img] Compared to spread spectrum modulation, OFDM also has the ability to resist spike and narrowband pulse interference. Furthermore, at the same transmission rate, OFDM occupies a narrower bandwidth than spread spectrum modulation, which is invaluable for distribution network carrier channels with only 500kHz of bandwidth. However, OFDM modulation and demodulation technology is relatively complex, and its implementation using DSP technology is difficult. Siemens uses ASIC technology to create dedicated chips, which also have channel characteristic testing functions. Through testing and analysis, it can indicate which frequency bands are suitable for transmission, which greatly facilitates the implementation of the technology. 3.2 Network Technology Distribution networks have many carrier channel branches and numerous connected devices, resulting in severe impedance mismatch, significant standing wave ratios, and substantial channel attenuation. Early development of distribution network carrier communication equipment focused solely on adopting new modulation principles to reduce the signal-to-noise ratio requirements and increase output power to address the problem of high channel attenuation, but the actual application results were not good. Distribution network communication networking combines communication technology with network technology, providing a new approach to solving the problem. As shown in Figure 2, if the communication unit of the distribution substation and the communication unit of switch S3 are far apart, making direct communication difficult, a segmented relay method can be used to transmit information to S3. After being relayed through S1 and S2, the data is then transmitted to the communication unit at S3. This effectively reduces the output power of the communication unit and overcomes the effects of high channel attenuation and fading, achieving efficient data transmission. This concept can be realized through token ring or token bus LAN technologies in network technology. Obviously, using token ring or token bus technology requires the communication unit to have a sufficiently high transmission rate; otherwise, with multiple relay links, the overall communication time will increase, exceeding the allowable delay range and failing to meet real-time requirements. The Siemens DC-3000 distribution network carrier communication unit, while occupying the same bandwidth, has a higher transmission rate and possesses the technical foundation for using token ring or token bus network protocols. [img=350,116]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/27-3.jpg[/img] [img=350,210]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/27-4.jpg[/img] [img=350,184]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/27-5.jpg[/img][img=350,139]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/27-6.jpg[/img] As mentioned earlier, one of the main problems that distribution network carrier communication equipment solves is how to ensure the smooth flow of data channels when a ground fault occurs in a processing phase. There are two methods to solve this problem: a) Using phase-to-phase coupling, which converts phase-to-phase coupling to phase-to-ground coupling when a fault occurs, ensuring the smooth flow of the channel. The drawback is that the communication cost will increase when there are many communication nodes and a large network scale. b) Using a token ring method, if a channel fault occurs, information can be transmitted through the other half of the loop path. The Siemens-DCS3000 distribution network carrier communication unit adopts this method. In addition, it has various networking functions, as shown in Figures 5-8, where BU represents the DCS-3000 distribution network carrier communication unit. 4. Conclusion We have analyzed the special characteristics of distribution network carrier channels and distribution network carrier communication devices. It can be seen that adopting new modulation principles combined with network technology is one of the effective ways to solve distribution carrier communication problems. The rapid development of Internet technology has placed increasingly urgent demands on "last-mile access." Distribution networks and low-voltage power supply networks are natural networks for "last-mile access." The development of distribution carrier technology not only brings convenience to distribution network automation but will ultimately contribute to "last-mile access."