1 preface
With the development of national economy and the improvement of people's living standards, people's demand for electricity is increasing day by day, and at the same time, higher requirements are put forward for the reliability and quality of power supply. Distribution feeder automation is one of the most direct and effective technical means to improve the power supply reliability of distribution system. In recent years, the country has stepped up the transformation of urban and rural power grids, and the investment of major domestic power supply bureaus in distribution network automation is also increasing. In the process of realizing distribution automation, we find that communication is a difficult problem. Here we only talk about the application of optical fiber communication in power distribution automation.
2 distribution network automation requirements for communication
Like dispatching SCADA system, distribution automation system also needs an effective communication network, and it has its own characteristics: a large number of terminals. The distribution network system has many switching stations, distribution transformers and electrode breakers. In order to monitor these devices, many FTU and TTU are needed. At the same time, these FTU are equipped with distributed devices, which are widely distributed geographically and have scattered communication nodes.
The scale, complexity and automation degree of distribution automation system determine that the communication system should meet the following requirements:
(1) Reliability:
There are many communication devices in the distribution network system exposed to the outdoor environment, which must be able to resist the invasion of high temperature, low temperature, sun exposure, rain and snow, hail, lightning and other natural environments. At the same time, avoid all kinds of electromagnetic interference as far as possible, ensure long-term stable and reliable work, and require the communication system to work normally when the line is cut off.
(2) Economy:
Considering the overall economic benefits of the distribution network system, the communication system investment should not be too large, and strive to make full use of the existing communication resources of the main network, make overall planning for the main network and the distribution network, and avoid repeated investment.
(3) Large addressing capacity:
Communication system should not only consider the needs of current and future data transmission, but also consider the requirements of system upgrade.
(4) Two-way communication:
In order to realize the functions of telemetry, remote communication and remote control, distribution automation must have the ability of two-way communication.
(5) Simple operation and maintenance-free.
According to the above requirements, with the decline of optical fiber price, optical fiber communication is widely used in power system at present.
3 optical fiber communication
Since the advent of laser and low-loss optical fiber, optical fiber communication system has risen rapidly with its unparalleled advantages in technology and economy, and has become popular all over the world. The system is a communication system that uses optical fiber as transmission medium and light as carrier signal to transmit information. The wavelength of applied light wave is 1.0 ~ 1.0 μm μ m. The whole system consists of electric terminal, optical terminal, optical cable and repeater. Optical fiber can be divided into single mode fiber (SMF), multimode fiber (MMF), long wavelength low astigmatism fiber (LMF), polarization maintaining fiber (PMF) and plastic fiber (POF). Commonly used are single-mode and multimode optical fibers. Multimode fiber transmits multiple light wave modes, while single mode fiber transmits only one light wave mode. The transmission distance of single-mode fiber is longer than that of multimode fiber. At present, optical signals can generally travel about 6 kilometers in multimode fiber and 30 kilometers in single-mode fiber. Therefore, the price of single-mode optical equipment is higher than that of multimode optical equipment. Practical optical fibers are usually transmission lines composed of multiple optical fibers, reinforced cores, protective materials and fixing materials.
Optical fiber modem can complete the conversion between optical signal and digital signal. Optical fiber modems usually have more than one data port for transmitting synchronous or asynchronous signals. The communication rate can reach 2Mbps or higher, and the communication rate commonly used in distribution networks is generally below synchronous N×64K or asynchronous 19200bps. Therefore, it is enough to meet the needs of distribution network communication. The connection diagram of optical fiber modem is as follows:
In addition, there is an optical fiber modem with double-ring self-healing function. This function greatly enhances the reliability of communication. Its functional schematic diagram is shown in Figure 2:
In fig. 2(I), points A, B and C are double-loop networks realized by self-healing optical modems. If point D fails, as shown in Figure 2(II), the optical paths of Station A and Station C are fused (looped back), so that communication will not be affected, and corresponding alarm and positioning signals will be sent to the main station, so that maintenance personnel can repair the failed optical cable in time.
4 characteristics of optical fiber communication
Optical fiber communication has the advantages of large communication capacity, small attenuation, lightning resistance, electromagnetic interference resistance, corrosion resistance, good confidentiality, high reliability and convenient laying, but the investment cost is relatively high, especially for the optical fiber laying of directly buried cable lines in urban areas, the construction cost will be even higher.
Realization scheme of optical fiber communication in distribution network
The networking mode of optical fiber communication is very flexible, which can be constructed into various topological structures such as star, chain, tree, mesh, single fiber network, double fiber network, multi-branch on the ring, multi-ring intersection, multi-ring tangency and so on.
According to the characteristics of distribution automation system, optical fiber network usually needs to form a ring network and connect with computer local area network to realize data sharing. Common networking methods are shown in Figure 3.
In fig. 3, "s" stands for network server, "W 1, W2, Wn" stands for workstation, "b" stands for substation, "k" stands for switching station and "t" stands for distribution transformer.
In practical engineering design, fully considering the complexity of the topological structure of power communication private network, SDH transmission system can adopt the design idea of 126 E 1(2M port) full cross connection, double main optical rings and multiple optical branches. The basic framework is that 1 ~ 3 SDH/STM- 1 double-fiber self-healing ring crosses or tangents, and it can be upgraded to SDH/STM-4 online by replacing the optical card if necessary. If the local area network of the local dispatching center is located in the geographical center of the network, it is suggested to design it as a tangent ring with the dispatching center as the tangent point, as shown in Figure 4. If the local area network of the local dispatching center deviates from the geographical center of the network, it is suggested to design it as an intersecting ring. Since the dispatching center is not at the intersection point, in order to transfer between rings reliably, at least two intersecting points of each ring protect each other, as shown in Figure 5.
6 concluding remarks
In the actual distribution automation communication system, it is necessary to establish a low-cost, high-efficiency two-way communication system and provide very high reliability and information flow performance at an acceptable cost. At the same time, because the distribution network automation system has too many functions to complete and the system is complex, it is unrealistic and uneconomical to use a single communication system to meet all functional requirements. Therefore, in the distribution network automation system, various communication modes should be applied, and the best combination should be selected according to the comprehensive economic and technical indicators. In power system, the common communication modes are digital microwave multiple access, digital radio, wireless spread spectrum, special cable, post and telecommunications local network, carrier wave, spread spectrum carrier wave and so on. , which can be selected for networking.