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1) Redundant configuration. Nowadays, all power plant DCS system backbone communication networks generally adopt redundant configurations, and some also realize multiple redundancy, so that the communication network can maintain high availability.
2) Use optical fiber as a transmission medium. As we all know, optical fiber not only has significant advantages such as long transmission distance, large communication bandwidth and corrosion resistance, but also has the outstanding advantages of strong anti-electromagnetic interference and low transmission error rate. These are conventional transmission media such as copper cables and twisted pairs. Incomparable, it has gradually been widely used in the backbone communication networks of communication systems and automation systems in recent years. It can be predicted that with the further decline in the price of optical fiber products and the physical dispersion of DCS systems, the use of optical fibers will continue to expand.
3) Solution configuration of the network. This measure not only effectively isolates data traffic at all levels and improves the real-time communication performance, but also isolates communication failures and limits the impact of failures to a minimum range as much as possible, thereby improving the reliability of the entire communication system. A more detailed discussion can be found at: The next content.
4) Automatic bumpless switching under network faults. All DCS communication systems have an on-line monitoring function. Once a working communication network fails, the system will automatically switch to a redundant network in a hot standby state to ensure continuous and undisturbed operation of the entire system. In some systems, redundant networks are Inter-periodically switch to prevent the hot-spare network from being discovered in time when it is faulty, thus losing the hot-spare role.
5) Data integrity check and automatic repeat function. In most systems, data integrity (such as CRC) is performed before and after all data messages are sent. Once an error occurs, data retransmission is automatically triggered.
6) Automatic disconnection and bypass of faulty stations. The failure of any one node will not affect the smooth flow of the entire network. The failed node should be automatically disconnected and bypassed. Once the fault is removed, the node can automatically restore the entire network of data traffic.
7) The communication processor and the control processor are separated. In some systems, the control processor and the communication processor are completely two independent processors. The two perform their duties independently of each other, so that the control speed of the control algorithm of the control unit can be guaranteed, and at the same time, Ensure the real-time and security of network communication, avoid the slow response caused by network communication failure to control, or affect the effective transmission of the network due to the complex control algorithm.
In addition, adopting an open network topology and communication protocol is also an important measure to improve the reliability of the entire system. In the early DCS systems, because common network systems and communication protocols were difficult to meet requirements, many manufacturers developed proprietary communication systems and proprietary communication protocols, but their biggest drawback was their specificity. Manufacturers were closed to each other and their reliability. The need for manufacturers to constantly improve and develop themselves has led to high development costs. With the rapid development of computer network technology, IEC8802.x, ((compared with the familiar IEEE802.x standard) series of standards have been introduced, the performance and reliability of general-purpose computer networks have been continuously improved, and the price has rapidly declined. Under the trend, major DCS manufacturers have all given up their efforts to develop their own unique network products and instead adopted general computer network architectures and related hardware and software products.The most typical example in this regard is the use of FDDI networks in control networks due to FDDI. Designed for use in backbone high-speed networks, the intrinsic dual-ring topology makes it not only redundant but also fault tolerant, allowing the entire system to maintain normal communication even when the dual ring is disconnected simultaneously. These FDDI standard features make Manufacturers do not need to design their own undisturbed switching systems between redundant control networks.At the same time, because FDDI networks are designed for backbone networks, there is a large amount of standard hardware interconnected with conventional Ethernet and token networks. It has a very good openness and high reliability.
The biggest benefit of the open network to DCS manufacturers is that the system's openness and reliability have been significantly enhanced. This is because the professional leaders in the communications field have strong technical strength, and the reliability and openness of the communications system hardware and software products are long-term. It has been fully verified in use. By adopting an open communication system, the DCS system provides the possibility of interoperability of software and hardware products from different vendors, which helps users to freely select related products in a wider range so as to achieve a better performance-cost ratio. The most successful example is the emergence of fieldbus technology and the application of Ethernet technology in industrial control. It can be said that the openness of the DCS system has gradually become an increasingly important indicator. There is an additional benefit of adopting an open system, which is to effectively reduce the R&D costs of manufacturers and accelerate the upgrading of products.
2 Layered structure of DCS communication network 2.1 Advantages and disadvantages of layered structure After a lot of analysis and discovery, based on openness, reliability and real-time transmission considerations, the communication network of each DCS system adopts a hierarchical structure without exception. Data collection is uploaded, and control data is downloaded. Different levels of communication networks can use different topologies and different communication protocols according to actual needs. The cooperation of all levels together constitutes the overall communication system. From the current point of view, each DCS system generally has a field-level fieldbus communication network or a remote communication network, a control network between electronic devices, and an operation management network in a control room. As shown in FIG. 1, a typical system according to this configuration has ABB's PROCONTROLP, Siemens's TELEPER MXP, Hitachi's HLACS5000, Bailey's Svmphony and Honeywell's TDC3000. In order to simplify the hierarchical structure of the system, some systems merge the control network and operation management network into one network, such as OVATION of Westinghouse, I/A of FOXBORO, and MAXl000+ of MCS. It should be noted that FIG. 1 is only a schematic diagram. The network topology of each layer may be any of a ring type, a bus type, and a star type. 
In the DCS communication system layered structure, the fieldbus layer is used to connect field smart meters, the remote bus is used to connect remote I/O cabinets, and the control network layer is mainly used for communication between each controller and process data server. The management network layer is used for communication between operator stations, engineering stations and process data servers. As for the whole plant information management layer (MIS), it is mainly used for production management, optimized dispatching and personnel management of the whole plant. Because MIS networks generally do not adopt redundant configuration, and the relative wiring is complex and the reliability is relatively low, the automation expert of the power plant proposes to add a real-time monitoring information network (SIs) between the operation network and the MIS network. Connect the plant-level public system, each unit DCS system and load distribution station, etc., for real-time monitoring and optimized scheduling of multiple units. In addition, there are internal communication networks in the automation cabinets. The differences between different systems are relatively large. However, deterministic master-slave communication networks or token communication networks are generally adopted, and will not be discussed here.
Considering that most of the data needs to be transmitted only in the underlying network, only a few need to be transmitted across the network through the upper layer network. Therefore, after the entire communication system is properly layered, on the one hand, the network traffic at each layer can be reasonably isolated. Improve the real-time communication at all levels, on the other hand is also conducive to the reliability of the network, through the layering can effectively isolate the network communication failures at all levels, such as operating the management network failure will not affect the normal operation of the control network. Another advantage of adopting a layered structure is that DCS manufacturers can select a reasonable network topology and communication protocol according to their needs, and minimize costs by satisfying the real-time and reliability of the entire communication system, thereby improving the overall system. Competitiveness, such as the operation of the network layer in Figure 1, through the use of the popular standard industrial Ethernet architecture and TCP/IP communication protocol, effectively improves the system's openness, and at the same time, because the Ethernet is very cheap, so that the overall cost is effectively reduced . It should be noted that in this layered system, the reliability of communication devices connecting different networks becomes a key factor, and if possible, a high quality standard gateway is used.
2.2 Further improving communication efficiency through segmentation Due to the low efficiency of Ethernet under heavy load, special attention should be paid to effectively reducing the communication load of Ethernet, especially when the Ethernet layer is used in the control layer. Compared with the control network, although the real-time requirements of the operation layer network are much lower (in general, due to human visual impact, the refresh rate of the operation screen data should not be higher than 1 second, or the operator may be fatigued), but the Ethernet Due to the inherent shortcomings of transmission uncertainties, it is always beneficial to effectively reduce the communication load. Therefore, three methods can be used for this purpose. The first is to use a higher speed Ethernet, such as 100Mbps Fast Ethernet; the other is to use a switched Ethernet. The network replaces the shared Ethernet to avoid the collision domain, thereby improving the transmission efficiency. The typical representative is the MAX1000+ system of MCS; the third is the use of reasonable segmentation technology. Each of these three methods has its own characteristics and is reflected in the existing DCS. Given that the first two methods are already familiar, we will not repeat them here. We only briefly discuss the third case here.
Because the hierarchical network architecture shown in Figure 1 has generally adopted the configuration of dual-server and multi-operator stations, the operational management network can be artificially divided into two relatively independent Ethernet networks. Two networks are connected through bridges. Two servers are respectively attached to two Ethernet networks, and the operator station is divided equally between two networks. For example, a general set of DCSs is currently equipped with five operator stations, so it can be distributed by 2, 3, as shown in Figure 2. Show. In this way, under normal circumstances, each server only serves its own corresponding operator station, thereby reducing the network transmission load and server load, and can significantly improve the response speed of the system. If there is an abnormal situation, such as a server crash, then the two Ethernets through the bridge become a whole. At this time, the working server serves all the operator stations at the same time. Although the response speed is slightly slower, it does not affect at all. Normal use. This method not only makes full use of the two servers, but also can significantly change the system response speed by adding only one bridge. Therefore, it is a "low input, high return" solution. According to the author's understanding, there are currently SYMPHONY of Bailey Corporation and PROCONTROI of ABB Corporation. Both P and Hitachi's HIACS 5000+ use this measure. 
3 Conclusion DCS data communication network is a very complex system, its reliability and real-time nature has always been a very important issue, and has always been an important indicator for continuous development and improvement of DCS manufacturers. This article only summarizes theoretically based on relevant data and does not fully explain the reliability of each system. This is because the reliability of the communication system is not only closely related to the reliability of other systems, but also to various DCSs. Manufacturers' hardware manufacturing technologies are also of great importance.
Measures to improve the reliability of DCS communication system
1 The main measures to improve the reliability of DCS communication system With the further improvement of the automation level of the power plant, the DCS system has increasingly played a key role in the entire production process system, and therefore the requirements for its stability and reliability are getting higher and higher. The reliability of the entire DCS system has been required to reach more than 99.9%, and the communication system as a key subsystem of the DCS system, its reliability and real-time direct impact on the entire DCS system. Therefore, in order to effectively improve the reliability of the DCS communication system, DCS manufacturers often adopt the following measures: After adopting the above scheme, the communication efficiency and reliability of the operation management network have been greatly improved. In fact, similar segmentation measures are also used in the control network layer. The most typical ones are Hitachi's HIACS3000/5000 system and FOXBORO's I/ASeries system. The schematic diagram is shown in Figure 3, and the control layer network is divided according to the system. For the combustion system, soda system and water supply system. By dividing, the communication load of each control network segment is effectively reduced, real-time communication is greatly improved, and reliability is also significantly improved. However, it should be noted that when this method is used, the control network segment is divided and the control cabinet is properly distributed. When possible, placing the control cabinet with frequent data exchange on the same control network segment can greatly improve the communication. real-time. For example, according to related information provided by Hitachi, in the Hitachi HIACS 3000/5000 system, the data transmission period in the cabinet is lOOms, and the data exchange between the control cabinets on the same control network requires 200ms. If it is necessary to exchange data between control cabinets on different control networks. It takes 400ms. The potential advantage of using this method is that it can better realize the physical dispersion of the control cabinet, because the control network segment can be flexibly configured as required, which is better reflected in FOXBORO's DCS practical application system.