Typical Application of Fieldbus Technology in Substation Automation System

Typical Application of Fieldbus Technology in Substation Automation System

Substation automation systems are developing in the direction of decentralization and networking. The substation environment is special, and a reliable, real-time, and easy-to-operate communication network is needed to connect the upper management network and the front-end measurement and control network. The fieldbus, which is mainly used for low-level equipment communication, is fully applicable to this occasion. Using fieldbus instead of traditional serial communication can not only fundamentally improve the performance of substation automation systems, but also make the system reliable, flexible, and easy to expand. This article introduces the concept and characteristics of fieldbus and the advantages of fieldbus in substation automation systems, as well as the introduction of several typical fieldbuses.

【Keywords】 Substation automation fieldbus FCS DCS network node IED equipment high-speed data communication network

0 Introduction
　　Substation automation system is developed on the basis of computer technology and network communication technology. Looking at the development history of substation automation system, it can be roughly divided into three stages: in the early 1990s, "centralized" structure substation automation products began to appear; in the mid-1990s, "layered distributed" system entered the market; and now it is the "decentralized distributed" system that dominates the market [1-3]. However, at present, most of the decentralized distributed substation automation systems, whether domestic or foreign, use serial communication (RS232, RS485, RS422 bus, etc.) between their components [4]. Serial communication has a slow transmission rate, is easily interfered with, has limited communication distance and inflexible communication mode, so it cannot meet the requirements of large-scale real-time data transmission.
　　Fieldbus is both an open communication network and a fully distributed control system. As a link between intelligent devices (IEDs), it connects intelligent devices hanging on the bus as network nodes into a network system, and further constitutes an automation system to realize the comprehensive automation functions of basic control, compensation calculation, parameter modification, alarm, display, monitoring, optimization and control integration [5]. It has become a development trend to use network methods, especially fieldbus technology, to solve the communication problems of substation automation systems.

1 Concept of Fieldbus Fieldbus
　　is a communication network for interconnecting field intelligent devices in the fields of process automation, manufacturing automation, building automation, etc., which was developed internationally in the late 1980s and early 1990s. As the basis of the factory digital communication network, it communicates the connection between the production process site and the control equipment and between them and the higher control management level. It is not only a grassroots network, but also an open, new fully distributed control system. This comprehensive technology, which mainly includes intelligent sensing, control, computer, digital communication and other technologies, has attracted worldwide attention and become a hot spot in the development of automation technology, and will lead to profound changes in the structure and equipment of automation systems. As the production department with the highest degree of automation in history, the power system has naturally become the forefront of the application of fieldbus, especially in the substation automation system, where it is more widely and deeply used. Many powerful and influential companies in the world have successively developed fieldbus technology and products to varying degrees. The working environment of fieldbus equipment is at the bottom of process equipment. As a basic communication network at the factory equipment level, it is required to have the characteristics of simple protocol, strong fault tolerance, good security and low cost; it has certain time certainty and high real-time requirements; it also has the characteristics of stable network load, mostly short frame transmission, frequent information exchange, etc. Due to the above characteristics, the fieldbus system has the characteristics of different upper-layer high-speed data communication networks from network structure to communication technology. The fieldbus system is generally called the fifth-generation control system, also known as FCS-fieldbus control system. People generally call the pneumatic signal control system PCS before the 1950s the first generation, the 4~20mA and other electric analog signal control systems the second generation, the digital computer centralized control system the third generation, and the distributed distributed control system DCS since the mid-1970s the fourth generation. As a new generation of control system, the fieldbus control system FCS, on the one hand, breaks through the limitation of the DSC system using a dedicated communication network, adopts a solution based on openness and standardization, and overcomes the defects caused by the closed system; on the other hand, it changes the DCS's centralized and decentralized distributed system structure into a new fully distributed structure, and completely delegates the control function to the site. It can be said that openness, decentralization and digital communication are the most prominent features of fieldbus systems.

2 Technical characteristics of fieldbus
　　a. Openness of the system. An open system means that the communication protocol is open, and the devices of different manufacturers can be interconnected and exchange information. The fieldbus developers are committed to establishing an open system of a unified factory bottom network. The openness here refers to the consistency and openness of relevant standards, emphasizing the consensus and compliance with standards. An open system can be connected to any other device or system that complies with the same standards. A fieldbus network system with bus function must be open. The open system gives the right of system integration to the user. Users can combine products from different suppliers into systems of any size according to their needs and objects.
　　b. Interoperability and interoperability. Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems, and point-to-point and point-to-multipoint digital communication can be implemented. Interoperability means that devices with similar performance from different manufacturers can be interchangeable and interoperable. 　　c. Intelligence and functional autonomy of field equipment. It distributes functions such as sensor measurement, compensation calculation, engineering quantity processing and control to field equipment. The basic functions of automatic control can be completed by field equipment alone, and the operating status of the equipment can be diagnosed at any time. 　　d. Highly decentralized system structure. Since the field equipment itself can complete the basic functions of automatic control, the fieldbus has become a new architecture of a fully distributed control system. It has fundamentally changed the existing DCS centralized and decentralized distributed control system, simplified the system structure, and improved reliability. 　　e. Adaptability to the field environment. Working at the front end of the field equipment and as the bottom layer of the factory network, the fieldbus is designed for the current serial communication with slow transmission rate, susceptibility to interference, limited communication distance and inflexible communication mode, so it cannot meet the requirements of large-scale real-time data transmission. 　　The fieldbus is both an open communication network and a fully distributed control system. As a link between intelligent devices (IEDs), it connects the intelligent devices hanging on the bus as network nodes into a network system, and further forms an automation system to realize the integrated automation functions of basic control, compensation calculation, parameter modification, alarm, display, monitoring, optimization and control [5]. Applying network methods, especially fieldbus technology, to solve the communication problems of substation automation systems has become a development trend. 　　1 Concept of Fieldbus Fieldbus 　　is a communication network for interconnecting field intelligent devices in the fields of process automation, manufacturing automation, building automation, etc., which was developed internationally in the late 1980s and early 1990s. As the basis of the factory digital communication network, it communicates the connection between the production process site and the control equipment and between them and the higher control management level. It is not only a grassroots network, but also an open, new fully distributed control system. This comprehensive technology, which mainly includes intelligent sensing, control, computer, digital communication and other technologies, has attracted worldwide attention and become a hot spot in the development of automation technology, and will lead to profound changes in the structure and equipment of automation systems. As the production department with the highest degree of automation in history, the power system has naturally become the forefront of the application of fieldbus, especially in the substation automation system, where it is more widely and deeply used. Many powerful and influential companies in the world have successively developed fieldbus technology and products to varying degrees. The working environment of fieldbus equipment is at the bottom of process equipment. As a basic communication network at the factory equipment level, it is required to have the characteristics of simple protocol, strong fault tolerance, good security and low cost; it has certain time certainty and high real-time requirements; it also has the characteristics of stable network load, mostly short frame transmission, frequent information exchange, etc. Due to the above characteristics, the fieldbus system has the characteristics of different upper-layer high-speed data communication networks from network structure to communication technology. The fieldbus system is generally called the fifth-generation control system, also known as FCS-fieldbus control system. People generally call the pneumatic signal control system PCS before the 1950s the first generation, the 4~20mA and other electric analog signal control systems the second generation, the digital computer centralized control system the third generation, and the distributed distributed control system DCS since the mid-1970s the fourth generation. As a new generation of control system, the fieldbus control system FCS, on the one hand, breaks through the limitation of the DSC system using a dedicated communication network, adopts a solution based on openness and standardization, and overcomes the defects caused by the closed system; on the other hand, it changes the DCS's centralized and decentralized distributed system structure into a new fully distributed structure, and completely delegates the control function to the site. It can be said that openness, decentralization and digital communication are the most prominent features of the fieldbus system. 　　2 Technical characteristics of fieldbus 　a. Openness of the system. An open system means that the communication protocol is open, and the equipment of different manufacturers can be interconnected and exchange information. The fieldbus developers are committed to establishing an open system with a unified factory bottom network. The openness here refers to the consistency and openness of relevant standards, emphasizing the consensus and compliance with standards. An open system can be connected to any other equipment or system that complies with the same standards. A fieldbus network system with bus function must be open. The open system gives the right to system integration to the user. Users can combine products from different suppliers into systems of any size according to their needs and objects. 　b. Interoperability and interoperability. Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems, and point-to-point and point-to-multipoint digital communication can be implemented. Interoperability means that equipment with similar performance from different manufacturers can be interchanged and interoperable. 　c. Intelligence and functional autonomy of field equipment. It disperses the functions of sensor measurement, compensation calculation, engineering quantity processing and control to the field equipment. The basic functions of automatic control can be completed by the field equipment alone, and the operating status of the equipment can be diagnosed at any time. 　d. Highly decentralized system structure. Since the field equipment itself can complete the basic functions of automatic control, the fieldbus has constituted a new architecture of a fully distributed control system. It fundamentally changes the existing DCS centralized and decentralized distributed control system system, simplifies the system structure, and improves reliability. 　e. Adaptability to the field environment. Working at the front end of the field equipment, as the bottom layer of the factory network, the fieldbus is designed for working in the field environment. It can support twisted pair, coaxial cable, optical cable, radio frequency, infrared, power line, etc. It has strong anti-interference ability, can use two-wire system to realize power transmission and communication, and can meet the requirements of intrinsic safety and explosion-proof. 　　3 The superiority of fieldbus in substation automation system 　Due to the above characteristics of fieldbus, especially the simplification of fieldbus system structure, the design, installation, commissioning, normal production operation and maintenance of substation automation system are superior. 　a. Save hardware quantity and investment. Since the intelligent devices dispersed in the front end of the interval equipment in the fieldbus system can directly perform a variety of sensing, control, alarm and calculation functions, the number of transmitters can be reduced, and separate controllers, calculation units, etc. are no longer needed. Functional units such as signal conditioning, conversion, isolation technology and their complex wiring of the DCS system are no longer needed. Industrial PCs can also be used as station control operation workstations, thus saving a large amount of hardware investment. Due to the reduction of control equipment, the floor space of the control room can also be reduced. 　b. Save installation costs. The wiring of the fieldbus system is very simple. Since a pair of twisted pairs, a pair of optical fibers or a cable can usually be connected to multiple devices, the amount of cables, terminals, trough boxes and bridges is greatly reduced, and the workload of wiring design and joint calibration is also greatly reduced. When it is necessary to add field IED equipment, there is no need to add new cables. It can be connected to the original bus cable nearby, which not only saves investment, but also reduces the workload of design and installation. According to the calculation data of typical test projects, the installation cost can be saved by more than 60%. 








 







　c. Save maintenance expenses. Since the field control equipment has the ability of self-diagnosis and simple fault handling, and sends the relevant diagnostic maintenance information to the control room through digital communication, the operation and maintenance personnel can query the operation of all bay-level equipment, diagnose and maintain information, so as to analyze the cause of the fault early and eliminate it quickly. The maintenance downtime is shortened, and the maintenance workload is reduced due to the simplified system structure and simple wiring.
　d. Users have a high degree of initiative in system integration. Users can freely choose equipment provided by different manufacturers to integrate the substation automation system. Avoid being "framed" by the selection of a certain brand of products, and will not be at a loss for incompatible protocols and interfaces in system integration, so that the initiative in the process of substation automation system integration is completely in the hands of users.
　e. Improve the accuracy and reliability of substation automation system. Due to the intelligence and digitization of fieldbus equipment, compared with analog signals, it fundamentally improves the accuracy of measurement and control and reduces transmission errors. At the same time, due to the simplified system structure, the number of equipment and wiring is reduced, and the internal functions of field instruments are strengthened: the round-trip transmission of signals is reduced, and the working reliability of substation automation system is improved.
　f. It is easy to realize equipment expansion and product modification. Substation automation systems based on fieldbus can be easily expanded and modified. When expanding the system, new IED devices can be connected to the existing fieldbus without adding any components or considering whether the fieldbus interface matches. Modifications to functions and parameters can be directly implemented through fieldbus login and query. Therefore, the cost required to implement new control strategies or product modifications is reduced.
　　4 Typical fieldbuses
　a. HART
　HART is the abbreviation of Highway Addressable Remote Transducer. It was developed by the American ROSEMOUNT company in 1986 as a transitional temporary communication protocol. However, it is now widely recognized and has become a de facto international standard.
● The HART protocol is based on the open system interconnection model (OSI) of the International Organization for Standardization (ISO) and uses OSI layers 1, 2, and 7, namely the physical layer, data link layer, and application layer. The physical layer uses FSK technology based on the Be11202 communication standard, so communication can be carried out through leased telephone lines. ●HART protocol uses FSK technology to superimpose a frequency signal on the 4~2mA process measurement analog signal. Logic 1 is 1200Hz, logic 0 is 2200Hz, and the baud rate is 1200bps. It successfully enables analog and digital bidirectional signals to be carried out simultaneously without interfering with each other. Therefore, when communicating with intelligent instruments, analog instruments, recorders and analog controls can also be used. It is an ideal solution to gradually realize digital performance (including digital process variables) without modifying the field instruments. This is a protocol for the transition from analog systems to digital systems. ●Three types of missions are specified in the application layer. The first type is general commands, which are commands that all devices can understand and execute; the second type is general behavior commands, and the functions it provides can be implemented in many field devices; the third type is special device commands, so as to implement special functions in some devices. This type of command can allow the company that develops such devices to be unique. In addition, it also provides users with a unified device description language DDL. ●HART supports point-to-point, master-slave response mode and multicast mode. ● Direct communication distance: 3000m for a single device with shielded twisted pair, and 1500m for multiple devices. When only one power supply is used, 15 intelligent devices can be connected. 　b. CAN 　　CAN is the abbreviation of Controller Area Network. This is a control area network developed by BOSCH in Germany. It is a serial communication network with high reliability and supports distributed real-time control. It was originally used as a serial data communication protocol for data exchange between a large number of control measuring instruments and actuators inside the car. Now it has gradually developed to the control of other industrial sectors, including machinery manufacturing, CNC machine tools, medical equipment, building management monitoring, and monitoring of substation automation equipment. The ISO/TC22 Technical Committee of the International Organization for Standardization has formulated the international standards ISO/DIS1 1 898 (communication rate = 1Mbps) and ISO/DIS11519 (communication rate = 125Kbps) for the CAN protocol. Among fieldbuses, it is currently the only fieldbus approved as an international standard. Intel, Philips, Motorola and other chip manufacturers all produce CAN bus chip products. In China, HAPCAN and other series of products have been launched. The fully distributed substation integrated automation systems DISA2 and DISA3 developed by domestic power system manufacturers adopt CAN bus. The internal bus of ABB RTU560 in the automation system of the 220kV substation at the top of Changsha Electric Power Bureau also adopts CAN bus. ●CAN protocol implements layers 1 and 2 of ISO/OSI model. The physical layer defines all electrical characteristics in the transmission process; the target layer and the transport layer include all functions of the data link layer defined by ISO/OSI. The functions of the target layer include confirming which information is to be sent, confirming the information received by the transport layer and providing an interface for it. The functions of the transport layer include frame organization, bus arbitration, error detection, error reporting, error handling, etc. ●CAN can transmit and receive data in several ways such as point-to-point, point-to-multipoint (grouping) and global broadcast, and can work in multi-master mode. Any node on the network can actively send information to other nodes on the network at any time, and can easily form a multi-machine backup system. ● Each node on the network can define different priorities to meet different real-time requirements. CAN uses non-destructive bus arbitration technology. When two nodes transmit information to the network at the same time, the node with low priority actively stops data transmission, while the node with high priority can continue to transmit data without being affected, effectively avoiding bus conflicts. ● The number of nodes on CANBUS is 2000 in theory and 110 in practice. The direct communication distance is 10km/5Kbps, 40 m/1MKbps. The transmission medium is twisted pair and optical fiber. ● CAN uses a short frame structure with a low probability of interference, and uses redundant check CRC and other error correction measures to ensure a very low information error rate. It also has an automatic bus shutdown function. In the case of serious errors, it can cut off its connection with the bus so that other operations on the bus are not affected. 　c. LONWORKS 　　LONWORKS is a local operation network, and LON is the abbreviation of Local Operation Network. The product was launched by ECHERON in the United States in 1991. Its application scope covers almost all areas of measurement and control applications, especially aviation/aerospace, building automation, energy management, monitoring of substation measurement and control equipment, factory automation, industrial process control, computer peripherals, electronic measurement equipment, etc. LONWORKS has also been widely used in China's power system. For example, Sifang Company's CSC2000 substation integrated automation system uses the LONWORKS bus. Within the Changsha Electric Power Bureau, six 110kV substations, including Xinkaipu, Guihua, Wuyi, Jingwanzi, Ansha, and Zhentou, have applied this system. ● LONWORKS's communication protocol LONTALK protocol follows the ISO/OSI reference model and provides all 7-layer services defined by OSI. This is the only fieldbus that provides all services. 














● The core of LONWORKS is Neuron chip, which contains three 8-bit CPUs: the first CPU is a media access control processor, which implements the first and second layers of LONTALK protocol; the second CPU is a network processor, which implements the third to sixth layers of LONTALK protocol; the third CPU is an application processor, which implements the seventh layer of LONTALK protocol and executes the user-written code and the operating system services called by the user code. LON-WORKS neuron chip has been produced by Motorola and Toshiba.
● Provide a set of development tool platforms LON Builder and Node Builder. With this set of tools, users can use neuron chips, LONTALK communication protocol and LONWORKS transceiver to develop the systems and products they need very conveniently and flexibly.
● The direct communication distance of LONWORKS is 2700m/78Kbps, 130m/1.25Mbps; the number of nodes is 32,000; the transmission media is twisted pair, coaxial cable, optical fiber, cable, etc.
　d. PROFIBUS
　　PROFIBUS is the abbreviation of Process Field Bus. In 1987, the German Federal Ministry of Science and Technology gathered 13 companies and 5 research institutes to formulate the German standard for field bus according to the ISO/OSI reference model. It was published in DIN19245 in April 1991 and officially became the German field bus standard, and then included in the European standard EN50170. PROFIBUS has been widely supported. From October 1996 to January 1997, 47,000 systems were installed in Germany alone. It has been widely used in the fields of processing industry, process automation, intelligent buildings, substation automation systems, etc. PROFIBUS has been widely used in power systems. For example, SIEMENS's SICAM and LSA substation automation systems use PROFIBUS, and the Dongtang 110kV substation within the Changsha Electric Power Bureau uses the LSA integrated automation system. The centralized control of the spillway gates of the Shanghai Yangshupu Power Plant and the Wuqiangxi Hydropower Station also uses PROFIBUS.
●PROFIBUS omits layers 3 to 6 according to the ISO/OSI reference model and adds a user layer. The first layer defines the physical transmission characteristics; the second layer defines the access protocol; the seventh layer defines the application functions. PROFIBUS introduces function modules (FB), object dictionaries (OD) and device description languages ​​(DDL) in the user layer, allowing users to perform complete internal operations on the device, thereby achieving device interoperability.

5. Conclusion
　　Applying fieldbus technology to substation automation system can fully meet the requirements of fast and efficient data communication on site, making the system more reliable, more open and less costly, greatly improving the overall level of substation automation system. References 1. Yang Qixun, "Development Trend of Substation Integrated Automation System" -- "Automation of Electric Power System". 1995.19.(10): 7~9 2. Jin Tao, Tang Tao, Que Lianyuan, et al., "Analysis and Discussion of Distributed Substation Automation System" -- "Automation of Electric Power System". 1997.21(10): 69~72 3. Tao Xiaonong, "Communication Technology Scheme of Distributed Substation Monitoring System" -- "Automation of Electric Power System". 1998.22(4): 51~54 4. Jin Wuqiao, "Development Strategy of Substation Automation System" -- "Automation of Electric Power System". 1999.23(22) 5. Yang Xianhui, "Fieldbus Technology and Its Application". Beijing: Tsinghua University Press. 1998