Optimization Design Ideas for Substation Automation Protection-EEWORLD

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1 Introduction

Substation automation technology has developed rapidly in recent years, and many substation automation systems have been put into operation in various places. Microcomputer protection, as an important part of substation automation system, has also achieved rapid development. Microcomputer protection devices have achieved remarkable results in terms of action speed, action accuracy, operation reliability and ease of use. Traditional analog protection is no longer comparable to it, but there is still great potential for development in terms of giving full play to the characteristics and advantages of microcomputers. In terms of realizing the relay protection function while also having other functions (such as control, measurement, communication, etc.) to form a microcomputer integrated protection device, and in terms of configuration and management between other secondary devices in the substation automation system, there is still a lot of room for optimization design. Based on the current status of microcomputer integrated protection in substation automation at home and abroad, this paper discusses its system composition and functional configuration with the idea of optimization design, and puts forward several ideas for its optimization design.

2 Functional requirements of substation automation system for microcomputer protection

2.1 Protection function should be relatively independent

Ensuring the safe and stable operation of relay protection is the basic requirement for designing substation automation system. Due to the particularity of relay protection, the automation system must not reduce the reliability of relay protection. Since the microcomputer protection device is required to make quick decisions and respond quickly, it needs higher reliability and sensitivity, so it still needs to maintain relative independence in function and role. In system operation, the reliability of the relay protection unit should only be related to its protection device itself, and has nothing to do with other devices related to the protection unit (such as communication ports, networks, monitoring units, etc.). This requires that in the function setting of automation, the protection unit is relatively independent, and even if the network connected to the protection device fails, it should not affect the reliability of the relay protection itself.

2.2 Information sharing

Under the premise of ensuring safe operation, information resource sharing should be achieved as much as possible in the design of substation automation system. In terms of measurement circuits, protection, wave recording, instrumentation, metering, telecontrol, etc. can be classified and optimized according to the different requirements of CT secondary characteristics, reducing measurement links, reducing secondary circuit load, and improving measurement accuracy.
From the perspective of the entire power system automation field, computer communication technology has driven various analog devices or systems to develop in the digital direction, and the application of digital signal processing has further promoted the informatization of these devices. The main feature of informatization is the effective use and sharing of information, which can greatly improve the performance-price ratio of the device or system. For relay protection, information sharing will be used to achieve the integration of multiple protection functions, and it is also conducive to the realization of comprehensive automation optimization design of substations.

3 Adopt object-oriented distributed design ideas

With the development of power systems, whether it is the analysis and application software of power systems, or the scale of management, monitoring and control systems, if the traditional function-oriented design method is still used, it will make the development, modification and expansion of these systems more and more difficult, unnecessary repeated settings will increase, and production efficiency will be difficult to improve. Object-oriented design methods are the best choice to solve these problems.
Due to the limitations of computer hardware level, early substations had separate and centralized implementation of protection functions and telecontrol functions, and basically divided functional units according to information type (i.e., by profession). With the development of process control architecture and the advancement of computer communication technology, the use of object-oriented distributed structures in substations to form field protection control systems has increasingly shown its strong vitality.
The so-called object-oriented distributed structure is to configure the field-level units according to the principle of corresponding primary equipment and secondary protection equipment. One primary equipment interval corresponds to one secondary protection monitoring unit. The protection is decentralized, which reduces the burden on CT, greatly saves secondary cables, further reduces the load on secondary circuits, and further reduces the area of the control room, making system configuration and operation more convenient, and improving the speed of system troubleshooting and adjustment operations. At the same time, each protection unit is connected to the station control-level equipment through the bus, and the protection engineer station is used to connect the upper and lower levels, so that the concept of the field maintenance process is clear and the responsibility is clear. In addition to completing the conventional automation protection management function, the substation can also realize the adaptive adjustment of the protection unit through the field high-speed bus; the open station-level network interface is set up to facilitate the establishment of a connection with the management level, fully ensuring the interactivity and scalability of the system. In this way, the important rapid decision-making function of relay protection can be realized at the field level, while the comprehensive protection involving multiple points and the adaptive functions related to the system structure are coordinated and completed by the engineer station and the field bus.

[page]4 Modular design of relay protection

Standardization and modularization are the basic requirements to ensure the unity of independence and scalability of substation automation systems. However, due to the lack of unified design solutions and product standards in China, this situation of a hundred flowers blooming has also caused duplication of work in the power research and production industry to a certain extent. Many products of different manufacturers are self-contained and lack good compatibility and scalability with each other, which brings great trouble to users. Therefore, the modularization and standardization of protection software and hardware design should be established to ensure the independence of the system itself and its compatibility with other systems.

4.1 Principles of module determination

4.1.1 Decomposition module with function as the core

Module is a product composition mode composed of general components (modules). Module is an independent unit with a certain function. Function is the basis for the composition of the module and the basis for system decomposition. Module decomposition should be hierarchical. For example: (1) Basic module (such as power plug-in). The functional component with plug-in as the unit is an independent unit with specific functions. (2) Plug-in level module. It is composed of plug-in modules with independent comprehensive functions. (3) Product level module. It can be composed of plug-in level modules and has a variety of protection control and measurement functions.

4.1.2 Modules should be typicalized

The requirements of many similar functions among various protection devices are simplified and unified, and merged into one or several typical functional units. Then these typical functional units are separated from the products, making them independent units with specific functions that can be used in various product systems in terms of functions, parameters and interfaces. The significance of this typicalization is to eliminate unnecessary duplication and diversity of module functions.

4.2 Modularization of hardware

The modularization of hardware starts with the basic module. When the system is decomposed into modules, the cohesion of the module should be the largest, and the decomposition point should be selected at the part with the least interface and the weakest part. This can simplify the interface structure and make the system easy to decompose and combine. The basic module is designed as a plug-in type, and the hardware can be used in different protections. In this way, the development cycle is shortened and the convenience of production, debugging and maintenance is ensured. The main plug-ins are: power plug-in, AC plug-in, CPU plug-in, open-in and open-out plug-in, etc. Different basic module combinations of hardware can constitute plug-in-level modules, and different plug-in-level module combinations can constitute product-level modules.

4.3 Modularity of software

Like hardware, the software of each processing module also adopts a modular structure. On the whole, the software module and the hardware module are set up in a matching manner. The advantages of high software development and replication rate, flexibility, good consistency and high reliability should be fully utilized to derive new products through software, that is, new varieties can be derived through software with the same hardware or to meet the special requirements of users. At the same time, the modularity of software is also conducive to shortening the software development cycle. For modules such as the main program, interrupt sampling program, human-machine interface program, startup interrupt program communication program and GPS synchronization program shared by protection, the protection device can be flexibly copied and utilized.
[page] It should be pointed out that the modular product system in practical application is dynamic. Modularization not only has a process of formation, improvement and maturity, but also with the continuous updating of technology, people's understanding is constantly deepening, and the reliability requirements of products are constantly improving. Therefore, the modular system is not only inevitable but also necessary for the implementation of innovation, timely modification and updating. However, in terms of modification and updating, it has inheritance in both circuit and mechanical structure, and has its unique advantages. This is also the advantage of modular design.

5 Communication technology

5.1 Analysis of information flow of substation microcomputer integrated protection

There are three flows of data related to substation and microcomputer integrated protection: the first flow is from protection equipment to the upper monitoring machine of the substation, which mainly transmits information reflecting the operating status of the primary system; the second flow is from the upper monitoring machine to the protection equipment, usually the upper monitoring machine issues commands to modify the setting parameters of the protection equipment and some other control information; the third flow is the horizontal data exchange between protection equipment. This type of data depends on the system design and functional division. For example, if synchronous sampling is required between protection equipment, the horizontal data exchange volume is large. At the current stage, considering the independence of the protection unit and reducing the system's dependence on communication, the communication between protection devices mainly considers the communication problem between the decentralized protection devices and the unit monitoring devices. Since the two devices are usually very close to each other, conventional serial communication technology can be used to solve the problem.

5.2 Communication system configuration principles

The structural configuration of the substation communication system should follow the design principles of distributed systems, try to arrange the computing and processing capabilities to the information source point, collect and process nearby, and control nearby. Only the necessary information involved in coordinated control is transmitted to each other to improve the system operation efficiency, reduce the system's dependence on communication, and improve the reliability of the overall system.

5.3 Communication method selection

The network topology of the local area network in the substation mainly adopts star and bus connection methods.
The star communication method takes the upper monitoring machine as the center point and connects to the control I/O devices and protection devices distributed on each switch cabinet through communication cables in a divergent manner. Its medium is generally optical fiber, so it has the characteristics of natural isolation, high anti-interference and high security of optical communication, which is suitable for use in harsh environments such as substations. In addition, each I/O unit and protection unit communicates independently with the station-level computer, monopolizes the communication line, and has good maintainability. Therefore, many substation automation systems currently use star communication system solutions. However, the total length of cables required for star connection is longer than that of other structures, especially when there are more station-level computer wiring, the construction is complicated; the horizontal communication between each I/O unit and protection device must be carried out through the station-level computer, which is complicated and inefficient; when using optical fiber communication technology, because the optical fiber itself is difficult to achieve T-shaped connection, the bus structure cannot be realized unless the optical fiber ring network technology is used. Therefore, active photoelectric conversion equipment must be used, which will inevitably cause the reliability of the communication system to be subject to the reliability of each connection point. Therefore, it is recommended to consider carefully when choosing a star communication system solution.
With the development of communication technology, more and more people have reached a consensus on field-level communication for the application environment of substation automation technology, that is, a multi-redundant system is formed by a bus network, and each node on the network can communicate with other nodes; the total length of the required cable is short, and the reliability is high, unlike the star-open connection method, which has a reliability bottleneck formed by a central control node (station-level computer). Here, the new generation of international field measurement and control network CAN is recommended. The technical solution based on the CAN network is easy to realize dual-network backup, can be integrated with I/O units and protection units to achieve high-speed data exchange, can ensure the real-time nature of emergency information, and has strong anti-interference ability, low cost and simple construction.
[page] It should be pointed out that at present, there is no unified interface standard for communication between equipment from different manufacturers in substations in China, and there are many types of communication protocols, which not only takes a lot of manpower for software and hardware development, but also brings many inconveniences to users' equipment selection, operation and maintenance. The quality of protocol selection not only affects the reliability and efficiency of the communication system, but also affects the normal functioning of the system function. Therefore, we should speed up the standardization of communication system protocols in substation automation systems, accelerate the formulation and publicity of national standards equivalent to the IEC870-5 series of international standards, avoid duplication of work, improve labor productivity, and give full play to the technical advantages of automation.

6 Expanding the comprehensive protection function of microcomputers

Microcomputer protection is the development direction of protection in the future. The existing domestic microcomputer protection devices have well completed the transformation of protection from analog to digital, but there is still great potential for development in giving full play to the advantages of microcomputers and enhancing protection functions.

6.1 Expanding the storage capacity of fixed values

In the power system, the operation mode of the power grid is often changed. In order to meet the reliability, flexibility and speed selectivity of the relay protection device under different operation modes, it is required that the protection device execute different setting values under different operation modes. If the method of temporarily inputting new setting values on site when the operation mode changes is adopted, it will bring many inconveniences, mainly reflected in (1) complex setting values require relay protection maintenance personnel to go to the substation site to change, which increases the maintenance workload; (2) the system takes a long time to change the setting value, which is not conducive to the safe operation of the system, especially when the setting value needs to be changed during the accident handling; (3) simple setting values are changed by operators, which is prone to errors.
Therefore, for microcomputer protection devices, it should be possible to pre-select and formulate a variety of the most commonly used and important operation modes, so as to formulate corresponding power grid protection setting value setting schemes, and store the values corresponding to these schemes in multiple protection setting areas. When the operation mode changes, the operator can change the setting value of the relay protection by adjusting the setting area code of the on-site protection device. This is fast and accurate, and solves the inconvenience of re-entering the setting value on site. Improve the coordination ability of protection under various operation modes and reduce the error rate of changing the setting value.

6.2 Record the working information of the protection device itself

After completing a certain logic judgment, the traditional analog relay protection can only record the output results or events that have occurred in the form of a signal light, and the information content is relatively small. Although the current microcomputer protection device can output protection action and self-test information, it is generally similar to the traditional analog protection in providing working information. For microcomputer protection, it has powerful computing, logical judgment and storage capabilities, making it possible to record the working information of the protection device itself in detail.
Recording the working information of the protection device itself has the following main functions: (1) By recording its own working information and combining it with fault recording, it is possible to determine the action behavior of the protection in a scientific, fair and realistic manner, rather than just inferring; (2) It is not only of great practical significance for the operation and maintenance of the protection device, but also of guiding significance for the research, development, improvement and improvement of product performance; (3) Even if the protection device operates correctly, its own working information is also of reference value, such as analyzing whether the action speed and action process of the protection meet the performance requirements of the product, and whether its protection action behavior is consistent with the fault development process.
The protection device needs to record the following details of its own work: (1) the actual program flow when a certain self-test or action result is generated (can be output through a printer or serial port as needed); (2) the fixed value area code executed when the protection action is performed, and the data of each protection function action point; (3) the action time of each main component or function and the input and output timing diagram during the action; (4) among multiple starting quantities, indicate which conditions are met for starting, etc. This information should not be lost after power failure and can be sent to the dispatch end through the built-in modem.

[page]6.3 External input information recording

Microcomputer protection can correctly make corresponding logical judgments according to different external information. Since these external information have a vital impact on the program flow and even the action behavior of the microcomputer protection during operation, it is necessary to record in detail and output the timing diagram of external input quantity with standard GPS time scale and accurate to milliseconds on demand in order to accurately analyze the action behavior of the protection, especially the incorrect action that may occur under complex working conditions. It should be possible to see when each external information is input, how long it lasts, whether there is any interruption in the middle, etc.

6.4 Use GPS to unify the synchronous clock

The global satellite positioning system (GPS) has a very wide range of uses. People are exploring various new methods of monitoring, protection and control of power systems by using its excellent time synchronization performance.
Two time signals can be obtained from GPS: one is the second pulse signal, and the other is the date and time code corresponding to the leading edge of each second pulse, that is, the time mark of the second pulse signal. In order to meet the needs of accident analysis, GPS should be used to obtain the synchronous clock, and the built-in GPS timing interface circuit should be used to provide standard time for the operation of the protection device, such as using the GPS timing pulse to control the protection sampling pulse, so as to ensure the consistency of the sampling data of the same model device in time throughout the network.

7 Conclusion

The current status and development of substation automation research require that the microcomputer integrated protection that constitutes automation can not only operate reliably and independently, but also be expanded into a larger system with stronger functions. Therefore, it is necessary to strengthen the standardized design of microcomputer protection.
Due to the rapid development of science and technology, the research and development cycle of new products is getting shorter and shorter. Therefore, in the development of new products, the reliability, advancement and economy of the products should be comprehensively considered, and new ideas, new principles and new technologies should be adopted to develop new microcomputer protection with high reliability and powerful functions. It can be predicted that with the development of China's power industry and the technological progress of related industries such as electronic technology and industrial automation information engineering, a new generation of microcomputer protection devices with better human-machine interface and safer and more reliable operation will develop rapidly.

References

1 Zhu Daxin, Liu Jue The content, functional requirements and configuration of the integrated automation system of substations "Automation of Electric Power Systems", 1995
2 Jiao Zhixian, Chen Yong, Li Shiyu, etc. The functions that microcomputer protection should have in the integrated automation of substations "Automation of Electric Power Systems" 1996
3 Yan Zeyou, Yang Qixun A new type of integrated automation system for substations "Relay" 1998
4 Zheng Sanbao, Shi Tiehong, etc. "Design and implementation of complete sets of microcomputer protection and management for substations"

Reference address：Optimization Design Ideas for Substation Automation Protection

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