1 Introduction With the development of social economy, the advancement of science and technology and the continuous improvement of people's living standards, people's demand and dependence on electricity is increasing, and the requirements for safe and stable power supply are getting stronger and stronger. However, due to the influence of the power system itself and external interference, grid accidents occur from time to time, which not only causes the economic benefits of the power management enterprises to be lost, but also has a serious impact on the power users and the whole society. Since the 1960s, large-scale blackouts have occurred in various countries around the world due to the stability and destruction of the power system. In July and August of 1996, there were two major blackouts in the western United States. The US President believed that the blackout had "endangered national security." In the second half of 2003, large-scale blackouts occurred in North America and Canada, London, Sweden, Denmark, and Italy, which shocked the world. In particular, on August 14, 2003, the power outage of the US and Canada that affected the power supply of 50 million people caused major economic losses and was the most serious blackout in the history of the United States.
In China, in the past 20 years, there have been more than 100 major blackouts in major power grids. Under the conditions of west-to-east power transmission and north-south interconnection, China will form a nationwide giant power system. If there is a major power system accident, its scale and losses may increase substantially. Therefore, ensuring the safety, stability and economic operation of large-scale interconnected power systems is a major and urgent issue that must be addressed as a major strategic issue.
2 Power system security issues
2.1 Safety issues of modern power systems The safety of power systems refers to the degree to which the system can maintain stable operation and normal power supply in the event of a fault. The traditional power system security is mainly to study the dynamic characteristics of the power system itself in the event of a fault, including the power angle stability, voltage stability, frequency stability, system unwinding, and thermal overload of the system. Such studies are generally directed at a single fault, while large-scale blackouts are often complex sequences of chain events.
With the development of modern communication technology and information technology, in order to ensure the safety and economic operation of large power grids, various information systems, such as dispatch automation (SCADA/EMS), distribution network automation system (DA) and substation integrated automation system (SA) ), the power market technical support system has been widely used in the field of power systems. Figure 1 shows the overall architecture of modern power systems. Power systems and information systems and communication systems have been integrated into highly integrated hybrid systems. The monitoring and control of power systems is increasingly dependent on the reliable operation of information systems and communication systems. The computer system in the information system is the core, and the improper maintenance of the computer system is one of the basic reasons for the blackout of the 8.14 US and Canada. Failure of a critical communication system can paralyze the entire system, losing controllability and observability. Therefore, the concept of power system security must be expanded.
Recently, some researchers have proposed the concept of power system vulnerability (Vulnerability) as a new framework for dynamic security assessment of power systems. The term vulnerability often appears in relevant literatures in the fields of environment, ecology, computer networks, etc., to describe that related systems and their components are susceptible to impact and destruction, and lack of resistance to interference and restoration of initial state (self structure and function). ability. They have different meanings in different disciplines. The vulnerability of the power system can be defined as: the dangerous state of the catastrophic accident in which the power system lurks in a large area due to human intervention, information, computers (software, hardware), communications, power system components, and protection and control systems. . System vulnerability is closely related to the level of system security and the trend of system security levels when system parameters change. In this concept, people set an acceptable reference value for them. When the system security status is evaluated, the system security level and its changing trend are determined. Whether the system is vulnerable depends on whether they are above or below the set reference value.
2.2 Factors Affecting Power System Security Issues There are many factors affecting the security of power systems. For infrastructures that make up modern power systems, they can be divided into internal and external factors.
(1) Internal factors:
1) Failure of main components of power system: failure of generator, transformer and transmission line;
2) Control and protection system failure: hidden fault of protection relay [4, 5], circuit breaker malfunction, control failure or misoperation;
3) Computer software and hardware system failure;
4) Information and communication system failures: loss of communication with the EMS system, automatic control and protection, failure of the information system (causing information loss or unreliable information) or congestion, external intrusion information/communication systems (such as hackers) Intrusion);
5) Factors in the competitive environment of the power market: competition and inconsistency among the participants in the electricity market, lack of initiative in replacing old control and protection systems or power generation devices;
6) Power system instability: static / transient / voltage / oscillation / frequency instability.
(2) External factors:
1) Natural disasters and climatic factors: earthquakes, hail, thunderstorms, storms, floods, heat waves, forest fires, etc.;
2) Human factors: operator misoperation, control and protection system settings, sabotage (including war or terrorist activities).
3 Power system security measures
3.1 Strengthening the construction of power grids and reducing the probability of accidents The power industry is an industry that needs long-term and advanced investment. The construction of large power plants takes 5-10 years and the service life is about 30 years. Therefore, the plant (power plant) network (grid) coordination, unified planning, advanced construction, and reasonable structure are required to ensure the safe operation of the power system. In particular, it is necessary to strengthen power grid construction (strengthening long-distance transmission grids, receiving power grids and secondary systems) to improve grid safety and reliability, reduce accident probability, and reduce power outage losses.
In the US-Canadian power outage accident that occurred on August 14, 2003, the US official proposed that the "old and old" power supply network, that is, the aging of equipment, is a serious hidden danger of power system failure. According to statistics, in developed countries, the life expectancy of power generation equipment has increased from 12% in 1990 to 31% in 2000, and is expected to reach 50% by 2010. Similarly, in the field of transmission and distribution, a large part of the infrastructure has a life span of nearly 70 years. On the other hand, many of the devices designed decades ago are no longer suitable for advanced digital technology. Therefore, the aging of power equipment is a common problem in developed countries. Replacing aging equipment requires new large-scale investments. However, after the marketization of the power industry, market participants are concerned about the interests of today and tomorrow, not the interests of 20 years or 30 years. In the past decade, investment in countries that have undergone reforms in the power industry has remained at a low level due to competitive pressures, market inefficiencies and regulatory uncertainties, in the United States and Sweden, for example. The peak reserve for power generation has fallen from 20% in 1990 to 10% in 2000. Therefore, it is necessary to have a comprehensive plan for the construction of the power system, and to establish a certain regulatory system and investment incentive mechanism so that the development of the power industry can meet the requirements for the operational safety of the power system.
3.2 Strengthening the monitoring and management of the power system The interconnection of the power system makes it possible to optimize the allocation of resources in a wide area, and to exchange and support each other. However, in a closely connected interconnected power system, a local fault can quickly propagate throughout the system, causing large-scale power outages. Therefore, in the handling of accidents, it is required to respond quickly and efficiently. Taking the United States as an example, judging from the two major blackouts in the western United States in 1996 and the blackouts on August 14, 2003, there are many areas for improvement in the US power system monitoring and management. In the United States, more than 3,000 power companies operate a power industry with a total capacity of about 900 GW in the vast North American land. Although the power grids of more than 3,000 power companies are interconnected to form a large power system in North America, their scheduling and Management is independent, and there is no organization or organization that monitors the national or a large regional interconnected power system to coordinate and coordinate the system-wide safe operation and post-accident fault handling. After a failure of a part of an interconnected power system, other parts of the interconnected power system are often unaware of the occurrence of the fault before the fault hits. Therefore, in an interconnected power system, unified power grid management, unified power grid dispatch, and establishment of a sound safety operation system are important conditions for ensuring the safe and reliable operation of the power system. Through regular training, we must continuously improve the quality of dispatching and operating personnel, especially the ability to respond to emergencies. In the R&D project of the US Grid2030 research project, it is proposed to build the National Grid Control Center in 2010, emphasizing the combination of the transmission and distribution grid and the communication information network.
In order to improve the operating environment of the power grid and reduce the damage caused by external forces and natural power systems, it is necessary to do daily maintenance work, for example, to trim the branches of the transmission corridor in time to avoid the occurrence of several large blackouts in the United States due to the occurrence of wires and branches. A large-scale blackout caused by flashover.
3.3 Strengthening the basic research closely related to power system safety The power system consisting of different capacity generators, transmission and distribution lines of different voltage levels and lengths, and different capacity and characteristic loads is a typical complex large system, showing high dimensionality and non- Linearity, time-varying, incomplete information, wide-area (large-scale spanning space-time) interconnectivity, and complex features of differential algebra. The spatio-temporal operation of this large system has always been a very difficult academic and engineering problem [6-16]. There is an urgent need to establish new theoretical and methodological systems (modeling, analysis, simulation, simulation, prediction, and control methods) to effectively solve key problems faced by complex power systems, such as long-term dynamic behavior analysis and simulation of cross-regional power systems. System cascading failure prevention and control and other topics to ensure the safe and reliable management and operation of the power system.
Early research and development of wide-area, intelligent, adaptive power system protection and control systems that integrate power systems, wide-area protection and control, and communications infrastructure (including gps technology) to deliver real-time critical and Extensive information, anticipate possible problems, quickly evaluate the weak links of the system, and timely complete the defensive measures such as self-healing and adaptive reconfiguration actions based on system analysis, which will form a powerful anti-accident capability of the complex complex power system nationwide. To avoid catastrophic accidents and ensure the safe and stable operation of the power system.
Countries around the world attach great importance to research on the safety of power systems. In the United States, the Complex Interactive Networks/ Systems Initiative (CIN/SI), jointly funded by the US Department of Defense's Army Research Office and EPRI, is a joint government and industry grant for five years. A $30 million large-scale research project involving 28 universities and 2 power units participated in the project. The project began in the spring of 1999 with the aim of exploring new theories and methods for the safe operation of power systems in new technological environments.
In this regard, Europe is conducting two major projects OMASES (Open Market Access and Security Assessment System) and EXaMINE (Power Security in the New Market Environment). Among them, OMASES is an industrial research project supported by the EU. It has been in operation since 2001. The participants include industry, research units, universities and system operators. The contents of this project are: transient stability assessment, training simulator for EMS operators, and market simulator. Prepare to access an existing EMS system or apply it in a new EMS. Field testing and verification is currently being carried out in Italy and Greece.
In July 2003, the US Department of Energy hosted a seminar on "American Power Transmission Technology Outlook". The meeting proposed the Grid2030 research plan for the development of the US power grid. The meeting mainly proposed to establish the national grid as the target of Grid2030. The R&D project also recommended the establishment of the National Grid Control Center in 2010, and reached a consensus on achieving national networking.
In China, among the first 10 major projects funded by the National Major Basic Research Program (973 Program), there are research projects on major scientific issues related to power system disaster prevention and economic operation [9, 10], and organize many domestic units. Research has been carried out and many achievements have been made. It has passed the acceptance test in 2003. At present, relevant departments are further organizing major research projects on the safety of power systems.
Basic research closely related to the safety of power systems requires long-term, sustained, and high-input investments. Establishing research funds (similar to funding for basic research) in national or power companies is an effective approach. The basic research will provide theoretical and practical results for the safe operation of power systems under normal and special conditions (such as war); it should be included in the national medium and long-term scientific research and development plan; international cooperation should be carried out to absorb from the accidents that have occurred. Useful lessons.
3.4 Studying the impact of natural and man-made damage (including war and terrorism) on the safe operation of power systems In modern society, power industry and disaster prevention systems, communication systems, military commands are closely related to social activities and people’s lives and electricity supply. As with control systems, public health systems, etc., national infrastructure with serious consequences should be included, and the safe and reliable operation of these systems is fundamental to the country's economy, safety and quality of life. Therefore, as one of the country's main (or key) infrastructures, it is necessary to study the impact of natural disasters and man-made damage (including war) on the safe operation of power systems, scientifically distinguish between various early warnings and emergencies, and establish correspondingly responsive, Efficient and unified response strategies and emergency measures.
4 A number of basic research directions closely related to the safety of power systems
4.1 Research on Modeling of Integrated Power Systems and Integrated Energy and Communication System Architecture (IECSA) Over the years, large-scale power system dynamic behavior analysis has received wide attention. However, with the interconnection of power grids, the emergence of multi-feed AC/DC hybrid transmission modes, and the application of high-power power electronic equipment, various dynamic behaviors and characteristics such as ultra-low frequency oscillations appearing in power systems seriously affect the safe operation of large power grids. There is an urgent need to further understand the mechanism and propose control methods and measures. The establishment of the electric load model is a key issue in this research field, including the identification theory and method of the complexity and uncertainty of the electric load model, and the macro structure identification of the electric load model.
With the development of information technology, power systems and information systems and communication systems have been integrated into integrated hybrid systems. Traditional methods of research on power systems have been difficult to deal with such complex systems, and new theoretical and methodological systems need to be established in modeling, analysis, simulation, prediction and control to effectively solve the critical problems faced by complex power systems. To ensure the safe operation of the power system. The interaction and integration of power systems, information systems, computer systems, and communication systems must be considered and studied simultaneously. The parallelism of multiple networks, the parallelism of multiple physical processes, and the parallelism of multiple types of components should be considered in modeling. To fully apply real-time measurement information, develop distributed real-time computing. The Grid2030 research program developed by the US power grid proposes the construction of the Integrated Energy and Communication System Architecture (IECSA) and has organized research as a major project.
For a long time, research on power system safety assessment has focused on the modeling of power systems themselves and the calculation of faults, without considering the closely related information systems and communication system models. This is because the models of information systems and communication systems have not yet been established, and the interaction between information systems and power systems is lack of systematic and in-depth research. It is urgent to apply the relevant theories of complex interactive systems and distributed artificial intelligence to cope with electricity. The complexity brought about by the continuous expansion of the system and the development of new power system safety assessment theory.
Multi-agent systems are expected to provide new avenues for solving these problems [17, 18]. For the power system, it mainly regards each member of the network as a distributed autonomy agent that can complete certain tasks independently, and then realizes the coordination of each member through the interaction and cooperation of multiple agents. Overall control objectives. The most typical power system grading method is to divide the system into systems such as power generation, transmission, distribution, and power. The multi-agent structure in the power market environment may be an independent power generator, a power transmission service provider, an auxiliary service provider, or the like. On the surface, this classification method is the extension and extension of traditional hierarchical control technology, but there are obvious differences, such as the priority authority of the control strategy and the definition of the final goal. There are also specialized modeling methods and tools for communication systems and information systems. If the communication system corresponds to a communication agent, the information system corresponds to an Information agent. This grading method lays the foundation for accurately simulating the interaction between layers in a multi-agent hierarchical structure. The multi-agent system has the characteristics of resource sharing, easy expansion, high reliability, flexibility, and real-time performance. It is very suitable for solving the modeling, control and analysis and evaluation tasks of complex systems such as large-scale power systems. It provides a new way to realize real-time analysis and global coordinated control of wide-area power systems.
4.2 Research on Information Theory and Application of Wide-area Power System The information distribution of wide-area power system is wide and numerous. It is necessary to have an advanced and reliable hierarchical and partitioned information system to enable timely and correct transmission of wide-area information. Guarantee and effectively process information to achieve real-time monitoring of the entire system. To this end, there must be a real-time parallel fault diagnosis system to diagnose and predict future possible or potential faults in a timely manner in a large amount of real-time information (including measurement information, equipment "health" status, etc.).
With the construction of the National Power Data Network (SPDnet), dispatch automation (SCADA/EMS) ((SA) development and popularization, various information management systems (such as production management systems, marketing management systems), geographic information systems (GIS) The application of power market technical support systems and other information systems for power grid operation in the field of power systems indicates that information technology has become more and more integrated into power systems. However, power systems are still relatively backward in information processing technology. The processing of information is still in the low-level digital signal processing stage, the information collection repeatability is large, the information optimization is not realized, and the complexity of hardware construction and the control loop are complicated; the application of information is too simple. As a complex large system The information processing technology should have the advantages and characteristics of multi-information, multi-level, multi-integration, etc. It can adapt to the application to the power system with large operation mode and complex system structure [19]. Change the current power system information processing mode. It will be a new topic in the field of power system research.
A power information system is a system with a very complex, large or unpredictable problem domain. The only solution is to develop a large number of modular components (agents) with special functions that are specifically designed to solve a specific aspect of the problem. In the event of interrelated problems, the agents in the system coordinate with each other and can handle this correlation correctly. The application agent improves the information fusion algorithm, increases the feedback algorithm of the system, and changes the original simple one-way environmental information and knowledge transmission from the lower layer to the upper layer, so that the upper layer can also transmit the planning and management information to the lower layer. This information fusion system has complete observation, integration, decision making and coordination functions. Therefore, distributed information processing technology based on multi-agent system is a promising research direction in this field.
At the same time, the negative impact of information technology also affects the power system. The invasion of hackers has added new connotations to the security of the power system, and its impact needs further research.
4.3 Conducting research on wide-area power system security prevention system In order to prevent large-scale blackouts that may occur in wide-area complex power systems, it is necessary to develop wide-area, intelligent, adaptive and hierarchical and global coordination with power systems. Protection and control systems. It integrates power system, wide area protection and control, and communication infrastructure to provide real-time critical and extensive information, anticipate possible problems, quickly evaluate system weaknesses, and complete self-healing and self-system based analysis in a timely manner Adapt to preventive measures such as reconfiguration actions to avoid catastrophic accidents. It differs from the traditional methods and techniques used in that the latter is based only on the local control actions of the local measurement signals and only on the state of the individual devices; the former is based on the safety assessment of the wide-area information, after the failure Localize the fault so that the fault does not develop into an important technical measure for large-scale power outages. Relay protection should extend from traditional component protection to system protection, and the impact of hidden failure of relay protection devices on cascading failures should be studied. Emergency control systems should implement online decision making to increase the speed and effectiveness of control.
To realize the wide-area power system security prevention system, real-time monitoring, real-time simulation and real-time control of the power grid must be realized based on the Wide Area Measurement System (WAMS). The main objectives of the wide area power system security system are:
(1) Accidental detection or prediction of system accidents. According to the state of the system, overload or transmission line sag, etc., using advanced algorithms for prediction. Real-time monitoring of the "health" status of power equipment and maintenance management according to the state of the power equipment to avoid system accidents caused by equipment aging or failure (failure). The thermal limits of the transmission line should be monitored online. The cause of several major blackouts was static power flow, which later exacerbated the system crash.
(2) Rapid response to system failures. Such as early isolation of faults, advanced overload conditions of overloaded components, and other measures to avoid accident expansion.
(3) Restore the system as soon as possible, that is, restore the power supply to the power supply as soon as possible, and return the system to normal operation. Pay attention to the preparation of the accident recovery plan, and consider accident recovery problems in the grid and power supply construction stages as much as possible, such as a complete "black start" program. A proper accident recovery plan can reduce the loss of power outages. The development of decentralized power supplies also creates conditions for rapid recovery after an accident.
(4) When the system is in normal operation, the operating performance of the system is corrected to maintain safety, that is, the ability to withstand failure is higher.
Recently, researchers have proposed the idea of ​​System Protection Terminal (SPT) [6]. As shown in Figure 2, the terminal is connected to the control system of the substation. In order to apply the time scale, a GPS function is required. The SPT has a high-speed communication interface, and performs power system data exchange between the terminal databases. The database has all the real-time refreshed measured values ​​and signals of the substation, and other SPT data required for the substation control.
Further develop a comprehensive multi-layer structure that combines phase measurement, protection and EMS to form a wide-area protection and monitoring network. As shown in Figure 3, a local protection center (LPC) is composed of several SPTs, and the System Protection Center (SPC) is the coordinating agency of the LPC. The design of this three-layer structure can be implemented in stages. The goal of the first phase is to develop monitoring capabilities (such as the wide-area measurement system WAMS). The main purpose is to improve the information and state estimation of the analysis operators after the accident. Developed by WAMS as a protection and control system. There is also another "flat structure" in which the SPT communicates directly with other SPTs. Such a structure is easily implemented with the structure of a multi-agent system.
5 Conclusion It is a major and urgent issue to ensure the safety, stability and economic operation of large-scale interconnected power systems. This paper introduces the safety issues of power systems and their research trends. From the analysis of various factors affecting the safety of power systems, combined with the current technical means, some measures are proposed on how to strengthen the safety of power systems and prevent and control large power outages. It also puts forward some views on the methods and research directions that should be adopted in the future research in this field.
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