What is the purpose of relay protection?

Relay protection maintenance refers to the automatic anti-accident measures to study the problems and abnormal working conditions that endanger the safe operation of power system, so as to explore its countermeasures. In its development process, power system and its components (generators, transformers, transmission lines, buses, etc.). ) is mainly maintained by contact relay, so it is called relay maintenance. The basic task of power system relay protection maintenance is: when there is a problem or abnormal working condition in the power system, the problem equipment will be automatically cut off from the system in the shortest time and in the smallest area, or a signal will be sent to the personnel on duty to eliminate the root cause of abnormal working condition, so as to reduce or avoid the damage of the equipment and the influence on the power supply in neighboring areas. A Brief History of Relay Maintenance1In the last quarter of the 9th century, fuse, as the earliest relay maintenance device, has been applied. With the development of power system, the structure of power grid is becoming more and more complex, and the short-circuit capacity is increasing. At the beginning of the 20th century, the electromagnetic relay maintenance device acting on circuit breakers came into being. Although electronic devices have been used in maintenance equipment since 1928, the large-scale popularization and production of electronic static relays were realized after the rapid development of transistors and other solid-state components in the 1950s. Static relay has the advantages of high sensitivity, fast action speed, simple maintenance, long service life, small size and low power consumption, but it is more susceptible to environmental temperature and external interference. 1965, digital relay maintenance using computer appeared. The rapid development of large-scale integrated circuit technology and the wide application of microprocessor and microcomputer have greatly promoted the development of digital relay protection technology. At present, the digital maintenance of microcomputer is in a rapidly changing research and test stage, and a small number of equipment have been put into operation. The basic performance of relay maintenance The correct work of relay maintenance not only effectively improves the safety and reliability of power system operation, but also correctly uses relay maintenance technology and devices, and may also reduce the investment of primary equipment on the premise of meeting the technical conditions of the system. In order to complete its function, relay maintenance must have the following five basic properties. ① Safety: The relay maintenance device should stop reliably when it should not be operated, that is, it should not be misoperated. ② Reliability: The relay maintenance device should act reliably during this action, that is, it should not refuse to act. ③ Rapidity: The relay maintenance device should be able to cut off or eliminate the faulty parts or abnormal working conditions from the system in the shortest time. ④ Selectivity: The relay maintenance device should remove the problem part from the system within the shortest possible time interval to ensure the maximum power supply to the problem-free part. ⑤ Sensitivity: indicates the ability of relay maintenance device to reflect problems. Usually expressed as sensitivity coefficient klm. There are two ways to express the sensitivity coefficient, that is, the maintenance sensitivity coefficient reflecting the rise of the problem parameters, klm= the minimum calculated value of the problem parameters when the metal short circuit occurs in the maintenance area/maintenance action parameters; Maintenance sensitivity coefficient reflecting the decline of problem parameters, klm= operation parameters of maintenance/maximum calculated value of problem parameters when metal short circuit occurs in maintenance area. The five characteristics of relay maintenance are closely related to each other. When choosing a maintenance plan, we should also pay attention to economy. The so-called economy not only refers to the equipment investment and operation and maintenance expenses of the maintenance device, but also must consider the losses caused to the national economy when the maintenance device fails or refuses to operate. Relay protection maintenance can be divided into the following four ways: ① According to the maintenance object, there are transmission line maintenance and major equipment maintenance (such as generator, transformer, bus, reactor and capacitor maintenance). ② According to the maintenance function, there are short-circuit maintenance and abnormal operation maintenance. The former can be divided into main maintenance, backup maintenance and auxiliary maintenance; The latter can be divided into overload maintenance, loss-of-excitation maintenance, out-of-step maintenance, low-frequency maintenance, open-phase operation maintenance and so on. (3) Signal classification, analog maintenance and digital maintenance based on comparison and operation processing of maintenance devices. All electromechanical, rectifier, transistor and integrated circuit (operational amplifier) maintenance devices that directly reflect the continuous analog quantity of input signals belong to analog maintenance; The maintenance device using microprocessor and microcomputer reflects the discrete digital quantity after sampling and A/D conversion, which is digital maintenance. ④ According to the principle of maintenance action, there are overcurrent maintenance, low voltage maintenance, overvoltage maintenance, power direction maintenance, distance maintenance, differential maintenance and high frequency (carrier) maintenance. The system maintenance equipment that realizes relay maintenance function is called relay maintenance device. Although there are many kinds of relay maintenance and different equipment, they all include the following main links: ① signal acquisition, that is, measurement; ② signal analysis and processing; ③ Judgment link; (4) Output link of action signal. The above is only limited to the relay maintenance of components (generators, transformers, buses, transmission lines, etc.). ) power system, the operation practice of power systems in various countries has proved that it is far from preventing the serious accident of long-term large-scale power outage of the whole power system by only configuring relay maintenance devices of various parts of the power system. Therefore, it is necessary to study the working conditions of the system, the characteristics of the system when it loses stability and how to restore the normal operation of the system as soon as possible after the corresponding relay maintenance device removes the faulty components. These are the contents that need to be studied in system maintenance. The task of system maintenance is to limit the influence scope to the minimum as far as possible and reduce the load outage time to the shortest when the normal operation of large power system is damaged.