Analysis of relay protection problems in electrical equipment

With the popularization of microcomputer relay protection applications, protection devices have gradually been equipped with corresponding data interfaces to realize remote data transmission of important information of protection devices. It has become a consensus to make full use of the technical characteristics of digital protection, realize the status maintenance of digital protection, change the current planned maintenance mode of protection devices, change preventive tests to predictive tests, and improve the safe operation level of equipment.

However, relay protection constitutes a system, not just the device itself, such as AC, DC, control circuits, etc., because some circuits have no monitoring means, it is impossible to conduct real-time technical analysis and judgment on the equipment status. For example, since the operation circuit has always been implemented by hardware, except for a small amount of hardware signals that can be uploaded through remote control or comprehensive self-equipment, there is no online monitoring means for the circuit, forming a blank spot in the protection monitoring circuit.

Therefore, in terms of the current application status of relay protection devices, strictly speaking, most protections do not meet the conditions for condition-based maintenance.

1. Overview of Condition-Based Maintenance

The equipment maintenance system is constantly evolving with the advancement of science and technology, from post-maintenance/fault maintenance to preventive maintenance. There are two main modes of preventive maintenance: time-based maintenance, regular maintenance with pre-set maintenance work content and cycle, or planned maintenance, and reliability-centered maintenance.

Condition-based maintenance, also known as predictive maintenance, was first proposed by DuPont in the United States. It uses the current working condition of the equipment as the basis for maintenance, and uses condition monitoring to diagnose the health of the equipment and determine whether the equipment needs maintenance or the best time for maintenance. The goal of condition-based maintenance is to reduce equipment downtime, improve equipment reliability and availability, extend equipment life, reduce operating and maintenance costs, improve equipment operating performance, and improve economic benefits. Condition-based maintenance is based on effective monitoring of equipment status, and arranges maintenance time and projects based on the results of monitoring and analysis and diagnosis. It mainly includes three links: equipment status monitoring, equipment diagnosis, and maintenance decision-making. Condition monitoring is the basis of condition-based maintenance, and condition monitoring is the basis for equipment diagnosis. Maintenance decision-making is to combine the online monitoring and diagnosis situation, and comprehensively consider the technical application requirements of equipment and systems to determine specific maintenance plans or strategies. The power system has long implemented a maintenance system that mainly relies on preventive planned maintenance. Maintenance items are determined mainly based on maintenance regulations. There is a situation where equipment with many defects is under-maintained, while equipment in good condition is over-maintained. This has led to blind maintenance to a certain extent. In fact, it is difficult to truly achieve the maintenance goal of "maintaining what needs to be maintained, and maintaining a good repair."

Electrical equipment can be divided into primary equipment and secondary equipment according to different functions. Among them, electrical secondary equipment mainly includes relay protection, automatic devices, fault recorders, local monitoring and telecontrol, etc. With the promotion of primary equipment status maintenance and the application of line non-stop maintenance technology, the power outage time caused by maintenance equipment will become shorter and shorter, which objectively puts forward new requirements for the maintenance of electrical secondary equipment. As an important part of electrical secondary equipment, relay protection undertakes the important function of ensuring the stability of the power grid and the safety of power equipment. In actual operation, system failures caused by relay protection often occur. Although the number of incorrect protection actions has been relatively reduced with the widespread use of digital protection devices, the absolute number of incorrect protection actions caused by various reasons such as manufacturing, design, construction, testing, and operation is still very large. As the main means to ensure that relay protection correctly performs the function of "silent sentinel" of the power grid, it is still based on the traditional "Regulations on the Inspection of Relay Protection and Power Grid Safety Automatic Devices" to maintain the availability of protection devices through regular maintenance. Obviously, this inspection regulation based on static equipment is no longer suitable for digital protection that is widely used in modern information technology.

Therefore, how to ensure the reliable operation of relay protection equipment to meet the requirements of safe operation of power grid through appropriate technical measures and means in terms of maintenance system, maintenance methods, inspection items, maintenance cycle, etc., and implement status maintenance of protection equipment will become an inevitable choice.

Protection status maintenance requirements #e#

2. Maintenance requirements for protection status

Traditional relay protection, safety automatic devices and secondary circuit wiring are to ensure that the device components are intact and function normally through regular inspections, and to ensure that the circuit wiring and setting values ​​are correct. If a protection device fails between two inspections, it can only be discovered when the protection device fails or the next inspection. If a power system failure occurs during this period, the protection will not work correctly. The previous protection inspection procedures were designed based on static relays and did not fully consider the technical characteristics of digital protection. It is not reasonable to use the maintenance cycle and items specified in the previous regulations for digital protection.

At the same time, the current main connection mode of the power grid has greatly limited the time for equipment outage and maintenance. For example, it is difficult to implement power outage maintenance for line protection with a half-circuit breaker connection mode, unless it is combined with line power outage maintenance; the double busbar connection mode has gradually cancelled the bypass switch, and it is difficult for transformer protection to require transformer power outage due to protection verification. The regular inspection arrangement of bus differential protection and failure protection is even more difficult. On the other hand, live verification protection has safety risks in implementation and personnel safety responsibility risks. Therefore, in actual operation, it is basically difficult to ensure that the protection equipment can effectively complete the inspection items in accordance with the requirements of the "Regulations on Inspection of Relay Protection and Power Grid Safety Automatic Devices"; in particular, the characteristics of digital protection depend to a large extent on software programming, which is not possible to find the deviation of protection characteristics through traditional inspection items. In fact, the rationality of the inspection items determined by the traditional inspection procedures has faced the challenge of new technology application. The implementation technology of digital protection enables the protection device itself to have a strong self-test function. Therefore, as the monitoring and diagnosis of the device itself has the possibility of realization, the determination of the maintenance decision of the protection device has a reliable basis. At the same time, the conceptual rationality and technical feasibility of condition-based maintenance of electrical equipment make the protection condition-based maintenance mode have a strong demonstration effect. The improvement of maintenance efficiency and equipment reliability will effectively improve the safety and availability of equipment and meet the needs of safe and stable operation of the power system.

3. Application difficulties that need to be solved

Unlike electrical primary equipment, the state monitoring object of electrical secondary equipment is not a single component, but a unit or a system. What is monitored is the dynamic performance of each component. The development of self-diagnosis technology of microcomputer protection and microcomputer automatic devices has laid a technical foundation for the state monitoring of protection equipment.

Although the digital protection device itself has the basis for implementing condition-based maintenance, the relay protection that serves as a safety barrier for the power grid includes not only the device itself but also AC input, DC circuit, operating control circuit, etc. If the scope of condition-based maintenance is limited to the device itself, it will be difficult to have a basis for implementation and promotion. The condition-based maintenance of protection must be considered as a systematic issue. In other words, if the status monitoring link of the protection can include AC input, DC, operating circuit, etc., the condition-based maintenance is more likely to be promoted in practical applications.

Therefore, the implementation of protection equipment condition maintenance should monitor: AC measurement system, including CT, PT secondary circuit insulation, circuit integrity, and measurement element integrity; DC system, including DC power, operation and signal circuit insulation, circuit integrity; logic judgment system: including hardware logic judgment circuit and software function. The protection device itself is easy to realize condition monitoring, but because the electrical secondary circuit is composed of several relays and cables connecting various devices, it is difficult to monitor the condition of relay contacts and the correctness of circuit wiring online, which may be one of the main reasons why the protection has not been able to effectively promote condition maintenance.

The electrical secondary operation circuit is a circuit for operating and controlling electrical primary equipment, and is an important part of relay protection. When the relay protection equipment requires status maintenance, the relay protection export control circuit operation box adopts a hardware structure, that is, the relay is directly connected through the secondary line in the 220V strong current circuit, the wiring is complicated, and it does not have the functions of self-test, online monitoring, and remote data transmission.