1. Introduction Condition-based maintenance is a hot topic in current power system research. With the opening of the power market, competition among power sectors will become increasingly fierce, making condition-based maintenance of electrical equipment imperative. The development of microelectronics, computer technology, and communication technology has made condition-based maintenance of electrical equipment possible. Currently, there are many research articles in China on condition-based maintenance technology for primary electrical equipment, but condition-based maintenance of secondary electrical equipment that protects, controls, and monitors primary equipment has been neglected. Traditional relay protection, according to the requirements of the "Regulations on Inspection of Relay Protection and Automatic Safety Devices for Power Grids," involves periodic inspections of relay protection, automatic safety devices, and secondary circuit wiring to ensure that device components are intact, function normally, and that circuit wiring and settings are correct. If a fault occurs in the protection device between two inspections, it can only be detected when the protection device malfunctions or during the next inspection. If a power system fault occurs during this period, the protection will not operate correctly. Abnormal protection devices are a very serious problem for the power system. Therefore, secondary electrical equipment also needs condition monitoring, implementing a condition-based maintenance model to keep pace with primary equipment and adapt to the needs of power system development. 2. Condition-Based Maintenance System 2.1 Evolution of Condition-Based Maintenance System Equipment maintenance systems have continuously evolved with the development of social productivity and the progress of science and technology. From the reactive maintenance/breakdown maintenance (BM) of the First Industrial Revolution to the preventive maintenance (PM) of the Second Industrial Revolution in the 19th century (CM), preventive maintenance has further developed over many years, resulting in different maintenance methods depending on the technical conditions and objectives. These mainly include time-based maintenance (TBM), also known as scheduled maintenance (SM), which pre-sets maintenance work content and cycles; reliability-centered maintenance (RCM), a maintenance method aimed at achieving the inherent reliability level of mechanical equipment at the lowest cost; and in 1970, DuPont first advocated condition-based maintenance (CBM), also called predictive diagnostic maintenance (PDM). This maintenance method is based on the current operating condition of the equipment. Through condition monitoring, it diagnoses the equipment's health status to determine whether maintenance is needed or the optimal time for maintenance. The goals of condition-based maintenance are: to reduce downtime (total maintenance), improve equipment reliability and availability, extend equipment life, reduce operating and maintenance costs, improve equipment performance, and increase economic efficiency. 2.2 Definition of Condition-Based Maintenance Condition-based maintenance can be simply defined as: a maintenance method that scientifically arranges maintenance time and projects based on equipment condition monitoring and the results of monitoring and analysis. It has three layers of meaning: equipment condition monitoring; equipment diagnosis; and maintenance decision-making. Condition monitoring is the foundation of condition-based maintenance; equipment diagnosis, based on condition monitoring, integrates historical equipment information and utilizes technologies such as neural networks and expert systems to judge the equipment's health status. For electrical equipment, condition-based maintenance includes not only online monitoring and diagnosis but also equipment operation and maintenance, live-line testing, preventative testing, fault recording, equipment management, equipment maintenance, and post-maintenance acceptance, among other tasks. Finally, it involves making maintenance decisions based on comprehensive information from equipment, operating conditions, and the power market. When making maintenance decisions for power plants and substations, it is necessary to consider the operating status of the power grid, such as peak and valley periods of electricity consumption, and the wet and dry seasons of power generation; it is also necessary to consider the operating status of other equipment in the unit system to which the equipment is located, as maintenance by system is more reasonable than maintenance of only a single piece of equipment; the needs of the electricity market must be considered; and a decision risk analysis must be conducted. Since the founding of the People's Republic of China, the power system has long implemented a maintenance system mainly based on preventive planned maintenance. This maintenance system has revealed problems such as insufficient maintenance of equipment with many defects, excessive maintenance of equipment in good condition, and blind maintenance due to reliance on maintenance procedures to determine maintenance items. With the development of the social economy and the improvement of science and technology, it is gradually transitioning to a predictive condition-based maintenance system. For example, the Dalian Power Supply Bureau began to pilot condition-based maintenance of electrical equipment in 1990. [b]3 Condition-based Maintenance of Secondary Electrical Equipment[/b] Electrical equipment can be divided into primary equipment and secondary equipment according to their functions. Secondary electrical equipment mainly includes relay protection, automatic devices, fault recording, local monitoring, and remote control. Their normal and reliable operation is a fundamental requirement for ensuring power grid stability and the safety of power equipment. In actual operation, system faults caused by secondary electrical equipment occur frequently. Table 1 summarizes the operation of relay protection devices in 220kV and above systems over four years. It can be seen that the number of incorrect protection actions has decreased relatively, but it is still quite high. The causes of incorrect protection actions involve protection personnel, operating personnel, design departments, manufacturing departments, natural disasters, and other unknown causes. With the widespread application of microcomputers in relay protection and automatic devices, the reliability and setting flexibility of relay protection have greatly improved. Maintaining secondary electrical equipment based on the traditional "Regulations for the Inspection of Relay Protection and Power Grid Safety Automatic Devices" is clearly outdated. Furthermore, the promotion of condition-based maintenance of primary equipment and the application of live-line maintenance technology will result in increasingly shorter power outage times due to equipment maintenance. This poses new requirements for the maintenance of secondary electrical equipment. Therefore, changes are needed in the maintenance system, methods, inspection items, and scheduled maintenance cycles of secondary electrical equipment. Condition-based maintenance of secondary electrical equipment should be implemented to ensure its reliable operation and meet the needs of power development. Condition-based maintenance of secondary electrical equipment uses equipment condition monitoring and self-diagnosis technologies, combined with historical data on equipment operation and maintenance, to accurately evaluate the condition of the secondary equipment and scientifically schedule maintenance time and items based on the evaluation results. [b]3.2 Content of Condition Monitoring for Secondary Electrical Equipment[/b] Condition-based maintenance is based on equipment condition monitoring. It involves monitoring the correctness and reliability of secondary equipment operation and estimating its lifespan. The main objects of condition monitoring for secondary electrical equipment include: AC measurement systems; DC operation and signal systems; logic judgment systems; communication systems; and shielded grounding systems. AC measurement systems include good insulation and integrity of the TA and TV secondary circuits, and the integrity of measuring elements; DC systems include good insulation and integrity of the DC power, operation, and signal circuits; and logic judgment systems include hardware logic judgment circuits and software functions. Unlike primary electrical equipment, the condition monitoring object of secondary electrical equipment is not a single component, but a unit or a system. It monitors the dynamic performance of each component, and the performance of some components still requires offline testing, such as the characteristic curve of the current transformer (CT). Therefore, offline testing data of secondary electrical equipment is also the basis for condition monitoring and diagnosis. 3.3 Condition Monitoring Methods for Secondary Electrical Equipment Compared with primary electrical equipment, condition monitoring of secondary electrical equipment does not rely excessively on sensors. Therefore, condition monitoring of secondary electrical equipment is easier to achieve both technically and economically. Conventional protection condition monitoring is relatively difficult to implement; existing measurement methods can be fully utilized without increasing new investment. Examples include monitoring of CT and TV line breaks; DC circuit insulation monitoring; and secondary fuse blowout alarms. The development of self-diagnostic technology in microprocessor-based protection and microprocessor-based automatic devices, and the improvement of substation fault diagnosis systems, have laid the technical foundation for condition monitoring of secondary electrical equipment. Each module within the protection device has self-diagnostic functions, performing inspection and diagnosis of the device's power supply, CPU, I/O interface, A/D converter, memory, and other components. Fault testing methods such as comparison method, coding method, verification method, watchdog timer method, and characteristic word method can be used. For protection devices, diagnostic programs can be loaded to automatically test each piece of equipment and component. 4. Several Issues in Condition-Based Maintenance of Electrical Secondary Equipment 4.1 Electrical Secondary Circuit Monitoring Issues Electrical secondary equipment can be structurally divided into electrical secondary circuits and protection (or automatic) devices. Currently, the microcomputerization of protection devices makes condition monitoring easy. However, electrical secondary circuits consist of numerous relays and cables connecting various devices, with many points scattered. Monitoring the condition of relay contacts and the correctness of circuit wiring online is difficult and uneconomical. For electrical secondary circuits, the focus should be on equipment management, such as equipment acceptance management and offline maintenance data management, combined with online monitoring to diagnose their condition. 4.2 Electromagnetic Interference Monitoring Issues of Power System Secondary Equipment Due to the widespread use of numerous microelectronic components and high-integration circuits in electrical secondary equipment, these devices are becoming increasingly sensitive to electromagnetic interference and are highly susceptible to it. Electromagnetic waves interfere with secondary equipment, causing sampling signal distortion, automatic device malfunction, protection malfunction or failure to operate, and even component damage. The International Electrotechnical Commission (IEC) and relevant domestic departments have formulated electromagnetic compatibility (EMC) standards for relay protection. However, currently, the monitoring and management of the on-site electromagnetic environment are not included in the maintenance scope, and there are no suitable monitoring methods. Conducting electromagnetic compatibility assessment tests on secondary equipment is a very important part of condition-based maintenance of secondary equipment. Interference sources, coupling paths, and sensitive devices in different plants and stations must be monitored and managed. For example, checking the shielding and grounding status of secondary equipment; managing the use of mobile communication devices (such as mobile phones) near microprocessor-based protection devices. [b]4.3 Relationship between Condition-Based Maintenance of Secondary Equipment and Condition-Based Maintenance of Primary Equipment[/b] The maintenance of primary equipment and secondary equipment is not completely independent. In many cases, secondary equipment maintenance can only be carried out when the primary equipment is de-energized for maintenance. When making decisions on condition-based maintenance of secondary equipment, the situation of the primary equipment must be considered, and a technical and economic analysis of condition-based maintenance should be conducted. The goal is to reduce power outage maintenance time, minimize economic losses caused by power outages, reduce the number of maintenance operations, and lower maintenance costs, while ensuring the reliable and correct operation of secondary equipment. [b]4.4 Relationship between Condition-Based Maintenance of Secondary Equipment and Equipment Management Information System (MIS)[/b] Many power plants have now established Equipment Management Information Systems (MIS) to manage equipment operation status, defect and fault conditions, and records of previous maintenance and tests using computers, achieving information sharing. This information is one of the important bases for making condition-based maintenance decisions. To achieve condition-based maintenance of secondary equipment, it is necessary to improve the Equipment Management Information System (MIS). 5 Conclusion Condition-based maintenance of electrical secondary equipment is a necessity for the development of the power system. The widespread use of self-diagnostic technologies in microcomputer protection and microcomputer automatic devices makes condition monitoring of electrical secondary equipment relatively easy to achieve both technically and economically. With the development of integrated automation systems, the number of secondary equipment and cables can be greatly reduced, overcoming the difficulties currently existing in conventional protection condition monitoring. The widespread use of Equipment Management Information Systems (MIS) in the power system provides information support for the implementation of condition-based maintenance of electrical secondary equipment. Condition monitoring of secondary electrical equipment will contribute to the development of integrated automation in substations.