I. Introduction With the deepening of power system reform, the power system needs to further improve the reliability of power equipment in the power market. Ensuring operational reliability and maintenance economy has become an urgent need for transmission and generation companies to adapt to market competition. Replacing the existing planned maintenance model with condition-based maintenance is a trend in the development of the power industry. According to the requirements of the "Guiding Opinions on Implementing Condition-Based Maintenance in Thermal Power Plants" issued by the State Power Corporation, power companies should select some suitable equipment for condition-based maintenance based on their own equipment conditions, personnel, and financial situation, and carry out the work within a certain scope first. 550kV, 220kV, and 110kV high-voltage switches, as well as plant-level high-voltage switches, have all been included in the scope of condition monitoring. The foundation for realizing condition-based maintenance of electrical equipment is online condition monitoring. Online monitoring can accurately and in real-time reflect the condition of electrical equipment and predict its service life, providing a basis for maintenance decisions. Electrical equipment condition monitoring is not only the foundation of the condition-based maintenance model, but also meets the development needs of unmanned substations—the electrical operation management model currently being implemented by China's power sector. This necessitates that power companies gradually develop an optimized comprehensive maintenance approach integrating fault diagnosis, periodic maintenance, condition-based maintenance, and proactive maintenance to improve equipment reliability and reduce electricity costs. II. Significance of Online Condition Monitoring for Circuit Breakers Online condition monitoring systems for high-voltage circuit breakers are used to monitor the mechanical performance, electrical insulation performance, contact life, and operating circuit status of high-voltage circuit breakers in substations, switching stations, and power plants. By comprehensively analyzing online monitoring data and relevant historical data, the current operating status of the high-voltage circuit breaker can be diagnosed, providing a basis for decision-making regarding condition-based maintenance of electrical equipment. In the power system, high-voltage circuit breakers are the most numerous, require the largest amount of maintenance, and are the most expensive. Statistics show that more than half of substation maintenance costs are spent on high-voltage circuit breakers, and 60% of that is spent on minor repairs and routine maintenance. Furthermore, statistics indicate that 10% of circuit breaker failures are caused by incorrect maintenance. Major overhauls of circuit breakers involve complete disassembly, which is time-consuming, expensive, and can introduce many new defects during disassembly and reassembly. In the current relatively conservative planned maintenance, the maintenance lacks a certain degree of targeting. Circuit breaker online condition monitoring systems can promptly understand the working status and location of defects of circuit breakers, reducing premature or unnecessary power outages for testing and maintenance, reducing maintenance workload, lowering maintenance costs, and improving the targeting of maintenance, thus significantly improving the reliability and economy of the power system. Online monitoring mainly includes mechanical online monitoring, contact electrical life monitoring, etc., and the monitoring content includes leakage current monitoring, gas density monitoring, breaking number monitoring, breaking current monitoring, vibration waveform monitoring, opening and closing coil current waveform monitoring, circuit breaker infrared imaging monitoring, and operating mechanism oil pressure monitoring, etc. III. Analysis of the Usage of High-Voltage Circuit Breakers 1. Usage Status of Circuit Breakers in the Domestic Power System According to statistics from various power group companies and provincial (regional, municipal) power companies (bureaus), as of the end of 1999, there were 273,153 6-220KV distribution switches operating in the national power system, an increase of 23,261 units compared to 1998, with an average annual growth rate of 9.3%, and it is projected to reach 390,000 units by 2003. The usage ratio of high-voltage switches at each voltage level, with 550kV installed as 100%, is as follows: 550kV : 363kV : 252kV : 126kV : 72.5kV : 40.5kV : 12kV, which is 100% : 51% : 1653% : 4491% : 816% : 6391% : 27711%. The types and voltage levels of circuit breakers installed by some provincial and municipal power companies are shown in Figure 1 and Figure 2, respectively. Figure 1: Statistical distribution of circuit breaker types [align=center]Figure 2: Statistical distribution of circuit breakers by voltage level (with 550kV installed as 100%)[/align] 2. Status of online monitoring of switchgear abroad According to information, the United States issued the "Guidelines for Insulation Diagnosis of Electrical Equipment" in 1995, and has now shifted to online monitoring as the main method, and has formulated relevant standards. Japan entered the era of predictive maintenance based on condition monitoring in the 1980s. Its research and application of this technology progressed rapidly, accumulating a wealth of data and experience, and gradually forming some standards and relatively mature methods. In 1998, GIS intelligent maintenance included monitoring internal corona, decomposition gases, sound conduction, withstand voltage characteristics, and temperature rise. The ion mobility spectrum analyzer (manufactured by Analytical Sensor Systems) developed by the Central Laboratory of the Institute for Spectral Chemistry and Spectral Applications and the Institute for Scientific Research and Development in Germany, as a tool for on-site monitoring of SF6 circuit breaker gas, has been successfully applied in the Brazilian power sector. In the United States, more than 50% of power companies have applied condition-based maintenance technology to implement condition-based maintenance for power generation equipment. CONSOLIDATED ELECTRONICS, INC. (CEI) has developed and manufactured the SM6 series circuit breaker monitor (US Patent No. 5,385,451), which can measure SF6 gas temperature and pressure online and continuously and in real-time calculate SF6 gas density. Simultaneously, it can monitor the operating status of the circuit breaker's internal heaters, operating current, gas/liquid pressure, and trip/close coils, predicting SF6 gas leakage trends and other potential faults, thus preventing accidents and improving the reliability of power system operation. This monitoring data can be obtained via a field PC or through remote communication via RS232/485 interface, and the data can be stored for up to 10 years. This product has already entered the Chinese market. ABB has already considered the status monitoring of some circuit breaker performance aspects during the production of high-voltage gas-insulated switchgear (GIS), such as the energy storage status, secondary circuit integrity, and opening/closing position status of the circuit breaker. ABB has also launched an intelligent centralized control/protection unit for high-voltage switchgear, combining control, signaling, protection, measurement, and monitoring functions, enabling the high-voltage switchgear to have continuous self-monitoring capabilities and direct connection to the power plant control system. Monitoring items include: auxiliary circuit monitoring, power supply monitoring of motor operating switch devices, trip coil integrity monitoring, circuit breaker energy storage spring status monitoring, mechanical operation frequency monitoring, gas pressure monitoring, cumulative switch operating time (in hours), and contact switching time (from closed to open). 3. A rapidly developing market for online circuit breaker monitoring: Domestic and international research shows that implementing condition monitoring for high-voltage circuit breakers has significant economic benefits. In 1995, the US Electrical Research Institute (ERI) partnered with Corn Edison Power Plant to install condition monitoring equipment on 10 generating units. Calculations showed that the economic benefits exceeded US$16 million in the first two years. In China, with the advancement of power system reform, the development of the power market requires a new maintenance model—condition-based maintenance. Currently, many power plants and power supply bureaus have implemented condition-based maintenance. For example, the condition-based maintenance work at Beilun Power Plant, Waigaoqiao Power Plant, Zouxian Power Plant, Dalian Power Supply Bureau, Guangzhou Power Supply Bureau, and Baoji Power Supply Bureau has achieved good results. In the coming years, the power system will require a large number of online condition monitoring devices. Based on an annual installation rate of 2% for circuit breakers, approximately 5463 (273153 × 2%) to 780 (390000 × 2%) circuit breakers will be equipped with local monitoring units (2% explanation: assuming 40% of units agree with online condition monitoring technology, of which 50% are prepared to install online condition monitoring systems on 10% of their circuit breakers). IV. Development Direction of Circuit Breaker Online Condition Monitoring Systems Currently, condition monitoring products primarily monitor the insulation status of electrical equipment in substations, with fewer monitoring other performance aspects. From the application of online monitoring systems, the following aspects should be considered: (1) Determination of fault diagnosis threshold. Determining the fault threshold is a prerequisite for correct diagnosis. Early fault diagnosis relies on practical experience or standards. By analyzing the changing trends of operating characteristics from the relevant action parameters and performance parameters provided by the online monitoring device, or by comparing historical records, the circuit breaker status can be judged. (2) There are three types of circuit breakers: three-phase linkage operation, three-phase split operation, and single-phase circuit breaker. The types of data collected are the same, but the number of data collected varies greatly. It is impossible to take all three cases into account when sampling. It is recommended that the design of the sampling board be based on three-phase linkage operation circuit breakers, while also taking into account single-phase operation circuit breakers. The monitoring of three-phase split operation circuit breakers should be studied in a timely manner. (3) When installing the monitoring unit outdoors, it is necessary to solve the problems of anti-interference, moisture protection, and long-term operational stability. The measurement error and response time of the sensor affect the measured quantity. For example, the response time of the Hall current sensor can reach 10μS, but the response time of the electromagnetic isolation sensor is 350RNS. Therefore, it is necessary to consider how to correct it. Although the development of the circuit breaker online monitoring system has been going on for several years, from the perspective of actual application, the future development should consider the following aspects: (1) Integration. Integrating multiple monitoring functions of high-voltage circuit breakers into one unit is conducive to the fusion of multiple information and supports networked centralized management. (2) Universality. Due to the different insulation media of circuit breakers (such as oil, SF6 gas, vacuum), the monitored items are different. In hardware design, a sufficient number of sampling channels should be considered to ensure the universality of the sampling board and meet the requirements of different types of circuit breakers. In software design, configuration technology is used to set the relevant parameters of different types of circuit breakers. (3) Expandability. The number of circuit breakers monitored by the monitoring system is unlimited; considering the compatibility between the circuit breaker local sampling unit and the substation online monitoring system, it can provide information for the substation integrated automation. (4) Combinability. The internal plug-in modules of the circuit breaker local sampling unit are modular. (5) Reliability. The monitoring system has good electromagnetic compatibility and self-testing function.