1. Introduction The reliability of power supply is crucial to the national economy and people's livelihood. Ensuring the safe and reliable operation of the power system has always been a critical issue for the power sector, and the safe operation of high-voltage equipment is the foundation of the entire system's safe operation. When high-voltage electrical equipment operates in the power grid, insulation defects caused by poor manufacturing, aging, or external damage can lead to insulation accidents that affect the safe operation of both the equipment and the power grid. Therefore, the traditional practice after equipment is put into operation is to periodically shut down power for preventative testing and maintenance to detect insulation defects and prevent insulation accidents. However, with the development of the national economy, society's demands for the reliability of power supply are increasing, and the power system is gradually expanding. The traditional practice of periodically shutting down power for preventative testing can no longer meet the high reliability requirements of the power grid. With the development of science and technology, the concept of online insulation monitoring for high-voltage electrical equipment has been proposed and welcomed by industry professionals, and the technology has developed rapidly. Several 500kV substations under our company's jurisdiction have been using this technology since 1998, achieving some experience and good results. According to the monitoring results of the online monitoring system, it was found that the dielectric loss of multiple current transformers in the 500kV-200kV range was seriously exceeded and the leakage current of one 500kV surge arrester was seriously exceeded. 2. Overview of the development of online monitoring technology for insulation of high-voltage electrical equipment . Many foreign power companies began to study and promote the application of online monitoring technology for substation equipment in the 1970s. The main purpose was to reduce the time and number of preventive power outage tests and improve the reliability of power supply. However, the equipment at that time was rudimentary, the testing methods were simple, and the level was low. With the rapid development of computer technology, online monitoring equipment products have been continuously updated and improved, and the level of online monitoring technology has been continuously improved. To date, many countries have widely used online monitoring technology. In recent years, at the International Society for High Voltage Engineering (ISH) and the Asian Conference on Insulation Diagnosis (ACEID), papers on online monitoring and condition-based maintenance of electrical equipment insulation accounted for a considerable proportion. The development of online insulation monitoring technology has generally gone through three stages. (1) Live testing stage. This stage began around the 1970s. At that time, people simply measured certain insulation parameters (such as leakage current) of electrical equipment directly in order to avoid power outages. The equipment was simple, the test items were few, and the sensitivity was poor. (2) Starting in the 1980s, various special live-line testing instruments appeared, which made online monitoring technology move from traditional analog quantity testing to digital measurement, getting rid of the traditional measurement mode of directly connecting the instrument to the test circuit. Instead, the measured parameters were directly converted into electrical signals by sensors. (3) Starting in the 1990s, with the popularization and use of computer technology, microcomputer multifunctional online insulation monitoring systems with computer processing technology as the core appeared. Using computer technology, sensing technology and digital waveform acquisition and processing technology, more insulation parameters can be monitored online. This online monitoring has a large amount of information and fast processing speed. It can display, store, print, transmit remotely and alarm for over-line in real time, realize the automation of online insulation monitoring, and represent the development direction of online insulation monitoring today. To date, a large number of online monitoring technologies have been widely used in the defect detection of power system equipment and have gained some experience. For example, online chromatographic analysis of transformer oil and infrared temperature measurement technology of electrical equipment are very mature and have played an important role in detecting the insulation performance of equipment. In China, the development and application of online monitoring technology began in the 1980s. Due to limitations in overall technology at the time, such as the low reliability of electronic components and the nascent stage of computer applications, the level of online monitoring technology was relatively low. The late 1980s saw the first application boom in China, but later, due to various reasons, it cooled down, reaching a low point in the mid-1990s. However, some manufacturers and research institutions did not slacken their research on this technology, and power supply departments in various regions gradually introduced online monitoring technology. After 2000, with the continuous maturation of online monitoring technology and objective needs, online monitoring technology began to be valued again, and currently, this work is being carried out in many regions of China. 3. Basic Principles 3.1 Basic Principles Online monitoring technology for high-voltage electrical equipment insulation monitors various characteristic parameters of insulation using the operating voltage of electrical equipment while it is in operation. Therefore, it can accurately reflect the operating conditions of electrical equipment insulation, thus making a relatively accurate judgment on the insulation status. The main parameter detected in online monitoring of high-voltage electrical equipment insulation is the dielectric loss value of the electrical equipment, and its measurement principle mostly uses hardware phase detection, i.e., zero-crossing comparison. Most current online insulation monitoring products use the Fast Fourier Transform (FFT) method to calculate dielectric loss. The standard voltage signal of the operating equipment PT and the leakage current signal of the equipment are directly sampled and converted by high-speed A/D and sent to the computer. The signal is analyzed by frequency spectrum analysis by software, and only the basic signal of 50HZ is extracted to calculate the dielectric loss. This method can effectively eliminate the interference of various high-order harmonics, the test data is stable, and it can reflect the insulation changes of the equipment well. For online monitoring of equipment physical quantities (such as transformer oil temperature, gas content, etc.), the signal is collected by placing sensor probes and converted into digital signals and sent to the computer for analysis and processing. 3.2 General functions of the system In recent years, the online insulation monitoring system for high-voltage equipment can measure the insulation characteristic parameters of the energized equipment in real time and analyze and process the acquired data. It generally has the following functions: (1) Measure the changes in capacitive current and resistive current of the surge arrester during operation, and understand the internal insulation moisture and valve plate aging. (2) Measure the leakage current and dielectric loss of capacitive equipment such as CVT, coupling capacitor, current transformer, and bushing to understand their internal moisture, insulation aging, and damage defects. (3) Measure the changes in the internal combustible gas of the insulating oil in oil-filled equipment to understand whether there are overheating, discharge, or other defects inside the equipment. However, for the entire online measurement system, to ensure its measurement accuracy and stable performance, it must achieve the following performance: a. Stable detection impedance, unaffected by strong electromagnetic interference from substations, self-protected under system operation overvoltage and lightning overvoltage, without performance changes or software damage. b. Good detection signal transmission, without distortion or influence on other nearby signals, and unaffected by interference from other signals. c. Has expert analysis function, intelligently judging the internal insulation status of the equipment. d. System analysis data can be remotely transmitted to achieve data sharing. 4. Key Points Analysis of Monitoring Equipment 4.1 Surge Arresters Most zinc oxide surge arresters used in substations no longer have series gaps. During MOA operation, a certain amount of leakage current always passes through the valve plates, accelerating valve plate aging. Moisture and aging are the main causes of MOA valve plate deterioration. Detecting the total leakage current and resistive current of the MOA can effectively reflect the insulation condition of the MOA. Current measurement reflects severe overall moisture. In early aging, the resistive current increases significantly, while the total current change is not obvious. Under normal operating conditions, the main current flowing through the surge arrester is capacitive current, with resistive current accounting for only a small portion, about 10%-20%. The resistive component mainly includes: surface leakage along the inner and outer surfaces of the porcelain bushing, surface leakage along the valve plates and its own nonlinear resistance component, leakage of the insulating support components, etc. When the valve plates age, the surge arrester is damp, internal insulation components are damaged, or the surface is severely contaminated, the capacitive current does not change much, while the resistive current increases significantly. The main cause of surge arrester accidents is that the increased resistive current leads to increased losses, causing thermal breakdown. Therefore, measuring the AC leakage current and its active component is the main method for on-site testing of surge arresters. The preventive test procedure also includes the measurement of the "operating leakage current" of zinc oxide surge arresters (MOA) in the pre-test items. 4.2 Measuring the dielectric loss tangent of capacitive equipment such as CVT, coupling capacitor, current transformer, and bushing is a highly sensitive test item. It can detect overall insulation dampness, insulation deterioration, and local defects of electrical equipment. Insulation dampness defects account for 85.4% of defects in capacitive equipment. This is because the capacitive structure is forcibly equalized through capacitance distribution, and its insulation utilization coefficient is high. Once the insulation is damp, it will often cause an increase in insulation dielectric loss, leading to breakdown. The development speed of insulation breakdown is very fast. However, insulation deterioration generally has the following basic characteristics: (1) The insulation dielectric loss value will increase, and the heat generated by this and other reasons may eventually lead to thermal breakdown of the insulation. Measuring the insulation loss tangent (tgδ) can detect changes in dielectric loss. (2) Partial discharge and dendritic electricity may occur in the insulation. Partial discharge with large discharge capacity usually only occurs during lightning strikes, switching overvoltages, and insulation damage. The resulting dielectric loss can be reflected by tgδ measurement. (3) The insulation characteristics are more affected by temperature changes. The insulation temperature coefficient is determined by the type, size, and condition of the insulation itself. For a specific voltage level and insulation design, the temperature nonlinearity and sensitivity of tgδ value will increase due to the increase in temperature coefficient caused by insulation deterioration. Therefore, all factors affecting insulation temperature (dielectric loss, ambient temperature, load changes, etc.) have a more significant impact on the tgδ value of aging insulation. For equipment with capacitive insulation, defects that are still in a relatively early stage of development can be found by detecting their dielectric properties. Studies have shown that in the initial stage of defect development, the results obtained by measuring the increase rate of current and the change of dielectric loss tangent are consistent and have high sensitivity; in the later stage of defect development, the phenomenon of current increase and capacitance change are consistent, making it easier to detect the development of defects. 5. Case Introduction Currently, our company manages 9 500kV substations and 13 lines. In the late 1990s, our company began installing online insulation monitoring devices for high-voltage equipment in some of the 500kV substations. Taking the 500kV Weifang substation high-voltage equipment condition monitoring system as an example, we will introduce its installation and operation. The 500kV Weifang substation high-voltage equipment condition monitoring system was put into operation in 1988. At the beginning of its operation, the system was not perfect, and some high-voltage equipment was not actually connected to the operation, resulting in unreliable operation and failure to play its due role. In 2001, we invested in the renovation of the system and installed a CIE-2000 type online insulation monitoring system. 5.1 System Introduction (1) The CIE-2000 type online insulation monitoring system is divided into three parts. The first part is the local signal acquisition unit, the second part is the front-end processing system, and the third part is the remote data analysis and transmission system. The front-end processing system controls the local signal acquisition unit through the industrial bus, displays it in various forms, intuitively compares the current and historical data curves, and transmits it from the internal local area network or to the remote data analysis and transmission system. The diagnostic software can remotely download current and historical data of the substation via WEB and can access the MIS system of Shandong Electric Power Ultra-High Voltage Company to assist relevant professionals in assessment and management. Voltage signal sensor, current signal sensor, voltage signal, insulation leakage current signal, A/D conversion, FFT algorithm test box, industrial control computer, local CRT, telephone controller, MODEM, CIE2000 background system (2) According to the different positions of the terminal board, it can be divided into bus type and branch type. Bus type means that the terminal board is placed in the terminal box and installed on the field. The output cables of the signal box are all connected to the terminal board. Only n buses are needed to lead to the main control room, making the machine screen in the main control room neater and saving cables. Branch type means that the terminal board is installed in the machine screen in the main control room. The cables must be introduced from the field to the machine screen disconnect bar in the main control room and then introduced to the terminal board. (3) Test accuracy of CIE-2000 type insulation online monitoring system: primary leakage current: 1%; equipment equivalent capacitance: 1.5%; system voltage: 1%; frequency: 0.05HZ; dielectric loss test accuracy: 0.1%; surge arrester active current: 10%; surge arrester total current: 5%. 5.2 Comprehensive upgrade and renovation of the main cabinet In early 2001, the upgrade and renovation of the 500kV Weifang substation high-voltage equipment condition monitoring system began, and the CIE-2000 type insulation online monitoring system was installed. The main cabinet contains computer (industrial control computer) system, signal input/processing, power supply and other components. The industrial control computer is the core component of control measurement. All equipment in the main cabinet was upgraded and replaced. The industrial control computer was replaced with a PENTIUM III800SIC class, which basically meets the requirements of data detection, processing and storage of the whole station; the front-end host was replaced with a CPU586, 16M memory, 1M display memory and 1G hard disk capacity; the working software uses the newly developed CIE2000 system. 5.3 Replace all probe sensors. Sensors are divided into insulation signal sensors and voltage signal sensors, which play a crucial role in acquiring and transforming signals in the system. Their performance directly determines the quality of the detection system. Replace all of them with ISS-80 type sensors with full epoxy resin casting and iron shell shielding. Replace the grounding wire of the final screen with a large cross-section multi-strand wire. Note that when installing the signal extraction box, the front of the box should be slightly higher than the rear to prevent water accumulation. The grounding of the signal extraction box should be welded with No. 10 steel bars to ensure reliable grounding. 5.4 Lay armored shielded cables. Replace the existing cables with new six-core armored shielded cables. Add high-strength insulating sleeves to the cable ends to prevent rodent and mechanical damage, as well as interference and for a neat and aesthetically pleasing appearance. 5.5 Connection and commissioning. Throughout the installation and construction phase, ensure a good grounding system with sufficient current-carrying capacity in the grounding wire. After the entire system equipment is installed, proceed to the online commissioning and trial operation phase. For any abnormal conditions that occur immediately, the machine is adjusted and processed in a timely manner. For example, if the grounding current of phase B of the No. 1 main transformer exceeds the standard and alarm is found, the actual measurement of the equipment with a clamp meter is normal. It is later found that the problem is with a phase probe, and it is corrected immediately. After repeated debugging and comparison, after a week of trial operation, the device tends to be stable. The following figure is the signal transmission diagram of the 500KV surge arrester of the No. 1 main transformer. It can be seen that the equipment operation analysis can be performed through images and data, and the automatic detection and manual judgment are implemented in parallel. 5.6 System characteristics (1) Design principle of signal acquisition unit The acquisition unit collects the voltage, end screen current and other signals of the monitored equipment on the spot and performs data processing to obtain its amplitude, phase and other parameters. Then, the microcomputer can calculate electrical parameters such as dielectric loss angle. (2) Design features of acquisition unit: DSP technology is used as the hardware platform; the sensor adopts a high permeability iron core, which can accurately measure the amplitude and angle of small signals. The shielding measures are complete and the interference is reduced; the forward amplification section adopts low temperature drift, high precision operational amplifier and high precision resistor to stabilize the analog amplification channel. (3) Requirements for the selection of monitoring system: Select an online monitoring system for insulation of high-voltage equipment. In accordance with the requirement that the installation of the detection equipment does not affect the operation mode of the substation equipment (especially the grounding of equipment components), the system adopts a single-turn core-type sensor that is not directly electrically connected to the high-voltage equipment; select a hierarchical distributed system to collect electrical parameters locally, and avoid long-distance transmission of microamp and milliamp level small current analog signals; the system should be easy to construct and install and have good maintainability; the local measurement of status data should be accurate and stable. 5.7 System operation status: The high-voltage equipment status monitoring system of this station selected transformer bushings, iron cores, capacitive voltage transformers, current transformers, and zinc oxide surge arresters as the main equipment to be measured. Among them, surge arresters measure the total leakage current and its capacitive and resistive components; transformer bushings, capacitive voltage transformers, and current transformers measure their leakage current and relative changes in dielectric loss; iron cores detect leakage current; and environmental parameters such as on-site temperature, humidity, and surface pollution current of porcelain skirts are monitored and recorded. After the entire system was debugged in the background and put into operation, it has been running normally and the measurement data is accurate. The data is basically consistent with the actual live and de-energized measurement data. After a year of actual operation, the data analysis system software function is relatively complete and the entire system is reliable. Operation practice shows that the online insulation monitoring system can effectively detect some early insulation faults. 6. Online monitoring and condition-based maintenance The traditional operation and maintenance work of the power system is to implement "planned maintenance". "Planned maintenance" means that the electrical equipment must be shut down for maintenance according to the test cycle specified in the preventive test regulations for high-voltage electrical equipment. Condition-based maintenance, on the other hand, is based on the actual operating conditions of the equipment. According to the changes in its insulation characteristic parameters under the operating voltage, it is determined by analysis and comparison whether the electrical equipment needs maintenance, and the items and contents to be maintained. It has strong pertinence and real-time nature. Therefore, it can be said that "condition-based maintenance" means "maintain what needs to be maintained and maintain it well". 6.1 Characteristics of planned maintenance (1) Periodicity. Planned maintenance is carried out according to the test cycle specified in the preventive test regulations. It must be maintained when the time comes. It has strong periodicity. The advantage is that it is convenient to arrange work plans. The disadvantage is that it does not care about the actual condition of the equipment, has a lot of blindness and coercion, and is prone to "over-maintenance" of the equipment, wasting a lot of manpower and material resources. At the same time, various withstand voltage tests may cause new damage to the insulation of the equipment, etc. (2) Temporary. Periodic preventive tests can only detect the insulation status of the equipment at a certain time. They cannot detect the insulation status of the equipment in a timely manner, cannot determine when the equipment will have insulation defects, and cannot detect the development of defects. In particular, defects that develop rapidly inside the equipment and are prone to causing major insulation accidents cannot be detected. (3) Low test voltage. The test voltage of periodic preventive tests is generally lower than the operating voltage of the equipment, so periodic preventive tests cannot accurately detect defects under the operating voltage of the equipment. (4) Reduced power supply reliability of the power grid. Since the periodic preventive tests of planned maintenance need to be carried out under power outage, the power outage time of the equipment is increased, which will inevitably affect the power supply reliability of the power grid. At the same time, the power supply department also suffers a loss of power supply. 6.2 Condition maintenance under the guidance of online monitoring (1) Real-time. High-voltage equipment online monitoring technology monitors the insulation status of equipment in real time, without being limited by the equipment's operating conditions and time. It can detect the insulation status of equipment at any time. Once a defect occurs in the equipment, it can be detected, tracked, and dealt with in a timely manner, which is more significant for ensuring the safety of the power grid. (2) Authenticity. Since online monitoring technology detects the insulation parameters under the operating voltage and conditions of the equipment, the detection results are consistent with the actual situation and are more authentic and comprehensive. (3) More targeted. The maintenance items, contents, and time can be determined according to the development and changes of insulation defects. The maintenance purpose is clear and more targeted. (4) Improved equipment power supply reliability. Due to the implementation of condition-based maintenance, the number and time of equipment power outages are reduced, the equipment power supply reliability is improved, power supply loss is avoided, and the overall labor productivity of the power sector is also improved. 6.3 The basis of condition-based maintenance is to grasp and understand the insulation parameters of the equipment under energized conditions in real time. Online monitoring technology is the only way to obtain the insulation parameters of the equipment under energized conditions. By monitoring various state parameters of insulation in real time under operating voltage and analyzing and comparing the changes of these quantities, it is determined whether the equipment needs to be maintained. In recent years, the manufacturing quality and level of high-voltage equipment have been greatly improved, which has also provided better conditions for condition-based maintenance, and the advantages of condition-based maintenance have become more obvious. 7. Discussion of problems The promotion and application of online monitoring technology for electrical equipment insulation has played a positive role in the safe operation of electrical equipment. Many power supply departments have actively promoted condition-based maintenance, which has reduced the workload of equipment maintenance and improved the reliability of the power grid. It is welcomed by power supply grassroots units and power supply employees. However, due to the complexity of the technology and the diversity of electrical equipment, there are still some problems worth studying and discussing. (1) The characteristics and quality of the sensor are the key to online monitoring. Currently, the commonly used sensor is the Rokovsky coil sensor, which is easily affected by external environment such as temperature, pressure, and impact. It is an important factor affecting the accuracy and stability of the test. Therefore, the development of high-precision and high-stability sensors is still a research topic for online monitoring. (2) Interference problem. Since high-voltage electrical equipment is in a strong electric field environment, the difficulty of collecting micro signals increases. (3) Requirements for online monitoring technology for equipment manufacturers. Currently, high-voltage electrical equipment does not consider the problem of online monitoring. Online monitoring equipment manufacturers design and install it according to the equipment conditions in the operating station. Some operating equipment can be installed and extract signals, while others cannot. For example, the monitoring of transformer core temperature, top oil temperature, and gas cannot be carried out because sensors cannot be installed. However, if this is taken into account during the transformer manufacturing and design, it is not a difficult task. (4) Accumulate operating experience, improve the expert system, and formulate monitoring standards. The insulation parameters of high-voltage electrical equipment online monitoring often have a "deviation" from the power outage test results, but this "deviation" often has a certain pattern. As long as data is accumulated and analyzed, it is not difficult to find out. Based on this, alarm values ​​can be set according to the preventive test standards. When the equipment insulation parameters exceed the alarm value, the system will automatically alarm. Improving the expert system, establishing a database, strengthening the analysis function, and formulating monitoring standards are still urgent problems to be solved. (5) Actively promote condition-based maintenance. On the basis of accumulating operating experience, condition-based maintenance can be implemented to improve the reliability of the power grid, reduce the workload of maintenance, and in turn promote the development of online monitoring technology. (7) Online detection equipment for ultra-high voltage power line insulators is still in the initial development stage. In the past few years, due to the limitation of communication conditions, online detection equipment for line insulators could not achieve data transmission and was not widely used. With the rapid development of communication technology, the GSM wireless communication network has basically covered urban and rural areas across the country, and the long-distance transmission of online detection data of line insulators is no longer a problem. The current research, development, and promotion of online detection equipment for ultra-high voltage power line insulators presents a good opportunity. 8. Conclusion (1) Online insulation detection technology for high-voltage electrical equipment can promptly detect and identify changes in the internal insulation state of equipment, address insulation faults in a timely manner, and ensure the safe operation of the power grid. (2) Online insulation monitoring technology for high-voltage electrical equipment is the foundation and only technical means for power supply units to implement condition-based maintenance. The use of online insulation monitoring technology should be further promoted, operational experience accumulated, and condition-based maintenance of electrical equipment actively implemented.