With the increasing complexity of power system operation environment and the deepening of power system reform, traditional power grids are accelerating the transformation to smart grids. The goal of equipment condition-based maintenance is achieved through real-time perception of equipment status by new sensors, reliable communication via modern network technology, and effective monitoring by background expert systems.
I. Analysis of Condition-based Maintenance Strategy
Condition-based Maintenance (CBM) refers to a maintenance mode that judges equipment abnormalities and predicts failures based on equipment status information provided by advanced condition monitoring and diagnostic technologies, and carries out maintenance before failures occur. That is, maintenance plans are arranged according to the health status of equipment. Compared with traditional periodic maintenance, the CBM strategy can timely detect hidden dangers and take corrective measures, avoiding the blindness and waste of human and material resources caused by maintenance based solely on time nodes. The prerequisite for implementing CBM is that equipment is equipped with perfect online monitoring devices, which can real-time monitor operating parameters and provide criterion support for condition-based maintenance. The online condition monitoring of medium-voltage switchgear includes main circuit temperature rise, mechanical characteristics of circuit breakers, service life of vacuum interrupters, and performance of key secondary components.
II. In-depth Analysis of On-line Temperature Rise Monitoring Technology
During the long-term operation of medium-voltage switchgear, the contact resistance at the engagement position of the moving and static contacts of the circuit breaker, the lap joint of the main busbar and power cable, and other parts often increases due to improper installation or poor contact, causing the temperature of the main circuit to rise. If such hidden dangers are not found in time, the continuous operation of switchgear will further aggravate the heating and oxidation of these parts, resulting in a vicious cycle, which may lead to consequences such as melting and falling off of contact fingers, burning of contacts, rapid degradation of adjacent insulation parts, and even malignant accidents such as breakdown and explosion.
The main circuit of medium-voltage switchgear is in a high-potential environment. If direct measurement is adopted, various problems such as high-voltage insulation and electrical isolation under high and low potentials need to be solved. At present, the following methods are mainly used in the market to directly or indirectly monitor the temperature rise of the main circuit: color-changing sheets, infrared imaging temperature measurement, optical fiber temperature measurement, wired built-in temperature measurement, wireless embedded temperature measurement, etc.
- Color-changing sheet temperature measurement: Advantages are low cost and water resistance; disadvantage is difficult observation due to limited installation position.
- Infrared imaging temperature measurement: Advantages are non-contact detection and no interference from visible light; disadvantage is that the material properties and surface condition of the measured object affect measurement accuracy.
- Optical fiber temperature measurement: Advantages are high voltage resistance, corrosion resistance, electromagnetic interference resistance, and automatic alarm function; disadvantage is complex construction and high procurement and maintenance costs.
- Wired built-in temperature measurement: Advantages are low price and automatic alarm function; disadvantage is easy false alarm due to electromagnetic interference, and difficult isolation between high and low potentials during wiring.
- Wireless embedded temperature measurement: Advantages are excellent cost-performance, high measurement accuracy, and wireless solution to the isolation problem between high and low potentials, avoiding damage from small animals or human abrasion compared with embedded data cables or optical fiber cables; disadvantage is short communication distance, requiring encryption to ensure reliable communication security.
The above schemes solve the problem of temperature measurement, but the temperature rise is also related to the magnitude of the passing current. Single temperature measurement without simultaneous current measurement cannot accurately reflect the real state of moving and static contact engagement or busbar lap joint, leading to false alarms or missed alarms. Therefore, the on-line temperature rise monitoring device also needs to cooperate with the background expert system for scientific analysis and diagnosis, which can judge whether the current temperature rise is abnormal according to the real-time load current and give processing suggestions accordingly.
III. Monitoring Technology Analysis of Mechanical Characteristics and Electrical Life of Vacuum Circuit Breakers
Vacuum circuit breakers are very important power equipment. According to statistics, more than half of the maintenance costs of substations are spent on high-voltage circuit breakers, and 60% of them are used for minor repairs and routine maintenance of circuit breakers. Frequent operations and excessive disassembly and maintenance will reduce the operation reliability of vacuum circuit breakers. Therefore, real-time on-line monitoring of vacuum circuit breakers helps to master their operation characteristics and change trends, shifting planned maintenance to condition-based maintenance.
The mechanical characteristic parameters of vacuum circuit breakers mainly include opening/closing time and speed, synchronism, contact pressure, overtravel, rebound amplitude, etc., which can be measured by linear displacement sensors, angular displacement sensors, pressure sensors and other devices. In the traditional method, the linear displacement sensor is installed at the bottom of the moving contact insulation pull rod of the vacuum circuit breaker, which requires a large space for linear movement, is not conducive to the miniaturization of equipment, and the sensor pull rod will have measurement errors due to wear or deformation. The new angular displacement sensor is installed at the mechanism spindle of the vacuum circuit breaker, which can accurately measure data such as overtravel, closing bounce time, opening rebound amplitude, closing speed, opening speed, closing time, opening time, etc., and the installation position is not easy to wear and convenient for maintenance. The contact pressure sensor is installed at the moving contact insulation pull rod, which can judge the state of the vacuum interrupter according to the change trend of the contact pressure value during opening and closing, and predict the remaining reliable electrical life of the vacuum interrupter by combining the analysis of historical switching load current conditions.
IV. On-line Monitoring Technology Analysis of Key Secondary Components of Vacuum Circuit Breakers
The on-line monitoring device for the state of secondary components of vacuum circuit breakers can realize the monitoring and data collection of the internal energy storage motor, opening coil, and closing coil of the circuit breaker. The most advanced current method is to use Hall elements to induce the magnetic field changes around the wires of execution equipment such as energy storage motors, opening coils, and closing coils, so as to achieve non-intrusive voltage and current measurement without worrying about the situation that the execution equipment cannot act due to the shutdown or damage of the monitoring device. Through real-time monitoring of the voltage and current of key secondary components of vacuum circuit breakers, operation and maintenance personnel can realize rapid diagnosis of potential faults of key secondary components through the analysis of fault waveforms and the comparison of front and back data. According to the diagnosis results, customers can formulate maintenance plans in advance to avoid serious impacts on power supply continuity after sudden faults.
V. Applications of Online Switch and Palmtop Switch
The online switch is a website system developed to provide remote condition monitoring, which can be accessed through any mainstream browser, including IE, Chrome, Firefox, Safari, etc. Based on a powerful cloud data center, the online switch filters, refines, and saves the large amount of status data obtained every day, then preliminarily filters various events according to threshold values and criterion algorithms, and issues alarms for suspected fault information. The online switch can set up perfect permission review and content grading designs to ensure the information security of user data.
The palmtop switch is a mobile terminal application developed specifically to provide remote condition monitoring. Based on Apple's advanced iOS system, it has powerful functions, high security, and is convenient for users to understand the operation status of medium-voltage switchgear anytime and anywhere, becoming a powerful assistant for maintenance workers.
Conclusion
With the development of intelligent monitoring technology and the popularization of the concept of condition-based maintenance, the on-line monitoring and on-line diagnosis schemes for medium-voltage switchgear are gradually improving and approaching maturity. After comprehensive application, they can effectively improve the comprehensive management and decision-making level of medium-voltage switchgear, realize standardized management and intelligent decision-making, and provide basic data support for the long-term safe and reliable operation and condition-based maintenance of medium-voltage switchgear. The continuous improvement of equipment management and decision-making levels will surely bring good economic and social benefits to the power industry. At present, however, the on-line monitoring devices for medium-voltage switchgear in China are of uneven quality, and it is necessary to conduct in-depth research on their principles, structures, and technical indicators, and select the optimal scheme to realize the on-line monitoring function of medium-voltage switchgear.
