How to monitor temperature with capacitors

Оптичен температурен сензор, Интелигентна система за мониторинг, Производител на разпределени оптични влакна в Китай

Fiber optic temperature sensors not only have wide applications in temperature measurement of switchgear, circuit breakers, and Трансформатори, but also have insulation, anti-interference, and high voltage resistance characteristics that other traditional temperature sensors cannot achieve in capacitor temperature monitoring.

The high-voltage parallel capacitor bank device is currently an extremely important reactive power source in the power system, playing a crucial role in improving the structure of the power system and enhancing power quality. The main function is to provide reactive power to the power system, reduce line losses, improve voltage quality, and increase equipment utilization. As a reactive power compensation device, power capacitors are usually used in substations through high-voltage centralized compensation. The compensation capacitors are connected to the 10kV or 35kV busbar of the substation to compensate for the reactive power on all lines and transformers on the busbar side of the substation. They are often used in conjunction with on load tap changers to further improve the power quality of the power system.

The impact of temperature rise fault on high-voltage capacitors

Capacitors often encounter various faults during operation, which pose a significant threat to the safe and normal operation of the power system. Common faults of capacitors in power operation include oil leakage, poor insulation, and burnt fuses. Among them, the most harmful and frequently occurring fault is capacitor failure caused by heating. The heating caused by capacitor faults is divided into heating at the busbar connection point and heating at the fuse outside the capacitor, with the latter being more likely to occur. През последните години, the 35kV high-voltage parallel capacitor bank has experienced abnormal temperature rise due to aging or high load current during daily operation due to long operating years and construction and installation processes. If such abnormal situations are not detected and dealt with in a timely manner, they can easily develop and expand, leading to damage to individual capacitors and even group explosions and injuries. The high failure rate directly threatens the safety of 500kV power equipment and the personal safety of operation and maintenance personnel, resulting in significant fluctuations in grid voltage, increased active and reactive power losses, reduced service life of capacitors, and affecting the normal and stable operation of the grid. Power capacitors are mainly used for reactive power compensation in power systems to improve power factor. In order to make it operate more reliably, the current industry mainly considers connecting internal components of capacitors in series with internal fuses. When a capacitor experiences complete component failure due to weak dielectric points, the internal fuse connected in series with the component will activate, isolating only a portion of the damaged components. The capacitor will continue to operate with only a slight decrease in power. At this point, the disturbance in the capacitor bank can be ignored, and the total capacity of the capacitor bank will not be significantly affected by the action of a fuse. The introduction of an internal fuse protects the capacitor components, but invisibly increases the fault points. Inside power capacitors, the internal fuse is the main heating point, but its volume and diameter are very small (about 135mm in length and 0.45mm in diameter), and it is generally hidden between capacitor components. Due to current measurement technology, it is difficult to accurately and objectively measure the surface temperature of the internal fuse under actual operating conditions.

Dry type capacitor temperature monitoring
Понастоящем, oil immersed capacitors and dry-type capacitors are commonly used in the field of high voltage. The latter has the advantages of environmental protection, material saving, ниска цена, simple process, леко тегло, small area, self-healing product, more reliable operation, good fire resistance, less likely to produce high voltage gas, and greatly reduced the possibility of explosive hazards.
A dry-type capacitor consists of a capacitor core, a casing, a sleeve, and other accessories. The capacitor core is composed of capacitor elements and insulating components. Capacitor components are made by winding thin film insulating media and aluminum foil electrodes with a certain thickness and layers, or by evaporating a layer of metal on the thin film to form a metalized film. After the components are rolled up, they are loaded into the component housing, and several capacitor components are connected in series or parallel to form the entire capacitor core.
Dry type capacitors are usually used indoors or underground with poor ventilation conditions, and the internal heat dissipation of capacitors can only rely on gas. Compared with oil immersed capacitors, the heat transfer coefficient of gas is lower, so the heat dissipation performance of dry type capacitors is poor. All of these have adverse effects on the operation of dry-type capacitors. Practice in power system operation has shown that the failure rate of capacitors is significantly higher from June to September each year compared to other months. In some regions, the power industry regulations stipulate that the hottest point temperature of the full film capacitor core shall not exceed 80 °C. Когато температурата надвиши 80 °C, the insulation performance of polypropylene film (PP film) as a dielectric will decrease.
Понастоящем, the temperature field of dry-type capacitors is generally measured by traditional temperature sensors to measure the temperature of the capacitor shell, and then the internal temperature is calculated. The temperature value obtained in this way has errors in the distribution of the internal temperature field of the capacitor, and cannot accurately obtain the true temperature of the highest temperature point.

Понастоящем, the temperature measurement method for internal protection fuses of power capacitors includes temperature rise test, but this test only estimates the temperature rise of the internal fuse by measuring the current and resistance of the internal fuse. Its accuracy is poor, and in the actual process of passing current to the internal fuse, the resistance of the internal fuse will change with the temperature. On the one hand, it is difficult to ensure its constant current flow. On the other hand, the correspondence between the resistance of the internal fuse and temperature is only applicable within a certain temperature range. Beyond this range, it will be difficult to obtain accurate results. Следователно, this indirect method of measuring the temperature rise of the internal fuse in capacitors has limitations and low accuracy. Освен това, the temperature rise of the internal fuse is measured through a thermal resistor, but due to the fact that the thermal resistor is much larger in both volume and diameter than the internal fuse, it will affect the actual temperature of the internal fuse during contact measurement, resulting in poorer measurement accuracy. In view of this, it is necessary to design a simple and feasible measuring device to accurately grasp the temperature of the fuse inside the capacitor under actual operating conditions, provide a basis for the design and selection of the fuse inside the capacitor, and effectively improve the reliability of the internal fuse protection action, ensuring that the temperature of the internal fuse will not cause damage to the internal insulation of the capacitor.

Disadvantages of infrared thermography for temperature measurement
Понастоящем, the heating maintenance of capacitors is mainly carried out through infrared imaging inspection. Обаче, infrared thermal imaging cannot test the temperature inside a closed environment, and the test results are affected by the season, time, and surface smoothness of the testing equipment. Infrared testing equipment is expensive and cannot continuously monitor the temperature of high-voltage electrical equipment for a long time. There is high voltage on the capacitor and strong electromagnetic interference around it, which often leads to false alarms and missed alarms in traditional detectors. For this purpose, highly reliable and high-performance temperature sensors are needed to monitor the temperature of capacitors in real-time and effectively, in order to avoid equipment burnout and power outages.

Освен това, current temperature measuring equipment cannot detect the specific temperature inside the capacitor. The existing capacitors are used in environments with significant temperature changes. Prolonged use of capacitors at abnormal temperatures can seriously affect their service life and increase their damage rate.

Capacitor fiber optic temperature measurement system
The FJINNO capacitor fluorescence fiber optic temperature measurement system not only solves the problem of traditional temperature sensors being unable to accurately measure the temperature of small internal fuses, but also solves the potential isolation between strong and weak electricity and the anti electromagnetic interference problem of data communication, providing a good solution for comprehensively and accurately grasping the hot spot temperature of the internal core of capacitors.

The fiber optic temperature monitoring host is equipped with temperature measurement alarm software, and the monitoring computer collects temperature information transmitted by the fiber optic temperature signal demodulator through the communication port. Real time display of temperature data for each temperature measurement point, temperature alarm software provides functions such as graded monitoring, temperature curve drawing, temperature distribution display, historical curve query, report generation, and printing;