How to measure the temperature of oil immersed transformers? The advantages of using fluorescent fiber optic temperature sensors to monitor windings

The temperature measurement methods for oil immersed transformers mainly include the following:

Direct measurement method:

Principle: Directly embed sensors in the winding, propagate signals through optical fibers, and achieve online and real-time measurement of the hot spot temperature of the winding under high voltage and high magnetic field conditions. The fiber optic thermostat determines the temperature by measuring the inherent parameter (attenuation time) of the phosphor alone, which does not change due to physical changes in the fiber and does not require verification. The temperature sensor is made of stable high-temperature resistant fluorescent material and directly attached to the end of the fiber optic probe. The probe is compatible with oil immersed transformers for a long time and has excellent electrical performance. When the light pulse emitted by the LED light source is sent to the temperature sensor in contact with the winding through the optical fiber, the pulse excites the fluorescent material of the sensor, causing it to produce longer wavelength fluorescence. The temperature of the sensor is measured based on the decay time of the returned fluorescence, and then processed to display the temperature value and related system parameters. The temperature information is transmitted to the control room.

Indirect calculation measurement method:

Principle: Indirect calculation based on the hot spot calculation formula in relevant standards (such as IEC 60076.7-2005 "Load Guide for Oil immersed Transformers"). It is a traditional classical estimation method that estimates based on a hypothetical transformer thermal model and practical empirical data. Among them, the steady-state temperature equation calculates the hot spot temperature under the assumption that the temperature rise of the oil and winding is linear, while the transient temperature equation considers the real-time variation of the load to calculate the temperature rise. For transformers cooled by natural oil circulation (ON), the hottest point temperature at any load is equal to the sum of ambient temperature, top oil temperature rise, and temperature difference between the hot point and top oil; For transformers cooled by forced oil circulation (OF), the calculation method is based on the bottom oil temperature and average oil temperature; For transformers with forced oil circulation guided (OD) cooling, the calculation is basically the same as the OF cooling method, but the variation of wire resistance with temperature needs to be considered, plus a correction factor. However, the calculation process is complex and highly theoretical, and the basic data in the formula needs to be collected with the help of temperature measuring devices, resulting in measurement and calculation errors. Moreover, there are few people who can proficiently master this theory, and it is rarely used in production sites.

Thermal simulation measurement method:

Principle: Currently, both domestically and internationally, the "thermal simulation" technology based on GB 1094.7-2008 "Load Guide for Oil immersed Power Transformers Part 7" is widely used to indirectly measure the temperature of transformer windings. This standard considers the transformer winding temperature (Tw) as the sum of the top oil temperature (To) of the transformer oil tank and the temperature difference between the transformer copper oil (Two), i.e. Tw=To+Two. This method obtains an additional current Ict proportional to the load current from the current transformer of the transformer, and converts it into Ih through a composite transmitter (Ih is the working current artificially given based on the copper oil temperature difference of the transformer after adjusting Ict). Ih heats a specially designed set of electric heating elements in the temperature controller, and the additional temperature rise proportional to the load current generated by this current on the electric heating element is the simulated copper oil temperature difference Two, which is superimposed on the top oil temperature To of the transformer, so that the winding temperature controller obtains the winding temperature of the transformer. However, during the calibration of the temperature controller, an additional analog current needs to be applied in the constant temperature bath, which makes the process of setting the current matching device, temperature transmitter, and temperature display device cumbersome. The introduction of load current and the installation of current matching devices make the secondary circuit complex, and temperature fluctuations or disturbances often occur due to external conditions during operation. The temperature measuring element of the winding thermostat is located at the top of the oil tank. Although compensation current is artificially introduced, there are still measurement errors and time delay errors, which cannot accurately and real-time display the hot spot temperature of the winding.

Oil surface temperature monitoring method:

Pressure sensing measurement method: The pressure sensing temperature controller mainly consists of three parts: elastic element, capillary tube, and temperature sensing package, forming a sealed system filled with temperature sensing medium inside. When the temperature changes, the volume of the temperature sensing medium changes accordingly, and the volume variable is transmitted to the elastic element inside the instrument through the capillary tube, causing it to produce corresponding displacement. After the displacement is amplified by the mechanism, it drives the pointer to indicate temperature changes and drives the micro temperature switch to control the corresponding cooling and protection systems, achieving the purpose of monitoring temperature and controlling temperature rise to protect the transformer. The latest temperature controller adopts composite sensing technology, and the Pt100 platinum resistance signal installed in the temperature chamber is remotely transmitted to the secondary instrument in the control room. It can synchronously display and control the top oil temperature of the transformer, and also provide 4-20mA standard current signal or 0-5V standard voltage signal to the computer. The pressure sensing temperature controller can realize the start stop cooling system, start protection system, and real-time monitoring of transformer oil surface temperature. It has no current matching device, low cost, and does not require additional analog current circuits for calibration, making it relatively simple. However, the temperature measuring element of the pressure type thermostat is located at the top of the fuel tank, which has measurement errors and time delay errors, and cannot accurately simulate heat.

Resistance thermometer measurement method: using the characteristic that the resistance of a metal changes with temperature to measure temperature.

Principle of Fluorescent Fiber Optic Temperature Sensor for Monitoring Oil Immersed Transformer Winding

The fluorescent optical fiber temperature measurement system for oil immersed transformers consists of internal optical fibers, flanges, external optical fibers, and a temperature measurement host. Internal optical fibers are arranged on winding coils, iron cores, oil surfaces, and other parts according to design requirements. They are connected to external optical fibers through penetrators on flanges, and the optical signal is transmitted to the temperature measurement host through the optical fiber to analyze the temperature at the measurement point. It is based on the principle of fluorescence temperature measurement. When the excited beam of light enters the winding, the fluorescent molecules will absorb the energy of the beam and emit a fluorescence signal. There is a relationship between fluorescence afterglow and winding temperature. By measuring the lifetime of the fluorescence signal, the temperature of the winding can be determined.

Advantages of Fluorescent Fiber Optic Temperature Sensor for Monitoring Oil Immersed Transformer Winding

High precision temperature measurement: Using the nonlinear relationship between fluorescence signal and temperature for temperature measurement can achieve high temperature measurement accuracy. Research shows that the accuracy of fluorescence temperature measurement for transformer windings can reach 0.1 degrees. This high-precision temperature measurement helps to more accurately grasp the temperature condition of transformer windings, detect small temperature changes in a timely manner, and thus prevent faults that may be caused by abnormal temperature in advance.

High voltage and electromagnetic interference resistance: Oil immersed transformers, especially high-voltage oil immersed transformers, operate in environments with high voltage, high current, and strong magnetic fields. The fluorescence fiber optic temperature sensor has strong anti-interference ability, avoiding the problems of contact thermal resistance and temperature measurement field limitations of traditional temperature measurement methods. This enables it to stably and accurately measure winding temperature in complex electromagnetic environments, without being affected by external electromagnetic interference, ensuring the reliability of measurement results.

Comprehensive temperature measurement capability: It can achieve comprehensive temperature measurement of transformer windings without being affected by local faults or complex working conditions. It can monitor the temperature of various parts of the winding in real-time and continuously. This way, the temperature distribution of the winding can be fully obtained, which helps to comprehensively evaluate the operating status of the transformer, timely detect local overheating and other abnormal situations, and is of great significance for ensuring the safe and stable operation of the transformer.

Good stability: Some fluorescent fiber optic temperature sensors do not require secondary calibration after installation, ensuring long-term stability and reliability. This feature reduces the maintenance workload of the sensor, while also ensuring the accuracy and consistency of measurement results during long-term use, reducing the risk of measurement errors caused by fluctuations in the sensor's own performance.

Capable of data storage and transmission: Some fluorescent fiber temperature sensors are equipped with a 1GB industrial storage card, which can store data locally. And the interface adopts standardized design, which facilitates connection with local area networks and wide area networks of various systems, integrates into the automation comprehensive control system, facilitates data management, analysis, and remote monitoring, and helps to achieve intelligent management and operation of transformers.

Online automatic detection function: When equipment malfunctions, the automatic detection circuit can issue warnings or abnormal device signals, promptly notifying operation and maintenance personnel for handling, improving the safety and reliability of equipment operation, and reducing serious consequences such as transformer damage caused by failure to detect equipment faults in a timely manner.

Comparison between Fluorescent Fiber Optic Temperature Sensor and Traditional Temperature Measurement Methods (such as Oil Surface Temperature Monitoring):

Measurement location: The fluorescent fiber optic temperature sensor can directly measure the winding and reflect the temperature inside the winding; Oil surface temperature monitoring can only measure the oil surface temperature and cannot directly reflect the hot spot temperature of the winding, which has limitations in determining the internal temperature condition of the winding.

Accuracy and sensitivity: Fluorescent fiber optic temperature sensors have the advantages of high accuracy and anti-interference, and can accurately measure changes in winding temperature; The monitoring of oil surface temperature is affected by factors such as oil temperature distribution, and the response to changes in winding temperature is not sensitive and accurate enough.

Functional features: The fluorescent fiber optic temperature sensor has comprehensive temperature measurement, data storage and transmission, online automatic detection and other functions; The main function of traditional oil surface temperature monitoring is to monitor the oil surface temperature, which is relatively single.

The development trend of temperature measurement technology for oil immersed transformers

High precision:

Accurate temperature measurement is crucial for the safe operation of oil immersed transformers. The accuracy of fluorescent fiber optic temperature sensors can reach 0.1 degrees, and technology will be continuously improved in the future to achieve higher accuracy.

Intelligence and automation:

With the intelligent development of the power system, the temperature measurement technology of oil immersed transformers will also move towards intelligence and automation. By leveraging technologies such as the Internet of Things, big data, and artificial intelligence, real-time collection, transmission, analysis, and processing of transformer temperature data can be achieved. Connect the temperature sensor of the transformer to the monitoring system through IoT technology to achieve remote monitoring and data sharing. Using big data technology to analyze a large amount of temperature data, establish a temperature prediction model, and predict the temperature change trend of transformers in advance, providing a basis for preventive maintenance. Artificial intelligence technology can be used for fault diagnosis, automatically identifying temperature anomalies in transformers and determining fault types through learning from historical temperature and fault data.

Multi functional integration:

Temperature measurement technology will not be limited to temperature measurement, but will develop towards multifunctional integration. For example, combining temperature measurement with other status monitoring functions of transformers, such as insulation status monitoring, analysis of dissolved gases in oil, etc.

By integrating multiple functions into a sensor or monitoring system, comprehensive assessment of transformer status can be achieved. This can reduce the complexity and cost of the equipment, improve its reliability and operational efficiency. Meanwhile, multifunctional integration also helps to achieve integrated management of transformers and improve the convenience of operation and maintenance.

Adapt to special environments and needs:

In some special application scenarios, such as harsh environments such as high temperature, high humidity, and strong corrosion, or for special types of oil immersed transformers such as large capacity and ultra-high voltage, higher requirements are placed on temperature measurement technology.

Temperature measurement technology is suitable for these special environments and needs. For example, high-temperature and corrosion-resistant sensor materials and packaging technology are used to ensure the normal operation of sensors in harsh environments. For large capacity and ultra-high voltage transformers, temperature measurement techniques that are more suitable for their electrical and structural characteristics can improve measurement accuracy and reliability.

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