1　There are shortcomings in traditional temperature measurement technology

       Power transformers are important equipment in substations, and they undertake important tasks such as step-up, step-down and voltage regulation of the power grid. With the continuous expansion of the scale of the power system, the continuous improvement of the voltage level and the continuous increase of the transformer capacity, the failure rate has also increased. The temperature of the main transformer (hereinafter referred to as the main transformer) is an important monitoring indicator of its operating conditions, especially when the transformer is running at high temperature and high load, it is more related to the safe operation of the main transformer. Therefore, conducting online diagnosis and understanding the real-time operating conditions inside the main transformer coil has made the main transformer temperature measurement system more attention. In the traditional main transformer temperature measurement technology, the most widely used and most representative are the thermistor bridge temperature measurement method and the platinum resistance temperature measurement method. Although traditional temperature measurement technology is widely used at this stage, it still has certain limitations. For example, the thermistor method, whose rate of change is nonlinear, is not suitable for measuring high temperature areas. Although Pt100 (platinum resistance) has high measurement accuracy and good linearity, it has slow thermal response and poor vibration and impact resistance. For another example, traditional temperature measurement technologies are all temperature measurement systems based on electrical signal sensors. However, due to the high voltage and strong electromagnetic field inside the transformer, such electrical signal sensors are extremely susceptible to electromagnetic interference. The reliability and accuracy of the data are It is difficult to be guaranteed.

2　 New technology of optical fiber temperature measurement

       In order to overcome the shortcomings of traditional temperature measurement technology applied in the field and promote the construction of smart grid, many equipment online monitoring technologies have developed rapidly, with excellent performance and increasingly perfect. The application of optical fiber temperature measurement technology in a strong electric field environment has achieved gratifying results. Its remarkable optical characteristics can electrically separate the sensor from other parts of the system, thereby improving the flexibility and safety of sensor design. At the same time, through online remote real-time measurement of the main transformer, the problem of monitoring the temperature blind zone of the hot spot of the transformer's internal coil is solved.

       2.1 The principle and characteristics of optical fiber temperature measurement

       Any material whose optical properties change with temperature changes basically constitutes the basis of an optical temperature sensor. Among them, the parameters that change with temperature include absorptivity or its transmission performance, reflectivity, refractive index, degree of polarization, and emitted fluorescence or phosphorescence. The optical fiber temperature measurement technology was originally used as a phosphor sensor by oxysulfide. As the temperature of the phosphorescent substance rises, the relaxation process of non-luminescence takes up a significant position, so the luminous intensity also decreases. The fluorescence emitted by rare earth element phosphors is composed of spectral lines produced by different excited states of third-order europium ions. The ratio of the luminous intensity of different spectral lines has nothing to do with the fluctuation of excitation conditions and parasitic losses, but is related to temperature. Therefore, the ratio can provide accurate temperature information. However, this technology is expensive and should not be adopted in actual engineering projects.

       With the development of optical fiber temperature measurement technology, a material that can exponentially decay fluorescence for a long time is currently used as the sensor. The material used has the advantages of high temperature resistance, stable chemical properties, no electromagnetic interference, and high data reliability. And reasonable cost performance, mass production, suitable for industrial temperature measurement field. The high-efficiency phosphor sensor uses magnesium fluorogermanate excited by tetravalent manganese ions as the sensor material. The red radiable phosphor sensor is excited by visible short-wave and ultraviolet radiation, as shown in Figure 1, and the transmission spectra at the two filters used to distinguish excitation and fluorescent radiation are also given.

Temperature can be measured by measuring the decay rate of fluorescence. The time between the initial signal level S1 and the decay signal St/e is the decay time, as shown in Figure 2. The composite change of the decay time with the temperature change is shown in Figure 3. The optical fiber temperature measurement system can use low-energy light-emitting diodes as Figure 2 "The relationship between the fluorescence intensity of the phosphor sensor and the decay time Figure 3 "The relationship between the decay time and the temperature of the magnesium fluorogermanate sensor The light source, when the photodetector receives the phosphor sensor After the fluorescence, the decay time is calculated by the microprocessor and the digital signal processor, and the returned fluorescence intensity is reflected in the form of temperature.

       2.2 Application of optical fiber temperature measurement technology

       Shenbian is the most influential transformer production unit in China, and online temperature measurement technology is the focus of its consideration. The real-time monitoring of equipment status can more effectively improve the safety of equipment operation. For this reason, Shenyang Transformer uses the new technology of optical fiber temperature measurement of the main transformer winding hot spot to monitor the conventional oil temperature of the main transformer and the hot spot temperature rise in the winding.

       2.2.1 "Optical fiber temperature measurement system integration

       The main transformer winding hot spot optical fiber temperature measurement system includes data sampling and analysis alarm functions (including hardware and software), and can be linked with the communication component of the intelligent component. After the monitoring data of the main transformer winding hot spot optical fiber temperature measurement system is connected to the intelligent device (IDE), it is uploaded to the data processing center of the station control layer, and the dynamic monitoring value of the main transformer winding hot spot is pre-diagnosed and analyzed. The station control layer is mainly composed of a data processing server, which provides a human-machine interface running in the station to realize the display of condition monitoring data (including real-time data, historical trends, etc.), comprehensive evaluation and analysis of equipment, and decision-making assistance. Data processing server and automation The monitoring system is integrated, and the software of the main transformer winding hot spot optical fiber temperature measurement system is also integrated into the state analysis system.

       2.2.2 Construction of optical fiber temperature measurement system

       Since the temperature measurement fiber needs to be wound inside the transformer and soaked in insulating oil for a long time, PTFE and Kevlar are used to make the fiber jacket, and high-power light-emitting diodes are used. In this way, the diameter of the optical fiber winding coil can be reduced, and the bending radius can be smaller, which facilitates the winding and installation of the optical fiber in the main transformer winding. After investigating the optical fiber temperature measuring equipment market, Shenbian adopts the optical fiber controller produced by the domestic Xi'an Heqi Optoelectronics Technology Co., Ltd. The company is a pioneer in the field of optical fiber temperature measurement in China, and has the installation experience and performance of hundreds of transformer winding hot spot optical fiber thermostats.

       1) The optical fiber thermostat has 4 channels, the temperature measurement range is -30～260℃, the temperature measurement accuracy is ±1℃, the temperature resolution is 0.1℃, the sampling speed is 50ms, and it can display 6 channels of data at the same time, simulation There are 6 outputs (4-20 mA), the ambient temperature is -40～70℃, the input AC power is 50Hz, 110～240V, the life of the light source is 30 years, and the communication protocol supports IEC 61850.

       2) The optical fiber sensor transmits the temperature signal inside the transformer, adapts to various harsh chemical environments, and is not affected by the high voltage of the transformer and electromagnetic interference. The temperature measurement range is -40～260°C, the response time is less than 0.25s, the temperature resolution is 0.1°C, the optical fiber sheath is protected by double Teflon sheath Kevlar, no electromagnetic interference, the sensor inner diameter is 1 .5mm, the diameter of the spiral jacket is 2.8mm, and the service life is 30 years.

       3) The material of the shell penetrator is 304 stainless steel, the maximum pressure is 689.6kPa, the maximum vacuum is less than 133.322×10-3Pa, and the maximum working temperature is 180°C.

       2.2.3 Installation of optical fiber temperature measurement system

       The correct installation of the optical fiber and the probe in the main transformer coil is very important to the normal operation of the optical fiber temperature measurement system. For the core type main transformer, first select the required radial spacer block and slot it diagonally, then use GORTEX to wrap the probe in the slot to enhance the safety and flexibility of the probe, and finally put the radial spacer block in The wire adjacent to the hottest part of the main transformer coil. When the assembly of the core type main transformer coil is nearly completed, install the radial spacer embedded in the probe into the coil, and fit the insulating paper tube tightly on the outside of the optical fiber inside the fuel tank to ensure that the position of the fuel tank wall penetrator is far away from the routine maintenance in the future Location.

       The main transformer winding hot spot optical fiber temperature measurement system is implemented to monitor the temperature of 4 points of the main transformer. The 4 optical fibers are led from the main transformer to the penetrator on the mounting plate, and the 4 optical fibers are led to the optical fiber thermostat through the wire groove, and the temperature is controlled. The temperature of 4 points is displayed on the monitor.

       The installation diagram of the main transformer winding hot spot optical fiber temperature measurement system is shown in Figure 4.

Figure 4 　 Temperature measurement system installation

The installation position of the optical fiber temperature measurement of the winding hot spot of the main transformer of the sinking transformer is determined as the high voltage winding, the medium voltage winding, the top oil temperature, and the bottom oil temperature according to the operating requirements and the technical characteristics of the temperature measurement equipment, covering all important areas of the main transformer , So that real-time monitoring and diagnosis functions can be truly realized, thereby reducing the probability of damage to the main transformer winding and extending the service life of the main transformer.

３　 Conclusion

       The hot spot optical fiber temperature measurement technology of the main transformer winding is a new technology developed in recent years. Optical fiber has a series of advantages such as small size, light weight, softness and winding, good electrical insulation performance, immunity to electromagnetic interference, and ability to work in flammable and explosive environments. Different from the traditional method of measuring the oil temperature of the top or bottom of the main transformer, the optical fiber temperature measurement technology can accurately measure the “hot spot” temperature on-line through the sensing probe directly placed in the main transformer winding, and it does not require calibration to provide power customers with A direct and fast dynamic measurement technology. With the improvement of the reliability of electronic sensor signal processing equipment and the reduction of the price of individual devices, the application of optical fiber winding temperature measurement technology in the power industry will inevitably become more and more extensive.

Released on February 12, 2015