Infrared diagnosis is an effective technical means and important content for online monitoring of equipment. It can timely and accurately detect and diagnose accident hazards and fault precursors of operating equipment, so as to take reasonable and reliable handling measures, reduce energy loss and waste caused by overheating of equipment, and reduce or avoid sudden equipment accidents caused by overheating faults. Due to the fact that infrared diagnosis is a non-contact non-destructive testing and fault diagnosis of equipment in operation, it can achieve scanning of a large temperature distribution field on the operating site of the equipment and fixed-point temperature measurement of local defects. The technology is advanced, the operation is convenient, and the detection is fast. The high sensitivity of infrared detection instruments, combined with computer technology, processes the infrared thermal images of the equipment, thereby achieving technical functions such as data statistics, analysis, display, and storage, facilitating the establishment of equipment status and equipment file management databases.

1 The Fundamentals and Principles of Infrared Technology

1.1  Principles of Infrared Technology

The principle of infrared technology is based on the fact that all objects above absolute zero in nature emit infrared radiation at all times. At the same time, this infrared radiation carries characteristic information of the object, which provides an objective basis for using infrared technology and distinguishing the temperature and thermal distribution fields of various measured targets. Infrared technology is a technology that studies the generation, transmission, and conversion of infrared radiation. It can clearly display temperature distribution images that cannot be seen by the human eye.

Infrared radiation has wavelengths between 0.76-1000 μ An electromagnetic wave between m, divided into near-infrared (0.76-3) according to wavelength range μ m) Mid infrared (3-6 μ m) Far infrared (6-15 μ m) Far infrared (145-1000) μ m) Four categories. It is located in the area between radio waves and visible light in the continuous spectrum of electromagnetic waves. The energy of infrared radiation from an object is closely related to its wavelength and temperature. The energy of infrared radiation can be measured by the surface temperature of the object. The larger the energy of radiation, the higher the surface temperature of the object; On the contrary, it indicates that the surface temperature of the object is lower. Therefore, the detection of infrared radiation from objects can achieve imaging of the surface thermal field of the object.

The commonly used temperature scales to measure temperature changes include Fahrenheit temperature (oF0F), Celsius temperature (℃ 0C), and thermodynamic temperature (K). The relationship between the three:

The wavelength corresponding to the peak energy density of temperature radiation decreases as the temperature of the object increases. According to Venn's law:

In the formula:
λ- Peak wavelength（ μ M)
T - Thermodynamic temperature of object (K)
From formula (2), it can be seen that for high temperature measurement, a shorter working wavelength is required, while for low temperature measurement, a longer working wavelength is selected.
The radiation power of an object is directly proportional to the surface temperature and occurrence rate of the object, which conforms to the Stephen Boltzmann law:

P=εσT⁴

In the formula:
T - Thermodynamic temperature of object (K)
ε- Infrared emissivity (radiation coefficient) of object surface
σ- Stephen Boltzmann constant
1.2 Infrared thermal imaging equipment
Infrared imaging equipment is a temperature measurement instrument developed based on infrared technology. Common equipment includes infrared thermometers, infrared thermal televisions, and infrared thermal imagers. The infrared thermal imager has been widely used in the power system since the 1980s and has now been widely used in the steel industry.
The use of thermal imagers involves three parameters: emissivity, emissivity, and reflectivity. The emissivity refers to the ratio of the actual radiation intensity of an object to the radiation intensity of a blackbody when the temperature is the same. There are many factors that affect the emissivity of an object, such as surface temperature, roughness, flatness, color, infrared wavelength, etc; Projection rate refers to the ratio of the energy transmitted through an actual object when a radiation is incident on it to the energy incident on that object; Reflectivity refers to the ratio of the energy emitted by an object to the energy incident on the object.
2. Temperature monitoring of high and low voltage distribution systems
The high and low voltage distribution system is the core equipment of steel enterprises. If there is damage or shutdown, it will cause unnecessary damage to the smooth operation of the equipment and the stability of production. Dynamic monitoring of the high and low voltage distribution system in use has become an urgent task for equipment operation. Among numerous detection methods, infrared thermal imager temperature detection has been proven to be an effective and easy to implement method. The high and low voltage distribution system has abnormal temperature, mainly reflected in equipment overcurrent or long-term operation within the rated current range of 200A or above; The second is oxidation or virtual connection of electrical contacts; The third issue is the non-standard wiring of electrical connection points, lack of installation of wiring noses, and weak crimping. These faults can be indirectly detected by monitoring the temperature changes of components in the high and low voltage distribution system cabinets. Shandong Laigang Yongfeng Iron and Steel Co., Ltd. has used thermal imaging technology to explore an effective method for monitoring the expansion of heating faults in electrical equipment.
3 Application Case Studies
3.1 Iron making 4 # old machine head dust removal 12DP cabinet load switch on the south side
On July 9, 2018, during a night inspection using an infrared thermal imager, it was found that the temperature of one phase on the south side of the load switch of the 12DP cabinet for dust removal at the 4 # old iron making machine head was 150 ℃ (see Figure 1).

On site coordination has been carried out to reduce the load on the electric field, and the secondary current of the 3 # and 4 # electric fields has been reduced from 860mA to 500mA. The temperature of the load switch has dropped to 82 ℃, and the frequency of temperature detection has been increased. When planning maintenance, redistribute the load and replace damaged load switches. After repairing and replacing the load switch, the temperature returned to normal (see Figure 2).

3.2  Detection of heating fault in the wiring terminals of the electric field in Room 2 and Room 3 of the dust removal distribution room of the 4 # old machine head in the ironmaking plant

On July 10, 2018, there was a local high temperature of 81 ℃ in the red phase upper terminal of the electric field (back of the cabinet) in Room 2 and Room 3 of the dust removal distribution room of the 4 # old machine head in the ironmaking plant. It was suspected that the heat source was at the connection part of the thyristor, and it needs to be thoroughly opened for maintenance (see Figure 3).

After maintenance and confirmation, it was found that the heat source was indeed present at the connection of the thyristor. After replacement, it was detected again with an infrared thermal imager and the temperature dropped to normal.
3.3 Detection of heating faults in the wiring terminals of the 1 # SVC phase reactor in the steelmaking plant

4  Conclusion

Infrared thermal imaging technology, as a non-contact detection method, has been widely used, especially in the operation of electrical systems in steel enterprises and the diagnosis of hot spot faults. The Equipment Engineering Department of Shandong Laigang Yongfeng Iron and Steel Co., Ltd. organized each branch to conduct a second night inspection and temperature state detection every month. 232 faults of over temperature of load switches, electrical contacts, and cable heads were discovered, and effective measures were taken, Preventing the occurrence of major malignant accidents.