Introduction of infrared basic principles
As long as its temperature is higher than the absolute temperature (-273 ° C), there will be irregular movement of molecules and atoms in all objects in nature, and the surface will continuously radiate infrared rays. Infrared is an electromagnetic wave with a wavelength range of 0.78 ~ 1000um, which is invisible to human eyes. An infrared imaging device is a device that detects infrared rays radiated on the surface of such an object that are not seen by human eyes. It reflects the infrared radiation field on the surface of the object, that is, the temperature field.
Note: The infrared imaging device can only reflect the temperature field on the surface of the object.
For power equipment, the basic principle of infrared detection and fault diagnosis is to detect the infrared radiation signal on the surface of the equipment to be diagnosed, so as to obtain the thermal characteristics of the equipment, and based on this thermal state and appropriate criteria, make whether the equipment is faulty and Diagnosis of fault attributes, occurrence location and severity.
In order to deeply understand the infrared diagnosis principle of power equipment faults and better detect equipment faults, the following will discuss the relationship and law between the thermal status of power equipment and the infrared radiation signal generated by it, the influencing factors and the working principle of DL500E.
One. Infrared radiation emission and its regularity
(1) Infrared radiation law of black body
The so-called black body is simply an object whose absorption rate of incident radiation of all wavelengths is equal to 1 under any circumstances, that is to say total absorption. Obviously, because any object that actually exists in nature has a certain reflection of incident radiation of different wavelengths (absorptivity is not equal to 1), the black body is just an idealized object model abstracted by people. However, the basic law of black body thermal radiation is the basis of infrared research and application. It reveals the quantitative relationship between the infrared thermal radiation emitted by the black body and the temperature and wavelength.
Below, I focus on introducing three of the basic laws.
1. Law of Spectral Distribution of Radiation-Planck's Law of Radiation
A black body with an absolute temperature of T (K), the radiant power (abbreviated as spectral radiance) Mλb (T) and the wavelength λ and temperature T that the unit surface area emits to the entire hemispherical space within the unit wavelength interval around the wavelength λ meets the following relationships with the wavelength λ and temperature T
Mλb (T) = C1λ-5 [EXP (C2 / λT) -1] -1
Where C1-first radiation constant, C1 = 2πhc2 = 3.7415 × 108w • m-2 • um4
C2-Second radiation constant, C2 = hc / k = 1.43879 × 104um • k
Planck's law of radiation is the basis for all quantitative calculations of infrared radiation. It is more abstract when introduced, so I won't go into details here.
2. The law of radiant power change with temperature-Stephen-Boltzmann law
Stephen-Boltzmann's law describes the change law of the total radiated power Mb (T) (abbreviated as total radiance) of all wavelengths emitted by the black body unit surface area to the entire hemisphere space with its temperature. Therefore, the law is obtained by integrating Planck's radiation law with wavelength:
Mb (T) = ∫0∞Mλb (T) dλ = σT4
Where σ = π4C1 / (15C24) = 5.6697 × 10-8w / (m2 • k4), it is called Stephen-Boltzmann constant.
Stephen-Boltzmann's law shows that all objects whose temperature is higher than zero degrees Kelvin will emit infrared thermal radiation spontaneously, and the total radiation power emitted per unit surface area of ​​the black body is proportional to the fourth power of Kelvin. Moreover, as long as there is a small change in temperature, it will cause a large change in the radiation power emitted by the object.
Then, we can imagine that if the total radiated power emitted per unit surface area of ​​the black body can be detected, can we not determine the temperature of the black body? Therefore, Stephen-Boltzmann law is the basis of all infrared temperature measurement.
3. Law of Spatial Division of Radiation-Lambert Cosine Law
The so-called Lambert cosine law is that the radiation intensity of the black body in any direction is proportional to the cosine of the angle between the observation direction and the normal of the radiation surface, as shown in the figure
Iθ = I0COSθ
This law shows that the blackbody has the strongest radiation in the normal direction of the radiating surface. Therefore, when actually doing infrared detection. It should be selected as far as possible in the normal direction of the measured surface. If it is detected in the direction of the angle θ with the normal, the received infrared radiation signal will be reduced to COSθ times the maximum value of the normal direction.
(2) Infrared radiation law of actual objects
1. Kirchhoff's law The ratio of the radiation exit M (T) and the absorption power α of M / α has nothing to do with the nature of the object, and is equal to the radiation exit M0 (T) of the blackbody at the same temperature. It shows that an object with large absorption power has a large emission power. If the object cannot emit radiation energy of a certain wavelength, it can never absorb radiation energy of this wavelength.
2. Emissivity experiments show that in addition to the temperature and wavelength, the radiance of an actual object is also related to factors such as the material properties and surface state of the object. Here, we introduce a radiation coefficient that varies with the material properties and surface state, and then the basic laws of blackbody can be applied to actual objects. This emissivity, often referred to as emissivity, or specific emissivity, is defined as the ratio of the radiation performance of an actual object to a black body at the same temperature.
Here, we do not consider the effect of wavelength, only study the total emissivity of the object at a certain temperature:
ε (T) = M (T) / M0 (T)
Then Stephen-Boltzmann law applied to real objects can be expressed as:
M (T) = ε (T) .σT4
(3) Emissivity and its impact on equipment status information monitoring
An object must have absorption, reflection and transmission for a given incident radiation, and the sum of the absorption rate α, the reflection rate Ï and the transmission rate Ï„ must be equal to 1:
α + Ï + Ï„ = 1
Moreover, its reflection and transmission parts remain unchanged. Therefore, under thermal equilibrium conditions, the radiant energy absorbed by the object must be converted into the radiant energy emitted by the object. It can be concluded that under thermal equilibrium conditions, the absorption rate of an object must be equal to the emissivity of the object at the same temperature:
α (T) = ε (T)
In fact, from Kirchhoff's law, we can also infer the above formula:
M (T) / α (T) = M0 (T)
ε (T) = α (T)
ε (T) = M (T) / M0 (T)
Then for an opaque object ε (T) = 1-Ï (T)
According to the above formula, it is not difficult to qualitatively understand the following factors that affect the emissivity:
1. Effects of different material properties Materials with different properties have different absorption or reflection properties for radiation, so their emission properties should also be different. Generally, when the temperature is lower than 300K, the emissivity of the metal oxide is generally greater than 0.8.
2. Effect of surface condition The surface of any actual object is not absolutely smooth, and will always show different surface roughness. Therefore, this different surface morphology will affect the reflectivity and thus the emissivity value. The magnitude of this effect also depends on the type of material.
For example, for non-metallic dielectric materials, the emissivity is less or unaffected by surface roughness. However, for metallic materials, surface roughness will have a greater impact on emissivity. As for wrought iron, when the surface condition is rough and the temperature is 300K, the emissivity is 0.94; when the surface condition is polished and the temperature is 310K, the emissivity is only 0.28.
In addition, it should be emphasized that in addition to the surface roughness, some human factors, such as the application of lubricating oil and other deposits (such as paint, etc.), will significantly affect the emissivity of the object.
Therefore, when testing, we should first clarify the emissivity of the measured object. In general, we do not understand the emissivity, so we can only use the phase comparison method to identify the fault. For power equipment, the emissivity is generally between 0.85-0.95.
3. Temperature influence
The influence of temperature on objects of different nature is different, and it is difficult to make a quantitative analysis.
Pay attention only during the testing process.
(4) The effect of radiation transfer between objects
We have discussed above that there must be absorption and reflection for a given incident radiation, and when the thermal equilibrium is reached, the radiation energy absorbed by it must be converted into radiation energy emitted outward. Therefore, when we detect any target in a substation, the detected temperature must be influenced by other nearby objects.
Therefore, when testing, we should pay attention to the direction and time of detection, so as to minimize the influence of other objects.
(5) Impact of atmospheric attenuation
The atmosphere has physical processes such as absorption, scattering, and refraction on the radiation of objects, which have an attenuation effect on the radiation intensity of objects. We call it extinction.
The extinction effect of the atmosphere is wavelength-dependent and has obvious selectivity. Infrared can be completely transmitted in three bands in the atmosphere. We call it the atmospheric window, which is divided into near infrared (0.76 ~ 1.1um), mid infrared (3 ~ 5um), and far infrared (8 ~ 14).
For power equipment, most of its temperature is relatively low, concentrated around 300K ~ 600K (27 ℃ ~ 327 ℃). In this temperature range, it can be deduced from the basic infrared law that the infrared radiation signal emitted by the equipment Infrared 8 ~ 14um interval accounts for the largest percentage, and the radiation contrast is also the largest. Therefore, most of the infrared detection instruments of the power system work within the wavelength of 8 ~ 14um.
However, please note that even when working in an atmospheric window, the atmosphere has an extinction effect on infrared radiation. In particular, water vapor has the greatest influence on infrared radiation. Therefore, when testing, the humidity is preferably less than 85%, the closer the distance, the better.
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