This paper studies on the emissivity of electrical distribution equipments using infrared thermography with 8-14 µm of wavelength. The infrared thermography camera is widely used for preventive maintenances propose in many industries.It can be captured a thermal image for identifying fault on electrical equipment and measuring the temperature as well.The most important factor to use the thermal image accurately is the emissivity of materials. Therefore, this project aims to define the unknown emissivity of electrical devices in order to apply with the infrared thermography camera.The heat chamber to control the operating temperature 30oC – 200oC has been created to maintain the temperature of material for measuring the emissivity. To ensure the emissivity measurement technique, the general used material such as iron, stainless steel, brass, copper and aluminum were done the experiment and compared with the standard emissivity table. The selected electrical equipments (new and old) are cable lug, P.G. connector, some part of fuse holder and disconnecting switch. Finally, the emissivity data of selected devices can be used to input to thermal image camera for a better accuracy of temperature measurement.
Keywords:Infrared Thermography, emissivity measurement
There many applications of infrared thermography for non-destructive testing. EvaWulsten et.al  measured droplet surface temperature in dry air from the droplet temperature predicted by acoustic levitation theory. J.M.Laskar et.al  report measurement of thermal diffusivity of solid samples by using a continuous heat source and infrared thermal imaging. Their technique, a continuous heat source is used for heating the front surface of solid specimen and a thermal camera for detecting the time dependent temperature variations at the rear surface.The incorporation of heat loss correction in the solution of heat equation provides the values of thermal diffusivity for aluminum, copper and brass. A. Brosse et.al proposed the inspection of grinding process. A perfect control of this process is thus necessary to ensure correct final parts and limit damage. The experience on this subject has shown that the main effects on ground surface are residual stresses or metallurgical change, which are directly linked with the temperature and the power absorbed during the process.These example applications are pointed that the advantage of thermal image camera but they did not explain about the accuracy of thermal image. The most important of heat radiation effect is emissivity of the measured object. Therefore the accurate emissivity is taken into account. P. Herve et.al  derived a direct measurement method for the total directional emissivity of various coatings of interest for satellites applications.The effective spectral range chosen the measurements covers 6–800 µm.The design of the measurement apparatus for several coatings was presented. Tadeusz Walachdescribed a special relationship of thermal image camera and signal. Conventional and unconventional methods of the emissivity measurements together with a detailed analysis of the accuracy of typical methods are presented in his paper.A criterion and a procedure of choosing the emissivity measurement method are also proposed.Xiaogang Sun et.al  investigated the temperatures and emissivities of the metallic thermal protection blanket at 900–1,300◦C experimentally by using a multi-wavelength pyrometer.A linear relation between the emissivity and true temperature at different wavelengths is assumed.Some experimental results for the practical data processing of measurements performed on the metallic thermal protection blanket show that the difference between the calculated temperature and the temperature measured by a standard thermocouple.This paper is presented the properties of heat radiation of the object (Emissivity), using thermal image camera (Thermal Image: TI) with 8 to 14 µm of wavelength in order to apply preventative maintenance monitoring the quality of material and equipment in the electrical distribution system.
2.PRINCIPLE OF THERMOGRAPHY
From the principles of the radiation, it is important to understand about to the laws of radiation. In this concept, each body at a temperature T emits a radiation with a wavelength inside the electromagnetic spectrum.The radiation emitted can be measured by a value called luminance and noted as L0. The value of L0 is given analytically by the Planck’s law as in equation 1.
However, for a real body at the same temperature T,the luminance measured will not be equal to L0 but L = ε L0. Where ε is called emissivity of the material and takes values in the range [0,1]. Most often the electrical distribution equipment emissivities range from 0.1 to 0.95 depending on the type of material and surface characteristic.The basic idea of thermography is then to measure the luminance of a body in a range of wavelength chosen to obtain the temperature of the body.Moreover, inside the electromagnetic spectrum there are many kinds of radiation from the radio waves to the X and γ rays, the principle of infrared thermography is to measure the radiation in the infrared domain where the luminance reached maximum value.
Radiosity (J, W/m2) accounts for all the radiant energy leaving a surface). It represents the rate at which radiation leaves a unit area of the surface. This radiation includes the reflected portion of the irradiation (G,W/m2), as well as direct emission from the surface (E,W/m2) (see Fig.1). It is generally different from the emissive power depending on the emissivity value.
The irradiation (G, W/m2) is defined as the rate at which radiation is incident on a surface per unit area of the surface. As shown in Figure2, portions of the radiation may be reflected, absorbed, and transmitted.From a radiation balance on the medium, it follows that
Where α, ρ and τ are defined as the fraction of the total irradiation absorbed, reflected and transmitted by a surface, respectively
Finally, the thermography method gives very reliable non-contact measurement with many advantages for preventive maintenance in electrical power and distribution system.
Fig.2 Absorption, reflection, and transmission processes associated with a semitransparent medium.
3.1 Experimental set-up
The basic concept of emissivity measurement using infrared thermography is measuring the surface temperature by standard thermometer and adjusting the emissivity on infrared thermography camera until the temperature being similar. The environment temperature has to be controlled constantly during measurement process. Therefore this research has designed the heat and environment control chamber. The chamber can be provided the temperature from room temperature to 300 oC.
Inside the chamber has a jig and fixture for hold the sample on the suitable position. The user can open the door and change the sample easily. The opposite side of the door is the hole with the gate for measuring the temperature by thermal image camera. The thermal image camera from Fluke company model TI32 has been used in this experiment with 8 to 14 µm of wavelength. Temperature measurement range is (not calibrated below -10 °C) -20 °C to +600 °C (-4 °F to +1112 °F). The refresh rate of Image capture frequency is 9 Hz or 60 Hz refresh rate depending upon model variation. Detector type is 320 X 240 Focal Plane Array,uncooled microbolometer with operating temperature at -10 °C to +50 °C (14 °F to 122 °F). The dual channel digital thermometer with thermocouple type K is used in the experiment as shown in figure 4.
Fig.4 Jig and fixture for hold the sample.
Fig.5 Experiment setup for thermal image camera
3.2 Measurement procedure
Step1 determine the background temperature
In the heat chamber, the background temperature (TBG) is emitted from inside environment such as stainless steel wall and fixture. Because of the background temperature generated from several source, measuring of TBG is defined from the average temperature by varying the emissivity of 0.1 to 0.95 on the thermal image camera.
Step2 determine the emissivity of the object
From step1, the background temperature is entered into the thermal image camera. The emissivity measurement using infrared thermography is measuring the surface temperature by standard thermometer and adjusting the emissivity on infrared thermography camera until the temperature being the same temperature.
The experiments were started by heating up temperature at 200 oC. Afterward, the temperature of material was maintained during measurement emissivity. To ensure the emissivity measurement technique, the general used material such as iron, stainless steel, brass, copper and aluminum were done the experiment and compared with the standard emissivity table as shown in table1. The emissivity of electrical equipment is illustrated on table2.
Emissivity of electrical equipment is up to the material properties of each type of equipment. The emissivity values were achieved by controlling the surface temperature of an object with constant temperature. The common used material such as iron,copper, stainless steel, aluminum, brass was selected for measuring and comparing to ensure the accuracy of this method. The selected electrical equipments (new and old) are cable lug, P.G. connector, some part of fuse holder and disconnecting switch. Finally, the emissivity data of selected equipment can be used to input to thermal image camera for a higher accuracy of temperature measurement. This research can be provided advantages for preventive maintenance in electrical power and distribution system.
This work was supported by the research and development funding of Provincial Electricity Authority,Thailand.
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