| The phenomenon of surface flashover seriously affects the safety of power equipment,thereby endangering the stable operation of power grid system.It is particularly important to accurately detect and diagnose the state of surface discharge.At the same time,surface discharge is a complex process involving sound,light,electric and heat with multiple species,multiple physical and chemical processes,and multiple time and space scales.Therefore,the comprehensive evaluation of discharge has the problem of how to select indicators and how to combine them.Previous contactless measurements mostly focused on ultraviolet image morphology,ultra high frequency signal,acoustic signal etc.,while there is little research on visible digital image.With the development of digital image technology,the chromaticity information in the discharge visible light images is becoming more and more abundant,and the light radiation information under long exposure time can overcome the randomness of the discharge phenomenon.Therefore,in this paper,the author used chromaticity characteristics and spectral chromaticity coordinates for state diagnosis,and combined them with infrared radiation images and electrical signal characteristics,so as to realize a comprehensive understanding and comprehensive evaluation of surface discharge under different time scales.The main achievements are as follows:A new method to diagnose the AC surface discharge status by using chromaticity information of optical images with high spatial resolution in air was proposed,and a set of chromaticity diagnostic indexes suitable for many applications were established.Optical images of surface discharge in dark environment were collected firstly,and then colorimetric parameters of RGB,HSI,CIE-XYZ and LAB color space were extracted by image processing technology to diagnose the developing status of surface discharge.The main results are :(1)In the RGB color space,the pixel near the discharge high voltage electrode was prone to oversaturation,and the P-value diagnosis method based on the frequency of blue zero gray level in the grounding electrode region was established,so that different development stages of surface discharge could be accurately judged according to the P-value.(2)The relative position of chromaticity coordinates in CIE-xy Y color space was used to diagnose the state of surface discharge.It was discovered that the chromaticity coordinates change along a straight line between the equal-energy white point and the specific color.The relative position on the line represents the discharge severity,and the change rule is not affected by material and color.After correcting for ambient noise,the color space of surface discharge in the chromaticity diagram is divided.(3)The combination of H value and I value in HSI color space can intuitively judge the development state of surface discharge,but it is greatly affected by the material color.(4)The color difference value of LAB color space directly reflects the amplitude change of the working state of the equipment,which could be used for effective early warning in engineering applications.The chromaticity parameters of different color spaces can meet the needs of different occasions,and as a new macroscopic index,this method could be complementary to the current waveform and the spectral method of observing the microscopic process,and realize the cross-scale correlation between macroscopic phenomenon(image)and microscopic process(spectrum);From the point of view of potential application,this method also provides a new simple method to meet the requirement of long time scale state monitoring in industrial application.The traditional visible light image chromaticity coordinate method was introduced into the field of discharge spectrum diagnosis,and the linear variation law of chromaticity coordinate was found,which was consistent with the change of chromaticity coordinate based on visible light image,and the cross-scale conversion between microscopic process(spectrum)and macroscopic phenomenon(image)was realized.Experiments and calculations of surface,corona and dielectric barrier discharges showed that: with the increase of voltage,the coordinates of spectral chromaticity coordinates of different discharge forms change from the equal energy white point to spectral color in a straight line,which indicates that the main wavelength(spectral color)of the color remains the same during the discharge process,and the saturation increases with the increase of voltage;The slope of chromaticity coordinate line can diagnose the type of gas discharge.The slope reflects the dominant wavelength of radiation color(spectral color)and expresses the color characteristics of photon radiation during the discharge process;By studying the thermal characteristics of the surface discharge process,it is found that the dielectric plate has obvious temperature rise effect in the surface discharge process,and due to the influence of thermal convection,the maximum temperature generally appears at a certain distance from the electrode head,and different materials have different temperature rise due to the difference of specific heat capacity;Through the correlation analysis of different characteristic signals of surface discharge,the development process and physical mechanism of surface discharge are understood more deeply.Visible light chromaticity information was used to diagnose the dry band arc in surface discharge and pollution degree of insulator surface.Through the verification of electrical signals,optical images and thermal characteristics,it was found that there are four development modes of short gap pollution surface discharge due to the difference of humidity: surface mode,leakage current-surface mode,local arc-surface-arc mode and local arc-arc mode.The stepwise increase of arc length caused by thermal effect is found in the local arc-surface-arc mode,which provides a new idea for the control of fouling development in engineering;Different stages of AC arc could be recognized by visible light photos with short exposure time.By comparing the chromaticity characteristics of AC arc in zero-off stage with that of DC corona,it is indirectly proved that there is still residual current in the zero-off stage of AC arc;The temperature distribution of the double finger electrode is different from that of the clean surface under the pollution condition,and there is no symmetry anymore;The fitting formula of saturation and pollution degree of the visible light image of polluted insulator was obtained.The fitting formula can be used to realize online monitoring of insulation pollution degree.And the fitting results are in good agreement with the experimental data.A simulation experiment model of surface discharge was established using confined space,and the thermal characteristics of surface discharge were studied through visible light image analysis,infrared thermal image measurement and numerical simulation.The thermal effects at different stages during the polluted surface discharge process play a key role in the development of the discharge.However,the surface discharge is in a non-thermal equilibrium state,and there is no uniform measurable temperature.Muchmore,the surface discharge path is randomly distributed,so the heat is not easy to accumulate;and the process of heat conduction,heat convection and heat radiation are mixed together,so it is difficult to establish a simple calculation model.A method was proposed to collect and obtain the heat of surface discharge in the confined space formed by glass tube,and an equivalent temperature method to characterize the thermal characteristics of surface discharge on a long-term scale was proposed,and the variation law is in good agreement with the calculation results of heat diffusion equation.In addition,corona discharge and dielectric barrier discharge are found in the initial stage of short gap discharge in confined space.With the increase of voltage,the positive corona discharge develops into a creaking discharge;It is found that the presence of glass tube affects the breakdown voltage and the breakdown path compared with the open atmosphere environment;The influence of surface charge on the breakdown voltage of glass tube was determined by numerical simulation;The particles in the confined space were used to simulate the discharge phenomenon in the contaminated surface layer,and it was found that the effects of particles in the confined space on the breakdown characteristics are bidirectional: when the particles are not enough to limit the discharge path,the charge effect on the surface of the particles will lead to a significant decrease in the breakdown voltage;when the particles are too many,the breakdown difficulty will be increased;When stable arc occurs in confined space,the temperature rise rate of different medium is different,and the arc is easier to maintain when the particle is salt. 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