Deterioration Process And Diagnosis Of Moistened Defect In Oil-impregnated Paper Bushing

With the continuous growth of power load demand and the large-scale construction of UHV power grid,higher insulation quality levels are used for the reliability level of power equipment.Oil-impregnated paper?OIP?bushing is widely used in power transformer.It is the prior trial-produced insulation structure in the development of UHV power system.The damp defect in an OIP bushing has become one of the main factors affecting the safe operation of power transformer.The inner insulation of an OIP bushing is a capacitive structure of multi-layer with series aluminium foils,which has the characteristics of less oil,slender and more aluminium foils.The difference of insulation structure leads to the difference between the moisture migration process of bushings and transformers.The existing research on oil-impregnated paper insulation cannot support the diagnosis of damp bushings.Moreover,there is no effective criterion for the diagnosis of dampness level in an OIP bushing.Standards and methods in the existing test procedures cannot effectively diagnose the damp defect of bushings.The existing XY model based on frequency domain spectroscopy?FDS?is mainly used for moisture assessment of oil-immpregnted pressboard structure.However,it is not clear for dampness diagnosis in multi-layer aluminium foil bushing insulation.In addition,X-wax often exists in the bushing with damp defects in inner layer of bushings.The research on its composition and molecular formula is not clear at home and abroad.In addition,the research on the detection and diagnosis of the X-wax in the bushing is not clear.Therefore,the existing diagnosis methods of damp defect cannot meet the actual requirements of defect diagnosis in bushings.It is very necessary to carry out research on the deterioration process and characteristics of damp defect in an OIP bushing,to provide a basis for the diagnosis of damp defect in a bushing.In order to study the damp process and phenomenon of OIP bushing,a bushing model with transparent sheath is designed in this paper,which is designed in accordance with the principle of consistency with the actual technology,field strength and materials.It ensures the equivalence between the bushing models and the actual bushing cores.A high voltage and high current test platform is set up with the function of real-time video of moisture ingress process,discharge phenomenon and gas production phenomenon,detection of partial discharge,tan?,capacitance and off-line detection of frequency domain spectrum?FDS?.It can work for the application of 1000 A current,100 kV AC voltage to two conventional 72.5 kV bushings.In order to study the process and phenomenon of moisture ingress into bushings,the water distribution characteristics of bushing models is studied by means of ultrasonic humidification.And the moisture distribution law of bushings was found:under the action of water concentration gradient,water molecules penetrate into the air cavity of bushing through the weakest point of seals,and invade the bushing capacitor core in three ways:condensation,dissolution and adsorption.First,moisture condenses into water droplets on the surface of rod and metal in the head of bushing,and then gradually deposits to the tail under the action of gravity,causing the bushing tail to be damp.Secondly,the water content in the oil increases first and dissolves with the surface of oil.Driven by water balance,the water content in the oil is gradually absorbed into the paper by the cores,resulting in the decreasing of water content in the oil and the reasing of water in capacitor core.Thirdly,the suspended water in the oil is adsorbed on the surface of the core,resulting in serious local moisture on the surface of the core.A large number of aluminum foils are built into the capacitance core,which can block the radial migration of moisture,thus making the inner layer of the core damp slowly.The maximum moisture content of capacitor core is random,and the average moisture content is correlated with the maximum moisture content.From the dimensions of tan?,capacitance,partial discharge,the characterization parameters of early and late state of damp process for bushings are obtained:in the early damp stage,the tan? increment at 1.2U_m voltage is more than 0.2%than that at 10kV;in the late damp stage,the phase distribution of surface discharge at 1.2 U_m voltage is remarkable.The characteristics of high voltage tan? and partial discharge spectrogram of damp bushing are verified on 72.5 kV test platform.In order to reveal the change of tan? and capacitance in the damp process of bushings,the method of quantitative moisture in capacitance core layers is used to study the bushing models from outer damp to inner damp under the same average moisture content.In addition,the increment of capacitance in the range of 10Hz to 110Hz leads to the decrease of tan? in the whole frequency range,which explains the reason for the decrease of tan? in power frequency during the moisture ingress process.It is proposed that tan?and capacitance at lower frequency range own better diagnostic representation ability for damp bushings.In order to study the process and phenomena of water deposition at the tail part of bushings,the method of quantitative water injection is used to simulate the moisture deposition at the tail of bushing and the quantitative water absorption at the tail core.It is found that the high electric field at 1.5U_m could stimulate the water deposition movement at the tail of bushing and trigger the discharge.The method of using existing regulations may result in misjudgment due to the state of water deposition.Under rated voltage,the capacitance of tail damp bushing increases with the damp time,while the tand values and partial discharge and remain unchanged.In the layers breakdown process of tail damp bushing,the capacitance value increases linearly.Moreover,the phase distribution of partial discharge shows the characteristics of surface discharge.The partial discharge starts at the edge of aluminum foil.The electric field distortion of foil edge caused by moisture is the main cause of discharge in damp bushings.The distribution of electric field at the foil edge in a 72.5 kV bushing is simulated and analyzed.The methods of single edge folding and laying semiconductor paper on the edge of the aluminum foils are proposed to reduce the electric field strength at the edge of the aluminum foils.In order to study the deterioration characteristics and diagnostic methods of poor-dryness and poor-impregnated defects in OIP bushings,the drying box and vacuum drying box were used to simulate the defects of poor dryness and poor impregnation of bushings respectively,and the partial discharge phase distribution characteristics of insufficient dryness defects of OIP bushing were obtained.The decaying rate C_s of C_1mHz/C10kHx with drying time are proposed to evaluate the drying process of bushing.The discharge characteristic of poor-impregnated defect in a bushing are obtained,while the gas producing process of capacitor core during discharge is observed at the same time.The discharge channel was located at the nearby area of aluminum foil edge after the observation of capacitance core.X-wax in a bushing is the product of discharging and deteriorating of oil gap in paper layer under the combined action of electric field,thermal field and moisture.In order to further diagnose the insulation defects of bushing,the composition and structure of X-wax in paper samples were studied by means of XPS,elemental analysis and infrared spectroscopy.The composition of wax in actual bushing was obtained:the main components were C and H,the average molecule was C₂₀H₃₅,and the main groups contained methylene and methylene alkanes,which were similar to those of yellow vaseline.The influence of wax weight on FDS and polarization current curve of 1 mHz-10kHz was studied by using double-layer paper model and bushing models.It was found that the ultra-low frequency tan?? in the range of 1 mHz?10mHz increased with the wax weight.The 100s polarizaition current showed an increasing trend,while the characteristic value of branch parameters of extended Debye equivalent resistance?1/Ri had a good linear relationship with wax weight.In order to diagnose quantitatively the moisture content in a bushing,the influence of aluminum foil on the frequency domain spectroscopy were studied by damp bushings with foils and no foils.It was found that the under the same damp,the increment of 1mHz complex capacitance in bushings with foils is more than its in bushings with no foils.Based on the FDS value of bushing models with aluminum foils with different water content,a new physical quantity C_1mHz=/C_10kHz for the diagnosis of moisture content of bushing is proposed,and a quantitative diagnosis model Mc for the average moisture content of bushing is established.A diagnosis quantity of moisture content of bushing based on branches parameter ?C/R,of extended Debye model is proposed.A method for predicting the location of damp layer of a bushing is proposed by the real-part ratio of distributed complex capacitance to distinguish the damp layer.The accuracy of the quantitative diagnosis model Mc for the average water content of 500 kV bushing is verified in site.