Preparation Of Vegetable Insulating Oils And Study On Their Performances

Mineral oil has been widely used in oil-immersed electrical equipment for a long time with its excellent electrical insulating property,high coolant capability and low cost.However due to low flash point and burning point,it hardly meets the requirements of high fire prevention performance of insulation materials.And once leaked,it would cause serious pollution to the water and environment owing to only about 30%biodegradation rate.In recent years,with the improvement in people's environmental awareness and severe shortage of petroleum resources,researchers have been trying to find a green and renewable liquid substitute for mineral oil.Recently,vegetable oils,as renewably natural ester fluids from biomass,have drawn the most attention because of their high flash point,wide source and fully degradability.Moreover,they also show good physical and dielectric properties after the refining processing.But compared with mineral oil,vegetable insulating oil has its inherent disadvantages such as poor oxidation resistance,high pour point and high viscosity.These defects restrict the application of vegetable oil in the liquid insulating material.Focusing on the problems discussed above,this paper studies preparation,properties characterization and performance improvements of vegetable insulating oil,with natural vegetable oil as raw material.Soybean oil,canola oil,peanut oil and cottonseed oil in RBD(refined,bleached,deodorized)grade were chosen as raw materials.Purified vegetable insulating oils were prepared by the treatments of adsorption,deacidification and anhydration.By removing impurities,the acid value,moisture content and dielectric loss factor of purified vegetable insulating oils could meet the requirements of the transformer oil.By the results of study on the optimization process,the optimum technological conditions for refining method of soybean insulating oil are as follows.The active white earth was used as the adsorbent to remove the irmpurities under vacuum technical at twice,with additive amount up to 3%and 2%respectively.The alkaline Al2O3 was used as the adsorbent to reduce acid value,and the best additive amount is 4%temperature 50?and time 60min.Then the oil was filtrated under vacuum technical with microfiltration membrane and at last deeply purified through vacuum distillation.The electrical performance test results of four purified vegetable insulating oils showed superior insulation performances.The breakdown voltage and dielectric constant are higher than that of mineral insulating oil while dielectric loss and volume resistivity is slightly worse.The test results of physical properties showed that their closed cup flash point reached up to 280?,far better than mineral insulating oil.Their viscosities(40?)are 32 mm2/s-40 mm2/s,which are higher than mineral oil and means poor liquidity.Their pour points show a larger gap because of the differences of fatty acid composition.The soybean oil and canola oil have the lower pour points,but still higher than mineral oil.In summary,purified vegetable insulating oils have good insulating property and high flash points,but high kinematic viscosity and poor low temperature performance.According to the characteristics,purified vegetable insulating oils are suitable for being used as high-temperature transformer oils.Considering the pour points,canola oil and soybean oil are more suitable for the raw materials of purified vegetable insulating oils.Emphasizing on the high pour points of purified oils and methyl oils,the study was carried out to improve low temperature performance by adding pour point depressant(PPD).The results showed that polymethacrylate(PMA)PPD had obvious effect on two types of vegetable insulating oils,and the best addition was 0.5 wt%.With the assistance of PPD,purified canola oil and methyl canola oil can reach the pour point of-25?,meet the requirement of 25#transformer oil,and all of other oils can meet the requirement of 10#transformer oil.Focusing on the poor oxidation stability of vegetable oils,the study was carried out by adding antioxidant.Through the comparison of three common methods,Rancimat method selected as the most suitable evaluation methods for oxidation stability of vegetable insulating oils.Three kinds of common antioxidants were investigated by this method.The results showed that tertiary butylhydroquinone(TBHQ)had the best effect with the addition of 0.5%.Focusing on the high viscosities of purified oils,the methyl vegetable insulating oils with low viscosities were prepared by traditional base-catalytic transesterification of the same vegetable oils and methanol.Evaluated by transesterification conversion,we investigated the effects of the amount of catalyst,alcohol-oil ratio,reaction temperature and reaction time for the preparation.The results indicated that the optimum conditions for preparation of methyl vegetable insulating oils by transesterification is 9:1 for alcohol-oil ratio,0.6%for the amount of catalyst,60for the reaction temperature and 75 min for the reaction time.Under these conditions,all conversions of four methyl oils had reached more than 97%.The test results of physical properties showed that methyl oils had excellent viscosity properties,only 65%of that of mineral insulating oil.However,the flash points were far lower than that of purified oils,and just a little higher than that of mineral oils.The pour points were even higher than that of purified oils,became of the biggest drawback.Thus,the key problem of methyl vegetable insulating oils using as low-viscosity liquid insulating materials lies in the improvement of low-temperature performance.Seen from the previous results,purified vegetable insulating oils have the advantages of high flash point,and methyl vegetable insulating oils have the advantages of low viscosity.In order to obtain the vegetable insulating oils with balanced performances,we prepared partial transesterification vegetable insulating oils by controlling the transesterification conversion rate.With the purpose of gaining the products of specific conversion,the experiments were performed according to central composite design(CCD)of response surface methodology(RSM)to investigate the relationship between the factors and conversion,and a quadratic model was established to determine and optimize the condition parameters for a target conversion.The results of analysis on variance and validation experiments indicate suitability of this model.By changing the conversion,the performance of partial transesterification vegetable insulating oils can be variable and controllable,which make the products satisfy a variety of requirements.It broadens the applications of renewable biomass energy as an alternative to mineral oil.Because vegetable insulating oil is a new kind of liquid insulating material,fault diagnosis technology is still missing.Thus,the dissolution properties of seven key gases(H2,CH4,CO etc.)in vegetable insulating oils were studied.This study was a preliminary step for the determination of dissolved gas content and the establishment of Dissolved Gas Analysis(DGA)for vegetable insulating oils.Thermal failure simulation test was conducted to investigate the gas production rules of vegetable oil and vegetable methyl ester under 200? to 700?.Unlike traditional mineral insulating oil,main key gases in vegetable-based oils are CO,CO2,and C2H6,while minor key gases are H2,CH4,and C2H4.Tiny amount of C2H2 was generated at 700 ?.This study provides preliminary work and data base for DGA technology of vegetable-based insulating oil,also has a certain reference and guidance value for current application of vegetable-based insulating oil.According to DGA of mineral oil,the phenomenon of single excessive hydrogen(SEH)had led to lots of maintenance work.However,no fault was found in those cases.And the cause of this anomaly has not been clarified.Thus,potential sources of hydrogen production were analyzed based on the chemical composition and the properties of materials.And possible causes were further calculated and analyzed according to the theory of chemical thermodynamics,dynamics,crystallology and catalysis principles.Then some simulation experiments were also performed to validate theoretical studies.It was confirmed that corrosion reaction of moisture in the insulating materials with active metals such as aluminum,zinc or iron is the only cause of SEH.Based on the cause,a linear model was established to describe the growth trend of the dissolved-hydrogen concentration in SEH.This study demonstrates that SEH has little correlation with latent faults.The lower limit of dissolved-hydrogen concentration in industry standards should be revised to reduce unnecessary maintenance work caused by this phenomenon.