The interturn paper insulation of oil-immersed power transformer windings,under the combined influence of electric fields,moisture,and conductor heating,will produce bubbles,which pose a significant threat to the insu...The interturn paper insulation of oil-immersed power transformer windings,under the combined influence of electric fields,moisture,and conductor heating,will produce bubbles,which pose a significant threat to the insulation system.However,there is limited research on the characteristics of bubble evolution in oil-paper insulation under the influence of electric fields and the subsequent dissolution process.Based on the continuous observation of bubble size using electron microscopes,experimental and theoretical investigations into the formation and dissolution of bubbles under electric field conditions are presented.The effects of different field strengths on bubble evolution and dissolution characteristics were studied.The results showed that the electric field promoted both the generation and dissolution of bubbles,with a more pronounced effect observed at higher field strengths(below partial discharge(PD)inception electric field,hereafter referred to as PDIE).However,when the field strength exceeded PDIE,the bubbles tended to shrink and gradually increase in size.The changes in bubble volume were related to not only gas diffusion but also oxygen consumption and fault gases generation due to PD.A better understanding of the formation and dissolution characteristics of bubbles under varying field strengths is achieved.Furthermore,it also provides a reference for assessing the risk of bubble generation and conducting bubble-related fault diagnosis during the overload operation of oil-immersed power equipment.展开更多
As a novel chemical marker,methanol has been widely applied to evaluate the ageing state of insulating paper in recent years.Methanol gas sensor(MGS)could be applied to detect the methanol content in transformer oil f...As a novel chemical marker,methanol has been widely applied to evaluate the ageing state of insulating paper in recent years.Methanol gas sensor(MGS)could be applied to detect the methanol content in transformer oil for its high portability and stability.However,it has the defects of a high limit of detection(LOD)and poor anti-interference performance.Therefore,a novel method of detecting methanol in transformer oil based on MGS with the solid-phase extraction(SPE)technology was investigated in this paper.A detection platform for methanol in transformer oil based on SPE was established,and the SPE experimental process and the crucial parameters were optimised.The results show that the SPE decreased the LOD of MGS from 0.4 to 0.2 ppm and effectively eliminated the interfering substances in oil.Although a part of methanol would be lost during the SPE process,the regression correction coefficient was proposed to correct the evaluation errors.The results indicate that the average relative errors decreased from 17.09%to 4.38%,and the proposed method has good applicability.展开更多
Moisture ingression accelerates the ageing and degradation of the oil-paper insulation.Rapid and accurate moisture assessment of power apparatus in the field is thus essential.In this paper,an innovative design of a m...Moisture ingression accelerates the ageing and degradation of the oil-paper insulation.Rapid and accurate moisture assessment of power apparatus in the field is thus essential.In this paper,an innovative design of a microstrip petal-like complementary split ring resonator(MPCSRR)is proposed for the moisture assessment of the oil-paper insulation in the GHz range.The MPCSRR has advantages in concentrating the electromagnetic field,significantly increasing the number of resonance peaks,and achieving a sensitivity of approximately 3-4 times higher than the traditional microstrip ring resonators(MRRs).A mathematical model for analysing the dielectric response is established,combining the theoretical analysis and equivalent circuit model.The effects of microstrip width and substrate thickness on impedance matching,as well as the incidence depth and test sensitivity of the resonator,are explored using simulation.The experimental results demonstrate that the MPCSRR achieves rapid moisture detection within tens of seconds and accurately characterises the wetness of the oil-paper insulation.A moisture assess-ment model based on Lasso regression achieves an accuracy of 4%,confirming the reliability and validity of the proposed method.The results also verify that the sensitivity of the MPCSRR is also higher than that of the traditional MRR,showing its potential for precise and efficient moisture detection in oil-paper insulation.展开更多
Frequency domain spectroscopy(FDS)is widely used for assessing the condition of oil-paper insulated electrical equipment.However,in low temperatures,measurement results often deviate from expected values.This study in...Frequency domain spectroscopy(FDS)is widely used for assessing the condition of oil-paper insulated electrical equipment.However,in low temperatures,measurement results often deviate from expected values.This study investigates the low-temperature dielectric response characteristics of oil-paper insulation under large temperature differences(-60℃ to 30℃),wide frequency ranges(1 mHz-5 kHz)and varying moisture contents(0.41%-3.91%).The mechanisms of low-temperature effects on oil-paper insulation relaxation polarisation are revealed.Further,a novel low-temperature normalisation method and a moisture evaluation method are proposed based on the improved Havriliak-Negami model.Compared to traditional methods,the new approach signifi-cantly enhances the accuracy.The goodness of fit R2 for temperature normalisation improves from 0.9526 to 0.9757,and the error in moisture content evaluation has reached 0.498%.Finally,the novel approach is applied to diagnose the condition of a damped bushing model,demonstrating high potential for practical applications.This study enables condition diagnosis of oil-impregnated electrical equipment in extremely cold environ-ments,filling a critical gap in the field.展开更多
The insulating paper of the transformer is affected by many factors during the operation,meanwhile,the surface texture of the paper is easy to change.To explore the relationship between the aging state and surface tex...The insulating paper of the transformer is affected by many factors during the operation,meanwhile,the surface texture of the paper is easy to change.To explore the relationship between the aging state and surface texture change of insulating paper,firstly,the thermal aging experiment of insulating paper is carried out,and the insulating paper samples with different aging times are obtained.After then,the images of the aged insulating paper samples are collected and pre-processed.The pre-processing effect is verified by constructing and calculating the gray surface of the sample.Secondly,the texture features of the insulating paper image are extracted by box dimension and multifractal spectrum.Based on that,the extreme learning machine(ELM)is taken as the classification tool with texture features and aging time as the input and output,to train the algorithm and construct the corresponding relationship between the texture feature and the aging time.After then,the insulating paper with unknown aging time is predicted with a trained ELMalgorithm.The numerical test results show that the texture features extracted from the fractal dimension of the micro image can effectively characterize the aging state of insulating paper,the average accuracy can reach 91.6%.It proves that the fractal dimension theory can be utilized for assessing the aging state of insulating paper for onsite applications.展开更多
The purpose of this work is to determine the impact of thermal aging on the dielectric and physicochemical properties of the oil/paper mixed insulation. We performed a comparative analysis of dielectric paper dipped i...The purpose of this work is to determine the impact of thermal aging on the dielectric and physicochemical properties of the oil/paper mixed insulation. We performed a comparative analysis of dielectric paper dipped in two cooling fluids: palm kernel oil methyl ester (MEPKO) and mineral oil (MO). Two types of dielectric paper were used: Thermally Upgraded Kraft paper (TUK) and Nomex-910 paper (NP-910). An accelerated aging test was realized at 110<span style="white-space:nowrap;">°</span>C during a total of 96 hours. Samples of oil and paper were collected after 0, 48, 72 and 96 hours for analyses purposes. The analyses performed included the measurement of the Breakdown voltage (BDV) of the dielectric papers, the Total Acid Number (TAN) and the Decay Dissolved Products (DDP) of the liquid dielectrics. The BDV of NP-910 is greater than the BDV of TUK. Concerning the type of oil, the BDV of dielectric papers impregnated with MEPKO is greater than the BDV of similar papers impregnated with MO, indicating a better preservation of paper when dipped in methyl esters. The analyses of TAN and DDP revealed that Nomex-910 improves the oxidation stability of MO, but reduces the oxidation stability of MEPKO. These results prove that methyl esters can be used as a substitute to replace mineral oils in power transformers. Furthermore, they show that NP can be used mainly in areas of transformer where solid insulation is subjected to high thermal and electrical stress, and TUK other places where solid insulation is required. Such combination could assure money savings and a better preservation of the oil viscosity.展开更多
There is an extremely urgent demand in the realm of power equipment,including power transformers,motors,and cables.Specifically,there is a pressing need for cellulose-based composite insulating paper that can exhibit ...There is an extremely urgent demand in the realm of power equipment,including power transformers,motors,and cables.Specifically,there is a pressing need for cellulose-based composite insulating paper that can exhibit high thermal conductivity,superior mechanical properties,and robust insulation characteristics.In response to this demand,this study adopted a‘simulation-guided experimental research’methodology.Firstly,based on molecular dynamics(MD)simulations,nano-kaolin(KL)/cellulose composite models with different contents were constructed.Then,according to the simulation results,the corresponding proportions of nano-KL/cellulose insulating paper were prepared.The simulation and experimental findings further reveal a sig-nificant effect of nano-KL.To be more precise,nano-KL can effectively fill the microscopic defects and voids within the cellulose structure.Moreover,nano-KL forms an orderly and regular thermal conductivity network in conjunction with cellulose.As a result,this network structure elevates the paper's overall thermal conductivity.Owing to its low-dielectric-loss characteristics,nano-KL reduces the microscopic charge polar-isation phenomenon within the composite structure.It curbs the migration of elec-trons,alleviates the concentration of electric field stress,and ultimately improves the electrical insulation performance of the modified insulating paper.Notably,the 4 wt%nano-KL/cellulose insulating paper exhibits optimal performance,and its tensile strength,thermal conductivity,volume resistivity,dielectric loss,and breakdown strength are 55.81 MPa,0.201 W·m^(−1)⋅K^(−1),4.58×10^(15)Ω·m,0.25%,and 57.81 kV/mm.This study demonstrates MD simulations'feasibility and effectiveness in providing theories and protocols for experiments.展开更多
Charge transport in oil impregnated paper impacts the insulation performance of a transformer.This paper proposes a simulation method for the charge transport in oil impregnated paper insulation.The transient upstream...Charge transport in oil impregnated paper impacts the insulation performance of a transformer.This paper proposes a simulation method for the charge transport in oil impregnated paper insulation.The transient upstream finite element method(FEM)is applied to the transport equations of bipolar charges for establishing a numerical simulation model of charge transport in oil impregnated paper insulation.The method is validated by experimental results.The charge transport and electric field distribution in single-layer oil impregnated paper insulation under different temperature gradients is simulated.The trends of the simulation results are seen to agree with the corresponding experimental results.This paper conducts exploratory research into the simulation of charge transportation phenomenon in oil impregnated paper,and is of importance to the design of oil impregnated paper insulation.展开更多
During the operation of power transformer,its oil-paper insulation is continuously subjected to various stresses,c.g.,the thermal,electrical,mechanical,and chemical stresses,which cause insulation aging gradually.It h...During the operation of power transformer,its oil-paper insulation is continuously subjected to various stresses,c.g.,the thermal,electrical,mechanical,and chemical stresses,which cause insulation aging gradually.It has been considered that the combined thermal and electrical stresses are the most important and unavoidable factors that induce insulation materials aging.In this work,accelerated aging experiments of oil-impregnated pressboards under combined thermal(130℃) and electrical stresses(4 kV/mm) are performed,while the aging experiments under single thermal stress are also carried out at the corresponding temperature(130℃).The electrical and physic-chemical properties of oil,including dielectric losses factor tanδ,resistivity,acid value and pH value etc.,are measured during the aging process.Dissolved gasses in oil and polymerization degree of cellulose are also measured.The relationship between these properties of oil-paper insulation and aging time is investigated.The results show that dissolved gases in oil,resistivity,tanδof oil under combined thermal and electrical stresses are obviously different from that tinder thermal stress during aging process while some other properties show similar changing trend.For cellulose, compared with the single thermal aging results,it even shows a slower degradation rate in the presence of electrical stress.展开更多
Because of the increase in the transmission voltage levels,the demand for insulation reliability of power transformers has increasingly become critical.Cellulose insulating paper is the main insulating component of po...Because of the increase in the transmission voltage levels,the demand for insulation reliability of power transformers has increasingly become critical.Cellulose insulating paper is the main insulating component of power transformers.To improve the insulation level of ultrahigh voltage transformers and reduce their weight and size,reducing the dielectric constant of oil-immersed cellulose insulating paper is highly desired.Cellulose is used to produce power-transformer insulating papers owing to its excellent electrical properties,renewability,biodegradability and abundance.The dielectric constant of a cellulose insulating paper can be effectively reduced by chemical or physical modification.This study presents an overview of the foreign and domestic research status of the use of modification technology to reduce the dielectric constant of cellulose insulating papers.All the mentioned methods are analyzed in this study.Finally,some recommendations for future modified cellulose insulating paper research and applications are proposed.This paper can provide a reference for further research on low dielectric constant cellulose insulating paper in the future.展开更多
基金State Grid Corporation of China,Grant/Award Number:5500-202116119A。
文摘The interturn paper insulation of oil-immersed power transformer windings,under the combined influence of electric fields,moisture,and conductor heating,will produce bubbles,which pose a significant threat to the insulation system.However,there is limited research on the characteristics of bubble evolution in oil-paper insulation under the influence of electric fields and the subsequent dissolution process.Based on the continuous observation of bubble size using electron microscopes,experimental and theoretical investigations into the formation and dissolution of bubbles under electric field conditions are presented.The effects of different field strengths on bubble evolution and dissolution characteristics were studied.The results showed that the electric field promoted both the generation and dissolution of bubbles,with a more pronounced effect observed at higher field strengths(below partial discharge(PD)inception electric field,hereafter referred to as PDIE).However,when the field strength exceeded PDIE,the bubbles tended to shrink and gradually increase in size.The changes in bubble volume were related to not only gas diffusion but also oxygen consumption and fault gases generation due to PD.A better understanding of the formation and dissolution characteristics of bubbles under varying field strengths is achieved.Furthermore,it also provides a reference for assessing the risk of bubble generation and conducting bubble-related fault diagnosis during the overload operation of oil-immersed power equipment.
基金supported by the State Grid Heilongjiang Electric Power Company Ltd.,Electric Power Research Institute Project(52243724000M)the China Postdoctoral Science Foundation(2020M670921)the National Natural Science Foundation of China(52307164).
文摘As a novel chemical marker,methanol has been widely applied to evaluate the ageing state of insulating paper in recent years.Methanol gas sensor(MGS)could be applied to detect the methanol content in transformer oil for its high portability and stability.However,it has the defects of a high limit of detection(LOD)and poor anti-interference performance.Therefore,a novel method of detecting methanol in transformer oil based on MGS with the solid-phase extraction(SPE)technology was investigated in this paper.A detection platform for methanol in transformer oil based on SPE was established,and the SPE experimental process and the crucial parameters were optimised.The results show that the SPE decreased the LOD of MGS from 0.4 to 0.2 ppm and effectively eliminated the interfering substances in oil.Although a part of methanol would be lost during the SPE process,the regression correction coefficient was proposed to correct the evaluation errors.The results indicate that the average relative errors decreased from 17.09%to 4.38%,and the proposed method has good applicability.
基金Ningbo Power Supply Company Science and Technology Program,Grant/Award Number:KJKY20230393。
文摘Moisture ingression accelerates the ageing and degradation of the oil-paper insulation.Rapid and accurate moisture assessment of power apparatus in the field is thus essential.In this paper,an innovative design of a microstrip petal-like complementary split ring resonator(MPCSRR)is proposed for the moisture assessment of the oil-paper insulation in the GHz range.The MPCSRR has advantages in concentrating the electromagnetic field,significantly increasing the number of resonance peaks,and achieving a sensitivity of approximately 3-4 times higher than the traditional microstrip ring resonators(MRRs).A mathematical model for analysing the dielectric response is established,combining the theoretical analysis and equivalent circuit model.The effects of microstrip width and substrate thickness on impedance matching,as well as the incidence depth and test sensitivity of the resonator,are explored using simulation.The experimental results demonstrate that the MPCSRR achieves rapid moisture detection within tens of seconds and accurately characterises the wetness of the oil-paper insulation.A moisture assess-ment model based on Lasso regression achieves an accuracy of 4%,confirming the reliability and validity of the proposed method.The results also verify that the sensitivity of the MPCSRR is also higher than that of the traditional MRR,showing its potential for precise and efficient moisture detection in oil-paper insulation.
基金National Natural Science Foundation of China,Grant/Award Number:52477027。
文摘Frequency domain spectroscopy(FDS)is widely used for assessing the condition of oil-paper insulated electrical equipment.However,in low temperatures,measurement results often deviate from expected values.This study investigates the low-temperature dielectric response characteristics of oil-paper insulation under large temperature differences(-60℃ to 30℃),wide frequency ranges(1 mHz-5 kHz)and varying moisture contents(0.41%-3.91%).The mechanisms of low-temperature effects on oil-paper insulation relaxation polarisation are revealed.Further,a novel low-temperature normalisation method and a moisture evaluation method are proposed based on the improved Havriliak-Negami model.Compared to traditional methods,the new approach signifi-cantly enhances the accuracy.The goodness of fit R2 for temperature normalisation improves from 0.9526 to 0.9757,and the error in moisture content evaluation has reached 0.498%.Finally,the novel approach is applied to diagnose the condition of a damped bushing model,demonstrating high potential for practical applications.This study enables condition diagnosis of oil-impregnated electrical equipment in extremely cold environ-ments,filling a critical gap in the field.
基金This work was supported by the Tianyou Youth Talent Lift Program of Lanzhou Jiaotong University,the Youth Science Foundation of Lanzhou Jiaotong University(No.2019029)the University Innovation Fund Project of Gansu Provincial Department of Education(No.2020A-036)the Young Doctor Foundation of JYT.GANSU.GOV.CN(No.2021QB-060).
文摘The insulating paper of the transformer is affected by many factors during the operation,meanwhile,the surface texture of the paper is easy to change.To explore the relationship between the aging state and surface texture change of insulating paper,firstly,the thermal aging experiment of insulating paper is carried out,and the insulating paper samples with different aging times are obtained.After then,the images of the aged insulating paper samples are collected and pre-processed.The pre-processing effect is verified by constructing and calculating the gray surface of the sample.Secondly,the texture features of the insulating paper image are extracted by box dimension and multifractal spectrum.Based on that,the extreme learning machine(ELM)is taken as the classification tool with texture features and aging time as the input and output,to train the algorithm and construct the corresponding relationship between the texture feature and the aging time.After then,the insulating paper with unknown aging time is predicted with a trained ELMalgorithm.The numerical test results show that the texture features extracted from the fractal dimension of the micro image can effectively characterize the aging state of insulating paper,the average accuracy can reach 91.6%.It proves that the fractal dimension theory can be utilized for assessing the aging state of insulating paper for onsite applications.
文摘The purpose of this work is to determine the impact of thermal aging on the dielectric and physicochemical properties of the oil/paper mixed insulation. We performed a comparative analysis of dielectric paper dipped in two cooling fluids: palm kernel oil methyl ester (MEPKO) and mineral oil (MO). Two types of dielectric paper were used: Thermally Upgraded Kraft paper (TUK) and Nomex-910 paper (NP-910). An accelerated aging test was realized at 110<span style="white-space:nowrap;">°</span>C during a total of 96 hours. Samples of oil and paper were collected after 0, 48, 72 and 96 hours for analyses purposes. The analyses performed included the measurement of the Breakdown voltage (BDV) of the dielectric papers, the Total Acid Number (TAN) and the Decay Dissolved Products (DDP) of the liquid dielectrics. The BDV of NP-910 is greater than the BDV of TUK. Concerning the type of oil, the BDV of dielectric papers impregnated with MEPKO is greater than the BDV of similar papers impregnated with MO, indicating a better preservation of paper when dipped in methyl esters. The analyses of TAN and DDP revealed that Nomex-910 improves the oxidation stability of MO, but reduces the oxidation stability of MEPKO. These results prove that methyl esters can be used as a substitute to replace mineral oils in power transformers. Furthermore, they show that NP can be used mainly in areas of transformer where solid insulation is subjected to high thermal and electrical stress, and TUK other places where solid insulation is required. Such combination could assure money savings and a better preservation of the oil viscosity.
基金National Natural Science Foundation of China,Grant/Award Number:52277138Guangxi Science Foundation for Outstanding Young Scholars,Grant/Award Number:202400489+1 种基金Guangxi Key R&D Programs,Grant/Award Numbers:Guike AB24010119,Guike AB25069468The Innovation Project of Guangxi Graduate Education,Grant/Award Number:YCBZ2024052。
文摘There is an extremely urgent demand in the realm of power equipment,including power transformers,motors,and cables.Specifically,there is a pressing need for cellulose-based composite insulating paper that can exhibit high thermal conductivity,superior mechanical properties,and robust insulation characteristics.In response to this demand,this study adopted a‘simulation-guided experimental research’methodology.Firstly,based on molecular dynamics(MD)simulations,nano-kaolin(KL)/cellulose composite models with different contents were constructed.Then,according to the simulation results,the corresponding proportions of nano-KL/cellulose insulating paper were prepared.The simulation and experimental findings further reveal a sig-nificant effect of nano-KL.To be more precise,nano-KL can effectively fill the microscopic defects and voids within the cellulose structure.Moreover,nano-KL forms an orderly and regular thermal conductivity network in conjunction with cellulose.As a result,this network structure elevates the paper's overall thermal conductivity.Owing to its low-dielectric-loss characteristics,nano-KL reduces the microscopic charge polar-isation phenomenon within the composite structure.It curbs the migration of elec-trons,alleviates the concentration of electric field stress,and ultimately improves the electrical insulation performance of the modified insulating paper.Notably,the 4 wt%nano-KL/cellulose insulating paper exhibits optimal performance,and its tensile strength,thermal conductivity,volume resistivity,dielectric loss,and breakdown strength are 55.81 MPa,0.201 W·m^(−1)⋅K^(−1),4.58×10^(15)Ω·m,0.25%,and 57.81 kV/mm.This study demonstrates MD simulations'feasibility and effectiveness in providing theories and protocols for experiments.
基金supported by the National Basic Research Program(973 Program)(2011CB209404)National Natural Science Foundation of China(51477120).
文摘Charge transport in oil impregnated paper impacts the insulation performance of a transformer.This paper proposes a simulation method for the charge transport in oil impregnated paper insulation.The transient upstream finite element method(FEM)is applied to the transport equations of bipolar charges for establishing a numerical simulation model of charge transport in oil impregnated paper insulation.The method is validated by experimental results.The charge transport and electric field distribution in single-layer oil impregnated paper insulation under different temperature gradients is simulated.The trends of the simulation results are seen to agree with the corresponding experimental results.This paper conducts exploratory research into the simulation of charge transportation phenomenon in oil impregnated paper,and is of importance to the design of oil impregnated paper insulation.
基金Supported by National High Technology Research and Development Program of China(2007AA04Z411)National Basic Research Program of China(2009CB724505-1)Open Fund of State Key Laboratory of Power Transmission Equipment & System Security and New Technology(2007DA10512709405)
文摘During the operation of power transformer,its oil-paper insulation is continuously subjected to various stresses,c.g.,the thermal,electrical,mechanical,and chemical stresses,which cause insulation aging gradually.It has been considered that the combined thermal and electrical stresses are the most important and unavoidable factors that induce insulation materials aging.In this work,accelerated aging experiments of oil-impregnated pressboards under combined thermal(130℃) and electrical stresses(4 kV/mm) are performed,while the aging experiments under single thermal stress are also carried out at the corresponding temperature(130℃).The electrical and physic-chemical properties of oil,including dielectric losses factor tanδ,resistivity,acid value and pH value etc.,are measured during the aging process.Dissolved gasses in oil and polymerization degree of cellulose are also measured.The relationship between these properties of oil-paper insulation and aging time is investigated.The results show that dissolved gases in oil,resistivity,tanδof oil under combined thermal and electrical stresses are obviously different from that tinder thermal stress during aging process while some other properties show similar changing trend.For cellulose, compared with the single thermal aging results,it even shows a slower degradation rate in the presence of electrical stress.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.51977114 and 51867003)Bagui Young Scholars Program(Grant No.2019AQ16)China Postdoctoral Science Foundation(2022MD713732).
文摘Because of the increase in the transmission voltage levels,the demand for insulation reliability of power transformers has increasingly become critical.Cellulose insulating paper is the main insulating component of power transformers.To improve the insulation level of ultrahigh voltage transformers and reduce their weight and size,reducing the dielectric constant of oil-immersed cellulose insulating paper is highly desired.Cellulose is used to produce power-transformer insulating papers owing to its excellent electrical properties,renewability,biodegradability and abundance.The dielectric constant of a cellulose insulating paper can be effectively reduced by chemical or physical modification.This study presents an overview of the foreign and domestic research status of the use of modification technology to reduce the dielectric constant of cellulose insulating papers.All the mentioned methods are analyzed in this study.Finally,some recommendations for future modified cellulose insulating paper research and applications are proposed.This paper can provide a reference for further research on low dielectric constant cellulose insulating paper in the future.
