The accurate identification of the oil-paper insulation state of a transformer is crucial for most maintenance strategies.This paper presents a multi-feature comprehensive evaluation model based on combination weighti...The accurate identification of the oil-paper insulation state of a transformer is crucial for most maintenance strategies.This paper presents a multi-feature comprehensive evaluation model based on combination weighting and an improved technique for order of preference by similarity to ideal solution(TOPSIS)method to perform an objective and scientific evaluation of the transformer oil-paper insulation state.Firstly,multiple aging features are extracted from the recovery voltage polarization spectrum and the extended Debye equivalent circuit owing to the limitations of using a single feature for evaluation.A standard evaluation index system is then established by using the collected time-domain dielectric spectrum data.Secondly,this study implements the per-unit value concept to integrate the dimension of the index matrix and calculates the objective weight by using the random forest algorithm.Furthermore,it combines the weighting model to overcome the drawbacks of the single weighting method by using the indicators and considering the subjective experience of experts and the random forest algorithm.Lastly,the enhanced TOPSIS approach is used to determine the insulation quality of an oil-paper transformer.A verification example demonstrates that the evaluation model developed in this study can efficiently and accurately diagnose the insulation status of transformers.Essentially,this study presents a novel approach for the assessment of transformer oil-paper insulation.展开更多
The chaotic characteristics of time series of five partial discharge （PD） patterns in oil-paper insulation are studied. The results verify obvious chaotic characteristic of the time series of discharge signals and t...The chaotic characteristics of time series of five partial discharge （PD） patterns in oil-paper insulation are studied. The results verify obvious chaotic characteristic of the time series of discharge signals and the fact that PD is a chaotic process. These time series have distinctive features, and the chaotic attractors obtained from time series differed greatly from each other by shapes in the phase space, so they could be used to qualitatively identify the PD patterns. The phase space parameters are selected, then the chaotic characteristic quantities can be extracted. These quantities could quantificationally characterize the PD patterns. The effects on pattern recognition of PRPD and CAPD are compared by using the neural network of radial basis function. The results show that both of the two recognition methods work well and have their respective advantages. Then, both the statistical operators under PRPD mode and the chaotic characteristic quantities under CAPD mode are selected comprehensively as the input vectors of neural network, and the PD pattern recognition accuracy is thereby greatly improved.展开更多
In order to study the linkage effects between degree of polymerization and frequency domain dielectric spectroscopy characteristics of oil-paper insulation, the frequency domain dielectric response test platform of oi...In order to study the linkage effects between degree of polymerization and frequency domain dielectric spectroscopy characteristics of oil-paper insulation, the frequency domain dielectric response test platform of oil-paper insulation is set up. Complex permittivity of oil-paper insulation respectively composed by new or aged oil and insulation paper with different DP are tested, and complex permittivity of oil-paper insulation respectively composed by insulation respectively composed by new oil and insulation paper with different DP and low or high moisture content are tested. The test results are analyzed, and the analysis results show that the degree of polymerization of insulation paper has an influence on complex permittivity of oil-paper insulation though influencing the distribution of moisture and acids between oil and paper.展开更多
Both the real part and imaginary part of complex permittivity approximately have a log-linear frequency dependency at low fre- quencies, especially at ultra-low frequencies under conditions of different moisture conce...Both the real part and imaginary part of complex permittivity approximately have a log-linear frequency dependency at low fre- quencies, especially at ultra-low frequencies under conditions of different moisture concentrations and temperatures, which is recognized as the low frequency dispersion (LFD). In order to explain this dispersion, a new mechanism of dielectric response of LFD of oil-paper insula- tion is proposed. A simplified one-dimensional mathematical model of concentration polarization carrier caused by slow migration is developed and solved, which indicates that ion mobility is closely related to the size of gap and the adsorption capacity of cellulose molecu- lar chains to ions. A stochastic statistical model of the carrier mobility induced LFD is also developed. Moreover, actual tests under 50 °C and 2% moisture content were put forward, as well as simulations with according current waveforms. The simulation results agreed well with the experimental data in that concentration polarization of carriers caused by slow migration is the probable cause of low frequency dispersion of dielectric response for oil-paper insulation diagnosis.展开更多
Identification of the aging condition and the failure probability of oil-paper insulation in transformer is important for improving the reliability of electric power transmission system and applying life cycle cost (L...Identification of the aging condition and the failure probability of oil-paper insulation in transformer is important for improving the reliability of electric power transmission system and applying life cycle cost (LCC) management to electrical equipment. Based on data obtained in a series of multi-accelerated-aging experiments, two approaches for calculating failure probability of oil-paper insulation were compared in aspects of degree of polymerization (DP) and condition ranking. In the experiments, mineral oil and cellulose paper are sub- jected to electrical and thermal stresses, and several parameters, including dissolved gases’ volume fraction, furfural content, moisture content, and degree of polymerization, are measured after the aging process. Results show that weight of carbon oxide, which has a close relationship with cellulose paper degradation, is much higher in DP model than in condition ranking model. Moreover, it is concluded that DP model is more practically accurate than condition ranking model, because aging of cellulose paper rather than mineral oil is the key and critical factor of oil-paper insulation aging.展开更多
Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performances. The insulation gradually degrades under combined thermal, electrical and chemical stress...Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performances. The insulation gradually degrades under combined thermal, electrical and chemical stresses during routine operations, mainly because of space charges inside. This work investigated the space charge characteristics in oil-paper insulation under oil aging circumstance. New trans- former oil samples are thermally aged to obtain different aging states, and their physical and chemical properties are analyzed. New Kraft papers are dried in vacuum and fully immersed in these different aged oil samples, and three kinds of oil-paper samples are obtained. We use the pulsed electro-acoustic (PEA) method to measure space charge under both DC voltage-on and voltage-off conditions at room temperature. The effect of oil aging state on characteristics of space charge injection, accumulation, and decay is analyzed and discussed. The results show that comparing with the DC voltage-off condition, more charges are injected into samples at the interface of electrode and dielectric when DC voltage is on. When the oil-aged state gets worse, more charges are induced at both cathode and anode, more space charges are accumulated in the bulk, the area of negative charges is larger, and local electric field is distorted more seriously. Moreover, for the voltage-off condition, aged oil is good for space charge decay, and trapped positive space charges decay faster than trapped negative charges.展开更多
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
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
As the ultra high DC voltage grid comes into an important stage in the ‘Twelfth Five-year Plan’ of China, the requirement on the operation safety of power equipment needs to be improved. Oil-paper insulation plays a...As the ultra high DC voltage grid comes into an important stage in the ‘Twelfth Five-year Plan’ of China, the requirement on the operation safety of power equipment needs to be improved. Oil-paper insulation plays a significant role as the main insulation material of many key power equipments. The space charge behaviors of oil-paper insulation not only affect the dielectric strength, but also relate to the stability and security of power transmission facilities. Until now, the research on the space charge behaviors of oil-paper insulation remains on the stage of objective description and phenomenon explanation, thus it is very urgent to make some feature extractions on the test results in order to further investigate the DC space charge behaviors of oil-paper insulation. In this paper, based on the large numbers of test data in the first stage, data mining and feature extractions were performed on the DC space charge behavior of oil-paper insulation. The variation laws of some key characteristic parameters obtained in different experiment conditions such as the total charge injection amount, apparent charge mobility, and trap energy distribution were analyzed. In addition, the distribution of trapping level inside oil-paper was calculated by using the PEA test results based on the mechanism of the isothermal decay current, which realized the approximate calculation of trap depth by charge decay characteristics. What’s more, the exponential injection and decay equations of DC space charge changing with time were acquired for the first time, and the physical meanings of related parameters were defined. This has laid a foundation for the prediction and simulation of space charge in the oil-paper insulation.展开更多
High electric field and temperatures can result in electrical property degradation of oil-paper insulation and accelerate its aging process.This paper describes the regulation of space charge in the process of electro...High electric field and temperatures can result in electrical property degradation of oil-paper insulation and accelerate its aging process.This paper describes the regulation of space charge in the process of electro-thermal aging.Electric field distortion and differences in characteristic parameters during electro-thermal aging are analyzed,along with the effects of electro-thermal aging on space charge from the perspective of trap energy.High temperature and an electric field are used to accelerate the aging of oil-paper,and space charge characteristics are measured using the pulsed electro-acoustic(PEA)method throughout the electro-thermal aging.Based on results obtained,it can be concluded that homo-charge injection occurs at the anode and the type of the charge injection at the cathode varies throughout the electro-thermal aging;the decrease of permittivity during aging allows for space charge injection to take place;growth of trap depth makes the space charge accumulate in the middle of the sample;and space charge accumulation after electro-thermal aging results in inner electric field distortion,which leads to large current and decreased breakdown in voltages.展开更多
To improve the accuracy and efficiency of the aging life prediction and assessment of transformer oil-paper insulation,and to make up for the deficiencies of traditional characterizers,such as 2-furfural,carbon monoxi...To improve the accuracy and efficiency of the aging life prediction and assessment of transformer oil-paper insulation,and to make up for the deficiencies of traditional characterizers,such as 2-furfural,carbon monoxide,and carbon dioxide,a method for the simultaneous determination of methanol,ethanol,n−propanol,and η-butanol in oil with a single injection is established by headspace-gas chromatography-mass spectrometry.The measured results show that the determination limits of the four alcohol characterizers can be controlled to 10μg/kg level.Based on this method,the change patterns of the above four alcohols with thermal aging time and degree of polymerization are obtained through thermal aging experimental research.Ethanol,η-propanol,and η-butanol in oil indicate nearly linear correlations with thermal aging time and degree of polymerization,similar to that of methanol.By analyzing 52 sets of measured data of 500 kV EHV transformers in operation,η-butanol is found to have excellent performance,and a new method to evaluate the aging state of oil-paper insulation employing η-butanol and methanol is proposed along with the aging attention value model.The measured data of 500 kV EHV transformers in operation indicate that the combination of η-butanol and methanol as the preferred characterizers can effectively compensate for the shortcoming of traditional characterizers in the early stages of aging,and the feasibility of the method is verified.Two possible pathways for the generation of η-butanol by cellulose cleavage during the aging of oil-paper are proposed from the chemical structure of cellulose.展开更多
Extreme cold weather seriously harms human thermoregulatory system,necessitating high-performance insulating garments to maintain body temperature.However,as the core insulating layer,advanced fibrous materials always...Extreme cold weather seriously harms human thermoregulatory system,necessitating high-performance insulating garments to maintain body temperature.However,as the core insulating layer,advanced fibrous materials always struggle to balance mechanical properties and thermal insulation,resulting in their inability to meet the demands for both washing resistance and personal protection.Herein,inspired by the natural spring-like structures of cucumber tendrils,a superelastic and washable micro/nanofibrous sponge(MNFS)based on biomimetic helical fibers is directly prepared utilizing multiple-jet electrospinning technology for high-performance thermal insulation.By regulating the conductivity of polyvinylidene fluoride solution,multiple-jet ejection and multiple-stage whipping of jets are achieved,and further control of phase separation rates enables the rapid solidification of jets to form spring-like helical fibers,which are directly entangled to assemble MNFS.The resulting MNFS exhibits superelasticity that can withstand large tensile strain(200%),1000 cyclic tensile or compression deformations,and retain good resilience even in liquid nitrogen(-196℃).Furthermore,the MNFS shows efficient thermal insulation with low thermal conductivity(24.85 mW m^(-1)K^(-1)),close to the value of dry air,and remains structural stability even after cyclic washing.This work offers new possibilities for advanced fibrous sponges in transportation,environmental,and energy applications.展开更多
The ionic transport process is an important counterpart to charge transport in oil-paper insulation,and it significantly impacts oil flow electrification at the oil-paper interface.Despite this,the dynamics of this ph...The ionic transport process is an important counterpart to charge transport in oil-paper insulation,and it significantly impacts oil flow electrification at the oil-paper interface.Despite this,the dynamics of this phenomenon and the underlying mechanisms remain unclear,particularly at the molecular level.To understand this fundamental aspect,we conduct Molecular Dynamics study on the transport behaviour of an impurity ion in different oil-paper insulation models under various external electric fields.Different influence factors,such as external electric fields,temperatures,and local structural characteristics,are investigated in relation to the corresponding ionic mobility in different polymer models.According to the simulations,ionic mobility and its response to electric fields are higher in weaker electrostatic models.As a result,mineral oil exhibits the highest ionic mobility and the most substantial enhancement of ionic mobility by external electric fields,followed by vegetable oil,oil-paper blends,and insulating paper.This significant deviation in ionic mobility between oil and paper leads to the formation of an electric double layer near the oil-paper interface.The underlying physical mechanisms of different ionic mobility and its response to the electric field in different polymer models could be explained by the different polymer structural influences in terms of interaction energy and coordination numbers in a static manner,as well as by the interactions between the impurity ion and its surrounding atoms in terms of lifetime correlation functions and velocity autocorrelation functions in a dynamic manner.In addition,the influence of temperature on ionic mobility in mineral and vegetable oil is examined,and their activation energies are calculated.Advancing in the fundamental understanding of the dynamics of the ion transport process in oil-paper insulation is vital to improving their insulating properties for oil-impregnated power transformers.展开更多
This study extensively investigated the ageing characteristics and lifetime model of oil-paper insulation for oil-immersed paper condenser bushing.The accelerated thermal and electrothermal ageing experiments of oil-i...This study extensively investigated the ageing characteristics and lifetime model of oil-paper insulation for oil-immersed paper condenser bushing.The accelerated thermal and electrothermal ageing experiments of oil-immersed paper condenser bushing were con-ducted respectively.The properties of oil-paper insulation during the ageing process were investigated.Results showed that descending speed of degree of polymerization of the innermost and the outermost layer paper was higher than that of middle layer paper,the difference in water content on different paper layers caused varying deterioration speeds.The aluminium foil contributed to a surge in aluminium dissolution in oil,further accelerating the ageing of insulation.Based on the obtained degree of polymerization data under thermal ageing at 90°C,100°C and 130°C,the lifetime model of oil-paper insu-lation for condenser bushing model was established.In addition,the surface breakdown of the scale model of bushing occurred after 133 days ageing,the properties of bushing decreased gradually during the ageing process and sharply after the breakdown.Degree of polymerization deviation of different paper layers in bushing between calculated value according to the above lifetime model and actual value of bushing after the breakdown was less than 6%,further verifying the validity of lifetime model.展开更多
Raman spectroscopy, with its specific ability to generate a unique fingerprint‐like spec-trum of certain substances, has attracted much attention in diagnosing the ageing degree of oil-paper insulation. In this study...Raman spectroscopy, with its specific ability to generate a unique fingerprint‐like spec-trum of certain substances, has attracted much attention in diagnosing the ageing degree of oil-paper insulation. In this study, the feature extraction and ageing diagnosis methods of oil-paper insulation Raman spectroscopy data are further studied. Based on the non‐linear analysis of Raman spectra of different ageing samples, kernel principal component analysis was applied to extract the spectral features, and the back‐propagation neural network was used to build a diagnosis model with high diagnostic accuracy. The results show that Raman spectroscopy combined with kernel principal component analysis and the back‐propagation neural network can diagnose the ageing state of oil-paper insu-lation, with a diagnostic accuracy of 91.43% (64/70). The proposed method provides an effective and feasible method for the ageing assessment of oil‐immersed electrical equipment.展开更多
The main part of oil-immersed power equipment is oil-paper insulation.At lower ambient temperatures,transformer oil will become viscous.As a result,compared to those at room temperature or operating temperature,the fr...The main part of oil-immersed power equipment is oil-paper insulation.At lower ambient temperatures,transformer oil will become viscous.As a result,compared to those at room temperature or operating temperature,the frequency domain dielectric properties of oil-paper insulation at low temperatures are drastically different.For evaluating the insulation condition of electrical equipment,it is essential to accurately determine their dielectric properties.This paper described the FDS test,the DC conductivity test,and the transformer oil viscosity test,which was carried out in a laboratory environment for different ageing of oil-immersed pressboard and transformer oil at 233–373 K.The ef-fects of temperature on the dielectric properties at the competing mechanisms of polarisation and conductivity loss were clarified based on variations of the FDS curves.Considering the viscosity change of ageing transformer oil at low temperatures,the Arrhenius and VFT viscosity equations were used to achieve a fitting calculation in the different temperature zones.Based on the molecular/ion transition model in the force field and electric field,the characterisation relationships between the dynamic viscosity,DC conductivity,and test temperature of transformer oil were established.The limita-tions of conventional transformer oil equivalent dielectric relaxation models over a wide temperature range were finally clarified by combining the test findings of the DC con-ductivity over a wide temperature range with observed and simulated calculations.This paper provides an essential theoretical reference when using the FDS test to diagnose the insulation performance of oil-immersed power equipment in extremely cold regions.展开更多
The shortcomings of oil-immersed transformer solid insulation like low high-temperature resistance of the insulation paper and the uneven distribution of electric field caused by the large difference of dielectric con...The shortcomings of oil-immersed transformer solid insulation like low high-temperature resistance of the insulation paper and the uneven distribution of electric field caused by the large difference of dielectric constants between insulation paper and transformer oil,restricted the development of transformer with smaller size and larger capacity.In view of this situation and the transformer’s demand for the dielectric physicochemical and mechanical property of insulation material,polycarbonate,polyester film and polyphenylene sulfide with high high-temperature resistance were chosen for the comparison analysis in this paper We did the 300-day thermal aging experiment on four samples in transformer oil under different aging temperatures of90°C,110°C and 130°C,then analyzed the changes of their dielectric properties.The experiment results are as follows;the permittivity and dielectric dissipation factor of the three polymer materials are apparently smaller than those of the paper both before and after 300-day aging;the volume resistivity and partial discharge inception voltage of the three materials are higher than those of the paper both before and after aging;and the breakdown field strength of polycarbonate is higher than that of the paper both before and after aging,while that of polyester film and polyphenylene sulfide are slightly lower than that of the paper.展开更多
基金supported by the Natural Science Foundation of the Fujian Province(2021J01109).
文摘The accurate identification of the oil-paper insulation state of a transformer is crucial for most maintenance strategies.This paper presents a multi-feature comprehensive evaluation model based on combination weighting and an improved technique for order of preference by similarity to ideal solution(TOPSIS)method to perform an objective and scientific evaluation of the transformer oil-paper insulation state.Firstly,multiple aging features are extracted from the recovery voltage polarization spectrum and the extended Debye equivalent circuit owing to the limitations of using a single feature for evaluation.A standard evaluation index system is then established by using the collected time-domain dielectric spectrum data.Secondly,this study implements the per-unit value concept to integrate the dimension of the index matrix and calculates the objective weight by using the random forest algorithm.Furthermore,it combines the weighting model to overcome the drawbacks of the single weighting method by using the indicators and considering the subjective experience of experts and the random forest algorithm.Lastly,the enhanced TOPSIS approach is used to determine the insulation quality of an oil-paper transformer.A verification example demonstrates that the evaluation model developed in this study can efficiently and accurately diagnose the insulation status of transformers.Essentially,this study presents a novel approach for the assessment of transformer oil-paper insulation.
基金supported by National Natural Science Foundation of China(No.50877064)
文摘The chaotic characteristics of time series of five partial discharge （PD） patterns in oil-paper insulation are studied. The results verify obvious chaotic characteristic of the time series of discharge signals and the fact that PD is a chaotic process. These time series have distinctive features, and the chaotic attractors obtained from time series differed greatly from each other by shapes in the phase space, so they could be used to qualitatively identify the PD patterns. The phase space parameters are selected, then the chaotic characteristic quantities can be extracted. These quantities could quantificationally characterize the PD patterns. The effects on pattern recognition of PRPD and CAPD are compared by using the neural network of radial basis function. The results show that both of the two recognition methods work well and have their respective advantages. Then, both the statistical operators under PRPD mode and the chaotic characteristic quantities under CAPD mode are selected comprehensively as the input vectors of neural network, and the PD pattern recognition accuracy is thereby greatly improved.
文摘In order to study the linkage effects between degree of polymerization and frequency domain dielectric spectroscopy characteristics of oil-paper insulation, the frequency domain dielectric response test platform of oil-paper insulation is set up. Complex permittivity of oil-paper insulation respectively composed by new or aged oil and insulation paper with different DP are tested, and complex permittivity of oil-paper insulation respectively composed by insulation respectively composed by new oil and insulation paper with different DP and low or high moisture content are tested. The test results are analyzed, and the analysis results show that the degree of polymerization of insulation paper has an influence on complex permittivity of oil-paper insulation though influencing the distribution of moisture and acids between oil and paper.
基金Project supported by National Nature Science Foundation of China （51107105）, Sichuan Science Fund for Young Scholars （2011JQ0009）.
文摘Both the real part and imaginary part of complex permittivity approximately have a log-linear frequency dependency at low fre- quencies, especially at ultra-low frequencies under conditions of different moisture concentrations and temperatures, which is recognized as the low frequency dispersion (LFD). In order to explain this dispersion, a new mechanism of dielectric response of LFD of oil-paper insula- tion is proposed. A simplified one-dimensional mathematical model of concentration polarization carrier caused by slow migration is developed and solved, which indicates that ion mobility is closely related to the size of gap and the adsorption capacity of cellulose molecu- lar chains to ions. A stochastic statistical model of the carrier mobility induced LFD is also developed. Moreover, actual tests under 50 °C and 2% moisture content were put forward, as well as simulations with according current waveforms. The simulation results agreed well with the experimental data in that concentration polarization of carriers caused by slow migration is the probable cause of low frequency dispersion of dielectric response for oil-paper insulation diagnosis.
基金supported by National High-tech Research and Development Program of China（863 Program）（2009AA04Z416） National Science Foundation of China（51021005） Scientific Innovation of Colleges and Universities（Project v-200704）
基金supported by National High-tech Research and Development Program of China（863 Program）（2009AA04Z416） National Science Foundation of China（51021005） Scientific Innovation of Colleges and Universities（200704）
基金Project supported by National Basic Research Program of China (973 Program) (2011CB 209404)
文摘Identification of the aging condition and the failure probability of oil-paper insulation in transformer is important for improving the reliability of electric power transmission system and applying life cycle cost (LCC) management to electrical equipment. Based on data obtained in a series of multi-accelerated-aging experiments, two approaches for calculating failure probability of oil-paper insulation were compared in aspects of degree of polymerization (DP) and condition ranking. In the experiments, mineral oil and cellulose paper are sub- jected to electrical and thermal stresses, and several parameters, including dissolved gases’ volume fraction, furfural content, moisture content, and degree of polymerization, are measured after the aging process. Results show that weight of carbon oxide, which has a close relationship with cellulose paper degradation, is much higher in DP model than in condition ranking model. Moreover, it is concluded that DP model is more practically accurate than condition ranking model, because aging of cellulose paper rather than mineral oil is the key and critical factor of oil-paper insulation aging.
基金Project supported by China National Fund for Distinguished Young Scientists (51125029)National High-tech Research and Development Program of China (863 Program) (2007AA04Z411)
文摘Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performances. The insulation gradually degrades under combined thermal, electrical and chemical stresses during routine operations, mainly because of space charges inside. This work investigated the space charge characteristics in oil-paper insulation under oil aging circumstance. New trans- former oil samples are thermally aged to obtain different aging states, and their physical and chemical properties are analyzed. New Kraft papers are dried in vacuum and fully immersed in these different aged oil samples, and three kinds of oil-paper samples are obtained. We use the pulsed electro-acoustic (PEA) method to measure space charge under both DC voltage-on and voltage-off conditions at room temperature. The effect of oil aging state on characteristics of space charge injection, accumulation, and decay is analyzed and discussed. The results show that comparing with the DC voltage-off condition, more charges are injected into samples at the interface of electrode and dielectric when DC voltage is on. When the oil-aged state gets worse, more charges are induced at both cathode and anode, more space charges are accumulated in the bulk, the area of negative charges is larger, and local electric field is distorted more seriously. Moreover, for the voltage-off condition, aged oil is good for space charge decay, and trapped positive space charges decay faster than trapped negative charges.
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supported by the National Basic Research Program of China ("973" Program) (Grant No. 2009CB724505-1)the Doctor Foundation of Southwest University (Grant No. SWU110038)the Fundamental Research Funds for the Central Universities of Southwest University (Grant No. XDJK2010C020)
文摘As the ultra high DC voltage grid comes into an important stage in the ‘Twelfth Five-year Plan’ of China, the requirement on the operation safety of power equipment needs to be improved. Oil-paper insulation plays a significant role as the main insulation material of many key power equipments. The space charge behaviors of oil-paper insulation not only affect the dielectric strength, but also relate to the stability and security of power transmission facilities. Until now, the research on the space charge behaviors of oil-paper insulation remains on the stage of objective description and phenomenon explanation, thus it is very urgent to make some feature extractions on the test results in order to further investigate the DC space charge behaviors of oil-paper insulation. In this paper, based on the large numbers of test data in the first stage, data mining and feature extractions were performed on the DC space charge behavior of oil-paper insulation. The variation laws of some key characteristic parameters obtained in different experiment conditions such as the total charge injection amount, apparent charge mobility, and trap energy distribution were analyzed. In addition, the distribution of trapping level inside oil-paper was calculated by using the PEA test results based on the mechanism of the isothermal decay current, which realized the approximate calculation of trap depth by charge decay characteristics. What’s more, the exponential injection and decay equations of DC space charge changing with time were acquired for the first time, and the physical meanings of related parameters were defined. This has laid a foundation for the prediction and simulation of space charge in the oil-paper insulation.
基金supported by National Basic Research Program of China(973 Program)under Grant 2011CB209400。
文摘High electric field and temperatures can result in electrical property degradation of oil-paper insulation and accelerate its aging process.This paper describes the regulation of space charge in the process of electro-thermal aging.Electric field distortion and differences in characteristic parameters during electro-thermal aging are analyzed,along with the effects of electro-thermal aging on space charge from the perspective of trap energy.High temperature and an electric field are used to accelerate the aging of oil-paper,and space charge characteristics are measured using the pulsed electro-acoustic(PEA)method throughout the electro-thermal aging.Based on results obtained,it can be concluded that homo-charge injection occurs at the anode and the type of the charge injection at the cathode varies throughout the electro-thermal aging;the decrease of permittivity during aging allows for space charge injection to take place;growth of trap depth makes the space charge accumulate in the middle of the sample;and space charge accumulation after electro-thermal aging results in inner electric field distortion,which leads to large current and decreased breakdown in voltages.
基金supported by Innovation Foundation of China Electric Power Research Institute(GY83-18-006).
文摘To improve the accuracy and efficiency of the aging life prediction and assessment of transformer oil-paper insulation,and to make up for the deficiencies of traditional characterizers,such as 2-furfural,carbon monoxide,and carbon dioxide,a method for the simultaneous determination of methanol,ethanol,n−propanol,and η-butanol in oil with a single injection is established by headspace-gas chromatography-mass spectrometry.The measured results show that the determination limits of the four alcohol characterizers can be controlled to 10μg/kg level.Based on this method,the change patterns of the above four alcohols with thermal aging time and degree of polymerization are obtained through thermal aging experimental research.Ethanol,η-propanol,and η-butanol in oil indicate nearly linear correlations with thermal aging time and degree of polymerization,similar to that of methanol.By analyzing 52 sets of measured data of 500 kV EHV transformers in operation,η-butanol is found to have excellent performance,and a new method to evaluate the aging state of oil-paper insulation employing η-butanol and methanol is proposed along with the aging attention value model.The measured data of 500 kV EHV transformers in operation indicate that the combination of η-butanol and methanol as the preferred characterizers can effectively compensate for the shortcoming of traditional characterizers in the early stages of aging,and the feasibility of the method is verified.Two possible pathways for the generation of η-butanol by cellulose cleavage during the aging of oil-paper are proposed from the chemical structure of cellulose.
基金supported by Young Elite Scientists Sponsorship Program by China Association for Science and Technology(No.2022QNRC001)the National Natural Science Foundation of China(No.52273053)the Chenguang Program of Shanghai Education Development Foundation and Shanghai Municipal Education Commission(No.21CGA41)。
文摘Extreme cold weather seriously harms human thermoregulatory system,necessitating high-performance insulating garments to maintain body temperature.However,as the core insulating layer,advanced fibrous materials always struggle to balance mechanical properties and thermal insulation,resulting in their inability to meet the demands for both washing resistance and personal protection.Herein,inspired by the natural spring-like structures of cucumber tendrils,a superelastic and washable micro/nanofibrous sponge(MNFS)based on biomimetic helical fibers is directly prepared utilizing multiple-jet electrospinning technology for high-performance thermal insulation.By regulating the conductivity of polyvinylidene fluoride solution,multiple-jet ejection and multiple-stage whipping of jets are achieved,and further control of phase separation rates enables the rapid solidification of jets to form spring-like helical fibers,which are directly entangled to assemble MNFS.The resulting MNFS exhibits superelasticity that can withstand large tensile strain(200%),1000 cyclic tensile or compression deformations,and retain good resilience even in liquid nitrogen(-196℃).Furthermore,the MNFS shows efficient thermal insulation with low thermal conductivity(24.85 mW m^(-1)K^(-1)),close to the value of dry air,and remains structural stability even after cyclic washing.This work offers new possibilities for advanced fibrous sponges in transportation,environmental,and energy applications.
基金National Natural Science Foundation of China,Grant/Award Numbers:51477128,51507125Natural Science Foundation of Shaanxi Province,Grant/Award Number:2021JM-450。
文摘The ionic transport process is an important counterpart to charge transport in oil-paper insulation,and it significantly impacts oil flow electrification at the oil-paper interface.Despite this,the dynamics of this phenomenon and the underlying mechanisms remain unclear,particularly at the molecular level.To understand this fundamental aspect,we conduct Molecular Dynamics study on the transport behaviour of an impurity ion in different oil-paper insulation models under various external electric fields.Different influence factors,such as external electric fields,temperatures,and local structural characteristics,are investigated in relation to the corresponding ionic mobility in different polymer models.According to the simulations,ionic mobility and its response to electric fields are higher in weaker electrostatic models.As a result,mineral oil exhibits the highest ionic mobility and the most substantial enhancement of ionic mobility by external electric fields,followed by vegetable oil,oil-paper blends,and insulating paper.This significant deviation in ionic mobility between oil and paper leads to the formation of an electric double layer near the oil-paper interface.The underlying physical mechanisms of different ionic mobility and its response to the electric field in different polymer models could be explained by the different polymer structural influences in terms of interaction energy and coordination numbers in a static manner,as well as by the interactions between the impurity ion and its surrounding atoms in terms of lifetime correlation functions and velocity autocorrelation functions in a dynamic manner.In addition,the influence of temperature on ionic mobility in mineral and vegetable oil is examined,and their activation energies are calculated.Advancing in the fundamental understanding of the dynamics of the ion transport process in oil-paper insulation is vital to improving their insulating properties for oil-impregnated power transformers.
基金National Natural Science Foundation of China,Grant/Award Number:51907014Natural Science Foundation of Chongqing Province of China,Grant/Award Number:cstc2019jcyj‐msxmX0070。
文摘This study extensively investigated the ageing characteristics and lifetime model of oil-paper insulation for oil-immersed paper condenser bushing.The accelerated thermal and electrothermal ageing experiments of oil-immersed paper condenser bushing were con-ducted respectively.The properties of oil-paper insulation during the ageing process were investigated.Results showed that descending speed of degree of polymerization of the innermost and the outermost layer paper was higher than that of middle layer paper,the difference in water content on different paper layers caused varying deterioration speeds.The aluminium foil contributed to a surge in aluminium dissolution in oil,further accelerating the ageing of insulation.Based on the obtained degree of polymerization data under thermal ageing at 90°C,100°C and 130°C,the lifetime model of oil-paper insu-lation for condenser bushing model was established.In addition,the surface breakdown of the scale model of bushing occurred after 133 days ageing,the properties of bushing decreased gradually during the ageing process and sharply after the breakdown.Degree of polymerization deviation of different paper layers in bushing between calculated value according to the above lifetime model and actual value of bushing after the breakdown was less than 6%,further verifying the validity of lifetime model.
基金Joint Funds of the National Natural Science Foundation of China,Grant/Award Number:No.U1766217the National Science and technology project of Power Grid Corp,Grant/Award Number:No.61605020。
文摘Raman spectroscopy, with its specific ability to generate a unique fingerprint‐like spec-trum of certain substances, has attracted much attention in diagnosing the ageing degree of oil-paper insulation. In this study, the feature extraction and ageing diagnosis methods of oil-paper insulation Raman spectroscopy data are further studied. Based on the non‐linear analysis of Raman spectra of different ageing samples, kernel principal component analysis was applied to extract the spectral features, and the back‐propagation neural network was used to build a diagnosis model with high diagnostic accuracy. The results show that Raman spectroscopy combined with kernel principal component analysis and the back‐propagation neural network can diagnose the ageing state of oil-paper insu-lation, with a diagnostic accuracy of 91.43% (64/70). The proposed method provides an effective and feasible method for the ageing assessment of oil‐immersed electrical equipment.
基金Heilongjiang Provincial Postdoctoral Science Foundation,Grant/Award Number:LBH-Z22202The technology project of the State Grid Corporation of China Headquarters,Grant/Award Number:5500-202330167A-1-1-ZN。
文摘The main part of oil-immersed power equipment is oil-paper insulation.At lower ambient temperatures,transformer oil will become viscous.As a result,compared to those at room temperature or operating temperature,the frequency domain dielectric properties of oil-paper insulation at low temperatures are drastically different.For evaluating the insulation condition of electrical equipment,it is essential to accurately determine their dielectric properties.This paper described the FDS test,the DC conductivity test,and the transformer oil viscosity test,which was carried out in a laboratory environment for different ageing of oil-immersed pressboard and transformer oil at 233–373 K.The ef-fects of temperature on the dielectric properties at the competing mechanisms of polarisation and conductivity loss were clarified based on variations of the FDS curves.Considering the viscosity change of ageing transformer oil at low temperatures,the Arrhenius and VFT viscosity equations were used to achieve a fitting calculation in the different temperature zones.Based on the molecular/ion transition model in the force field and electric field,the characterisation relationships between the dynamic viscosity,DC conductivity,and test temperature of transformer oil were established.The limita-tions of conventional transformer oil equivalent dielectric relaxation models over a wide temperature range were finally clarified by combining the test findings of the DC con-ductivity over a wide temperature range with observed and simulated calculations.This paper provides an essential theoretical reference when using the FDS test to diagnose the insulation performance of oil-immersed power equipment in extremely cold regions.
基金supported by the National Natural Science Foundation of China(Grant No.50977025)
文摘The shortcomings of oil-immersed transformer solid insulation like low high-temperature resistance of the insulation paper and the uneven distribution of electric field caused by the large difference of dielectric constants between insulation paper and transformer oil,restricted the development of transformer with smaller size and larger capacity.In view of this situation and the transformer’s demand for the dielectric physicochemical and mechanical property of insulation material,polycarbonate,polyester film and polyphenylene sulfide with high high-temperature resistance were chosen for the comparison analysis in this paper We did the 300-day thermal aging experiment on four samples in transformer oil under different aging temperatures of90°C,110°C and 130°C,then analyzed the changes of their dielectric properties.The experiment results are as follows;the permittivity and dielectric dissipation factor of the three polymer materials are apparently smaller than those of the paper both before and after 300-day aging;the volume resistivity and partial discharge inception voltage of the three materials are higher than those of the paper both before and after aging;and the breakdown field strength of polycarbonate is higher than that of the paper both before and after aging,while that of polyester film and polyphenylene sulfide are slightly lower than that of the paper.
