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
The valve side windings of converter transformers bear AC, DC, impulse, and reversal-polarity voltages during operation, which could result in serious insulation problems of the equipment. By performing experiments wi...The valve side windings of converter transformers bear AC, DC, impulse, and reversal-polarity voltages during operation, which could result in serious insulation problems of the equipment. By performing experiments with surface discharge model of oil-paper insula- tion at 80 ℃ under combined AC-DC voltage for 200 h, we studied the spectrums and statistical parameters of partial discharges at different discharge stages. Furthermore, some fingerprint parameters were calculated in order to estimate the development situation of par- tial discharge, while the characteristic gases dissolved in the transformer oil were measured by gas chromatography. The surface discharges in the experiments were observed using a high speed camera, and a full discharge process could be marked off into four stages as follows. ①The elementary stage. When a partial discharge occurs near electrodes, electrical charges are injected into the region near electrodes and causing bubble generation. ②Due to their high resistivity and low dielectric constant, the bubbles would bare the major part of the voltage applied to samples. Therefore, discharge happens inside the small bubbles, and it emits a lot of light. ③Micromolecules of gas are produced in discharge, and further ionization in the transformer oil takes place simultaneously when high-energy electrons collide with oil molecules. ④The carrier charge moves forward to electrodes driven by the applied electric field, till they neutralize with the charge from electrodes, and hence discharge channels are formed subsequently.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
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
The paper provides a general overview of chemical processes leading to the degradation of oil-paper insulation in oil-immersed electrical current transformers. Previous knowledge available in literature is complemente...The paper provides a general overview of chemical processes leading to the degradation of oil-paper insulation in oil-immersed electrical current transformers. Previous knowledge available in literature is complemented by new results placing a specific emphasis on the physicochemical factors which affect the copper release in the insulation oil and the oil oxidation kinetics. It is demonstrated that various ageing processes interact with each other, with one or another process dominating under specific conditions. Comprehensive but disjoint studies focusing on separate sub-processes may produce rather misleading results, and occasionally, lie behind rather irrelevant quality demands imposed on the insulating liquids.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
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
Looking at the problem of electrical discharge development in mineral oil, it is easy to see that most studies in this area focused on the setups of bare high voltage electrodes having a point-plane electrode arrangem...Looking at the problem of electrical discharge development in mineral oil, it is easy to see that most studies in this area focused on the setups of bare high voltage electrodes having a point-plane electrode arrangement. The setups with insulated electrodes are the mar- gin of these studies but it seems to be important to find the dependences between the paper insulation on high voltage electrode and parameters of the discharges initiated in the vicinity of this electrode. Hence, in this paper the results of researches intended to reveal the role of insulation wrapping on a HV electrode in the mechanism of electrical discharges in transformer oil under lightning impulse of both polar- ities are presented. This role is determined by analysis of the parameters characterizing the discharges (onset voltage, propagation velocity, time to initiation, rise-time of light impulses) and also by observation of their spatio-temporal development and oscillograms of the light emitted by their channels. The research was performed for two model electrode configurations: an electrode with paper insulation and a bare electrode which had the same outer dimensions as the insulated one. The most essential conclusion from performed experiment is related to times to initiation. These times, equal in the case of insulated electrodes and model bare electrode, indicate that the source of "weak points" of the paper-oil insulation system is the oil, not the surface of insulation wrapping or the metal.展开更多
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Project supported by National Basic Research Program of China(973 Program) (2011CB 209400)Program of State Key Laboratory of Power Systems for ±1 100 kV UHVDC Technology(SKLD10M09)
文摘The valve side windings of converter transformers bear AC, DC, impulse, and reversal-polarity voltages during operation, which could result in serious insulation problems of the equipment. By performing experiments with surface discharge model of oil-paper insula- tion at 80 ℃ under combined AC-DC voltage for 200 h, we studied the spectrums and statistical parameters of partial discharges at different discharge stages. Furthermore, some fingerprint parameters were calculated in order to estimate the development situation of par- tial discharge, while the characteristic gases dissolved in the transformer oil were measured by gas chromatography. The surface discharges in the experiments were observed using a high speed camera, and a full discharge process could be marked off into four stages as follows. ①The elementary stage. When a partial discharge occurs near electrodes, electrical charges are injected into the region near electrodes and causing bubble generation. ②Due to their high resistivity and low dielectric constant, the bubbles would bare the major part of the voltage applied to samples. Therefore, discharge happens inside the small bubbles, and it emits a lot of light. ③Micromolecules of gas are produced in discharge, and further ionization in the transformer oil takes place simultaneously when high-energy electrons collide with oil molecules. ④The carrier charge moves forward to electrodes driven by the applied electric field, till they neutralize with the charge from electrodes, and hence discharge channels are formed subsequently.
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 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）
文摘The paper provides a general overview of chemical processes leading to the degradation of oil-paper insulation in oil-immersed electrical current transformers. Previous knowledge available in literature is complemented by new results placing a specific emphasis on the physicochemical factors which affect the copper release in the insulation oil and the oil oxidation kinetics. It is demonstrated that various ageing processes interact with each other, with one or another process dominating under specific conditions. Comprehensive but disjoint studies focusing on separate sub-processes may produce rather misleading results, and occasionally, lie behind rather irrelevant quality demands imposed on the insulating liquids.
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.
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.
文摘Looking at the problem of electrical discharge development in mineral oil, it is easy to see that most studies in this area focused on the setups of bare high voltage electrodes having a point-plane electrode arrangement. The setups with insulated electrodes are the mar- gin of these studies but it seems to be important to find the dependences between the paper insulation on high voltage electrode and parameters of the discharges initiated in the vicinity of this electrode. Hence, in this paper the results of researches intended to reveal the role of insulation wrapping on a HV electrode in the mechanism of electrical discharges in transformer oil under lightning impulse of both polar- ities are presented. This role is determined by analysis of the parameters characterizing the discharges (onset voltage, propagation velocity, time to initiation, rise-time of light impulses) and also by observation of their spatio-temporal development and oscillograms of the light emitted by their channels. The research was performed for two model electrode configurations: an electrode with paper insulation and a bare electrode which had the same outer dimensions as the insulated one. The most essential conclusion from performed experiment is related to times to initiation. These times, equal in the case of insulated electrodes and model bare electrode, indicate that the source of "weak points" of the paper-oil insulation system is the oil, not the surface of insulation wrapping or the metal.
