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The accurate evaluation of corona onset criterion in air gap under high voltage has important theoretical and practical significance, but it is calculated and measured inaccurately at present because of the distortion caused by space charge. In this study, space charge density is measured by contactless acoustic device and it can directly determine the corona onset voltage and surface electrical field. The direct detection of space charge can eliminate the distortion of charge when other parameters are used to measure corona onset criterion. The experimental onset voltage in this work are compared with that calculated by Peek's formula, corona current and photon number. Results demonstrate the applicability and accuracy of this method. In the other hand, maximum space charge ratio is adopted to analyze the change rule of surface electrical field. It is found that the surface field strength is not constant after corona occurs and should be lower than the onset electrical field. This study provides a feasible approach to further study the effect of space charge on corona onset criterion in air gap.

The accuracy of surface electric field (SEF) is important for the research of corona discharge, both in numerical simulation and actual application. However, it is widely considered that the SEF is invariable after corona occurs (Kaptzov’s assumption). This theory makes corona study more convenient, but slightly inconsistent with the actual situation. In this work, the influence of air pressure on SEF intensity and distribution of space charge density under negative DC corona discharge is studied in corona cage via sound pulse method and maximum charge density ratio. The most important finding in this work is that when the voltage gradient is greater than the onset voltage, contrary to popular perception, SEF is not a fixed value and lower than the onset field intensity. And a linear negative correlation between the space charge density and the SEF intensity after the corona initiates is found for the first time. In addition, the SEF intensity has a decreasing trend with the applied voltage rising after corona initiates, and rises slightly with the increase of air pressure. The change of space charge density with applied voltage and air pressure is opposite to the change of electric field.

A method was developed for the determination of four protease inhibitors (saquinavir, ritonavir, nelfinavir and indinavir) in chicken using ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The samples were extracted by shaking with 30% (v/v) acetonitrile aqueous solution (containing 1% (v/v) trichloroacetic acid), and purified by using mixed-mode cationic-exchanger (MCX) cartridges. The samples were separated on a Luna

2-furfuraldehyde (2-FAL) is widely accepted as a chemical marker to indicate the degradation of cellulose insulation in oil-impregnated transformers. However, the application of 2-FAL in the aging diagnosis of transformers in operation is ineffective. The excessive dispersion of 2-FAL concentration measured in transformers is caused by many influence factors. In addition, the 2-FAL concentration in oil cannot indicate the aging of the most serious point given the temperature distribution along the winding height, that is, the insulation at the hot spot of a winding. To optimize the assessment, an improved method for the hot-spot insulation assessment based on the 2-FAL concentration in oil is proposed in this work. The 2-FAL generation in cellulose chain scission is firstly analyzed theoretically, and a new linear relationship between ( 1 / D P t - 1 / D P 0 ) and 2-FAL total generation is obtained. Moreover, the 2-FAL partitioning between an insulation paper and mineral oil influenced by temperature, moisture, and aging status in total is considered. Second, by considering the linear relationship and the 2-FAL partitioning, an equation between the insulation at hot-spot temperature and the 2-FAL concentration in oil is established for the thermal aging under unequal temperature distribution condition. Finally, to verify the proposed method, a testing platform is introduced to conduct the thermal aging experiment with temperature gradients. The proposed method demonstrates better assessment results than the conventional thermal aging test at constant temperature.

Surface electric field (SEF) strength plays an important role in studying corona discharge and is widely utilized in the research of corona loss and ion flow field in transmission line projects. The electrical field invariant theory after corona occurs is widely adopted to describe the SEF, but it is not consistent with the actual corona discharge situation. Thus, how to accurately evaluate the change of SEF is crucial for the design of overhead lines. Space charge caused by corona around the wire surface is closely related to the SEF; therefore, it is feasible to use the space charge to characterize SEF. In this work, a system for space charge density measurement is established in the laboratory to analyze the distribution of space charge under different voltages and temperatures. The validity of the experimental results was verified by finite element method (FEM), and the maximum charge density ratio is proposed to correct the variation formula of SEF. The experimental results show that SEF is not equal to the onset field intensity, and displays a decreasing trend after corona initiates, which can be used to rectify the electrical field invariant theory.

The effects of furfural generation and partitioning on the aging assessment of oil-paper insulation of power equipment were investigated. Oil-paper samples with various furfural contents were prepared using an accelerated thermal aging experiment. The mass of the furfural generated during the aging of insulation paper was calculated and used to establish the quantitative equation between the accumulated furfural and the degree of polymerization (DP) of the insulation paper. Results showed that the reciprocal of the DP value was linearly related to the amount of furfural generated by the insulation paper. Effects of temperature, moisture, and temperature on the furfural partitioning ratio between oil and paper were analyzed using a furfural partitioning experiment, and a furfural partitioning model was established. Results showed that the furfural partitioning ratio in oil was positively correlated with temperature and moisture but negatively correlated with the DP of the insulation paper. A method for assessing the age of an insulation paper by integrating the furfural generation and partitioning models was proposed and applied on a 500-kV reactor that was returned for factory maintenance. The application result indicated that the method could accurately assess the overall aging condition of the oil-paper insulation of the power equipment.

The furfural concentration in oil is a well-accepted chemical marker indicating the aging condition of power transformers. However, recent researches suggest that the furfural concentration in oil ...

The load of transformers shows higher volatility and uncertainty than do the system-level and substation-level loads. This paper proposes a two-stage short-term load forecasting (STLF) model for power transformers. 1) Three state-of-the-art technologies are applied to predict the aggregated substation-level load by taking the historical load, weather, and calendar data as inputs. In this stage, no specific STLF model needs to be developed, which allows the forecasters to select the most accurate prediction results for transformer-level load forecasting. 2) The load distribution factor (LDF) is defined as the ratio of the transformer load to the substation load. The relationship between LDF and substation load is captured by nonlinear regression functions under different substation operating conditions, and the load of each parallel transformer is predicted using these nonlinear regression functions. Each nonlinear function can be accurately established even if the historical load data are scarce under some irregular operating conditions. Three application examples show the effectiveness and rationality of the proposed method. The third example demonstrates that STLF of transformers is necessary because it provides important information for optimizing substation operating schemes and equipment maintenance plans.

Furfural content in oil has been shown to be a powerful indicator of the deterioration of cellulose paper. The furfural content in oil may increase greatly when the moisture content increases in transformers. The increase of furfural content in oil can be attributed to various factors, such as aging of cellulose paper with the increase of moisture content and the difference in furfural production mechanism. In this research, thermal aging tests of oil-paper system with different moisture contents are conducted under laboratory conditions to determine the cause of such increase. Furfural content, both in insulation oil and in insulation paper, and the degree of polymerization values of insulation paper were regularly measured. Results show that the moisture content plays a significant role on the production of furfural, and the high initial moisture content in insulation paper leads to large amount of furfural produced when DP decreases to the same values. The difference in the moisture content apparently affects the distribution of furfural between oil and paper, and the increase in the moisture content in oil-paper system increases the diffusion of furfural from the insulation paper to the oil.

The sealing performance of transformer has important influence on its safe operation. However, sealing of the equipment is related to the pressure that applied. In this paper, gas content test of mineral oil under different pressures were conducted to simulate the situation under different sealing degree, and the two-parameter Weibull distribution was used to analysis the data. Composite model of mineral oil and air have been built and balanced under different pressures. The amount of gas molecules in mineral oil and the interaction between gas molecules and mineral oil were calculated. The test results show that the gas content of mineral oil increases gradually with the increase of pressure. When the pressure increased from 6.3kPa to 101.3kPa, the gas content in mineral oil with a probability of 63.2 % increased from 4.26 % to 8.99 %. When the pressure increased by 16.08 times, the gas content in mineral oil increased by 2.11 times. The repeatability of the three measurements was good. The simulation results show that with the increase of pressure, the interaction energy between mineral oil and gas molecules increases gradually, and the number of gas molecules entering mineral oil increases. When the pressure increases from 10.1kPa to 1013kPa, the interaction energy between mineral oil and gas molecules increases from 68.61 kcal/mol to 104.98 kcal/mol. The number of gas molecules entering the mineral oil increased from 110 to 161. The experimental results are in good agreement with the simulation results.

Lithium iron phosphate batteries have become the main choice for energy storage units in electrochemical energy storage due to their high safety, excellent electrochemical performance, long cycle life, and environmental friendliness. However, lithium-ion batteries inherently have safety risks. The thermal runaway of a single battery in a closed space may cause a chain reaction of surrounding batteries, and may ignite the generated combustible gas, causing serious explosion accidents. In this paper, the explosion characteristics under different initiation points of pressure relief plates are studied. The results show that the peak overpressure variation range of different detonation points in the prefabricated chamber is 1∼1.6 times the hatch opening pressure, where the peak overpressure of the detonation at the near end of the hatch is lower than that of the detonation at the distal end, reflecting the role of the detonation when the explosion energy is released to the outside of the chamber earlier. The above study can provide a reference basis for the safe operation of prefabricated cabin type energy storage power plant and the promotion of its application.

At present, infrared detection technology is widely used to diagnose the thermal fault of substation equipment, and it is needed to use computers to intelligently handle the progressively increasing infrared images. A model based on improved Bayesian optimization to diagnose thermal fault is proposed in this paper. Firstly, in order to utilize the effective network structure of image classification, ResNet10 and ResNet14 with much smaller parameter size are created based on CNN and residual block. Secondly, Bayesian optimization based on Gauss process and expected improvement is adopted to select hyperparameter, and the constrains of validation accuracy and parameter size are set to improve the performance of the algorithm. Experiment result shows that the improved Bayesian optimization can converge faster and obtain a smaller converge value. Compared with Alexnet, VGG-16, ResNet18, the proposed model has a highest accuracy and smallest parameter size, which can diagnose the thermal fault more effectively.

The existing research on furfural analysis mainly focuses on the relationship between the furfural content in oil and the degree of polymerization (DP) of the paper, which are always obtained by accelerated thermal aging at high temperatures. However, the operating temperature of a winding is relatively low, approximately 60°C to 90°C. Thus, the relationship established at high temperature may be not suitable for the operating temperatures. In this research, aging tests of oil-paper systems at different temperatures ( from 100°C to 130°C) were carried out, with the DP, furfural content in oil and furfural content in paper being measured regularly to analyze the effect of temperature variation on the production and diffusion behavior of furfural in oil–paper insulation. The results show that temperature influences the production of furfural, i.e., there is no obvious difference in the amount of furfural produced by paper in the initial aging periods; however, a higher temperature results in more furfural being produced when the DP decreases to values below 600. An obvious diffusion process occurs after furfural is produced in paper in accelerated aging experiments, resulting in most of the furfural content remaining in the cellulose paper in the initial aging period.

The accuracy of surface electric field (SEF) is important for the research of corona discharge phenomenon, both in numerical simulation and actual application. However, it is widely considered that the SEF is invariable after corona occurs (Kaptzov’s assumption). This theory makes corona study more convenient, but slightly inconsistent with the actual situation. In this work, SEF for the negative corona discharge in a coaxial cylinder’s model is investigated through the space charge density. Space charge density is measured by sound pulse method, which is a non-contact measurement method. It can avoid the introduction of interference in the study of SEF. By selecting the maximum charge density (charge density around the conductor) closely related to the SEF as the research objective, the variation of SEF intensity can be obtained. Results show that the values of charge density increase with the rise of applied voltage and the diameter of the conductor. Through the maximum charge density near the wire, the variation formula of SEF can be obtained. It shows that SEF is not a fixed value and not equal to the onset field intensity, and it has a decreasing trend with the applied voltage rising after corona initiates.

The main function of the transformer bushing is to support and connect the transformer lead-out line. In recent years, bushing faults have occurred frequently but there is not good online monitoring method for bushing. The ultra-high frequency (UHF) detection method has the characteristics of strong anti-interference ability and no influence on equipment operation while achieving non-contact measurement. This paper has studied the phenomenon of the bushing by artificially setting the partial discharge fault on the 35kV bushing, and the improved time difference algorithm is used to locate the partial discharge source, forming a preliminary analysis method and research ideas for the bushing. The UHF signals of Phase Resolved Pluse Sequence (PRPD) of the bushing corona discharge is mainly concentrated between 30°~80° and 210°~260°, and the amplitude is relatively dispersed, and the stable discharge in one cycle with the tap suspended discharge has a wider distribution range and a smaller relative amplitude, since the tap is closed to the flange, partial discharge occurs at a lower voltage, and the signals are distributed between 0° and 100°, 170°~250° and 330°~360°, but the discharge charge is relatively small. The analysis of the received signal of the UHF sensors can determine the time difference of the signal reaching different sensors, and the specific position of the PD source can be calculated by the time difference with a few centimeters error from the actual position.

The main function of the transformer bushing is to support and connect the transformer lead-out line. In recent years, bushing faults have occurred frequently but there is not good online monitoring method for bushing. The ultra-high frequency (UHF) detection method has the characteristics of strong anti-interference ability and no influence on equipment operation while achieving non-contact measurement. This paper has studied the propagation characteristics of two different UHF electromagnetic waves generated by partial discharge inside the oil impregnated paper (OIP) by simulations of finite element analysis. The attenuation of TEM mode is small, and higher modes below its cutoff frequency is exponentially attenuated. The electromagnetic wave leakage path generated by partial discharge inside the bushing is mainly in the OIP and the oil channels on both sides and the variation trend of the Poynting vector is the same. The externally received field strength spectrum shows that the signal strength is relatively high in the range of 1-2 GHz, considering the equipment used in the actual situation, such as amplifiers it provides a reference for subsequent experiments.

With the development of power transmission system, the synchronous condenser with large capacity has become the optimal choice to meet the dynamic reactive power demand of the power system, and has gradually become one of the research hotspots. The rotor windings inter-turn short circuit fault is a common fault in synchronous condenser because the field winding is exposed to various mechanical and thermal stress. This paper proposes a fault diagnosis strategy to cope with the rotor windings inter-turn short circuit fault of synchronous condenser, which can be used for the online fault monitoring. This paper analyze the characteristic of the rotor field winding fault and develop a park model based on synchronous condenser’s rotor field winding inter-turn short circuit firstly. Secondly, the characteristic of the synchronous condenser is analyzed, and the relationship between the number of short-circuit turns and the field current of the rotor windings is deduced. Finally, the proposed strategy is verified by simulated results.

In recent years, due to the high concentration of solid particles in transformer insulating oil, some large power transformers domestic and abroad have caused insulation fault. The influence of metal particles on the breakdown voltage of transformer oil is self-evident, relevant research shows that the damage of metal particles to the insulation performance of transformer oil is much higher than that of non-metallic particles such as cellulose particles. In this paper, a discharge test tank was designed and manufactured, and a lightning impulse discharge test platform for oil-paper insulation was built based on shadow imaging principle. The preparation method of insulating oil sample containing fixed metal particle concentration was put forward, a streamer development characteristic test scheme had been worked out. The effect of copper particles on the discharge characteristics of oil-impregnated insulating paper under two different field arrangement was studied. Discharge streamer imaging experiments of two insulating paper models were carried out under pre-breakdown voltage, and the streamer morphology characteristics were compared with the pure oil gap. When the insulating paper is placed along field lines, the streamer tends to develop along the surface of the insulating paper as a whole, and the insulating paper will speed up the development of streamer in oil. When the insulating paper is placed perpendicular to field lines, the paper will not affect the development of the streamer between the oil gaps, but will prevent the final formation of the main discharge channel of the streamer.

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual furfural content in oil is influenced by temperature because furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different furfural contents were prepared through an accelerated thermal aging test. A furfural equilibrium experiment was conducted at different temperatures to construct a furfural partitioning diagram. The mass fraction of furfural in oil and paper was calculated. Results showed that the mass fraction of furfural in oil increases with temperature rising and aging deepening, which means that furfural tends to migrate from paper to oil at high temperatures and deep aging conditions. The change in furfural partitioning between oil and paper at different temperatures causes a deviation in paper aging assessment. To correct the temperature effect on furfural partitioning, a furfural correction factor was developed to normalize furfural contents in oil at different temperatures to a reference temperature. The furfural correction factor was verified by experimental data and then introduced into the relation between degree of polymerization of paper and furfural content of oil. Verification results showed that the corrected equation can enhance the accuracy of paper aging estimation at different temperatures. The findings of this work indicated temperature effect on furfural partitioning should be considered when determining the furfural thresholds and evaluating the aging condition of power transformers.

Estimating the moisture content of insulation paper is important in transformer condition evaluation, the results of which are affected by thermal aging. This study investigates the independent effects of aged oil and aged paper on moisture equilibrium charts to determine which of the two primarily causes the inaccuracy in moisture estimation of aged transformers. A thermal aging experiment was conducted. At five aging degrees, four kinds of oil–paper insulation systems were formed: new oil–new paper, aged oil–new paper, new oil–aged paper, and aged oil–aged paper. Moisture equilibrium curves were drawn for each insulation system. The relative moisture content of oil and that of paper were calculated. Results showed that the moisture content of insulation paper decreased with aging, which led to the migration of the moisture equilibrium curves and further influenced the moisture estimation results. A comparative analysis showed that the migration of the moisture equilibrium curves was mainly cause...

Furfural content in oil is a common chemical marker indicating the aging condition of cellulose insulation paper in power transformers. However, the accuracy of aging estimation is affected by the furfural distribution ratio between oil and paper. Moisture is considered an important factor that influences the furfural distribution behavior in oil-paper system. In this study, oil-paper samples with different furfural and moisture contents were prepared. For samples with different moisture contents, furfural contents in oil and paper were measured and furfural mass fractions were calculated. Results showed that the oil-paper samples with high moisture contents exhibited a high furfural mass fraction in oil. This finding implies that high humidity promoted the furfural diffusion from paper to oil. The effect of moisture on aging assessment of insulation paper was analyzed. Results showed that the increase of moisture content in oil-paper system would significantly affect the accuracy of insulation paper aging assessment, leading to a lower assessment result than the real value.

Furfural analysis is considered as a reliable method to estimate aging of cellulose insulation paper and is widely applied in assessing the condition of field transformers. However, the actual concentration of furfural in insulating oil is influenced by oil replacement, and the correction method is seldom studied. In this research, an accelerated thermal aging test of oil-paper samples was conducted at 130 °C for 35 days. Insulating oil was replaced at early, middle, and late aging stages to investigate furfural loss during the oil replacement process and its effects on assessing the aging of insulation paper. Results showed that furfural in oil obviously declined after oil replacement, which caused a severe error in paper aging evaluation. To correct furfural loss, the furfural correction factor, which is defined as the ratio of the furfural content in oil after oil replacement to the furfural content before oil replacement, was introduced into the furfural analysis method. The parameters and influencing factors of the furfural correction function were researched. The furfural correction function was verified using experimental data obtained under laboratory conditions and field data collected from 44 in-service transformers. The results showed that the furfural correction function effectively corrects furfural loss during oil replacement and improves the accuracy of estimating paper aging. Finally, the furfural correction function was extended to the operating conditions of field transformers.

Moisture equilibrium curves are well accepted methods for moisture estimation of insulation paper in field transformers. The thermal aging effects on moisture partition diagrams of mineral oil-paper and a new aging-resistant mixed oil-paper insulation system were studied. The purpose was to assess the moisture content accurately, particularly in transformers used for long-term operations. Thermal aging experiment was conducted at 130 °C for 80 days, and moisture equilibrium curves at five different aging degrees and three different temperatures were constructed. Results showed that under the same oil moisture content, the moisture content in paper decreased as the aging condition aggravated. The mass fraction of moisture content in oil and paper was calculated at each aging degree. Comparative results revealed that the moisture absorption capacity of insulation paper had an age-related decreasing tendency, whereas the moisture absorption capacity of insulating oil had an age-related increasing tendency. Moisture partition diagrams of mineral and aging-resistant mixed oil-paper samples were compared. Results showed that with the same oil moisture concentration, the moisture of insulation paper impregnated in mixed oil was remarkably lower than that impregnated in mineral oil.

Furfural content in oil is a valuable indicator of cellulose paper degradation and is widely utilized in aging assessment for power transformers. However, with the same aging degree of insulation paper, the actual 2-furfural content in oil is influenced by temperature because 2-furfural partitioning behavior in the oil-paper insulation system of power transformers changes with temperature. Thus, correction of the temperature effect on 2-furfural partitioning is crucial for accurate assessment of the aging of power transformers. In this research, oil-paper samples with different 2-furfural contents were prepared through an accelerated thermal aging test. A 2-furfural equilibrium experiment was conducted at different temperatures to construct a 2-furfural partitioning diagram. Results showed that 2-furfural partitioning behavior is influenced by temperature. At high temperatures, 2-furfural tends to migrate from paper to oil. To correct the temperature effect on 2-furfural partitioning, a 2-furfural correction factor was developed to normalize 2-furfural contents in oil at different temperatures to a reference temperature. And the 2-furfural correction factor was verified by experimental data.