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We develop a Hybrid Bidirectional Long and Short-Term Memory based on attention mechanism (HBiLSTM-Attention) model and a BiLSTM-Attention model for multiclass flare forecasting within 24 h. We construct a new data base containing 10 separate data sets with magnetogram images and magnetic field parameters. Based on the same data base, for the first time we compare the multiclass forecasting performance of our proposed HBiLSTM-Attention model, BiLSTM-Attention model, and three other deep-learning models based on Convolutional Neural Network (CNN-based) from two aspects of categorical performance with the true skill statistic (TSS) and probabilistic performance with the Brier skill score (BSS). The major results are as follows. (1) The TSS values of our proposed model are 0.692 ± 0.042, 0.475 ± 0.038, 0.642 ± 0.043, 0.754 ± 0.062, 0.692 ± 0.042, and 0.708 ± 0.052 for No-flare, C, M, X, ≥C, and ≥M class, respectively, which are better than those of the BiLSTM-Attention model, and much better than those of the three other CNN-based models. (2) Our proposed model achieves the scores of BSS  = 0.498 ± 0.061, 0.202 ± 0.037, 0.209 ± 0.050, −0.271 ± 0.180, 0.498 ± 0.061, 0.268 ± 0.056 for No-flare, C, M, X, ≥C, and ≥M class, respectively, outperforming the other four models in every class except for X class. (3) To our knowledge, HBiLSTM-Attention is the first multiclass flare forecasting model based on magnetic field parameters and deep learning, and achieves promising prediction performance. Moreover, this is the first attempt to investigate the reliability of probabilistic prediction for multiclass flares. [ABSTRACT FROM AUTHOR]

Press-pack insulated gate bipolar transistor (IGBT) devices have multiple chips paralleled to enhance their power density. However, the current imbalance among chips limits the increase of the power, which is mainly attributed to external factors, like stray inductance, temperature, and mechanical pressure. Besides, for the difficulties in decoupling multiple external factors, the current imbalance in press-pack IGBT devices has been extensively studied by simulation, and lack of experimental investigations. In this article, a multifactor decoupling experimental approach for the current sharing characteristics is proposed. The stray inductance, temperature, and mechanical pressure of the two parallel branches can be adjusted independently for investigating chips’ current sharing. Then, the experimental platform for the parallel 3.3 kV/50 A IGBT chips under differentiated external factors is implemented. Measured results show that the stray inductance affects the transient current sharing during turn-on greatly, and the maximum current imbalance can reach 25.46%. The temperature mainly affects the steady state and the maximum current imbalance is 11.7%. Otherwise, mechanical pressure is negligible on the steady and transient current imbalances, which are less than 3%. Finally, the influence mechanisms of the external factors are proposed to explain the current sharing characteristics of parallel chips. [ABSTRACT FROM AUTHOR]

We propose a novel hybrid Convolutional Neural Network (CNN) model with one-versus-one approach to forecast solar flare occurrence with the outputs of four classes (No-flare, C, M, and X) within 24 h. We train and test our model using the same data sets as in Zheng, Li & Wang, and then compare our results with previous models using the true skill statistic (TSS) as primary metric. The main results are as follows. (1) This is the first time that the CNN model in conjunction with one-versus-one approach is used in solar physics to make multiclass flare prediction. (2) In the four-class flare prediction, our model achieves quite high mean scores of TSS = 0.703, 0.489, 0.432, and 0.436 for No-flare, C, M, and X class, respectively, which are much better than or comparable to those of previous studies. In addition, our model obtains TSS scores of 0.703 ± 0.070 for ≥C-class and 0.739 ± 0.109 for ≥M-class predictions. (3) This is the first attempt to open the black-box CNN model to study the visualization of feature maps for interpreting the prediction model. Furthermore, the visualization results indicate that our model pays attention to the regions with strong gradient, strong intensity, high total intensity, and large range of the intensity in high-level feature maps. The median gradient and intensity, the total intensity, and the range of the intensity for high-level feature maps increase approximately with the increase of flare level. [ABSTRACT FROM AUTHOR]

PEEK (Polyetheretherketone) and N2 are used as the frame material and insulation gas in the packaging of high voltage power electronic devices, respectively. Considering the IGBT device in normal operation, the chip will produce a large amount of Joule heat, which will increase the temperature of the insulation material in sub-modules. However, there has been no investigations on the influence of temperature on the surface discharge characteristics and mechanisms under positive repetitive square voltage (PRSV). In this paper, surface discharge current pulses and optical photos of surface streamer propagation of PEEK in N2 at different temperatures are collected under PRSV. The effects of different temperatures on discharge current pulse, charge quantities of discharge and time delay of discharge are analyzed in detail. Besides, the surface trap parameters at different polarization temperatures are measured by TSDC. The effects of temperature on the effective ionization rate, surface trap depth and densities are revealed. A reasonable model is used to explain the role of surface traps in the streamer propagation along the surface of PEEK. Besides, the time delay theory is introduced to quantitatively analyze the effect of temperature on the time delay of forward discharge (FD) and backward discharge (BD). The influence mechanism of temperature on specific pulse parameters of discharge current, time delay of discharge and charge quantities of discharge are all analyzed on the basis of the trapping and detrapping of surface electrons and ions at different temperatures. [ABSTRACT FROM AUTHOR]

We present a novel deep Convolutional Neural Network model with one-against-rest approach (OAR-CNN) and modify the hybrid Convolutional Neural Network (H-CNN) model of Zheng et al. (2019) for multiclass flare prediction to forecast whether an active region generates multiclass flare within 24 h. Additionally, in the OAR-CNN and H-CNN models, we employ the decision strategies of majority voting and probability threshold, respectively, comparing the prediction outcomes of these two strategies. Our models undergo training and testing on the same 10 cross-validation datasets as employed by Zheng et al. (2019) , and then compare the results with previous studies using forecast verification metrics, with a focus on the true skill statistic (TSS). The major results are summarized as follows. (1) This is the first attempt to utilize the decision strategies of majority voting and probability threshold in the OAR-CNN model for multiclass solar flare prediction. (2) In both the OAR-CNN and H-CNN models, the predictive results with the probability threshold decision strategy are higher than those with majority voting across all six classes (i.e., No-flare, C, M, X, ⩾ C, and ⩾ M class), except for a slight decrease in the C class in the OAR-CNN model. (3) The OAR-CNN and modified H-CNN models with the probability threshold decision strategy demonstrate comparable statistical scores across all categories and outperform previous studies. (4) In the prediction of four-class flare, our proposed OAR-CNN model with the probability threshold decision strategy achieves relatively high mean TSS scores of 0.744, 0.429, 0.567, and 0.630 for No-flare, C, M, and X class, respectively, surpassing or comparable to results from prior studies. Furthermore, our model achieves high TSS scores of 0.744 ± 0.042 for ⩾ C–class and 0.764 ± 0.089 for ⩾ M-class predictions. [ABSTRACT FROM AUTHOR]

Nickel sulfides are perfect anode materials for high-capacity and low-cost lithium-ion batteries (LIBs); however, with the shortcoming of polysulfide intermediate dissolution, volume expansion exceeding the limit during cycling also restricts their development. Herein, NiS/C composite materials are successfully anchored on chestnut shell fluff (CSF)-derived biochar by a glucose-auxiliary hydrothermal method along with an annealing treatment. The CSF biochar acts as an effective electron transmission channel for the rapid lithiation/delithiation of NiS and as a fixed sulfur carrier for inhibiting the dissolution of polysulfide. Glucose restrains the accumulation of NiS particles and then transforms into uniform amorphous carbon during annealing, which is more effective in buffering for rapid volume variation. Moreover, the CSF-NiS/C electrode exhibits a remarkable specific capacity of 1522.8 mAh g-1 (0.1 A g-1) and distinguished rate performance with 295 mAh g-1 capacity (3 A g-1), which are better than those of the pure NiS/C anode material displays. Researchers may be inspired by both of these reasonable design and synthesis strategies that are beneficial for the development of high-performance nickel-based sulfide anode materials for LIBs. [ABSTRACT FROM AUTHOR]

In this paper, the surface discharge current pulses of PEEK (polyether-ether-ketone) under positive repetitive square voltage are measured. The effects of voltage rise time and fall time on pulse parameters of discharge current, instantaneous voltage of discharge and time-lag of discharge are analyzed in detail. The results show that the amplitude of discharge current and the instantaneous voltage of discharge decrease, and the time-lag of discharge and the dispersion of time-lag increase with the increase of voltage rise time. With the increase of voltage fall time, the amplitude of discharge current decreases and the instantaneous voltage of discharge, the time-lag of discharge and the dispersion of time-lag increase. To analyze the influence mechanism of voltage rise time and fall time on surface discharge, the accumulation of surface charge under repetitive square voltage is regarded as a periodic process. Besides, the correlation between forward discharge and back discharge in the process of discharge is analyzed, and the development of discharge is analyzed by using the time-lag theory. Furthermore, the relationship between instantaneous discharge voltage and voltage slew rate is revealed, and the surface charge accumulation processes under different voltage rise times and fall times are compared to reveal the influence mechanism of rise time and fall time on the surface discharge under positive repetitive square voltage. [ABSTRACT FROM AUTHOR]

The focus of this study is on thermal bubbles which limits the application of phase-change cooling technology in high electric fields. In order to investigate the impact of the boil-induced bubble flow on the partial discharge (PD), a PD test chamber is set up which is modified to adjust the intensity of the bubble flow. PD signals and characteristics of the bubble flow are recorded under different heating conditions and electric fields. It is found that thermal bubbles have negligible impact on the PD characteristics under a non-uniform field, but the discharge frequency decreases as the boiling intensity increases. A deviation occurs in the trajectory of bubbles, which makes discharge in bubbles impossible. It is also found that the heat induced fluid affects the density of dielectric liquid and ionization coefficient of the discharge environment, which reduces PD frequency. [ABSTRACT FROM AUTHOR]

Efficient forecasting of solar flares is of significant importance for better risk prevention. Currently, there is relatively rare research on multi/four-classification of flares, and the influence of the number of time steps and data feature dimensions on the prediction performance of multi-class models has not been considered. In this study, we utilize the Space-weather HMI Active Region Patch (SHARP) data to develop two categories of models for multiclass flare prediction within 24 hr, including direct output four-classification models and four-classification models using a cascading scheme. The former encompasses Random Forest (RF) model, Long Short-Term Memory (LSTM) model, and Bidirectional LSTM (BLSTM) model, while the latter includes BLSTM Cascade (BLSTM-C) model and BLSTM Cascade with Attention Mechanism (BLSTM-C-A) model. These two categories of models are employed to contrast the impact of different numbers of time steps and the predictive performance in solar flare multi/four-classification. Additionally, we conduct, for the first time, feature importance analysis for multi/four-classification solar flare prediction using deep learning models. The main results are as follows: (1) As the number of time steps increases, the True Skill Statistic (TSS) scores of the four deep learning models improve, showing an overall upward trend in predictive performance. The models achieve their optimal performance when the number of time steps reaches 120. (2) Among the direct output four-class models, deep learning models (LSTM and BLSTM) outperform traditional machine learning model (RF). In both multi-class and binary-class predictions using deep learning, the BLSTM-C model performs better than other deep learning models (LSTM, BLSTM, and BLSTM-C-A). (3) In the feature importance analysis, the top-ranked important features include SAVNCPP and R_VALUE, while the least important features include SHRGT45 and MEANPOT. [ABSTRACT FROM AUTHOR]

The research on solar flare predicting holds significant practical and scientific value for safeguarding human activities. Current solar flare prediction models have not fully considered important factors such as time step length, nor have they conducted a comparative analysis of the physical features in multiple models or explored the consistency in the importance of features. In this work, based on SHARP data from SDO, we build 9 machine learning-based solar flare prediction models for binary "Yes" or "No" class prediction within the next 24 hours, and study the impact of different time steps and other factors on the forecasting performance. The main results are as follows. (1) The predictive performance of eight deep learning models shows an increasing trend as the time step length increases, and the models perform the best at the length of 40. (2) In predicting solar flares of ≥C class and ≥M class, the True Skill Statistic(TSS) of deep learning models consistently outperforms that of baseline model. For the same model, the TSS for predicting ≥M class flares generally exceeds that for predicting ≥C class flares. (3) The Brier Skill Score (BSS) of deep learning models significantly surpasses that of baseline model in predicting ≥C class flares. However, the BSS scores of the nine models are comparable for predicting ≥M class flares. For the same model, the BSS for predicting ≥C class flares is generally higher than that for predicting ≥M class flares. (4) Through feature importance analysis of multiple models, the common features that consistently rank at the top and bottom are identified. [ABSTRACT FROM AUTHOR]

In this paper, spectral characteristics of the corona-generated audible noise (AN) on a dc conductor are studied through a quadrate corona cage structure. To reduce the influence of background noise, all experiments are conducted in a semi-anechoic chamber. Different from previous investigations, time-domain waveforms of corona-generated AN are recorded and the frequency spectra are calculated based on the time-domain waveforms through the calculation method of A-weighted sound pressure level (SPL). The results confirm that the frequency spectra describe different characteristics at different applied voltages and have a close relationship with pulse amplitude and pulse repetition frequency. In addition, the AN from a negative corona discharge contains more high-frequency components than those from a positive corona discharge. The components with the frequency above 400 Hz contribute most to the resultant total A-weighted SPL. The differences of AN from positive and negative corona discharges are interpreted on the basis of the corona discharge physics. Furthermore, some correlation frequency components in the frequency spectrum are found and can be used to calculate the total A-weighted SPL by adding correction values. Finally, the influences of pulse amplitudes and repetition frequency on the frequency spectrum of AN are discussed on the basis of the time-domain stochastic model. [ABSTRACT FROM AUTHOR]

Corona-generated audible noise is induced by the collisions between space charges and air molecules. It has been proven that there is a close correlation between audible noise and corona current from DC corona discharge. Analysis on the correlation between audible noise and corona current can promote the cognition of the generation mechanism of corona discharge. In this paper, time-domain waveforms of AC corona-generated audible noise and corona current are measured simultaneously. The one-to-one relationship between sound pressure pulses and corona current pulses can be found and is used to remove the interferences from background noise. After the interferences are removed, the linear correlated relationships between sound pressure pulse amplitude and corona current pulse amplitude are obtained through statistical analysis. Besides, frequency components at the harmonics of power frequency (50 Hz) can be found both in the frequency spectrums of audible noise and corona current through frequency analysis. Furthermore, the self-correlation relationships between harmonic components below 400 Hz with the 50 Hz component are analyzed for audible noise and corona current and corresponding empirical formulas are proposed to calculate the harmonic components based on the 50 Hz component. Finally, based on the AC corona discharge process and generation mechanism of audible noise and corona current, the correlation between audible noise and corona current in time domain and frequency domain are interpreted qualitatively. Besides, with the aid of analytical expressions of periodic square waves, sound pressure pulses, and corona current pulses, the modulation effects from the AC voltage on the pulse trains are used to interpret the generation of the harmonic components of audible noise and corona current. [ABSTRACT FROM AUTHOR]

The corona-generated electromagnetic interference commonly known as radio interference (RI) has become a limiting factor for the design of high voltage direct current transmission lines. In this paper, a time-domain measurement system is developed to measure the time-domain characteristics of corona-generated RI from a single corona source under a positive corona source. In the experiments, the corona current pulses are synchronously measured through coupling capacitors. The one-to-one relationship between the corona current pulse and measured RI voltage pulse is observed. The statistical characteristics of pulse parameters are analyzed, and the correlations between the corona current pulse and RI voltage pulse in the time-domain and frequency-domain are analyzed. Depending on the measured corona current pulses, the time-domain waveform of corona-generated RI is calculated on the basis of the propagation model of corona current on the conductor, the dipolar model for electric field calculation, and the antenna model for inducing voltage calculation. The well matched results between measured and simulated waveforms of RI voltage can show the validity of the measurement and calculation method presented in this paper, which also further show the close correlation between corona current and corona-generated RI. [ABSTRACT FROM AUTHOR]

Solar radio flux is an important indicator of solar activity and solar UV burst. Accurate prediction of solar radio flux plays a crucial role in preventing and mitigating the impact of solar activity on human productivity. We propose a novel approach for the first time to predict short-term radio flux trends using a bidirectional long short-term memory (BLSTM) network. This approach aims to address the unique characteristics of temporality and nonlinearity observed in solar radio flux data. Our model takes into account various frequency characteristics that impact radio flux. This allows it to learn temporal patterns within the data, ultimately enabling accurate predictions of radio flux for the next 30 minutes. The proposed method is experimentally applied to the radio flux dataset of the US Radio Solar Telescope Network (RSTN). The results show that, in most frequency bands, the BLSTM model exhibits superior prediction accuracy and greater sensitivity to peak responses compared to the LSTM model, LSTM-Attention (LSTM-A) model, BLSTM-Attention (BLSTM-A) model, and persistence model (PM). Consequently, the BLSTM model is better equipped to accurately forecast changes in radio flux for the next 30 minutes. [ABSTRACT FROM AUTHOR]

The sound pressure level (SPL) has been generally accepted in the measurement and evaluation of degree of nuisance to the corona-generated audible noise from transmission lines. Quite many scholars have studied the spectrum and statistical characteristics of the SPL from transmission lines through the long-term frequency measurement. In fact, the corona sources on the transmission lines usually act as sound sources emitting sound pressure pulses with random amplitudes and random time intervals, yet the statistical characteristics of the random sound pressure pulses and their correlation to the SPL have barely been investigated. In this paper, a series of experiments with single positive corona source in a coaxial cylindrical structure were carried out to investigate the statistical characteristics of the pulse amplitudes and time intervals of sound pressure pulses. In the experiment, time-domain measurement for the audible noise was employed. Based on the statistical results, a stochastic model was developed to simulate the sound pressure pulses. The validity of the stochastic model was testified by comparing the simulated and measured A-weighted sound pressure levels. On the basis of the stochastic model, the influence of random pulse amplitudes and time intervals of the sound pressure pulses on the resultant SPL was demonstrated. [ABSTRACT FROM AUTHOR]

Audible noise and corona current are two main DC corona effects, and they may both highly depend on the characteristics of space charges generated during corona discharge. However, relatively little investigation has previously been done on the correlation between them especially in time-domain. In the present work, the timedomain waveforms of audible noise and corona current generated by single positive corona source were recorded simultaneously. Their characteristics were carefully analyzed and found to be closely related. It was found that both the audible noise and the corona current consisted of repetitive pulses, and one-to-one relationship in time could be observed and confirmed between them. Furthermore, a linear correlation between the peak-to-peak values of audible noise pulses and the amplitude of corona current pulses was obtained. Empirical formulas with the same forms for them were derived to further prove the linear correlation. The close correlation between the audible noise and the corona current was interpreted qualitatively in the presented work and found to be highly related to the generated space charges. [ABSTRACT FROM AUTHOR]

This paper aims to investigate time-domain characteristics of the audible noise caused by single corona source under positive corona discharge. The obtained time-domain waveform of the audible noise consisted of a series of bipolar sound pressure pulses. After the background noise was successfully removed, the characteristics of audible noise pulses, such as amplitudes of each pulse, pulse rate and apparent duration time, were analyzed in this paper. Using the applied voltage and the onset voltage as variables, empirical formulas were derived from the measured results to predict the pulse rate and the amplitudes of the pulses. Furthermore, a typical waveform in the form of segmented double exponential function was developed to simulate the audible noise pulse. The rationality of the typical waveform was confirmed both in time domain and in frequency domain. Finally, some qualitative explanations for the generation of the bipolar pulse waveform of audible noise were given. Based on the remaining space charges characteristics, the influences of the applied voltage on the amplitude and pulse rate of the audible noise pulses were studied qualitatively. [ABSTRACT FROM AUTHOR]

In this paper, we develop five machine learning models, neural network (NN), long short-term memory (LSTM), LSTM based on attention mechanism (LSTM-A), bidirectional LSTM (BLSTM), and BLSTM based on attention mechanism (BLSTM-A), for predicting whether a ≥C class or ≥M class flare will occur in an active region in the next 24 hr. We use the data base provided by the Space-weather Helioseismic and Magnetic Imager Active Region Patches of Solar Dynamic Observatory, including 10 magnetic field features of active regions from 2010 May 1 to 2018 September 13. The samples are labeled flare information (i.e. No-flare/C/M/X) using solar flare events catalogue provided by the Geostationary Operational Environmental Satellite and Solar Geophysical Data solar event reports. In addition, we generated 10 cross-validation sets from these data using the cross-validation method. Then, after training, validating, and testing our models, we compare the results with the true skill statistics (TSS) as the assessment metric. The main results are as follows. (1) The TSS scores for ≥C class are 0.5472 ± 0.0809, 0.6425 ± 0.0685, 0.6904 ± 0.0575, 0.6681 ± 0.0573, and 0.6833 ± 0.0531 for NN, LSTM, LSTM-A, BLSTM and BLSTM-A, respectively. The TSS scores for ≥M class are 0.5723 ± 0.1139, 0.6579 ± 0.0758, 0.5943 ± 0.0712, 0.6493 ± 0.0826, and 0.5932 ± 0.0723, respectively. (2) For the first time, we add an attention mechanism to BLSTM for flare prediction, which improves the performance of the model for ≥C class. (3) Among the five models, the prediction model based on deep learning algorithms is generally superior to the model based on the traditional machine learning algorithm. The performance of the LSTM models is comparable to that of the BLSTM models. In general, LSTM-A for ≥C class performs better than other models. In addition, we also discuss the influence of 10 features on LSTM-A, and we find that removing the least significant feature will result in better performance than using all 10 features together, and the TSS score of the model will improve to 0.7059 ± 0.0440. [ABSTRACT FROM AUTHOR]

Nickel-rich layered oxides are the most promising cathode materials for lithium-ion batteries owing to their high theoretical mass and volumetric energy densities. However, voltage attenuation caused by rupture of the secondary particle structure during charging and discharging severely precludes their practical application. Using LiNi 0.8 Co 0.1 Mn 0.09 Al 0.01 O 2 (NCMA) as a model system, we developed a strategy that simultaneously removed inherent lithium residues and formed a boron-containing lithium compound coating using an H 3 BO 3 solution and subsequent annealing. NCMA obtained after boric acid washing displayed a high specific capacity (220.6 and 200.8 mAh•g−1 at rates of 0.1 and 1 C, respectively) and good cycle stability (capacity retention of 85.66% after 200 cycles). This method removed excess LiOH and Li 2 CO 3 and formed a stable coating on the material surface. Consequently, voltage attenuation caused by the disintegration of transition metals was prevented, while simultaneously enhancing the structural stability and electrochemical performance of the material. [ABSTRACT FROM AUTHOR]

In the pursuit of enhancing the energy density of lithium-ion batteries, cathode prelithiation emerges as a pivotal approach, introducing additional irreversible active lithium ions to augment performance. Li 5 FeO 4 (LFO) stands out as an excellent option, boasting an impressive theoretical specific capacity and the additional advantages of cost-effectiveness in raw materials and low preparation cost. However, the impact of extra lithium incorporations on compositional changes at the electrode-electrolyte interface as well as on cell stability has impeded further researches. Herein, a Mn doping strategy is proposed and predicted by first-principles calculations to synthesize partially Mn-substituted Li 5.125 Fe 0.875 Mn 0.125 O 4 (LFMO). And the prelithiation with LFMO improved the interfacial composition of LiFePO 4 (LFP) batteries compared to LFO, achieving a charging capacity of 213 mAh∙g−1 in the first cycle at 0.05 C. The subsequent treatment demonstrated superior cycling stability, maintaining a discharge specific capacity as high as 155.3 mAh∙g−1 after 100 cycles at 0.2 C, and exhibiting a higher capacity retention in the full cell. These discoveries could contribute to enhancing the regulation of battery interface during the utilization of LFO, potentially promoting the development of cathode lithium additives. • First-principle calculations predict the feasibility of Mn-substituted Li 5 FeO 4. • Prelithiation with modified lithium additives triggers LiF formation at interfaces. • Mn Modified lithium additives improve the energy density and cycle stability. [ABSTRACT FROM AUTHOR]

Partial discharge which often occurs at channel stopper of high voltage chip will cause insulation damage of chips which is harmful to the device application. Thus, it is important to improve the whole design to promote chip insulation capacity. In this brief, a novel chip surface shielding structure named U-protector is proposed which can promote the insulation capability of high voltage chips. Through the simulation, it can be found that the maximum electric field strength of chips can decrease 8% with adding U-protector. Moreover, verified by experiments, the partial discharge initial voltage (PDIV) of chips with U-protector can be promoted about 6.1%–12.7% of rated voltage. Furthermore, the universality and practicality of this structure are also investigated which can be used in multiple high voltage chips. [ABSTRACT FROM AUTHOR]

In the present study, a test chamber filled with perfluorohexane (FC-72) is set up to investigate partial discharge (PD) in phase-cooling bubbles. Under the non-uniform electric field with needle point-to-hemisphere electrodes, thermal bubbles are generated from the needle point. The difference between PD with and without the bubbles is analyzed to identify features of the discharge in the bubbles. It is observed that the bubbles reduce the PD inception and intensify the discharge. A model of bubble motion is established to investigate bubble behavior in an electric field. Based on the observation of the flow and streamer patterns, a mechanism of PD in bubble flow is established and confirmed by phase-distribution of PDs with bubble transit time. [ABSTRACT FROM AUTHOR]

Polyetheretherketone is used in the packaging of high voltage power electronics as a frame material, and the surface discharge behaviors of polyetheretherketone are the main concerns for the packaging design. Pulse width modulation voltage, which shows a variable duty cycle, is applied to polyetheretherketone at working conditions. The surface discharge behaviors of polyetheretherketone under a positive repetitive pulse voltage of 50 Hz with different duty cycles were investigated in this paper. It is generally considered that the surface discharge inception is mainly related to electric field distribution; however, the pulse width of voltage could significantly affect the surface discharge inception characteristics. In this study, the surface discharge inception voltage, time-integrated streamer images, and cumulative surface discharge patterns under a positive repetitive pulse voltage of 50 Hz were obtained. It was found that the surface discharge inception voltage decreased with the increase in the duty cycle. This relationship was explained by the role of time-lag. Time-lag distribution analysis was performed, and the time-lag distribution was mainly determined by stochastic time-lag. The mean stochastic time-lag was obtained by the Laue plot, and the relationship between the mean stochastic time-lag and applied voltage could be well fitted by the inverse power law. Furthermore, the relationship between the surface discharge inception voltage and duty cycle could be exactly fitted by the inverse power law, which was derived from the relationship between the stochastic mean time-lag and applied voltage. The closely related relationships demonstrate that time-lag plays a critical role in the surface discharge inception at different duty cycles. [ABSTRACT FROM AUTHOR]

The surface discharge behaviors of polyetheretherketone (PEEK) in nitrogen under both positive and negative semi-square voltage were investigated using the current pulse detection method by non-inductive resistor and optical signal detection method by photomultiplier and digital camera. A 50Hz positive or negative semi-square voltage was applied to a rod-plane electrode system, and two types of surface discharge in a voltage cycle were found: one forward discharge and one back discharge. The characteristics and initiation mechanism of the two types of surface discharge were explored. The experimental results showed that surface discharges between cycles were independent. Based on the independence, the time-lag characteristics of surface discharge were obtained. An improvement on Laue-Zuber time-lag theory was made to fit the time-lag characteristics of forward discharge and back discharge. The mean time-lag decreased with the increase of voltage for both forward discharge and back discharge, and the relation between mean time-lag and applied voltage could be modeled by inverse power law. Comparative analysis on the characteristics of back discharge and forward discharge under opposite voltage was performed, and the characteristics of back discharge and forward discharge under opposite voltage were homogenous. In addition, the initiation mechanism of the two types of discharge was discussed. The initiation of forward discharge is believed to be mainly determined by the time-lag and the electric field caused by the applied voltage, while the initiation of back discharge is believed to be mainly determined by the time-lag and the electric field induced by the residual surface charge left by the forward discharge in the same cycle. Verification tests under positive semi-square voltage with different fall time were conducted to validate the initiation mechanism. [ABSTRACT FROM AUTHOR]

Due to the lack of alternating electric fields and alternating magnetic fields in HVdc transmission system, there has been no sustainable and stable energy harvesting method compatible with them up to now, hindering the application of HVdc sensors. In this letter, we proposed a novel dc energy harvesting method using corona current, and an energy harvesting circuit is constituted with space ion flow generated by dc corona. The feasibility of the proposed method is increased by adding an artificial corona electrode and placing the load before the corona electrode. In addition, it can flexibly operate at any position in an HVdc transmission system according to actual demand. Then the output power is further maximized by adopting a capacitor with high working voltages. The detailed measurement results based on the experimental system validate the effectiveness of the proposed method, with a stable output power as high as 10.15 mW in a dc system of 22 kV. A detailed discussion of the overall performance is finally carried out. [ABSTRACT FROM AUTHOR]

Turn-off failure under clamped inductive load, is one of the most concerns in insulated gate bipolar transistor (IGBT) chips. Besides, this turn-off failure can be attributed to two causes, i.e., the dynamic latch-up and the secondary breakdown. Despite great simulation work investigated previously, the experimental research should be further focused. Up to now, the typical features and differences for these two turn-off failures still remain unclear. In this article, two failures’ waveforms are first compared in experiment. Moreover, their internal current, electric field, and temperature distributions are compared by simulation in detail. Then, two different failure features, focusing on the failure cell structures, are summarized from the scanning electron microscope (SEM) results for the first time. The marked signatures of these failures, such as the current filament path and the mixture region, are further analyzed in this article. According to the SEM results, the lateral and vertical current paths can be observed in dynamic latch-up chip and secondary breakdown chip, respectively. These failure signatures in waveforms and SEM results can contribute to the distinguished method to the failure causes in IGBT device, and enhance the chip's performance more effectively. [ABSTRACT FROM AUTHOR]

This article investigates the discharge characteristic of the bubble flow in FC-72 under unipolar repetitive pulsed voltage, which supports the insulation design of two-phase cooling in an HV power electronic device. The effect of thermal bubble flow on the insulation of FC-72 refrigerant was evaluated. According to the partial discharge (PD) characteristics, it is found that there are two stages in the discharge characteristics of the bubble flow as voltage rises. Combining the PD inception voltages of FC-72 vapor under unipolar pulse voltage and dc voltage, it reveals the discharge thresholds of the vapor inside the bubble under the transient electric field and the steady electric filed, respectively, which proves the mechanism of the staged PD evolution. Based on the motion calculation of the bubble, the PD probability of the bubble flow within a voltage period is analyzed. It simulates the phase-resolved PD pattern, which reveals the PD mechanism of the bubble flow. The research provides a reference for the insulation design for the two-phase cooling technology in power electronics. [ABSTRACT FROM AUTHOR]

Threshold voltage (${V}_{\text {TH}}$) hysteresis affects the reliability of silicon carbide (SiC) MOSFETs. To evaluate the ${V}_{\text {TH}}$ hysteresis effect on switching characteristics, this article first investigates the ${V}_{\text {TH}}$ hysteresis in the static characteristics of three SiC MOSFETs with different gate structures. The results illustrate the density of the interface states in different gate structures. Then, the effect of ${V}_{\text {TH}}$ hysteresis on dynamic characteristics under varying OFF-state starting voltages (${V}_{\text {G}}^{ \mathrm{\scriptscriptstyle OFF}}$) is evaluated by experiment. Furthermore, the effect mechanism of ${V}_{\text {TH}}$ hysteresis and ${V}_{\text {G}}^{ \mathrm{\scriptscriptstyle OFF}}$ on switching characteristics is analyzed. Under the effect of the ${V}_{\text {TH}}$ hysteresis, a smaller ${V}_{\text {G}}^{ \mathrm{\scriptscriptstyle OFF}}$ reduces ${V}_{\text {TH}}$ when the device turns on. This phenomenon leads to a reduction in the turn-on delay and consequently lowers the turn-on loss. Therefore, the ${V}_{\text {TH}}$ hysteresis is a significant factor for gate driver design of SiC MOSFETs. [ABSTRACT FROM AUTHOR]

The printed circuit board Rogowski coil (PCB RC) has been widely applied in the complex electromagnetic environment, such as power electronic devices, where it is placed outside but very close to current-carrying conductors; thus, this brings great challenges to the anti-interference ability of it. Generally, the anti-interference ability of PCB RC is greater with more turns; however, the number of turns is limited by two aspects: the one is the bandwidth of PCB RC, which is reduced as the turns increase, and the other is the minimum distance between vias, which is determined by process technology. This article proposed a method of segmented turns arrangement of PCB RC to acquire better anti-interference ability almost without lowering the bandwidth and get closer to the technical limit of distance between vias. The segmented turns are designed to be stair-stepping rather than rectangular used in traditional integral turns, which is equivalent to splitting a complete rectangular turn into several small rectangular turns so that the turns density is increased with the anti-interference ability improved. As an example of the method, a square PCB RC with proposed segmented turns is designed and compared with the PCB RC with traditional integral turns, and the results of calculation and experiment show that the anti-interference ability of PCB RC with proposed segmented turns is enhanced to be doubled and very great even near the corner of the PCB RC. [ABSTRACT FROM AUTHOR]

Plasma extraction transit time (PETT) oscillation will cause electromagnetic interference, which is harmful to the device operation. Thus, it is important to estimate the PETT oscillation frequency accurately to suppress this oscillation. While the existing formula for PETT oscillation frequency is approximate due to its divergent problem, in this letter, a modified formula for PETT oscillation frequency is proposed for insulated gate bipolar thyristor devices. The modified formula considers the acceleration process, which is complex but does not change with the electric field. Therefore, the divergence of the formula can be well solved by introducing the modified term. Moreover, the modified formula can be obtained straightforwardly and well match the measured oscillation frequency, which verifies the accuracy of the proposed formula. [ABSTRACT FROM AUTHOR]

Plasma extraction transit time (PETT) oscillation occurs when insulated gate bipolar transistor (IGBT) devices turn off, whose frequency can reach hundreds of MHz. This high-frequency oscillation can induce electromagnetic interference problems and may exceed relevant IEC limits for electromagnetic emission. As one of the electromagnetic disturbance sources from internal IGBT, the PETT oscillation has not attracted much attention in the electromagnetic compatibility (EMC) problems of application and development of high-voltage IGBT devices yet. In this article, the detailed characteristics of PETT oscillation, i.e., oscillation frequency, duration time, and oscillation peak, in high-voltage IGBT devices are systematically investigated by experiments. From the results, oscillation frequency increases with the increase of reverse voltage, whereas decreases with the increase of the temperature. Then, more complex dependencies of the PETT oscillation characteristics on IGBT's commutation conditions are presented. It is shown that its characteristics vary greatly under different reverse voltage, forward current, and temperature. The hazard of PETT oscillation to electronic components is discussed as well. PETT oscillation occurs during the turn-on process, which is first reported. Based on the experiment results, the EMC test related to PETT oscillation is suggested to be performed under different commutation conditions. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed analysis on the observed streamer characteristics of PEEK (polyetheretherketone) surface in nitrogen at atmospheric pressure driven by positive repetitive pulse voltage, including the forward discharge streamer and the back discharge streamer. Fast oscilloscopic technique and digital photographic technique were employed to observe the streamers of surface discharges, and current pulse waveform analysis technique and image analysis technique were used to analyze the characteristics of the observed streamers. The experimental results show that the current pulse of forward discharge streamer has a shorter rise time, a higher current amplitude, a faster current rising rate, a shorter fall time, a faster current falling rate and a smaller charge quantity than those of back discharge streamer at the same voltage amplitude. Besides, the forward discharge streamer has a longer stopping length, a faster propagation velocity and a stronger maximum light intensity than back discharge streamer at the same voltage amplitude. Furthermore, the correlation between the electrical characteristics and optical characteristics of the streamer was analytically derivated, and the current rising rate of the streamer is proportionally correlated to the product of the streamer stopping length and the streamer velocity square. [ABSTRACT FROM AUTHOR]

The crossing of two-circuit high-voltage direct current (HVdc) overhead lines will happen with the increasing number of HVdc transmission projects. The 3-D calculation is needed to predict the ion-flow field near the crossed HVdc lines. In order to decrease the difficulty of meshing near the bundle conductors, the equivalent single conductors are applied in the modeling. A roughness coefficient converting approach for equivalent conductor in Peek’s formula is presented. Thus, the corona degree of bundle conductors can be reflected by the equivalent single conductor. Afterward, the effectiveness of equivalent corona conductors in ion-flow field calculation is checked by a series of experiments and simulation. Finally, the 3-D ion-flow field distribution near the crossing of ±800 and ±500 kV HVdc overhead lines is calculated using equivalent corona conductors. [ABSTRACT FROM AUTHOR]

MYB proteins represent one of the largest transcription factor (TF) families in plants, some of which act as key transcriptional regulators of secondary cell wall (SCW) biosynthesis. Cotton (Gossypium hirsutum) fiber is thought to be an ideal single-cell model to study cell elongation and SCW biosynthesis. However, little knowledge regarding the TFs controlling fiber SCW biosynthesis, particularly for R2R3-MYBs is known. By far, no comprehensive genome-wide analysis of the secondary wall-associated R2R3-MYBs has been reported in cultivated tetraploid upland cotton. In this study, we identified 419 R2R3-MYB genes by systematically examining the cotton genome. A combination of phylogenetic, RNA-seq and co-expression analyses indicated that 36 R2R3-MYBs were either preferentially or highly expressed in 20 day post anthesis (dpa) fibers and are putative SCW regulators. Among these MYB genes, 22 MYBs are homologs of known SCW MYB proteins and the other 14 MYBs are novel proteins without prior reported SCW biosynthesis-related functions. Finally, we highlighted on the roles of two MYBs named GhMYB46_D13 and GhMYB46_D9, both of which displayed the highest expression in 20 dpa fibers. Expression of GhMYB46_D13 or GhMYB46_D9 individually in Arabidopsis resulted in ectopic SCW deposition in transgenic plants. Furthermore, both GhMYB46_D13 and GhMYB46_D9 were able to activate the cotton fiber SCW cellulose synthase gene promoters. Thus, we have identified 36 R2R3-MYBs as potential SCW regulators in cotton fibers that represent strong candidates for further functional studies during fiber development and SCW thickening. [ABSTRACT FROM AUTHOR]

The 3D upstream FEM is applied to calculate the ion flow field near the crossing of two circuit ±800 kV UHVDC transmission lines. In order to deal with the interaction of two circuit lines, an improved space charge density correcting treatment on the conductor surfaces is proposed. Compared with the traditional method, the conductor surface electric fields could be closer to the corona onset electric fields by using the improved correction treatment. Then, an experimental platform is established in the laboratory to testify the calculation method. Comparison of the calculated and measured ground-level ion current density proves the validity of calculation method. Finally, the distribution characteristics of space charge density and electric field strength near the crossing area are analyzed. The crossing structure of bipolar HVDC lines improves the corona discharge level in the opposite polarity crossing area significantly. [ABSTRACT FROM AUTHOR]

Micro-shocks may happen when the human body is close to high-voltage transmission lines. Experiments near the actual operating ±1100 kV high-voltage direct current (HVDC) lines with a metal human body model (HBM) are conducted. The voltages on the HBM and the micro-shock currents are measured. A micro-shock simulation method in the ion flow field is presented according to the measured results. Based on the simulation method, the variations of current, charge and energy on the HBM during the micro-shock process are investigated. The influences of resistance and capacitance parameters on micro-shock currents are analysed. Furthermore, a simplified simulation method is given to estimate the micro-shock currents for the potential demand of engineering practice. [ABSTRACT FROM AUTHOR]

The existence of residential houses will distort the ion flow field under high-voltage direct current transmission lines. High efficiency electric field computational method is required during the engineering design. In this paper, the surface charge method is applied in the space-charge-free electric field computation, and a half-analytical integration strategy is presented to accelerate the computational speed. Then, ion flow field around the house is computed based on Deutsch assumption. The calculation methods are validated by the experiment in the laboratory. Electric field intensity around the top edge of steeple rooftop is larger than other areas, and the field distribution characteristics on the steeple rooftop are significantly different from flat rooftop. [ABSTRACT FROM AUTHOR]

In this paper, a wire-cylinder electrode system was designed to study the negative DC corona pulses due to the influence of AC voltage applied to an adjacent conductor. The corona onset voltage and distribution pattern and the parameters of the corona current pulses such as the rise time, pulse duration and the average pulse amplitude are systematically analyzed. The AC voltage is found to have an interesting modulation effect on the time interval of the current pulses. At last, based on a simplified ion cloud model, a full explanation is given to reveal the mechanism behind the influence of the AC voltage on negative DC corona. [ABSTRACT FROM AUTHOR]

The corridor of the high-voltage direct current transmission lines in China is very complex and there may be buildings near the lines. The conductivity of the building material may influence the electric field in the building. In this paper, based on the upstream FEM, a method is proposed to solve the coupled problem of the ion flow field in the air and the current field in the building model. The numerical results coincide with the analytic results and measurement results. The electric field in and on the building model is analyzed considering different conductivities. [ABSTRACT FROM PUBLISHER]

Current pulse because of positive corona discharge is a main source of radio interference (RI). An accurate measurement of the original corona pulse is necessary to study corona‐generated RI. However, the corona pulse is likely to be distorted by either LC oscillation or reflection. In this study, an experiment system with single corona source is built which can provide a nearly free‐reflection path for the current pulses, and measurements are carried out in the system. The magnitude of the measured waveform is close to one half of the original pulse, which proves the accuracy of the measurement method. In order to obtain the original current pulse, a comprehensive model for the current pulses' path, taking the stray capacitance and stray inductance into consideration, is established. Based on the model, an inversion method to derive the original pulse is proposed. In combination with the obtained original pulse, the model gives the calculated results for other experimental conditions. The calculated results agree well with the measured results, which provide solid validation for the accuracy of the measurement. Another advantage of the proposed measurement method is that the long cables enable the accurate measurements to be achieved under any terminal conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel washing and coating technique for NCA has been developed. • NCA cathode materials with phosphate nanocoating has been constructed successfully. • Phosphate surface restrains unwanted reactions on NCA during cycling. Air humidity sensitivity of LiNi 1− x − y Co x Al y O 2 (NCA, (1 − x − y) ≥ 0.8) cathode materials deteriorates their safety and electrochemical properties due to the rapid formation of high alkali surface. Water washing is the most used method to remove those residues alkali efficiently, however, excessive loss of Li+ and Al3+ during washing increases the resistance greatly and worsens the reversibility of cathodes eventually. In this paper, saturated Li 2 CO 3 solution is introduced for eliminating toxic LiOH by-products selectively, while inhibiting the dissolution of lattice Li+. As a result of full-cell test, the washed LiNi 0.815 Co 0.15 Al 0.035 O 2 exhibited upgraded electrochemical performance with an initial discharge capacity of 184.5 mAh g−1 at 0.2C and an upgraded capacity retention rate of 86.2 % after 500 cycles at 1C rate. In order to further improve the stability of Ni-rich lithium storage structure, LiH 2 PO 4 is introduced as a washing additive to construct a protective phosphate coating layer with high stability instead of Li 2 CO 3 on the outer surface of NCA. The stable phosphate skim improves the electrochemical stability of NCA effectively, and the modified cathode shows an excellent cycle stability with a high capacity retention rate of 90.3 % after 500 cycles at 1C. [ABSTRACT FROM AUTHOR]

With more HVDC transmission lines being built in China, the situation of the plastic greenhouses existing underneath the HVDC lines is becoming more common when the transmission lines pass over the farmland. The ionic space charges generated by corona discharge of HVDC transmission lines strengthen the electric field at the ground level. And with the accumulation of the ionic space charges on the surface of the plastic greenhouses, the ion-flow fields are highly perturbed. In this paper, a set of single conductor experiments is performed in the laboratory. Extensive measurements are carried out for different voltages and polarities with different types of plastic greenhouse models. The main goal of this work is to make it clear that whether or not the presence of greenhouses would affect the ion-flow fields and ion current density inside the greenhouse models. And through the analysis and discussion, we find out the differences of the ion-flow fields when the greenhouse models exist or not, which is the foundation of the future research. [ABSTRACT FROM AUTHOR]

The reduced-scale model of HVDC transmission lines in the laboratory is widely used to investigate the ionized fields. This paper is aimed at analyzing impact factors of the ion-current density measurement system based on Wilson plates in the laboratory. Both experimental and numerical simulation methods are used to obtain the characteristics of the measurement system. The results show that measurement data can be calibrated with different Wilson plates, and suitable parameters are proposed in this paper to meet the requirement of measurements. The measurement system described in this paper can be used in other similar applications. [ABSTRACT FROM PUBLISHER]

As ultra-high-voltage direct current (UHVDC) projects are constructed in China, the electromagnetic environment problem is widely concerned. One of the most important problems is the ion-flow field including the human body under the UHVDC lines. But it has not been analyzed sufficiently. In this paper, the 3-D upstream finite-element method, a new method to analyze the 3-D ion-flow field under HVDC lines is proposed and verified by experiments. Then, the method is used to calculate the ionized electric field and the ion current of the human body model under the UHVDC lines. The ionized electric field is significantly distorted by the human body. According to the results, the ionized electric field and the ion current of the human body remarkably increase with the height of the human body model. The results could provide a reference for evaluating the electromagnetic environment and the personal feeling test under the UHVDC lines. [ABSTRACT FROM PUBLISHER]

The HVDC and HVAC transmission lines running on the same tower is a promising technology to improve the transmission capacity of an existing corridor. The ions generated by corona discharge of HVDC transmission lines will strengthen the ground-level electric field, which may endanger human exposure. To investigate how the presence of HVAC transmission lines changes the ion-flow field, experiments are carried out on an indoor test model with bipolar dc and three-phase ac lines. The experiment results are then compared with the simulation results. The experiment and simulation results show that the HVAC transmission lines have a shielding effect on the ion-flow field from the HVDC transmission lines running above. Finally, the ground-level electric field and ion current density of the \pm 800–kV HVDC and 500-kV HVAC transmission lines running on the same tower are calculated. [ABSTRACT FROM PUBLISHER]

This paper aims at investigating the mechanism of hybrid ac/dc corona which is usually accompanied with the hybrid HVAC/HVDC power transmission lines. A coaxial wire-cylinder corona cage is designed in this paper, and the combined ac/dc voltage is applied to the wire and cage. The low-frequency component of corona current from the hybrid ac/dc corona in this corona cage with five types of wires is measured. To obtain the detailed characteristic of the interaction between the space charge and ionized electric field, a 1-D method is developed to simulate the hybrid ac/dc corona in the symmetrical corona cage. This method is validated through the comparison with measurement results. Based on this simulation method, analysis of the evolution of space charge and ionized electric field in one time cycle is presented. The regular pattern of charge injection from the discharge electrode is summarized, where the charge injection is found not linearly dependent on the applied voltage. Finally, the corona loss of hybrid ac/dc corona is calculated, and the similar result with the quadratic law in ac corona is found from the calculation results. This research provides useful knowledge for the engineering computation of the effects of hybrid ac/dc corona. [ABSTRACT FROM AUTHOR]

Abstract: 14-3-3 proteins are phosphoserine-binding proteins that regulate the activities of a wide array of targets via direct protein–protein interactions, and may play an important role in response to biotic and abiotic stresses. In this study, two cDNAs (designated as Gh14-3-3b and Gh14-3-3c) encoding putative 14-3-3 proteins were isolated from cotton cDNA libraries. Gh14-3-3b gene encodes a protein of 268 amino acids, while Gh14-3-3c gene encodes a protein of 261 amino acids. Real-time RT-PCR analysis revealed that both the Gh14-3-3b and Gh14-3-3c genes were preferentially expressed in roots. The transcript levels of both the genes were the highest in 3-day-old roots, and then dramatically decreased to relatively low levels in parallel with root development. In addition, the expression of these Gh14-3-3 genes in roots was significantly up regulated by salt treatments, suggesting that they may be involved in the signaling pathways in response to salt stress in cotton. [Copyright &y& Elsevier]