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• A reduced-order model for assessing the transient DC-link voltage oscillation stability of VSC-HVDC systems is proposed. • Lyapunov direct method is applied to assess the transient DC-link voltage oscillation stability of VSC-HVDC systems. • The conservative stabilizing conditions based on two kinds of Lyapunov functions are derived. • Limit cycle of the converter-tied DC system is found and the asymptotic stability boundary is mapped based on limit cycle theory. • The system stability can be improved by tuning the system parameters. Lyapunov direct method is utilized in this paper to reveal the transient voltage oscillation stability mechanism for VSC-HVDC systems. Firstly, a reduced analytical nonlinear model for transient voltage oscillation stability assessment is proposed, which is a Liénard system. Based on the proposed model, a novel Lyapunov function is constructed and the conservative stability boundary is depicted, which reveals that the transient voltage oscillation can be well damped when the operating points are within the domain of asymptotic stability. In addition, it is found that there exists one limit cycle for VSC-HVDC systems. Once any system operation point exceeds the limit cycle, the divergent oscillations are triggered, and the aperiodic instability will eventually appear if the perturbations failed to be cleared in time. Finally, according to the limit cycle theory, one sufficient instability condition is derived and the transient voltage instability risk of the VSC-HVDC systems can be well evaluated. [ABSTRACT FROM AUTHOR]

The dominant modes of the system can be described by the critical eigenvalues, which reflect the stability and stability margin. This paper proposes a dominant mode identification (DMI) method to assess the stability of the grey-box grid-tied converter. First, the analytical relationship between the eigenvalues and the transfer function is clarified based on the small-signal model. Furthermore, the critical eigenvalues are estimated by the system's transfer function. On this basis, the discrete transfer function of the grey-box grid-tied converter is constructed utilizing the frequency sweeping technique. Finally, the critical eigenvalues are determined using the measured data based on the customized particle swarm optimization algorithm. The method can apply to different grid connection scenarios of converter-based components and is easy to implement. The DMI method can not only assess the stability margin of grey-box grid-tied converters in operation, but also predict the stability of black-box converters after being connected to the grid in the planning stage. • The relationship between the frequency responses and the eigenvalues is clarified. • The stability is analyzed according to the dominant mode of the system. • The method can pre-analyze the stability of the black-box converter. • The PSO algorithm for identifying dominant modes is easy to implement. [ABSTRACT FROM AUTHOR]

• The analytical expressions of fault traveling waves are derived for MMC-HVDC grids. • The curvatures of fault traveling waves are proposed as the protection criterion. • The protection method based on curvatures has good sensitivity to fault resistance. • A protection platform has been developed and tested with RTDS platform. The existing protection techniques for high-voltage direct-current (HVDC) grids suffer from several shortcomings such as high sampling frequency, poor robustness, and reliance on simulation for threshold setting. To solve these problems, this paper proposes a non-unit protection method for modular multilevel converter (MMC)-based HVDC grids using the curvatures of backward traveling waves. To this end, the propagation characteristics of traveling waves and the boundary characteristics of DC lines are first studied, then the analytical expressions of backward traveling waves are derived. Moreover, the curvatures of backward traveling waves are analyzed. On this basis, a non-unit protection method is proposed, including zone selection, disturbance identification, and pole selection. At last, with a protection platform and a real-time digital simulator (RTDS) platform of the MMC-HVDC grid, the accuracy and the robustness of the proposed protection method are verified. The results show that the protection method can correctly identify faults with different distances and resistance in 1 ms and has strong robustness against transition resistance, sampling frequency, boundary value, noise, system topology, and line parameters. [ABSTRACT FROM AUTHOR]

The wind farm (WF) provides fast frequency regulation (FFR) has become a consensus in low-inertia power systems. However, the optimal power sharing within the large-scale WF during the FFR process is intractable. Inspired by the intrinsic similarity between the information flow and the power flow, this paper proposes a distributed power sharing control for the FFR of the large-scale WF based on the idea of equation-based congestion control (EBCC). Concretely, a global congestion index, reflecting the deviation between the output power and the FFR order of the WF, is introduced for power tracking at the WF level. Besides, a local congestion index, which implies the operating cost of each WT, is constructed to reduce the operating cost of the WF. Then, the power sharing of each WT is determined locally. The distributed control significantly reduces the computational and communication burden, which makes the proposed method suitable for the large-scale WF, as well as meeting the demand of FFR. Moreover, the robustness to varying operating conditions makes the method practical and attractive. Finally, the effectiveness of the proposed method is verified under various scenarios by simulations. • The concept of congestion control is introduced in the WF power sharing issue. • It achieves distributed and non-iterative optimal power sharing among WTs. • It has a lightweight computational and communication burden. • It tracks various FR orders rapidly and smoothly with less operating cost. • Optimization objectives is flexible and can be customizable. [ABSTRACT FROM AUTHOR]

With the development of PMUs in power systems, the response-based real-time emergency control becomes a promising way to prevent power outages when power systems are subjected to large disturbances. The first step in the emergency control is to start up accurately and fast when needed. To this end, this paper proposes a well-qualified start-up scheme for the power system real-time emergency control. Three key technologies are proposed to ensure the effectiveness of the scheme. They are, an instability index, a Critical Machines (CMs) identification algorithm, and a two-layer single machine infinite bus (SMIB) equivalence framework. The concave–convex-area-based instability index shows good accuracy and high reliability, which is used to identify the transient instability of the system. The CMs identification algorithm can track the changes of CMs and form the proper SMIB system at each moment. The new two-layer SMIB equivalence framework, compared with conventional ones, can significantly reduce the communication burden and improve the computation efficiency. The simulations in two test power systems show that the scheme can identify the transient instability accurately and fast to restore the system to stability after the emergency control. Besides, the proposed method is robust to measurement errors, which enhances its practicality. [ABSTRACT FROM AUTHOR]

In the modern power system, current source converted high‐voltage direct current (CSC‐HVDC) system is playing an increasingly important role due to the advantages such as flexible control, asynchronous interconnection, and long distance transmission. However, the conventional differential current protection has exposed the shortages in operation speed and fault identification. This study proposes a novel differential current protection scheme, which is composed of the differential current unit and blocking unit. Both of them are designed based on the differences in the newly defined differential current between the internal fault and external fault. A ± 800 kV UHVDC simulation system is established to verify the performance of the proposed protection. Simulation test results show that the proposed protection can accurately distinguish the internal fault from external fault in the whole fault period. The identification performance is not influenced by the fault location or fault resistance. Besides, the requirement of sampling rate is low and the calculation amount can be negligible, providing a favourable condition for the proposed protection scheme to be implemented into the real HVDC control and protection system. [ABSTRACT FROM AUTHOR]

The UHVDC transmission system has been increasingly used because of the advantages in long distance and large capacity transmission. However, the existing traveling-wave-based protection for UHVDC transmission lines is not sensitive enough to properly detect the internal fault with high fault resistance. A novel traveling-wave-based protection scheme for bipolar UHVDC lines is proposed in this paper. The discriminative characteristic between the internal and external fault, embodied in the backward traveling wave, is extracted to formulate the main protection criterion. The faulty line is identified by the polarity of the voltage fault component. In addition, only the single-pole voltage and current of rectifier side are required for the proposed protection scheme. The simulation and field data tests show that the proposed protection scheme can make a correct identification of the fault section and selection of the faulty line in the $\pm $ 800 kV UHVDC system and $\pm $500 kV HVDC system. Besides, the proposed protection can correctly detect the grounding fault with fault resistance up to 500 $\Omega $. [ABSTRACT FROM PUBLISHER]

The kinetics of the Diels–Alder (D–A) reactions of N-ethylmaleimide with 2,3-dimethyl-1,3-butadiene and of 9,10-dimethyl-anthracene with diethyl maleate in two critical binary solutions n -tetradecane + dimethyl carbonate and n -nonane + diethyl maleate have been studied by the three-wavelength UV-spectrophotometry. It was found that neither speeding up nor slowing down effect for the first reaction was observed, which obeyed the Griffiths–Wheeler rule; while a very weak slowing down effect was detected for the second reaction, but significantly weaker as compared with the prediction by the Griffiths–Wheeler rule. It was attributed to that only less than 0.002% of diethyl maleate changed to the products during the reaction, hence the concentration of diethyl maleate may be considered as a fixed thermodynamic density in the critical reaction system, resulting in no thermodynamic slowing down being detected. [ABSTRACT FROM AUTHOR]

Power transformer rupture and fire resulting from an arcing fault inside the tank usually leads to significant security risks and serious economic loss. In order to reveal the essence of tank deformation or explosion, this paper presents a 3-D numerical computational tool to simulate the structural dynamic behavior due to overpressure inside transformer tank. To illustrate the effectiveness of the proposed method, a 17.3MJ and a 6.3MJ arcing fault were simulated on a real full-scale 360MVA/220kV oil-immersed transformer model, respectively. By employing the finite element method, the transformer internal overpressure distribution, wave propagation and von-Mises stress were solved. The numerical results indicate that the increase of pressure and mechanical stress distribution are non-uniform and the stress tends to concentrate on connecting parts of the tank as the fault time evolves. Given this feature, it becomes possible to reduce the risk of transformer tank rupture through limiting the fault energy and enhancing the mechanical strength of the local stress concentrative areas. The theoretical model and numerical simulation method proposed in this paper can be used as a substitute for risky and costly field tests in fault overpressure analysis and tank mitigation design of transformers. [ABSTRACT FROM AUTHOR]

The liquid–liquid coexistence curves for the binary solutions of the room temperature ionic liquid (RTIL) 1-methyl-3-octylimidazolium tetrafluoroborate ([C 8 mim][BF 4 ]) in 1-pentanol and 1-hexanol were measured in the critical region. The critical exponent β and the critical amplitude B were deduced and the former was found to be consistent with the 3D-Ising value. The complete scaling theory was applied to describe the asymmetry of the diameters of the coexistence curves. Moreover, the dependences of the critical temperature, the critical mole fraction, the reduced critical temperature and the reduced critical density on the permittivity of alcohol for a series of binary solutions of RTIL in alcohols were discussed. [ABSTRACT FROM AUTHOR]

Current transformer (CT) saturation is one of the dominant causes of relay protection devices’ malfunctions, which pose a threat to the safe operation of the power system. To address this problem, we propose a hybrid physical model- and data-driven method. The method firstly detects the CT saturation and then compensates it to reproduce the real waveform. Considering the multi-factor and strong nonlinearity of CT saturation, a data-driven model, namely the Fully Convolutional Network (FCN), is built to detect the operation status of CT. As for the compensation, a physical model of short-circuit current is used for its conciseness and universality. Through tactfully integrating the data model and the physical model, the proposed method is endowed with two major merits: the arduous adjustment of universal thresholds and parameters in existing methods is avoided, and the deficiency in generalization and interpretability of the data-driven method is assuaged. Simulation and experimental results verify the effectiveness of the proposed method. Furthermore, its application potential to future protection is explored. [ABSTRACT FROM AUTHOR]

In recent years, the sub‐/super‐synchronous oscillation (SSO) caused by grid integration of direct‐drive permanent magnet synchronous generators (D‐PMSG) has attracted wide attention. However, most of the existing stability analysis methods are either complex or cannot quantitatively analyze the influence of parameter changes on system stability. To address the problems, this paper proposes a novel stability analysis method for SSO in the D‐PMSG integrated power system based on closed‐loop transfer function poles. Firstly, the impedance modeling method was introduced to analyze the stability of the D‐PMSG power system. Then, a stability criterion was proposed based on the poles of the closed‐loop function. The proposed criterion is simple and has a clear physical background. The real part of the poles represents the damping while the imaginary part of the pole reflects the oscillation frequency. Finally, simulation results verify the accuracy and effectiveness of the proposed method. Moreover, its application in influence factors for quantitative analysis was also validated. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

We have used three-wavelength UV-spectrophotometry to study the reaction of the alkaline fading of phenolphthalein in the critical solution of 2-butoxyethanol + water. It was found that when the temperature was far away from the critical point, the values of the natural logarithm of the rate constant k and the natural logarithm of the chemical equilibrium K determined in our experiments had good linear relationships with the reciprocal of temperature, which served as the backgrounds and were used for correcting k and K in the critical region. The critical slowing down of the reaction and the critical anomaly of the chemical equilibrium were detected near the critical point. The value of the critical exponent characterizing the slowing down effect of the reaction rate was obtained to be 0.156, which was close to the value 0.11 associated with the heat capacity divergence and agreed with the theoretical prediction. The experimental result also confirmed the theoretical prediction of 0.11 for the critical exponent characterizing the weak divergence of the singularity of the chemical equilibrium. [ABSTRACT FROM AUTHOR]

The lack of fault data is the major constraint on data-driven fault detection and isolation schemes for DC microgrids. To solve this problem, this paper develops an adversarial-based deep transfer learning model that can detect and classify short-circuit faults in DC microgrids without using historical fault data. In this transfer learning framework, the knowledge of faults is extracted from the transient features of line currents during normal operating disturbances, which is adversarially augmented and then transferred to a target domain as the labels of faults. With the transferred knowledge, a deep learning model combining convolutional neural network and attention-based bidirectional long short-term memory is trained, which is strengthened by attention and soft-voting ensemble mechanisms. In verification tests, this model reaches a high accuracy of over 90 % in classifying various short-circuit faults in a multi-terminal DC microgrid model within a short response time of less than 1 ms. Moreover, it is robust against measurement noises and adaptive to system configuration changes. The test results prove the effectiveness of the proposed method in the protection of DC microgrids without prior knowledge of faults. • No historical fault data needed for the proposed fault classification model. • Fault knowledge obtained from non-fault disturbances through transfer learning. • High accuracy (> 90 %) and short response time (< 1 ms). • Good robustness against measurement noises and changes in system configurations. [ABSTRACT FROM AUTHOR]

This paper presents an ultra-high-speed (UHS) pilot protection of transmission lines based on directional comparison of the transient energy (TE). The proposed method can easily determine the fault direction by calculating the TE during a very short period. The obtained fault directions at both ends of the protected line will be compared to discriminate the internal faults. In contrast to the conventional UHS protection using initial traveling waves, this method is able to correctly operate even when the fault inception angle is at zero crossing. In order to correctly indicate the fault direction and improve its sensitivity, the data-processing window has been reasonably chosen to obtain TE by analyzing the polarity variation of the transient power. A large number of various fault scenarios have been simulated on the 750-kV system using PSCAD/EMTDC, and the results show that the proposed method is able to discriminate the internal fault very rapidly and reliably. [ABSTRACT FROM AUTHOR]

In this paper, the holomorphic embedding power flow method (HELM) is designed to solve the AC/DC hybrid power flow equations. According to the power flow model of the high voltage direct current systems (HVDC), an appropriate embedding technique with three recursive algorithms is developed for three different control modes, which is compatible with existing achievements of HELM for AC systems. In this method, the advantages of HELM, such as non-iterative, deterministic and non-ambiguous, are fully retained. In addition, a new calculation strategy based on Bauer's Eta algorithm is designed for the implementation of Padé approximations and the recursive process to avoid redundant calculations without reducing accuracy. In order to verify the reliability and effectiveness of the proposed extended HELM, several power flow cases with different scale are studied by HELM and traditional iterative method. [ABSTRACT FROM AUTHOR]

In order to ensure safe lane change and avoid traffic accidents, an effective lane change assist system is required. In a lane change assist system, it is very important to obtain the following elements in time, such as actual distance between vehicles, minimum safety distance, and warning signal. To this end, this paper analyzed four kinds of lane change angle collision scenes. Initial position, initial velocity, acceleration, heading angle, and kinematics of vehicles were used to calculate the position of potential angle collision points between lane change vehicle and obstacle vehicles. Then, actual distance model was constructed based on potential angular collision points. The minimum safety distance model was also established under the two most unfavorable conditions. In order to achieve the lane change warning, three early warning rules were formulated. We verified the validity of models and early warning rules using vehicle driving video data of Interstate 80 in California. Models and early warning rules constructed in our research can be applied to the advanced active safety systems of vehicle, such as vehicle lane change assist system and active collision early warning system, which can improve the active safety and reduce traffic accidents. [ABSTRACT FROM AUTHOR]

This paper proposes a unified power flow algorithm based on the Newton–Raphson (NR) method to obtain a feasible solution for hybrid AC/DC grids incorporating voltage source converters (VSCs). The power flow model of hybrid AC/DC grids is composed of an AC grid model, a VSC station model, and a DC grid model. Detailed converter losses are involved in VSC station modeling. In the DC grid model, a unified dc control expression is applied to adapt to different control methods. To obtain sparse Jacobian matrix, the mismatch equations and unknown variables are classified into three categories corresponding to AC, VSC, and DC models. Then the proposed algorithm achieved quadratic convergence with the NR method. Once the solution is infeasible (variable out-of-limit) or diverging, the constrained load flow (CLF) method is applied automatically to achieve a feasible solution by revising the references of ac/dc grids. Simulations were carried out for the hybrid AC/DC test system composed of two AC grids and two DC grids. The results under different DC voltage control modes show that the proposed algorithm is able to obtain a feasible solution with high accuracy and convergence speed. [ABSTRACT FROM PUBLISHER]

In this paper, a simplified method for channel capacity estimation has been derived based on the transmission-line theory under the condition of high-bandwidth utilization. This method decouples the channel capacity into several independent components which may help to investigate the effects of backbone and branch, respectively. Analysis of backbone effects with respect to the available bandwidth reveals the applicable condition for the simplified method. Single branch effects are investigated considering branch length together with the loads. Besides, coupling effects caused by branch interactions are studied in a statistical way. The simulation results show great consistencies by using the proposed method and the conventional one and reveal the way that branches affect the channel capacity in different power networks. [ABSTRACT FROM AUTHOR]

[Display omitted] • Plant aboveground biomass (AGB) showed a positive and concave–down relationship with species richness (SR). • Precipitation and edaphic factors were correlated with AGB and SR, but temperature was not. • Drought resulted in low soil weathering degree and limited the productivity of alpine grasslands. The relationship between plant productivity and diversity is central to understanding changes in ecosystem functions. However, this relationship remains controversial due to vegetation types and environmental differences. For example, understanding how soil development involving mineral weathering and pedogenic development processes shapes plant productivity and diversity remains limited, especially in alpine grasslands. Here, we evaluated how plant species richness (SR) and aboveground biomass (AGB) are related to one another, and their drivers, by conducting investigations in 169 alpine grassland sites along a 3000 km-transect in the Qinghai-Tibet Plateau. We found that AGB and SR both exhibited a positive and concave–down relationship. The mean annual precipitation explained 23% of the variance in Vegetation characteristics, followed by altitude (14%), soil N content (13%), soil cation exchange capacity (11%), K content (7%), P content (6%), weathering index of Parker (5%). Precipitation was correlated with AGB and SR, but temperature was not. Drought conditions led to slow soil development with high soil pH, and a lack of soil fertility and water jointly limited plant productivity. Our results demonstrate that conservation of diversity within vulnerable ecosystems is critical to maintaining the proper functioning of grassland systems, especially in areas where drought and low soil development are a problem. Soil weathering degree should be considered in efforts seeking to improve projections of alpine grassland ecosystem functioning under future climate change. [ABSTRACT FROM AUTHOR]

The rehabilitation of disaster-prone areas can enhance the stability of soil structure and is a common way to increase organic carbon storage. The response of soil carbon sequestration pathways to different recovery modes is not clear, especially in mountain soils. After 11 years of recovery, we evaluated soil organic carbon (SOC) sequestration pathways in plantations (dominated by Olea europaea 'Leccino'), croplands [ Zea mays (L.) ] natural shrublands (Lycium chinense Mill), and natural grasslands [ Setaria viridis (L.) Beauv ]. The physical and chemical properties of the soil and the 13C natural abundance of each aggregate and its density components were studied. The results showed that, during the restoration process, the soil organic carbon content of natural grassland increased the most, while the δ13C value of soil of natural shrubs was the highest. The natural abundance of 13C was used to reveal the pathway of C flow in soil organic matter (SOM), as follows: free light fractions (ρ < 1.6 g cm−3)→mineral fractions (ρ > 2.0 g cm−3)→dense occluded fractions (ρ from 1.6 to 2.0 g cm−3) (in plantation, natural shrubland, and grassland). However, in cropland soil, C flowed as follows: mineral fractions→free light fractions→dense occluded fractions. Specifically, the SOC content decreased with aggregate particle size, and after entering the soil, plant litter was first stored in large aggregates and then decomposed into the free light fraction. The study revealed the mechanism of organic carbon sequestration in the restoration area, emphasizing that artificial restoration treatment can change the carbon conversion pathway, and reduced the sequestration of organic carbon. • The pathways of organic carbon turnover had been clearly determined by 13C natural abundance. • Natural grasslands have the highest soil organic carbon content under the same years of restoration. • δ13C was more enriched in micro-aggregates than in large and small macro-aggregates. • Artificial restoration altered the carbon flow pathway of transformation and reduced the sequestration of organic carbon. [ABSTRACT FROM AUTHOR]

Mixedsurfactant solutions are studied to understand their synergisticeffects. Here we report the micellization properties of mixed surfactantsolutions of the gemini surfactant N,N′-bis(dimethyldodecyl)-1,2-ethanediammoniumdibromide (12–2–12)with the imidazolium ionic liquid 1-dodecyl-3-methylimidazolium bromide(C12mimBr) investigated by isothermal titration calorimetry(ITC), conductometry, fluorimetry, and dynamic light scattering (DLS).A two-parameter Margules model was successfully used to correlatethe cmc values and calculate the compositions and activity coefficientsof the two components in the mixed micelle phase for 12–2–12/C12mimBr aqueous solutions with different overall surfactantcompositions. The dissociation degree of counterion, the thermodynamicquantities of micellization, and the excess thermodynamic quantitiesfor the mixed micelle were calculated and discussed. The ITC experimentin the low overall surfactant composition region showed a second phasetransition. A thermodynamic model was proposed to explore phase behaviorsof two different types of micelle and their solution, which was furtherconfirmed by fluorescence and DLS studies. [ABSTRACT FROM AUTHOR]