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• Building green view index (BGVI) was proposed to quantify 3D greenery visibility. • 76.10 % of the study area had a higher BGVI than the street view green view index. • Marginal effects of influencing factors on BGVI were explored using random forests. The vertical expansion of urbanization has increased the morphological heterogeneity of the urban landscape, affecting the physical and emotional wellbeing of urban dwellers by obstructing the view of greenery. In this study, multisource spatial data was used to calculate the Building Green View Index (BGVI). Baidu Street View (BSV) images were collected for comparison with the corresponding BGVI results. A random forest model was used to analyze the contributions and marginal effects of multiple influencing factors on BGVI. The results indicated that approximately 76.10 % of the sampled sites had a higher BGVI than the street view green view index, indicating buildings' superiority of visible greenery in height. The western edge of the research region frequently had the highest BGVI. Meanwhile, the hotspot regions were primarily located in the west, which was more consistent with the distribution of high value zones. The green area within the maximum visible distance, the maximum visible distance, and the average height were the most influential factors of BGVI according to marginal effects analysis with the highest IncNodePurity. As a quantitative measure of urban dwellers' visual accessibility to green space, the BGVI will contribute to urban green planning and the development of landscape architecture. [ABSTRACT FROM AUTHOR]

• The equivalent circuit of a 1100 kV UHV transmission system with Hybrid Reactive Power Compensation (HRPC) is developed. The severity is selected as the index of the interrupting characteristics of the circuit breaker, which is analyzed by the calculation method. • The waveforms of the drop-out current and transient recovery voltage (TRV) of circuit breaker are simulated before and after installation of HRPC to verify the validity of the calculation results. • The impact of different fault locations and compensation degrees on the interrupting characteristics of the most common faults in the power grid was analyzed. The interrupting characteristics of circuit breaker under different fault was compared. For extremely large ultra-high-voltage (UHV) power transmission and distribution lines with typical project using hybrid reactive power compensation (HRPC), single-phase earth fault (1LG), three-phase ground fault (3LG), phase-to-phase fault (2LS), and two-phase grounding fault (2LG) often occurs. Circuit breaker is one of the most critical equipment of UHV transmission system. And the interruption performance is crucial to circuit breakers' breaking capability. In this study, formal expressions of the indicators for the interruption performance of the circuit breaker are deduced in case of different faults based on the equivalent-valued parameter circuit based on the structure a UHV demonstration project. The simulations are used to validate the accuracy of formal expressions. Then, the influence of such factors as the location of the fault and the degree of HRPC on the interruption performance of the 1LG, 2LG, 3LG, and 2LS is analyzed. Finally, the interruption performance of circuit breakers under different faults are compared. A quantitative evaluation of the influence of changes in the relevant indicators on the interruption performance of circuit breakers can provide a technical basis for choosing the appropriate design of the circuit breaker in light of the application as well as equipment development and assessment. © 2017 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

Highlights • First, based on the general form of Maxwell's equations, the field source is transformed into an equivalent form with a constraint source, and a full-current PEEC model with a surface charge component element is established. • Second, a fast and accurate calculation of the inductance parameters of SCSR components involving multiple integrals is carried out using the geometric mean distance and complex component division. • Third, the equivalent structure and circuit equations for the SCSR internal windings are given, and the variation of winding resistance and inductance parameters with VFTO frequency is analyzed. Abstract Stepped Controlled Shunt Reactors (SCSRs) have advantages of flexible control and reliable compensation and are likely to become widely used in Ultra-High-Voltage/Extra-High-Voltage (UHV/EHV) transmission systems. Very fast transients that occur in such systems will generate a Very Fast Transient Overvoltage (VFTO) in the SCSR. This may cause damage to the SCSR and lead to system losses. In this paper, the electric field strength and magnetic induction intensity are taken as the basic physical variables when these transients occur. Maxwell's equations are used to separate the constraint sources, such as the conduction current, magnetization current, and polarization current, from the field source. Then, an all-current Partial Element Equivalent Circuit (PEEC) model for these component elements that considers the surface charge is established. Fast and accurate calculations of the inductance parameters, which involve multiple integrals, are performed using geometrical mean distances and a complex component segmentation method. The equivalent structures and circuit equations of the SCSR windings are presented, and the voltage distribution of the SCSR windings under VFTOs of different frequencies is obtained. The results show that, the voltage waveform in the SCSR windings more rapidly attenuates as the VFTO frequency increases. The peak value of the oscillating voltage in the high-voltage winding of the SCSR is higher than that in the low-voltage winding. [ABSTRACT FROM AUTHOR]

Hybrid reactive power compensation (HRPC) consists of a stepped controlled shunt reactor (SCSR) and a series compensation (SC), which will find applications in future ultra high-voltage (UHV) power grids to resolve the problems due to the frequent change of reactive power and bulk power transmission. However, very fast transient currents (VFTCs) are inevitably generated during switching, which would lead to insulation breakdown. In the present work, we first develop the equivalent model for HRPC, following which we deduce the expression of VFTCs in the time domain by using an inverse Laplace transform. The analysis indicates that the amplitude and frequency of VFTCs are both affected by the capacitance of the SCSR and of the SC, as well as the line length, stray capacitance, etc. The oscillating frequency, peak, and amplitude of the main frequency of the VFTCs in the substation can be modified by adjusting the silicon-controlled rectifiers in the SCSR when the disconnecting switch in gas-insulated switchgear is switched on. When the disconnecting switch in the SC is switched on, the VFTC oscillation frequency in SCSR decreases with increasing stray capacitance of SC, but the frequency and peak of the VFTC remains quite large. Increasing the line length between the SC and the SCSR suppresses the VFTC in UHV power systems. These results lay the foundation for developing HRPC methods to suppress VFTCs in UHV substations. [ABSTRACT FROM AUTHOR]

Abstract: Permanent-Magnet-Biased Saturation Based Fault Current Limiter (PMFCL) may find early applications in high-voltage and large capacity power systems due to prominent advantages such as compactness, small size, reliable and safe operation, and zero reset time. The prevailing topologies and the operating principles of the PMFCLs are elucidated in the paper. Theoretical analysis on both the static and dynamic characteristics of different modes of PMFCLs is carried out comparatively, in which new concepts of effective magnetic flux density and magnetic field intensity are proposed to describe the overall impacts of the magnetic field on the current limiting capacity of the PMFCLs. Based on theoretical analysis and mathematical deduction, equivalent magnetic circuits and equations are established, which presents invaluable fundamentals for further study on the operating mechanism of different modes of PMFCLs as well as optimization of the fault current limiting topology design. [Copyright &y& Elsevier]

• The equivalent circuit of a UHV transmission system with Hybrid Reactive Power Compensation (HRPC) is developed. The waveforms of the interrupting current and transient recovery voltage (TRV) of a CB are simulated before and after installation of HRPC. • The interrupting severity is proposed to evaluate the interrupting characteristics. • Several factors of the circuit breakers, including the peak value of the TRV, the rise rate of the transient recovery voltage, and the interrupting current, will change the interrupting characteristics of phase-to-phase fault under various operating conditions. This paper presents a theoretical study of the interrupting characteristics of circuit breakers in the event of phase-to-phase faults in UHV transmission lines with hybrid reactive power compensation (HRPC). The study uses the transmission line parameters of the Jindongnan–Nanyang–Jingmen UHV AC pilot demonstration project in China and establishes a simplified model for the distribution parameters of transmission lines. Mathematical expressions describing the relationships between circuit breaker interrupting parameters and compensation degrees of HRPC are derived. Based on these mathematical expressions and their comparisons with simulation results, the effects of HRPC on circuit breaker interrupting characteristics and on the parameters of the phase-to-phase fault circuit are determined, in which case the validity of the analytical method is verified. The results of this study will provide a theoretical foundation for practical implementation of HRPC in UHV transmission. [ABSTRACT FROM AUTHOR]

• The vegetation indices RVI, EVI, and LAI are good predictors for grassland AGB estimation. • Grassland AGB has a negative correlation with summer temperature. • Grassland AGB has a positive correlation with summer cumulative precipitation. • Livestock density is not a dominant factor to the interannual variation of Hulunbuir Grassland AGB. • Population density rarely showed significant correlation with Hulunbuir Grassland AGB. In the past 10 years, extreme weather phenomena have increased, and global warming has markedly advanced; moreover, the intensity of human activity has gradually increased. These have an impact on the growth of global vegetation. Related studies have focused on the Tibetan Plateau and some northern provinces of China to estimate grassland AGB at a large scale but a low resolution. Hulunbuir Grassland is an important supplier of livestock products, and therefore, it is important to precisely map AGB and explore the response of Hulunbuir Grassland AGB to climate change and human activities at a high resolution to identify complex spatial details. In this study, we selected vegetation indices from Landsat 8 OLI and topographic indices and used multiple linear regression and machine learning algorithms to estimate the spatial distribution of AGB from 2013 to 2022. Then, we analyzed the correlations between AGB and cumulative precipitation and daily average temperature in summer and between population density and livestock density at the pixel level. Our results demonstrated that the RF model performed well, with an RMSE of 28.23 and R2 value of 0.74; the AGB was positively correlated with the cumulative precipitation in summer in 94.45 % of the area and negatively correlated with the daily average temperature in summer in 96.32 % of the area. We suggest that it is necessary to reduce grazing activities in future warm and drought years and to adjust sources of income to adapt to the decrease in AGB under global warming conditions. This study will provide reference for the countries or regions that depend on temperate grasslands for husbandry. [ABSTRACT FROM AUTHOR]

• The bipolar charge transport and hydrodynamic drift-diffusion models were used. • The PD waveforms under the assessed conditions were unimodal. • Pressboard surface charge can form a shield layer that hinders streamer evolution. • The streamers have voltage-dependent shapes. • The time relationships between PD pulse and streamer evolution were found. A better understanding of the complicated characteristics and interrelationships between the charge distributions and partial discharge (PD) pulse waveforms occurring in oil-pressboard insulation can be used to characterise the PD mechanism as a basis for the optimisation of the insulation structure to minimise PD activity. These characteristics were studied by using a non-inductive resistor to measure the PD pulse waveforms produced in oil-pressboard insulation by a needle-plate at positive DC voltage and through additional simulations based on bipolar charge transport and hydrodynamic drift-diffusion models. It was found that the PD pulse waveforms are unimodal, and with increased applied voltage, their rising and falling edges and equivalent times decrease, and their apparent charge amplitudes and equivalent frequencies increase. These parameters have a saturation state at high voltages (>45 kV) and the oil streamers have voltage-dependent shapes and evolution velocities (v s); the higher the voltages, the higher the values of v s , and the wider are the streamer bodies. Accumulated charge can form a shield layer that hinders streamer evolution. Analysis of the PD mechanism in terms of the relationships between charge distribution and PD pulse waveform reveal that the rising and falling edges of a PD pulse correspond to streamer propagation in the oil gap and charge accumulation at the pressboard surface (i.e., the charging process in the pressboard), respectively. The peak value of the PD pulse waveform is shown to be related to the conduction current in the coupling loop as reflected by the current density at the streamer head. [ABSTRACT FROM AUTHOR]

Abstract Partial discharge (PD) of high voltage equipment detected by online monitoring is regarded as a key indicator of insulation status. Threshold estimation exerts an important influence on the denoising effects of PD signals after wavelet-based denoising. In order to improve the adaptive performance of wavelet-based denoising and reduce the distortion of denoised signal, an approach of hybrid particle swarm optimization (HPSO) adaptive wavelet threshold estimation (HPSOTE) is presented in this paper. To obtain the global optimum thresholds, a novel hybrid particle swarm optimization algorithm merging crossover mutation and chaos is proposed in HPSOTE which could solve the premature convergence problem of common threshold selection methods. Genetic algorithm (GA) and particle swarm optimization (PSO) algorithm are adopted to compare the effects with HPSOTE. The denoising results of simulated and actual PD signals show that HPSOTE has a faster convergence rate and more effective global optimization ability than the others and significantly improves the credibility and calculation speed. It gives better mean square error (MSE) and amplitude error performance. This HPSOTE can remove the noise effectively and has comparatively high application value in PD online monitoring for smart grid based on big data. [ABSTRACT FROM AUTHOR]