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Mode instability (MI) has become a major factor that restricts the brightness enhancement of fiber laser. This paper studies the MI threshold characteristics of the fiber oscillators with refrigerated ytterbium-doped fiber (YDF). In the experiments, two fiber lasers that can operate at low temperature were built. We rigorously tested the MI threshold of the lasers with the YDF placed in two different environments. At room temperature environment, the MI threshold of the oscillator was measured with the temperature of the YDF water cooling plate ranging from 25 °C to 5 °C. When the cooling temperature of the YDF drops from 20 °C to 5 °C, the MI threshold of the laser rises gradually, but the increase is small. In another experiment, the YDF is assembled in a constant temperature test chamber, in which the cooling of the YDF is more efficient. It is found that as the cooling temperature drops, the MI threshold of the laser has an obvious upward trend. In the process of the cooling temperature of the YDF dropped from 20 °C to −10 °C, the MI threshold of the laser was increased from 867 W to above 931 W, an increase of nearly 10%. It is the first detailed experimental demonstration which shows that decreasing the cooling temperature of the gain fiber can indeed increase the MI threshold of the fiber oscillator. This work can clarify the influence of cooling temperature on the laser thermal effect, which is conducive to perfecting the theoretical model of the MI effect. [ABSTRACT FROM AUTHOR]

Topology identification and parameter estimation form the basis for the operation and control of the distribution network with little or no observability. This paper proposes a method to estimate the admittance matrix for open-loop distribution networks based only on active/reactive power injection and voltage magnitude data. We derive the linear relationship between the admittance matrix and the measurements based on decoupled linear power flow (DLPF) equations. A total least squares regression method is proposed based on analysis of the characteristics of the open-loop grid admittance matrix. Case studies on the IEEE 33-bus system demonstrate the proposed method's effectiveness, accuracy, and efficiency. [ABSTRACT FROM AUTHOR]

In practical microgrids, the inhomogeneous initial values are widely appeared due to soft-starting operation. If traditional model order reduction approaches are applied, the input-output maps error between the original system and reduced-order system is large. To address this problem, this paper proposes a reduced-order aggregate model based on balanced truncation approach to provide the preprocessing approach for the real-time simulation of large-scale converters with inhomogeneous initial conditions in DC microgrid. Firstly, the standard linear time-invariant model with inhomogeneous initial conditions is established through non-leader multiagents concept. To end this, it is convenient for scholars to build complex system modeling with switched topology. Furthermore, the full system is divided into two components, i.e., the unforced component with nontrivial initial conditions and forced component with null initial conditions. Moreover, this paper presents an aggregated approach that involves independent reducing component responses and combining reducing component responses. Based on this, the input-output maps error is reduced. Then, the approximated error estimate of the reduced-order aggregate model regarding large-scale converters in DC microgrid is first provided, which provides prior knowledge and theoretical basis for DC microgrid designers. Finally, the simulation results illustrate the accuracy of the proposed approach. [ABSTRACT FROM AUTHOR]

A decentralized improved I-V droop control strategy for battery-supercapacitor (SC) hybrid energy storage system (HESS) is proposed in this paper. The dynamic power sharing between battery and SC is realized by replacing the constant droop coefficient in I-V droop control with virtual impedance, i.e. virtual inductance for battery side converter and virtual resistance for SC side converter. Besides, by injecting the virtual inductance in the battery side converter, negligible DC bus voltage deviation can be achieved without extra voltage compensator. Moreover, the state-of-charge (SoC) recovery is also considered to extend the service life of the HESS. Furthermore, in the proposed regulated power system, since the power allocation, DC bus stability and SoC recovery are decoupled from each other, the design of control parameters is simple. The corresponding design guideline is demonstrated in this paper. Finally, to verify the accuracy and feasibility of the theoretical analyses, hardware in the loop simulations have been conducted. [ABSTRACT FROM AUTHOR]

Fashion products typically feature in compositions of a variety of styles at different clothing parts. In order to distinguish images of different fashion products, we need to extract both appearance (i.e., “how to describe”) and localization (i.e., “where to look”) information, and their interactions. To this end, we propose a biologically inspired framework for image-based fashion product retrieval, which mimics the hypothesized two-stream visual processing system of human brain. The proposed attentional heterogeneous bilinear network (AHBN) consists of two branches: a deep CNN branch to extract fine-grained appearance attributes and a fully convolutional branch to extract landmark localization information. A joint channel-wise attention mechanism is further applied to the extracted heterogeneous features to focus on important channels, followed by a compact bilinear pooling layer to model the interaction of the two streams. Our proposed framework achieves satisfactory performance on three image-based fashion product retrieval benchmarks. [ABSTRACT FROM AUTHOR]

Ambient backscatter communication (AmBC), a promising solution to future Internet of Things (IoT), has attracted increasing attention from both academia and industry in recent years. AmBC systems enable communications between RF-powered devices (e.g., tags and sensors) and AmBC receivers by utilizing ambient RF source signals from existing communication systems. Thus it offers a low-cost and energy-efficient means of communications for future IoT applications. In the current AmBC research, the impact of using channel coding on existing primary networks in terms of the bit-error-rate (BER) performance of the AmBC systems are mostly ignored. Low-density parity-check (LDPC) codes that offer fast encoding and decoding process have been proposed as promising channel codes in the fifth-generation (5 G) networks. In this paper, we investigate the BER performance of an AmBC system that uses LDPC-coded RF source signals. We have analytically derived the BER upper and lower bounds of both the RF source and tag signals for our proposed AmBC system. Simulation results show that the BERs of both signals decrease significantly, compared to conventional AmBC systems with uncoded RF source signals. [ABSTRACT FROM AUTHOR]

This article presents a novel reduced order average model for dual-active-bridge converter which can be applied to all modulation methods, such as single-phase-shift modulation, dual-phase-shift modulation, extended-phase-shift modulation and triple-phase-shift modulation. This model considers conduction, inductance, and transformer power losses. Furthermore, the input–output filters are suitable for system performance analysis. Based on the large-signal average model, the small-signal model and the output transfer function are derived. The detailed model for predicting the large-signal transient is established and the small signal analysis in frequency domain is carried out. This model can accurately predict the steady-state and transient-state response of the system. The experimental results agreed well with the simulation results which proved the accuracy and correctness of the model. [ABSTRACT FROM AUTHOR]

In this paper, we propose a set of new sounding reference signals (SRSs) for time-division duplexing (TDD) massive MIMO systems with channel aging. It is analytically shown that the proposed set of SRSs exhibit zero correlation zone (ZCZ) property, which guarantees sparse and marginal interference between the SRSs. Therefore, all SRSs in the set can be transmitted concurrently in a cell with a short SRS period per user to alleviate the channel aging effect. We also analytically derive the peak-to-average power ratio (PAPR) and show how the SRS should be constructed to achieve low PAPR. Finally, simulation results are provided to show that the proposed SRSs result in larger downlink throughput than the 5G New Radio SRSs. [ABSTRACT FROM AUTHOR]

This brief proposes a novel generalized Fractional Folding (FF) architecture for digital signal processing integrated circuits. With this new structure, a Fractional Folding based enhanced Parallel Inversionless Berlekamp-Massey (FF-ePIBM) Reed-Solomon Decoder is presented of which the number of processing element (PE) can be reduced to only one, resulting in ultra-low hardware complexity. The FF-ePIBM VLSI architecture can greatly reduce the hardware cost by about 60% compare to the fully expanded parallel ePIBM architecture. With FF-ePIBM architecture, syndrome calculation (SC) block and Chien search & error evaluation (CSEE) block are able to operate at a lower frequency resulting in about 14.8% power saving. A polyphase clock signal is needed in order to achieve fractional folding function according to specific fractional-factor. It is generated from Delay Locked Loop (DLL) at little or no extra cost in different SoCs. To maximize the throughput of decoder, pipelined architecture is adopted to optimize critical path delay. The RS (255, 239) decoder with FF-ePIBM architecture is finally implemented with 40nm CMOS technology. The synthesized results demonstrate that the decoder has a gate count of 12.9k and can operate at 3.1GHz to achieve a highest throughput of 24.8Gb/s and a best TSNT FoM value of 592 to this date. [ABSTRACT FROM AUTHOR]

Due to global climate change, the effect of extreme weather on power systems has attracted extensive attention. In the prior-art grid resilience studies, the hurricanes or wildfires are mainly defended in terms of expected line damages, while they are prone to trigger short-circuit fault (SCF) evolved with dynamic influence in reality. In this paper, a fragile model is developed to evaluate the nodal SCF probability considering the insulation aging of equipment and extreme weather condition. Then, a response framework for extreme weather events is developed for a transmission system to defend the cascading impacts of expected SCFs. Specifically, switches are shifted to restrain the out-of-range short-circuit currents (SCCs) so that to ensure the SCFs can be removed by circuit breakers, generation rescheduling and load shedding are arranged to maintain the post-fault system transient stability. The above measures are optimized simultaneously by an integrated Mixed-Integer Nonlinear Programming (MINLP). Considering the error or uncertainty of weather event forecasts, a multi-state model is established to provide the most cost-effective grid resilience enhancement scheme, in which the expected urgent adaptions of the initial scheme subject to weather state transition is included in the overall cost. The proposed model and techniques are validated using the IEEE 39-bus New-England test system and realistic meteorological data. [ABSTRACT FROM AUTHOR]

In this article, we presented a bio-impedance analysis method (BIAM) based on the human hand anatomy and propose a feasible and flexible BIAM system to obtain the bio-impedance signals for different gestures to achieve a high classification accuracy in hand gesture recognition with flexibility in electrode arrangement and fewer electrodes. To verify the proposed method, 11 gestures, including two sets: hand gestures and pinch gestures, were selected for the experiment. Based on the functional structure of the human hand, we identified appropriate electrode positions and placed five electrodes on the hand surface for bio-impedance signal measurement. Compared with the electrical impedance tomography (EIT) method, which uses a band with the same number of electrodes wrapped around the wrist, the proposed method achieved 98.7% recognition accuracies on the hand gesture set and 97.8% on the pinch gesture set, while the EIT achieved only 97.1% and 86.3%, respectively. In particular, the proposed method demonstrated the advantage of distinguishing gestures with similar muscle contractions. [ABSTRACT FROM AUTHOR]

This article presents a loosely coupled visual/inertial hybrid system for relative attitude determination on a moving platform. The inertial measurements are accomplished by two gyroscopes, of which the slave gyroscope removes the redundant motion of the platform from the total motion sensed by the master gyroscope. To correct integration drift error and compensate for the gyroscope biases, we fuse the inertial and visual measurements fused together through the error-state Kalman filter (ESKF). Furthermore, an improved ESKF is proposed by utilizing the variational Bayesian (VB) inference to adaptively estimate the process and measurement noise covariance. To realize direct estimation of the process noise covariance, a latent variable is introduced and inferred together with the noise covariances and error-state. The practical experimental setup is built up and experiments of two different motion trajectories are conducted. The ESKF, the cubature Kalman filter (CKF), the extended Kalman filter (EKF), the proposed filter, and the other two adaptive filters are implemented for comparison to verify the effectiveness of the proposed filter. The results demonstrate the proposed filter show superiority in reducing both the root mean square error (RMSE) and maximum error (ME) and improving the filter consistency. [ABSTRACT FROM AUTHOR]

Multiple ship tracking (MST) is an important task in marine surveillance and ship situational awareness systems. Considerable work has been conducted on multiple object tracking in recent years, but it has focused primarily on pedestrians and automobiles, leaving a gap in studies on MST due to the particularities of complex marine scenes, such as ship scale variations, the long-tailed distribution of ships, and long-term occlusions caused by ship movements. In this article, we present a robust deep affinity network (RoDAN) for MST. To overcome the above difficulties in MST, we start with the basic deep affinity network (DAN) and improve it in three aspects: scale, region, and motion. For the scale dimension, we integrate an atrous spatial pyramid pooling (ASPP) module to improve the modeling ability for multiscale ships. For the region dimension, we propose the joint global region modeling (JGRM) module, which further strengthens the modeling ability of DAN and exploit it to overcome the long-tailed distribution property of ships. For the motion dimension, we propose the motion-matching optimization (MMO) module to fine-tune the tracking results and make our tracker more robust, less reliant on the front-end detector, and ameliorate long-term occlusions. The experimental results demonstrate that our MST method outperforms the state-of-the-art methods. In particular, it reduces the number of ID switches (IDSs) and trajectory fragmentations (FMs), achieving holistically preferable performance. Meanwhile, our method achieves a comparable speed. [ABSTRACT FROM AUTHOR]

Multipass cell (MPC), with a long effective optical path length (OPL), is an effective method to improve measurement sensitivity in laser absorption spectroscopy for trace gas sensing. We present a space equation (SE) method to design MPC, which can solve inaccuracy caused by using paraxial-matrix (PM) method. The location and dimension of light spot pattern can be calculated accurately and intuitively through SE method. Meanwhile, actual effective OPL can be accurately calculated, SE method can reduce the interference effect, the direction of each ray can be obtained accurately, and, hence, designing the angle of outgoing ray is simple. By adjusting the mirror curvature, mirror space, incident angle, and incident position, we realized four dense spot patterns with different multiple reflections in MATLAB simulation. We calculated the shape changes of light spot after multiple reflection and its influence on the design of outgoing ray hole size accurately. We also considered the evolution of dense pattern and found a method easy to obtain multiple ring pattern. We verified the validity of the proposed model and built a compact MPC with seven rings dense spot pattern. Such MPC offers an effective OPL of 36.48 m with 198 times of reflection. We verified the tunable diode laser absorption spectroscopy (TDLAS) technology to measure 610-ppmv standard methane. In contrast to PM method, the improvement of the signal-to-noise ratio (SNR) of 1.36 times is achieved when the volume of MPC is reduced by 3 times. In addition, the SNR can be improved to 6.6 times, while the volume of MPC is the same. Results validated the ability of the SE method to design MPC. [ABSTRACT FROM AUTHOR]

The droop control is an advantageous approach for stand-alone supply systems consisting of multiple batteries, allowing among various inverters without intercommunication. The droop coefficients of batteries always vary with their state-of-charge (SoC) and charge/discharge mode, resulting in small-signal instability. Nevertheless, the existing impedance-based approaches can only assess the droop coefficients stability point, but not the stability region. Therefore, this article proposes a droop coefficients stability region analysis approach. First, the charge/discharge SoC-based droop controlled battery, the P&Q controlled distributed generator and the constant power load are separately discussed. Meanwhile, the state matrix and return-ratio matrix are established, respectively. Furthermore, the novel forbidden region criterion based on the return-ratio matrix is constructed, which reduces conservatism compared with norm-based impedance criteria and partial forbidden region criteria. Such a forbidden region criterion is first switched to the Hurwitz identification problem regarding the equivalent return-ratio matrix. Combined the state matrix and the equivalent return-ratio matrix, the generalized incidence matrix is constructed to simultaneously identify subsystem stability and interactive stability. Based on the generalized incidence matrix, an adaptive step search strategy is proposed to obtain the droop coefficients coordinated stability region. Finally, the simulation and experimental results illustrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

In this article, a novel bidirectional dc–dc converter (BDC) consisting of an active switched-inductor (A-SL) cell, a zero current ripple cell and an auxiliary capacitor cell is proposed for the battery energy storage system. The proposed BDC integrates with the advantages of high voltage conversion ratio, low power switch voltage stresses, zero ripple current on the low voltage side (LVS), and constant potential difference between the grounds of LVS and high voltage side. Thanks to the use of synchronous rectification technology, the efficiency of the proposed BDC is increased. The operating principles, the characteristics analysis and the parameter design of the proposed BDC are given. In addition, to show the merits of the proposed BDC, a comparison study involves the proposed converter and other BDCs is demonstrated. Finally, some experimental results obtained using a 1000 W prototype are presented to valid the effectiveness of the proposed BDC. 96.43% and 96.67% maximum conversion efficiencies can be achieved in the step-up and step-down modes, respectively. [ABSTRACT FROM AUTHOR]

In dc microgrid, energy storage system (ESS) plays a crucial role to provide short-or-long term and high-quality electric energy. The different control strategies for bidirectional dc-dc converter (BDC) of ESS in grid-tied and islanded modes pose challenges to the coordination control of the dc microgrid. This paper proposes an autonomous control scheme for the BDC in dc microgrid. The proposed control scheme is based on $V^2-P$ droop control and unifies bus voltage regulation and power regulation in a single control structure. Thus, global smooth transition between various operation modes can be achieved without any control strategy changes, which avoids various mode switch detection mechanisms and improves system stability. Furthermore, the proposed control scheme is fully decentralized which reduces the reliance on communication, and enhances the reliability of the microgrid. On the other hand, $V^2-P$ droop approach eliminates the negative effect of widespread constant power loads (CPLs) in dc microgrid. The stability of the proposed control method is illustrated, and the design guideline of some critical control gains is addressed. Finally, the effectiveness of the proposed control scheme is validated by the real-time hardware-in-loop (HIL) platform. [ABSTRACT FROM AUTHOR]

Seeking consistent point-to-point correspondences between 3D rigid data (point clouds, meshes, or depth maps) is a fundamental problem in 3D computer vision. While a number of correspondence selection methods have been proposed in recent years, their advantages and shortcomings remain unclear regarding different applications and perturbations. To fill this gap, this paper gives a comprehensive evaluation of nine state-of-the-art 3D correspondence grouping methods. A good correspondence grouping algorithm is expected to retrieve as many as inliers from initial feature matches, giving a rise in both precision and recall as well as facilitating accurate transformation estimation. Toward this rule, we deploy experiments on three benchmarks with different application contexts, including shape retrieval, 3D object recognition, and point cloud registration. We also investigate various perturbations such as noise, point density variation, clutter, occlusion, partial overlap, different scales of initial correspondences, and different combinations of keypoint detectors and descriptors. The rich variety of application scenarios and nuisances result in different spatial distributions and inlier ratios of initial feature correspondences, thus enabling a thorough evaluation. Based on the outcomes, we give a summary of the traits, merits, and demerits of evaluated approaches and indicate some potential future research directions. [ABSTRACT FROM AUTHOR]

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics. [ABSTRACT FROM AUTHOR]

Accurate equivalent model can be efficient to analyze the dynamic properties of active distribution network (ADN) as well as assess their impacts on stabilities of interconnected power system. However, due to the stochastic nature of renewable resources and time-varying configurations of load conditions, traditional ADN equivalent model may not be robust enough to different operation conditions. To overcome the limitations, this article proposed a robustness-improved method for dynamic equivalent modeling of ADN. To sketch out the most representative operation conditions of ADN, two-step clustering method with fisher discriminant analysis are used for grouping of operation conditions featured characteristic data sets. With the key parameter based identification technique applied, the multiple solution issue in parameter identification process could be effectively avoided. To further enhance the robustness of equivalent model, long short-term memory neural network is adopted to generalize the identified parameters. The performance of proposed modeling method is comprehensively evaluated by an actual ADN based verification cases. [ABSTRACT FROM AUTHOR]

This paper presents the surface modification and charge transportation mechanism of thin polyimide films. Here, we demonstrate that the surface modification of these films, by using the fluorine gas-phase, improves the charges mitigation and, thereafter, suppress the space charges. The experimental results indicate that the thermal step current for fluorine coated films reduce significantly in comparison to pure polyimide films. The thin layer of fluorine coating helps to release the surface charge, reduce the charge injection from the electrode, provide hindrance for charges to penetrate into the bulk of the films and the higher degree of charge disappearing helps to reduce the space charge growth. [ABSTRACT FROM AUTHOR]

The high penetration of renewable energy sources always leads to the fluctuation of the droop coefficients which are designed in inverse proportion to their rated capacity, and power electronics devices are prone to static instability. Although the impedance-based approaches have been widely studied to deal with this problem, the stability-oriented droop coefficients region identification due to the fluctuation of renewable energy sources is not provided. Thus, this article proposes an impedance-based approach to assess the droop coefficients stability region in the power system consisting of numerous distributed generators (DGs). First, the modified phase margin and opposing argument (MPMOA) forbidden criterion is constituted to acquire the complementary space of the droop coefficients stability region. The MPMOA forbidden criterion is first transformed into the condition that the generalized return-ratio matrix is Hurwitz through mirror, rotation, and translation mapping. In comparison with the previous simplified stability criteria, the conservatism of the proposed criterion is reduced a lot. Thereafter, the droop coefficients stability region is directly calculated by the generalized return-ratio matrix and guardian map theory. Eventually, the simulation and experimental results are provided to validate the conservatism and effectiveness of the impedance-based stability region identification approach. Therein, the simulation results illustrate that the proposed droop coefficients stability operation criterion has lower conservatism than these of the previous simplified stability criteria. Furthermore, the simulation and experimental results illustrate that the proposed stability-oriented droop coefficients region identification approach can provide an effective parameter stability region. [ABSTRACT FROM AUTHOR]

A correlation-based digital background calibration technique to correct the capacitor mismatch error in successive approximation register (SAR) analogue-to-digital converters (ADCs) is proposed. To facilitate the calibration technique, an energy-efficient detect-and-skip (DAS) algorithm is exploited to perform the pseudorandom noise (PN) injection without adding extra hardware overhead. By randomly injecting the error weights and correlating the ADC output with the PN sequence in the digital backend, the real bit weights are identified. In addition, only the error weights are injected into the residue. Therefore, the residue increment caused by the injection is negligible, and the redundancy overhead is minimized. The proposed calibration technique is verified in a 12-bit SAR ADC. Behavioral simulations show that, with calibration, the signal-to-noise and distortion ratio (SNDR) and the spurious-free dynamic range (SFDR) are increased from 55.6 to 71.2 dB and 64.2 to 90.3 dB, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a novel and applicable approach to calculate the reactive power loss of converter transformer under dc bias. By using this proposed method, only general information of the converter transformer and the network is needed for the calculation within a certain dc current range. Specifically, a three-dimensional model of ± 800 kV converter transformer model for simulation test is established by the finite element method. Through simulation test, the electromagnetic characteristics and reactive loss characteristics of converter transformer under dc bias can be obtained, and the relationship between the bias current and the reactive loss of ± 800 kV converter transformer is quantified. Simulation results validate the feasibility of proposed algorithm. [ABSTRACT FROM AUTHOR]

A novel single-phase pulsewidth modulation (PWM) direct ac–ac converter based on two-level nondifferential dual-buck ac chopper legs with inverting and noninverting operations is first proposed in this article. It has the ability to resolve both voltage sag and swell problems at the same time when used as distributed flexible voltage conditioner. Compared to the traditional ac–ac converter, it has much enhanced system reliability thanks to no shoot-through problems even when all switches of each ac chopper legs are turned on, and therefore, the PWM dead time is not needed leading to improve the utilization of the duty cycles. Only half of the switches in the proposed converter is switched at high frequency during a switching period at most, which significantly reduces the total switching loss. In particularly, the converter has two greatest advantages that it retains the common sharing ground of the input and output and has the same buck/boost operation process for noninverting and inverting modes. In order to fully testify the performance of the proposed converter, a 500-W experimental prototype is built and tested at different conditions. [ABSTRACT FROM AUTHOR]

Due to the influences of imaging conditions, spectral imagery can be coarse and contain a large number of mixed pixels. These mixed pixels can lead to inaccuracies in the land-cover class (LC) mapping. Super-resolution mapping (SRM) can be used to analyze such mixed pixels and obtain the LC mapping information at the subpixel level. However, traditional SRM methods mostly rely on spatial correlation based on linear distance, which ignores the influences of nonlinear imaging conditions. In addition, spectral unmixing errors affect the accuracy of utilized spectral properties. In order to overcome the influence of linear and nonlinear imaging conditions and utilize more accurate spectral properties, the SRM based on spatial–spectral correlation (SSC) is proposed in this work. Spatial correlation is obtained using the mixed spatial attraction model (MSAM) based on the linear Euclidean distance. Besides, a spectral correlation that utilizes spectral properties based on the nonlinear Kullback–Leibler distance (KLD) is proposed. Spatial and spectral correlations are combined to reduce the influences of linear and nonlinear imaging conditions, which results in an improved mapping result. The utilized spectral properties are extracted directly by spectral imagery, thus avoiding the spectral unmixing errors. Experimental results on the three spectral images show that the proposed SSC yields better mapping results than state-of-the-art methods. [ABSTRACT FROM AUTHOR]

In the face of extreme events, e.g., hurricanes, the transmission systems, especially the transmission lines, are affected across time and space. To mitigate these impacts on the day-ahead market from a probabilistic perspective, a resilient unit commitment (UC) problem is formulated as a two-stage distributionally robust and robust optimization (DR&RO) problem. In the first stage, the commitment, energy, and reserves of generators are pre-scheduled to minimize the operational cost, responding to the worst load forecasting and line failure scenario in the operating day. The operating status of transmission lines are depicted by a novel uncertainty set with a distributionally chance constraint considering the repair of failed lines. This chance constraint is reformulated to its deterministic equivalence. Using both load shedding and generation curtailment, recourse problems are formulated in the second stage considering the time-varying operating status of transmission lines. The formulated DR&RO problem is solved using a hybrid Benders decomposition and column-and-constraint generation scheme. Simulations are conducted on the modified IEEE reliability test system (RTS) and two-area IEEE RTS-96 under hurricanes. Results verify the effectiveness of the proposed method, in comparison with prevalent two-stage stochastic and robust optimization methods. [ABSTRACT FROM AUTHOR]

In this article, a wide-angle space filter (SF) with absorption function based on 1-D magnetized plasma photonic crystals is proposed, which has a good rectangularity of greater than 0.95. Since the angle range (AR) of the TM polarization is larger than that of the TE, they can be split at specific angles, possessing the potential to be a polarization splitter (PS). The effects of plasma density ne and magnetic induction intensity B on the AR of the SF are discussed based on the tunability of the magnetized plasma. The increase in ne shrinks the AR of TE and TM polarizations at the same time, but that of the TM decreases faster, resulting in a narrower AR of PS. If B is augmented, only the AR of the TM wave will be reduced, which will also lead to the reduction in the AR of the PS. In addition, the impedance matching theory is used to explain the cause of the generation of SF. Compared with the traditional angular filter only with the transmission function, the SF we proposed with the absorption function is relatively rare, and it may have important application possibilities in the antenna field. [ABSTRACT FROM AUTHOR]

The hybrid fuel cell/supercapacitor (FC/SC) system is a promising onboard power supply system for more electric aircraft (MEA), where system stability is a critical issue due to the high penetration of constant power loads (CPLs) in MEA. This article proposes a composite finite-time controller for decentralized power sharing and stabilization of the hybrid FC/SC system with CPLs. It consists of an integral droop (ID) + finite-time controller for the SC converter and a proportional droop (PD) + finite-time controller for the FC converter. First, the coordination of PD and ID achieves decentralized power sharing between FC and SC such that SC only compensates fast fluctuations and FC provides smooth power at the steady state. Then, a finite-time observer is designed to provide feedforward compensation for the disturbances and enables accurate tracking with fast dynamics. Finally, a composite finite-time controller is constructed following a nonrecursive synthesis procedure with a rigorous large signal stability analysis. The proposed controller guarantees finite-time convergence even under large signal variations and can be easily implemented with a practical gain tuning procedure. Simulations and experiments are conducted to verify the proposed technique. [ABSTRACT FROM AUTHOR]

Terahertz (THz) transmission technologies constitute a promising candidate for supporting ultra-broadband short-range next generation communications. Hence, we analyse the performance of unmanned aerial vehicle (UAV) in the THz networks. The coverage probability is derived as well as the area spectral efficiency (ASE) and a pair of Line-of-sight (LoS)/non-line-of-sight (NLoS) probability models, namely the macrocell LoS/NLoS probability model and picocell LoS/NLoS probability model are adopted. Furthermore, the lower-bound of the network performance are derived via homogeneous Poisson point process (HPPP) analysis, as well as the upper-bound. The simulation results match the analytical results well, which show that the coverage probability of the network first increases upon increasing the THz UAV BS density, and then decreases beyond the maximum. Given the severe path loss experienced by THz signals, a higher UAV density is required for a certain coverage probability than at lower carrier frequencies. [ABSTRACT FROM AUTHOR]

This article investigates the current sharing and voltage regulation problem of a DC microgrid by a distributed supervisory control method. In the existing secondary control methods, current sharing feedback control loop and voltage regulation feedback control loop are linearly integrated to shift the droop equation. However, the linear combination of two loops may easily offset the control effect of each other, as the control dimension is less than the objective dimension. This fact motivates us to design a distributed supervisory secondary control (DSSC) scheme to achieve a more flexible configuring of each converter. The DSSC consists of two candidate controllers and a supervisor. The two controllers refer to current sharing configuring and DC bus voltage regulation respectively, and the supervisor is to orchestrate the switching between candidate controllers based on the system environment of DC microgrid. Under the proposed DSSC, each converter selects to configure the current sharing or regulates the DC bus voltage according to its need. Theoretical stability analysis is rigorously conducted. Finally, both simulation and experiment results are presented to demonstrate the effectiveness of DSSC. [ABSTRACT FROM AUTHOR]

In this article, penalty factor threshold and time step bound in discontinuous Galerkin time method based on vector wave equation (DGTD-WE) method are well estimated. Based on the semidiscrete form of the DGTD-WE method, properties of the system matrices are studied and the stability condition related to the penalty factor is derived. By decomposing the global system matrices of the DGTD-WE method into the local ones and developing an efficient iteration procedure, the lower bound threshold of the penalty factor is well estimated to guarantee the positive semidefinite property of the global system matrices. With the calculated penalty factor, the maximum time step is analytically determined by approximating spectral radius of the local system matrix. Both the penalty factor bound and the maximal time step are computed element-wise instead of a global system matrix operation, and thus, the proposed method can be efficiently applied into the large-scale meshes with different types of the basis functions and boundary conditions. Numerical examples are presented to demonstrate the validity and good performance of the proposed methods. [ABSTRACT FROM AUTHOR]

The linear transformer driver (LTD) is widely used in the field of pulsed power, especially for generating intense electron beam. The design of an LTD with quasi-rectangular voltage output is presented. The Blumlein pulse-forming network (BPFN) with L-type configuration replaced traditional capacitors to produce a quasi-rectangular voltage waveform; the adding voltage is achieved through a linear transformer (LT) that couples the voltage from multiple primaries to a single secondary. Since the output waveforms of BPFN and LT determine the output waveform of LTD, the effects of the configuration of the BPFN and the parameters of the LT on the output voltage waveform are studied. At the same time, according to the results, the output voltage waveform is improved by optimizing the inductance of the BPFN and selecting the appropriate parameters of the LT. After that, an electron beam generator based on six-stage LTD cavities is developed, and a well-shaped quasi-rectangular waveform is obtained at a planar diode load. [ABSTRACT FROM AUTHOR]

The problems of feedback dissipativity and stabilization of switched positive systems are investigated via the co-design of both the combined switching strategy and the state feedback control. In particular, the proposed technique guarantees a minimal dwell time between the adjacent switching even when all subsystems are not dissipative. The time-varying multiple linear co-positive storage functions are first adopted and sufficient conditions of rendering the considered system to be dissipative are also established. Moreover, stability is obtained when the multiple linear supply rates are non-positive, and asymptotic stability is further achieved under certain redesigned controller and the detectivity property of all subsystems. Finally, simulation studies verify that the proposed control scheme is valid. [ABSTRACT FROM AUTHOR]

The interleave boost converter (IBC) has been used as the interface converter between the low-voltage electrical energy sources, such as lithium-ion battery banks, solar panels, and fuel cells, and the dc bus of dc microgrids. With increasing penetration of tightly regulated power electronic loads, which behave as constant power loads, the stability of microgrid dc-bus voltage that is fed by the IBC is threatened by the loads’ negative incremental impedance feature. To ensure the stability of the bus voltage, this article proposes a nonlinear disturbance observer (NDO)-based sliding-mode control algorithm. The proposed algorithm has excellent robustness, low computational burden, and no extra hardware cost. A generalized average state-space model is proposed to facilitate the control design. In addition, an NDO is employed to estimate the output power of IBC rapidly and accurately. To verify the effectiveness of the proposed algorithm, simulation and experimental results are presented. [ABSTRACT FROM AUTHOR]

To be able to describe more complex fuzzy uncertainty information effectively, the concept of q-rung orthopair fuzzy sets (q -ROFSs) was first proposed by Yager. The q-ROFSs can dynamically adjust the range of indication of decision information by changing a parameter q based on the different hesitation degree from the decision-makers, where q ≥ 1, so they outperform the traditional intuitionistic fuzzy sets and Pythagorean fuzzy sets. In real decision-making problems, there is often an interaction phenomenon between attributes. For aggregating these complex fuzzy information, the Maclaurin symmetric mean (MSM) operator is more superior by considering interrelationships among attributes. In addition, the power average (PA) operator can reduce the effects of extreme evaluating data from some experts with prejudice. In this paper, we introduce the PA operator and the MSM operator based on q-rung orthopair fuzzy numbers (q-ROFNs). Then, we put forward the q-rung orthopair fuzzy power MSM (q-ROFPMSM) operator and the q-rung orthopair fuzzy power weighed MSM (q-ROFPWMSM) operator of q-ROFNs and present some of their properties. Finally, we present a novel multiple-attribute group decision-making (MAGDM) method based on the q-ROFPWA and the q-ROFPWMSM operators. The experimental results show that the novel MAGDM method outperforms the existing MAGDM methods for dealing with MAGDM problems. [ABSTRACT FROM AUTHOR]

In hybrid ac/dc microgrids, a bus converter system involving the use of a high-frequency dc transformer (DCT) plays a vital role to transmit power between dc and ac subgrids. However, the multiple control tasks and power transfer mode switching of bidirectional dc/ac converter create big challenges for the systematic controller design of DCT. To address this concern, a comprehensive coordinated frequency control (CCFC) is put forward in this article for the symmetrical CLLC-DCT to adapt the complicated hybrid ac/dc microgrid operation. By establishing the mathematical model on the voltage conversion gain (VCG) and active power transmission ratio (APTR), four operation modes are proposed based on power transfer conditions. The corresponding operators are derived based on the circuit feature to avoid the redundant manipulations. Frequency threshold is determined to avoid both overvoltage and low-voltage issues based on the known allowable range of VCG. The proposed CCFC scheme is detailedly realized by combining both the open-loop and closed-loop schemes to facilitate the design procedure. The prototype DCT demonstrates the proposed CCFC scheme can achieve a peak efficiency of 98.2% and satisfied performances on both VCG and APTR. [ABSTRACT FROM AUTHOR]

The multilevel boost converter (MBC) has been widely adopted in the dc microgrid systems due to its high voltage gain and simple structure. In recent years, the power electronic loads, which usually behave as constant power loads (CPLs), are penetrating in microgrids. The incremental negative impedance of CPLs degrades the stability of microgrid systems. To ensure effective power flow and guarantee system stability, eliminating the undesired effects of CPLs is a necessity. In this article, a novel assorted nonlinear stabilizer, which is integrated with an extended nonlinear disturbance observer (NDO) and an adaptive backstepping controller, is proposed for stabilizing the MBC-fed microgrid system with CPLs. First, the reduced-order MBC model is transformed into the Brunovsky's canonical form using the exact feedback linearization technique. Second, owing to the NDO, fast system dynamic responses are achieved. Using the estimations of NDO, an adaptive backstepping controller is developed to strictly guarantee the microgrid bus voltage stability in the sense of a large signal. Simulation and experiment results are presented to verify the effectiveness and feasibility of the proposed stabilizer. [ABSTRACT FROM AUTHOR]

Due to the geographic features and fruitful water resources, the distributed generation of Small Hydropower Generator Stack (SHGS) are vigorously developed in southwest China. As the more concerns about security and stability of power grids with the large amount of SHGS feed into the grid, the accurate models are critical to analyze those potential impacts. With the development of online monitoring technologies in smart grid, PMU based dynamic equivalent modeling for SHGS drawn many attentions. However, field applications reveal that traditional SHGS equivalent model may not be robustness enough to adapt the different disturbances. To overcome this deficiency, method to improve the robustness of SHGS dynamic equivalent modeling is proposed. In the paper, a coherency identification based multi-machine modeling method is introduced to enhance the accuracy of equivalent model. Sensitivity and correlation analysis based key parameters selection scheme is implemented to select the critical parameters, meanwhile avoiding the multiple solutions problem. Moreover, multi-objective optimization algorithm is designed for key parameters identification of the equivalent model. Additionally, hybrid dynamic simulation has adopted into the equivalent process to improve the computational efficiency. The effectiveness of the proposed method has been validated with an actual case from China Southwest grids. [ABSTRACT FROM AUTHOR]

Energy systems, e.g., power systems and natural gas systems, are increasingly interdependent, which brings benefits under normal conditions and introduces risks under extreme weather events, e.g., hurricanes. A comprehensive method is proposed to reflect the spatial-temporal hurricane impacts on interdependent power and natural gas systems. Mechanism analysis based and data based component failure models are used to map hurricane effects on exposed component failure probabilities based on which system states are generated using Monte Carlo simulation. For system states with failed components, the integrated energy flow is developed to determine minimum joint load shedding considering inherent properties of each system and characteristics of interdependence coordinately. Operational metrics and infrastructure metrics are employed to quantify the service adequacy and network resistance of the interdependent energy systems, respectively. The modified IEEE 24-reliability test system and the 12-node natural gas system under a simulated hurricane are used to validate the effectiveness of the proposed method. Simulation results show the method can identify weak parts and quantify the performance of interdependent power and natural gas systems under hurricanes to provide information for effective and coordinative preparedness. [ABSTRACT FROM AUTHOR]

5G physical random access channel (PRACH) will use new preamble formats based on short Zadoff-Chu (ZC) sequences to enable new use cases. These formats result however in a PRACH capacity shortfall where the number of sequences available for network planning may just be enough to support few cells without sequence reuse. To overcome this issue, two enlarged constant-amplitude sequence constructions that include ZC sequences, and thus being backward-compatible with the current 5G PRACH design, are presented. Both constructions feature a desirable subset structure from which follows a natural cellular network sequence allocation where each cell-specific sequence subset has a controllable low-correlation zone similar to ZC sequences, and thus enables a similar detection performance inside a cell. The two extensions are meanwhile structurally different and offer different network allocations, as for one, new sequences are distributed among all cells, while for the other, they are in separated cells. With both proposed supersets and the new 5G PRACH formats, hundred-times more cells without sequence reuse can be supported compared to ZC sequences for a moderately larger inter-cell correlation than intra-cell correlation. Simulation results confirm that using the proposed supersets instead of reusing sequences in the network can significantly improve detection performance. [ABSTRACT FROM AUTHOR]

The Newton-Raphson (NR) method is a powerful tool in nonlinear equations. However, a well-known disadvantage of this method is that the initial iteration value must be chosen sufficiently close to a true solution in order to guarantee its convergence. The Kantorovich theorem is virtually the only known sufficiency condition for convergence of NR method, and gives very conservative bounds. This letter analyzes the convergence of the NR method in feasible power-flow for direct-current (DC) network, and an explicit convergence condition is obtained. Comparing with the existing results, the proposed convergence condition is more concise and less conservative. Moreover, according to the proposed condition, one can easily set the initial iteration value to guarantee the convergence of the NR method. Case studies verify the correctness of the presented analysis. [ABSTRACT FROM AUTHOR]

This article presents a polarization-reconfigurable wideband high-gain antenna based on liquid metal tuning. The antenna consists of a center-feed circular patch antenna with a C-shaped slot as the driven element. To broaden the bandwidth and improve the realized gain, a parasitic circular disc and 13 circular directors are employed. The direction of polarization can be tuned by changing the location of the liquid metal alloy contained in the tube. The relationship between the dimensions and the maximally achievable gain has been analyzed. The antenna has exhibited a −10 dB impedance bandwidth from 4.3 to 5.3 GHz (a fractional bandwidth of 21.2%). A 14.4 dBi gain is achieved at 5.3 GHz. The simulation and measurement results show good agreement with each other. [ABSTRACT FROM AUTHOR]

The penetration level of photovoltaic (PV) keeps increasing in modern distribution networks, which leads to various severe voltage limits violation problems. This article aims to aggregate and utilize the PV inverters for voltage regulation by a fully distributed two-level Volt/VAr control (VVC) scheme. In the lower-level VVC (real-time scale), the rooftop PV inverters are aggregated via consensus algorithms and then governed by droop controllers in medium-voltage networks. The droop controller adjusts the reactive power output of each PV aggregator in real-time from its dispatched value depending on the bus voltage variations. In the upper-level VVC (15-min timescale), the reactive power of PV aggregators is dispatched for power loss minimization, where control signals are set as base values for PV aggregators. This problem is formulated as second-order cone programming and solved by the alternating direction method of multipliers. The simulation results demonstrate the effectiveness of the proposed method in both short-term and long-term scenarios with different system scales. [ABSTRACT FROM AUTHOR]

Root-cause identification of faults in a distribution network is conducive to fault inspection as well as reducing injuries and damage. In this article, a novel method based on correlation dimension and average resistance is proposed to identify the root cause of permanent single line-to-ground fault in urban small current grounding systems. First, the reliability of the grounding fault and whether the fault occurred on a dry or wet surface can be identified by the correlation dimension. Then, the average resistance of the fault is used to classify the specific root cause, including wet land, dry land, wet concrete, dry concrete, and water. Experimental results show that this method can realize the classification of root causes and is robust to sampling rate, harmonics, and line impedance. [ABSTRACT FROM AUTHOR]