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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]

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

In this paper, a modified one-cycle-control (OCC) scheme is proposed for regulating three-phase pulsewidth-modulated (PWM) rectifiers operating under unbalanced and distorted supply conditions. Mathematical analysis is first presented to show the dominant harmonic-voltage components appearing across the dc link of the PWM rectifier when it is controlled by an existing OCC scheme. Such dc-link harmonics when produced may aggravate harmonic currents flowing into the input terminals of the three-phase rectifier. To address these interrelated unbalance and harmonic concerns, the concept of reconstructing the appropriate reference signals in the OCC control core is introduced, before modifications are recommended for altering the key control equation. The improved OCC controller eventually developed is proven to exhibit minimal input-current distortion and a smoother dc-link voltage without using large capacitance. Other advantages exhibited by the traditional OCC scheme, such as no phase-locked loop, no frame transformation, and constant switching frequency, are also retained by the improved controller, whose theoretical findings are fully verified by experimental results obtained from a 1.2-kW three-phase PWM rectifier built in the laboratory. [ABSTRACT FROM PUBLISHER]

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]

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]

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]

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]

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]

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]

In this article, a fixed and smooth-switch-sequence modulation strategy is proposed to equalize the dc-link voltages among the cascaded rectifier. The structure of a single-phase three-level neutral-point-clamped cascaded rectifier is studied. Three core steps of this strategy are analyzed respectively, including carrier wave disposition calculation, fixed pulse generator, and voltage rank function. In addition, a criterion of the degree of the load unbalance is established in order to measure the voltage balancing boundary of the presented approach. The superiorities of the proposed control strategy embody the smooth switch sequence, the strong voltage balance capability, as well as the fixed switch sequence. Owe to these advantages, the cascaded rectifier could effectively reduce the switching frequency, achieve the goals of voltage balancing, and reduce the calculation burden of the controller when the number of cascaded modules is large. The effectiveness of the proposed strategy is verified by simulation and experiment results. [ABSTRACT FROM AUTHOR]

Different from single-bus dc microgrids (MGs), a multibus MG normally bridge multiple dc buses via a complex line impedance network. This intricate configuration together with tickling constant power loads may cause severe stability issues. For system stability improvement, conventional ways include designing small-signal stabilizers in a decentralized way and constructing large-signal stabilizers by solving Lyapunov equation in a central controller. Note that centralized methods may be vulnerable to single point of failures, while decentralized patterns are commended to reinforce MG reliability and scalability. To implement large-signal stabilization in decentralized mechanisms, which has rarely been investigated in the existing literature, a novel uniform nonsmooth control scheme (UNCS) is proposed, in this article. UNCS has the following fourfold advantages. First, UNCS helps to express commonly used dc/dc converters, boost, buck, and buck–boost, into a uniform model. The model substantially facilitates a uniform stabilizer design that accommodates the preceding three converters despite their differences in topology. Second, UNCS assigns each converter based distributed generator (DG) with a composite large-signal stabilizer comprising a nonsmooth observer and a nonsmooth controller. The controller neutralizes the disturbances estimated by the observer, and it also rigorously stabilizes the internal states in DG uniform model. Third, the stabilizer also provides an additional control degree of freedom, a finetuning factor, to refine DG dynamic responses to meet high requirements for voltage and power qualities. Fourth, when numerous DGs with the proposed stabilizer congregate to build a multibus MG, the overall system large-signal stabilization is autonomously attained without communication links, which is mathematically justified by Lyapunov stability analyses. As such, DGs are allowed to randomly plug in/out without worrying about impairing system level stability. Simulations help to properly select relevant key parameters. Experiments show that UNCS outperforms the standardly designed PI controls, and it will stabilize the multibus MG in a large operation range. [ABSTRACT FROM AUTHOR]

In this article, a graphic processing unit (GPU)-based acceleration implementation of a hybrid discontinuous Galerkin time domain method based on Maxwell’s equations and Helmholtz vector wave equation (HDGTD) has been developed. The computational domain is discretized by tetrahedrons and the resultant meshes are categorized into two regions solved by Maxwell’s equations and Helmholtz vector wave equation, respectively. The hierarchical vector basis functions are used to expand the unknowns in the HDGTD method, and a universal matrix technique is proposed to decompose the geometry-dependent matrices in each tetrahedron into the summation of universal matrices defined in barycentric coordinates, thus giving rise to a great decrease of the memory usage. A local time stepping (LTS) method based on a simple interpolation is introduced in the proposed HDGTD method to achieve highly efficient solution of multiscale problems. Two kinds of compute unified device architecture (CUDA)-based mapping techniques, i.e., 1-D and 2-D blocks, are implemented to achieve a tradeoff between the parallel speedup and the memory usage. With the 1-D block mapping, over 590 times speedup can be achieved, and in the case of the 2-D block mapping, over 150 times acceleration and 13 times memory reduction are obtained. Some practical complex examples are given to demonstrate a good performance of the proposed parallel method. [ABSTRACT FROM AUTHOR]

Deep learning (DL) architecture, which exploits multiple hidden layers to learn hierarchical representations automatically from massive input data, presents a promising tool for characterizing fault conditions. This paper proposes a DL-based multi-signal fault diagnosis method that leverages the powerful feature learning ability of a convolutional neural network (CNN) in images. The proposed deep model is able to learn from multiple types of sensor signals simultaneously so that it can achieve robust performance and finally realize accurate induction motor fault recognition. First, the acquired sensor signals are converted to time–frequency distribution (TFD) by wavelet transform. Then, a deep CNN is applied to learning discriminative representations from the TFD images. Since then, a fully connected layer in deep architecture gives the prediction of induction motor condition based on learned features. In order to verify the effectiveness of the designed deep model, experiments are carried out on a machine fault simulator where both vibration and current signals are analyzed. Experimental results indicate that the proposed method outperforms traditional fault diagnosis methods, hence, demonstrating effectiveness in induction motor application. Compared with conventional methods that rely on delicate features extracted by experienced experts, the proposed deep model is able to automatically learn and select suitable features that contribute to accurate fault diagnosis. Compared with single-signal input, the multi-signal model has more accurate and stable performance and overcomes the overfitting problem to some degree. [ABSTRACT FROM AUTHOR]

The utilization of voltage-source converter-based multi-terminal HVDC (VSC-MTDC) technology has been envisaged as a cost-effective solution to transmit bulk renewables from multiple sending ends to receiving-end AC systems. However, with complex configurations and various types of components, the reliability evaluation of a VSC-MTDC system is more complicated than that of a conventional HVDC system. In addition, due to stochastic failures and complex operating modes, the application of a VSC-MTDC system can have a significant influence on the reliability of hybrid AC/DC power systems. This paper presents a new technique for evaluating the nodal reliability of a hybrid AC/DC power system based on the VSC-MTDC system. The proposed technique is incorporated within a multi-state model for the VSC-MTDC system. Then, an optimization model for contingency management of a hybrid AC/DC power system is formulated. The model is established based on the optimal power flow technique for a VSC-MTDC based hybrid AC/DC power system. The objective of the model is to minimize the total system cost when determining the load curtailment and generation re-dispatch results for each contingency state. Nodal reliability indices for hybrid AC/DC power systems are proposed to evaluate the reliability performance of customers at each node. The effectiveness of the proposed methods is validated using a modified IEEE reliability test system. [ABSTRACT FROM AUTHOR]

Power loss and voltage regulation are two fundamental and challenging issues in DC distributed generation system. In this paper, a multi-objective optimization problem is established to achieve trade-offs between reducing power loss and decreasing voltage deterioration by adjusting the weighting coefficients. This problem is proven to be a convex optimization under constant impedance load and constant current load. A distributed algorithm based on random coordinate descent method is developed to solve this problem without acquiring the information of line impedance. Thus, the optimal operating point of the system can be found automatically in real-time, and the deviation of optimal solution caused by the fluctuation of network parameters is avoided. In addition, the complexity of this algorithm is low and its computational requirement is reduced in practice. Finally, case studies are implemented to verify the effectiveness of the proposed algorithm under the scenarios of line parameter fluctuation, load variation, communication line failure, and plug-play capabilities. [ABSTRACT FROM AUTHOR]

SIN is constructed to support real-time data acquisition, massive data transmission and processing, and systematized information services. Different from static networks, both the network topologies and the available network resources are dynamic in SIN. However, the existing networking technologies treat SIN as segmented static networks, and ignore the relationship among different segmented subnetworks, which results in low resource utilization and poor transmission QoS. This article exploits the time-varying graph theory to characterize and allocate the dynamic resources of SIN. We first introduce the overall SIN architecture, and discuss its key technologies in terms of network slicing and dynamic routing. Then, we construct a novel STAG to precisely depict the multi-dimensional time-varying resources and reveal their connection relationships in SIN. With STAG, we propose the dynamic network slicing strategy to build the dedicated network slices for different services, and design the intra-slice routing scheme to guarantee the service transmission QoS. Simulation results demonstrate our methods can complete more space missions with higher resource utilization. Finally, we discuss the promising future of the time-varying graph theory. [ABSTRACT FROM AUTHOR]

This paper proposes a semi-consensus strategy for multi-functional hybrid energy storage systems (HESSs) in DC microgrids. Batteries in a HESS are regulated by conventional V-P droops and supercapacitors (SCs) are with integral droops (ID). Only batteries are assigned with local distributed compensators which exchange information through sparse communication links. Those SCs are exempted from data exchange process, which would save system investment costs. Within the semi-consensus scheme, the most essential function is the cooperation of V-P droop and ID that helps to naturally allocate low frequency components of load power to batteries and high frequency components to SCs, thus prolonging the overall life time of HESS. In addition to the transient power allocation function, there are other three functions endowed by the proposed strategy, which are autonomous DC bus voltage recovery to its nominal level, spontaneous SC state of charge (SOC) restoration, autonomous power sharing and SOC balancing among batteries. It is the simultaneous realization of above four functions with limit communications that makes up the main contributions in this paper. A generic mathematical modeling of HESS with the semi-consensus strategy is established. The model allows for dynamic analyses to theoretically validate the effectiveness of proposed method in both frequency and time domains. In-house experimental results are shown fully consistent with the dynamic analyses and also effectively corroborate the intended HESS multi-functional operations. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) control based on adaptive virtual inertia is proposed to improve dynamic frequency regulation of microgrid. When the system frequency deviates from the nominal steady-state value, the adaptive inertia control can exhibit a large inertia to slow the dynamic process and, thus, improve frequency nadir. And when the system frequency starts to return, a small inertia is shaped to accelerate system dynamics with a quick transient process. As a result, this flexible inertia property combines the merits of large inertia and small inertia, which contributes to the improvement of dynamic frequency response. The stability of the proposed algorithm is proved by Lyapunov stability theory, and the guidelines on the key control parameters are provided. Finally, both hardware-in-the-loop and experimental results demonstrate the effectiveness of the proposed control algorithm. [ABSTRACT FROM AUTHOR]

Electromagnetic pulse (EMP)-induced nonlinear degradation of an L-band gallium arsenide-based low-noise amplifier (LNA) is studied by an injection experiment, failure analysis, and mixed-mode simulation. The experimental results indicate that the real-time response, radio frequency, and direct current characteristic of LNA samples under an EMP show nonlinear and permanent degradation features. In detail, the characteristics first degrade and then recover slightly as the injection power increases. In addition, the nonlinear degradation shows a close dependence on the pulse properties. Failure analysis reveals that the degradation of the LNA is attributed to latent failure defects under and around the gate metal strip of the first-stage high electron mobility transistor, with recovery well interpreted by an EMP-induced “annealing effect.” Simulation results show that if the pulse repetition frequency is smaller than 2.5 kHz, a pulse repetition-induced thermal accumulation effect will not occur and that the pulsewidth τ will mainly determine the actual action duration and the temperature of the “annealing effect.” Based on the above mechanism, the dependence of the pulse properties on nonlinear degradation is well explained. [ABSTRACT FROM AUTHOR]

Effective detection of multifaults in bearings and gears is a challenging issue in rotary machinery health monitoring. As such, a generalized Vold–Kalman filtering (GVKF)-based compound faults diagnosis method is presented in this paper. The technique includes four main steps: 1) a time–frequency ridge is separated from the time–frequency representation (TFR) of the vibration signal using a peak search method; 2) according to the time–frequency ridge, GVKF parameters corresponding to all the fault characteristic frequencies (FCFs) are estimated; 3) the fault feature components are obtained using the generalized demodulation transform (GDT) and the VKF with the GVKF parameters; and 4) the spectra obtained by the fast Fourier transform (FFT) are used to fault detection. The main contributions of the proposed method are as follows: 1) the influence of speed fluctuations and the unrelated harmonic components are removed through the integration of the GDT and the VKF and 2) the tachometerless GVKF parameters are defined and calculated to quantitatively detect different fault types, which avoids missed diagnosis and misdiagnosis. The proposed multifault diagnosis algorithm is verified by both simulation and experiment data. Comparison with other commonly used techniques has shown the advantage of the new method. [ABSTRACT FROM AUTHOR]

Considering the high cost and complexity of multiple-channel blind source separation, single-channel solutions have elicited a significant amount of interest in source separation. Joint maximum likelihood sequence estimation implemented by the Viterbi algorithm (VA) is a seemingly ultimate solution to the problem of data transmission over dispersive channels. However, both joint reduced-state sequence estimation (JRSSE) and joint delayed-decision feedback sequence estimation (JDDFSE) have numerous shortcomings when applied to single-channel blind signal separation. Hence, joint double-direction DDFSE (JDD-DDFSE) is proposed in this article. JDD-DDFSE employs joint VA (JVA) of the low-order channel twice. The estimated symbols from the initial JVA are utilized to equalize the causal and non-causal taps of the channel beyond the trellis state in the second JVA. A method that can derive the log likelihood ratio of the estimated symbols is provided; the complexity of this method is considerably lower than that of soft-output VA or the BCJR algorithm. Hence, the entire computation complexity of the iterative separation structure basically remains unchanged compared with that of the no-iteration structure. For the non-causal channel, the proposed algorithm has the lowest computation complexity and demonstrates a separation performance similar to that of conventional JVA, JRSSE, and JDDFSE algorithms. [ABSTRACT FROM AUTHOR]

With ground-based synthetic aperture radar (GB-SAR) discontinuous observation, the effect of rail error, caused by multiple installations, on imaging is difficult to eliminate with difference correction. In this article, the impact of rail error on each point, which is related to the incident angle of each point, is theoretically analyzed. This is the first attempt to introduce a baseline in the application of GB-SAR microdeformation to build a model that estimates the incident angle of each point in the image. Then, the rail error correction model is established using the incident angle, and the influence of the rail error on the image point is weakened using this model. The experimental results show that the precision of the estimated incidence angle cosine is 0.0132, which is comparable to the same angle cosine value measured by the total station. By applying the correction method in this article, different impacts of each point in the image, caused by rail error from multiple instrument installations, can be weakened to below 0.1 mm, meeting the requirement of GB-SAR microdeformation measurement submillimeter accuracy. [ABSTRACT FROM AUTHOR]

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results. [ABSTRACT FROM AUTHOR]

Memristors with small size, fast speed, low power, CMOS compatibility and nonvolatile modulation of device resistance are promising candidates for the next-generation data storage and in-memory logic computing paradigm. In comparison to the binary logics enabled by memristor devices, multi-valued logics can provide higher computation efficiency with simple operation scheme, reduced circuit complexity, and smaller chip area. In this contribution, we demonstrate that all the 27 univariate ternary logic operations can be realized with a single ZnO three-state resistive switching memristor in at most three steps. The nonvolatile modulation characteristics of the memristor allow the read step to be independent of the logic operation and capacitate logic-in-memory applications. The present methodology could be beneficial for constructing future high-performance computation architectures. [ABSTRACT FROM AUTHOR]

This paper introduces a novel H-bridge structured ac–ac pulsewidth modulation (PWM) converter topology based on two-level non-differential ac chopper legs, which can work at the non-inverting and inverting modes for the utility voltage compensation. Compared to the traditional bipolar-type H-bridge ac–ac converter, no bidirectional switches are switched in a complementary manner to overcome the commutation problem due to the delayed response of electronic circuits and semiconductor switching devices. The detailed PWM control signals and operational principles are presented to regulate the output voltage in a bipolar manner. Especially the main advantage is that the proposed ac–ac converter has the same buck/boost operation process for non-inverting and inverting modes, which ensures the continuously average current supply to the low-voltage output side without a high-value capacitor to support power. Additionally, due to the common sharing ground of the input and output, the feature that output can reverse or maintain phase angle with input is supported well. Then, the detailed analysis, design conditions, and experimental verification based on a 1 kW experimental prototype are presented. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) generators have continuously increased in recent years, whose power is usually controlled through the perturbation and observation (P&O) method. In essence, the P&O method is nonlinear and discontinuous. Hence, the conventional small-signal stability analysis is not suitable anymore when the influence of the P&O-based power control is considered. Focusing on this problem, this paper adopts the nonlinear describing function (DF) method to conduct the accurate stability analysis of PV generators with consideration of P&O-based power control. The detailed procedures about the DF method are introduced, and then the related influence factors like perturbation size, filters, and so on are analyzed quantitatively. Furthermore, the comparison with the conventional stability analysis methods is made, which suggests that the DF method can effectively enhance the accuracy of the stability analysis. All the conclusions are verified by the real-time hardware-in-loop tests. [ABSTRACT FROM AUTHOR]

Recently, hyperspectral image (HSI) classification approaches based on deep learning (DL) models have been proposed and shown promising performance. However, because of very limited available training samples and massive model parameters, DL methods may suffer from overfitting. In this paper, we propose an end-to-end 3-D lightweight convolutional neural network (CNN) (abbreviated as 3-D-LWNet) for limited samples-based HSI classification. Compared with conventional 3-D-CNN models, the proposed 3-D-LWNet has a deeper network structure, less parameters, and lower computation cost, resulting in better classification performance. To further alleviate the small sample problem, we also propose two transfer learning strategies: 1) cross-sensor strategy, in which we pretrain a 3-D model in the source HSI data sets containing a greater number of labeled samples and then transfer it to the target HSI data sets and 2) cross-modal strategy, in which we pretrain a 3-D model in the 2-D RGB image data sets containing a large number of samples and then transfer it to the target HSI data sets. In contrast to previous approaches, we do not impose restrictions over the source data sets, in which they do not have to be collected by the same sensors as the target data sets. Experiments on three public HSI data sets captured by different sensors demonstrate that our model achieves competitive performance for HSI classification compared to several state-of-the-art methods. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) is the main sign of the insulation deterioration; therefore, the online PD localization of the power transformers can provide the equipment degradation characteristics and has great value for the power grid. Because of some unique merits, acoustic PD detection is one of the best options for such applications. However, when discharges happen inside the windings, precise PD localization by acoustic method becomes very challenging. As the acoustic PD signal can be reverberated and distorted along its propagation, the type and number of sensors, with their installation, should be carefully studied and selected. In this paper, the acoustic wave propagation inside the transformer is explored via numerical simulation. Based on the understanding of wave propagation, a novel-structure fiber-optic acoustic sensor array is properly designed and installed into one phase winding, which is from a real 35 kV transformer. Experimental verification shows that, by using this proposed design, PD localization with less than 5-cm error can be achieved. Therefore, the novel sensor array developed in this paper, together with the installation and localization method, unveils a novel access to accurate online PD localization, especially for those PDs which happen inside transformer windings. [ABSTRACT FROM AUTHOR]

The digital control system of modular multilevel converters (MMCs) normally synchronizes with the switching actions. The switching frequency of an MMC is generally selected as low as possible from an efficiency point of view, which leads to an unavoidable and relatively long time delay in the control system. Such a time delay might deteriorate the MMC performance and even reduce the stability margin by introducing infinite uncertain eigenvalues into the system state functions. Without a proper time-delay model and quantitative system stability analyses, MMCs might suffer from uncertainties and unstable risks during their operations, especially when the switching frequency is low. This paper derives an MMC model considering the time delay in the digital control system. A Padé-based stability analysis method is then proposed, which consists of three steps: information integration and critical eigenvalue extraction, and trajectory visualization and critical angle (CA) calculation, and quantitative assessment and extreme value determination. By the proposed model and method, the critical eigenvalue can be effectively extracted. With the Hopf bifurcation, the time delay in the digital control system induces a pair critical conjugate eigenvalues into MMC systems, which is detected as an instability cause. The CA and the critical impact are defined to quantitatively assess the stability, while selecting the controller parameters and minimum switching frequency. The proposed method for the MMC stability assessment is experimentally verified. [ABSTRACT FROM AUTHOR]

Modern aircrafts are evolving toward more electric aircraft (MEA), resulting in greater reliance on the electrical system for safe flight. On-board power system of MEA integrates a large number of power electronic converters, and it is reported that semiconductor devices and electrolytic capacitors in power converters are the most vulnerable links impacted by loading conditions; thus, reliability becomes a critical concern in an MEA power system. This paper proposes a hierarchical approach for systematic reliability modeling and evaluation for the on-board power system of MEAs. It consists of three hierarchical levels (HLs): component level (HL1), subsystem level (HL2), and system level (HL3). In HL1, failure rates of power electronic components are modeled considering relevant inner structure and loading conditions; in HL2, the reliability of individual subsystems such as converters are constructed; in HL3, the system reliability is quantified based on the network architecture and reliability of the subsystems. The impacts of different parts (components/subsystems) on the overall system are assessed effectively with the identification of the vulnerable parts. This also provides a guideline for reliability enhancement by using thermal control techniques, adding redundancies or performing maintenance on the vulnerable parts to ensure the satisfactory of system reliability requirements. The proposed method is demonstrated on the future MEA power system architectures (hybrid ac–dc architecture and HVdc architecture). [ABSTRACT FROM AUTHOR]

This paper introduces a compensation strategy to deal with both current and voltage measurement errors in the three-phase pulsewidth modulated rectifier system. The dc offset and scaling errors in the voltage and current measurements cause the injection of undesired dc and unbalanced currents into the three-phase input current and subsequently lead to voltage ripple at the dc output voltage. This paper proposes a compensation scheme for current measurement error where the dc offset and scaling errors in the current measurement are estimated from the characteristic of the dc output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, an advanced current controller designed with a proportional integral plus two resonant controllers tuned at the fundamental grid frequency $({{\omega _s}})$ and $2{\omega _s}$ in the synchronous (d–q) reference frame is suggested to reject the impact of the dc offset and scaling errors in the voltage measurement. The proposed compensation method is developed without the need of extra hardware circuit, sensor, or precise information of system parameters so that it can be considered as more cost-effective and robust solution. The effectiveness of the proposed solution is verified by experimental results. [ABSTRACT FROM AUTHOR]

The rapidly developing data center industry results in a large amount of energy consumption. Considering its unique demand characteristics, it becomes desirable to improve its energy efficiency by reusing its waste heat. In this paper, an integrated energy management scheme is proposed to optimize the operation cost for a data center microgrid. The waste heat recovered from data center operation is optimally scheduled with other resources in the integrated energy management model to minimize the operation cost of data center microgrid. Taking the uncertain nature of renewable supply, power and heat demand into consideration, a two-stage stochastic programming model is formulated and analyzed under different test cases. The numerical study results demonstrate the effectiveness of the proposed integrated energy management model. [ABSTRACT FROM AUTHOR]

CLLC-type dc transformer (CLLC-DCT) is very popular in the hybrid ac–dc microgrid thanks to its high-power density advantage and good bidirectional power transfer capacity. In the hybrid ac/dc microgrid, the open-loop control is always utilized by the CLLC-DCT to cooperate with the bidirectional interlinking converter to realize the power and voltage conversion between the ac and dc bus. This paper first studies the circuit parameters design of the open-loop controlled CLLC-DCT with consideration of such a realistic problem: The real inductors/capacitors values are actually different with their theoretically designed values due to the operation power and temperature variation. To solve this problem, a robust circuit parameters design scheme is proposed for the CLLC-DCT in this paper. With the proposed scheme, the designed CLLC-DCT exhibits good power transmission and voltage regulation ability in the hybrid ac/dc microgrid even when its actual inductors/capacitors values vary with the practical power and temperature. The robust design method is experimentally verified in a hybrid ac/dc microgrid prototype. [ABSTRACT FROM AUTHOR]

Internet data centers are booming in the last few years, leading to gigantic amount of energy consumption and greenhouse gas emission. In this paper, a data center microgrid including conventional units, energy storage, and renewable generation is modeled to optimize its electricity bill and carbon footprint. Considering the randomness of electricity price, renewable output and workload distribution, the volatility risk is incorporated to measure the associated operation risk. Then, a day-ahead emission-aware stochastic resource planning scheme is formulated to decide the strategy on power procurement, energy storage operation, batch workload allocation, and unit commitment of conventional units. Simulation results show the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

In this paper, we propose and develop a leaky-wave antenna (LWA) by periodically loading shunt radiation element pairs along a host transmission line (TL). Each pair of radiation unit consists of one capacitive element and one inductive element, which are susceptance-complementary with each other. The complementary radiation element loading leads to an effective open-stopband (OSB) suppression, thereby facilitating a continuous leaky-wave beam scanning from backward, through broadside, to forward. Compared with the previously studied OSB-suppressed LWAs containing only one type of susceptive radiation elements, the proposed LWA features two unique properties: constant radiation efficiency over the operation band, and unit cell period only related to the host TL regardless of parameter values of the radiation elements. The aperture synthesis of the proposed LWA is hence straightforward as the radiation elements can be designed to be equally spaced. Experimental prototypes are designed and fabricated based on microstrip line structure as a proof of concept. Desired measured results agree well with the theoretical predictions. [ABSTRACT FROM AUTHOR]

A novel high-efficiency resonant light emitting diode (LED) backlight driver with passive current balancing and dimming is proposed and analyzed in this paper. The proposed LED driver achieves uniform brightness of each LED string by the charge balance of the resonant capacitor, which benefits from high efficiency. By controlling the output current of one LED string, the output currents of other LED strings can be controlled via current balancing of resonant capacitors, which makes its control simple. The proposed LED driver uses only one active switch, which decreases circuit volume and simplifies the control. Furthermore, a wide load range can be achieved by controlling the turn-on duty ratio of the single power switch of the LED driver with a fixed frequency. Finally, a prototype of a high-dimming-ratio small-size three-string LED driver is provided to verify the studied results. [ABSTRACT FROM PUBLISHER]

A high-performance three-phase pulse width modulation (PWM) converter with reduced dc-link capacitor under unbalanced ac voltage conditions is proposed in this paper. The unique feature of the converter lies in that sinusoidal symmetrical ac currents and ripple free dc voltage can be achieved simultaneously without using a bulky capacitor, which was thought to be a dilemma in the past. The control scheme is implemented in an α–β frame, so the complex rotate transformations are avoided, and it does not require the extraction of positive and negative sequence currents, which simplifies the control algorithm. A two-quadrant operation active power compensator together with its controller is developed to suppress the voltage ripple in the dc-link. Compared with the passive compensation method, the compensator is equipped with a smaller capacitor to reduce the system size, weight, and cost. Moreover, the compensator could be integrated with the conventional three-phase three-leg PWM converter seamlessly. The effectiveness of the proposed control schemes is verified by the experiment results. [ABSTRACT FROM PUBLISHER]

The traditional finite-control-set model predictive control (FCS-MPC) method for a cascaded H-bridge (CHB) rectifier has two main issues: heavy computational burden and low steady-state current performance. In this paper, a novel FCS-MPC method has been proposed for a single-phase CHB rectifier. The proposed method solves the optimization problem of FCS-MPC for one cell by one cell, like a “pipeline.” In the proposed method, the sampling period is divided into equal intervals by the number of cells. At the beginning of the first interval, the first cell selects its switching state to be applied. Then, the following cell selects its switching state to be applied at the beginning of next interval. Finally, the selected switching state of last cell will be applied at the beginning of the last interval. A cost function presenting the control objectives of common source current error and itself dc-link voltage error is evaluated for each cell. A single-phase three-cell CHB rectifier controlled by a DSpace DS1104 is tested and the experimental results show that a significant reduction in computational time, an improved steady-state current performance, and a comparable dynamic response are achieved in the proposed method in comparison with the traditional FCS-MPC method. [ABSTRACT FROM AUTHOR]

This paper proposes a modified one cycle control (OCC) for active power filter (APF) to compensate only harmonic components of load currents. The conventional OCC-based APF cannot differentiate between the harmonic and reactive components of load currents, which leads to the increment in device current rating because of compensation of large reactive currents besides harmonics. The reference signals of the OCC control core have been reconstructed and the associated control equations have been developed, following the control philosophy of simplicity. The modified scheme is easy to implement and only three more sensors are required for measuring load currents compared with the conventional OCC. This modified OCC guarantees the excellent performance of harmonic compensation alone and retains the advantages of the conventional OCC, such as no phase locking loop and the constant switching frequency. The feasibility and performance of the proposed OCC has been validated by experimental studies on a 5 kVA APF hardware platform developed in laboratory. [ABSTRACT FROM PUBLISHER]

We have evaluated the total-ionizing-dose response of graphene-based X-ray detectors fabricated from graphene oxide (GO) via laser scribing. Raman characterization shows that the resulting structures exhibit the desired conversion of GO to graphene; both high- and low-resistance detectors are formed. The detectors are quite radiation sensitive, as demonstrated by fast pulse testing. The response time of the detectors degrades with total-ionizing dose exposure as a result of electron trapping at residual oxygen from unreacted GO and/or adsorbed oxygen on the graphene surface. [ABSTRACT FROM PUBLISHER]

For hybrid energy storage system in dc microgrid, effective power split, bus voltage deviation, and state-of-charge (SoC) violation are significant issues. Conventionally, they are achieved by centralized control or hierarchical control methods with communications. This paper proposes a simple and effective strategy to solve the problem in a decentralized manner. A high-pass filter-based droop (HPFD) controller is proposed to regulate the battery converter, and a virtual capacitance droop (VCD) controller is implemented for a supercapacitor (SC) converter. The cooperation of HPFD and VCD first achieves autonomous power split that high-frequency fluctuation is buffered by SC and low-frequency power is supplied by battery. Meanwhile, the bus voltage deviation induced by the droop-based power sharing is eliminated automatically at steady state. The resulted bus voltage restoration simultaneously enforces SC SoC back to its nominal value, and, thus, ensures continuous operation of SC as a power buffer without the violation of its SoC boundary. A design guideline is developed to ensure expected system dynamics. The effectiveness of the proposed method and analytical results are validated by simulations and experiments. [ABSTRACT FROM PUBLISHER]

In this brief, a low-temperature microwave annealing (MWA) method is demonstrated for the formation of ohmic contact to AlN/GaN high electron mobility transistors (HEMTs) for the first time. Compared with the traditional rapid thermal annealing (RTA) technique, MWA-HEMT can achieve a comparable low ohmic contact resistance with much smoother surface of ohmic contacts. Transmission electron microscopy results show that no direct current path connecting the 2-D electron gas and the metal is formed. Temperature-dependent contact resistance measurement indicates that field emission tunneling dominates the current transport mechanism in ohmic contact formation. The maximum dc output current density of 1.4 A/mm and the peak extrinsic transconductance ( G\textsf {m} ) of 270 mS/mm are measured on 2 \times 8 \,\,\sf \mu $m^2$ gate MWA-HEMTs. Besides, MWA-HEMTs have higher {I}_{ \mathrm{ ON}}/{I}_{ \mathrm{ OFF}} ratio due to the lower gate leakage current than that of RTA-HEMTs. [ABSTRACT FROM AUTHOR]

Objective: To model inductive coupling of endovascular coils with transmit RF excitation for selecting coils for MRI-guided interventions. Methods: Independent and computationally efficient FEM models are developed for the endovascular coil, cable, transmit excitation, and imaging domain. Electromagnetic and circuit solvers are coupled to simulate net B1 + fields and induced currents and voltages. Our models are validated using the Bloch–Siegert B1 + mapping sequence for a series-tuned multimode coil, capable of tracking, wireless visualization, and high-resolution endovascular imaging. Results: Validation shows good agreement at 24-, 28-, and 34-μT background RF excitation within experimental limitations. Quantitative coil performance metrics agree with simulation. A parametric study demonstrates tradeoff in coil performance metrics when varying number of coil turns. Tracking, imaging, and wireless marker multimode coil features and their integration is demonstrated in a pig study. Conclusion: Developed models for the multimode coil were successfully validated. Modeling for geometric optimization and coil selection serves as a precursor to time consuming and expensive experiments. Specific applications demonstrated include parametric optimization, coil selection for a cardiac intervention, and an animal imaging experiment. Significance: Our modular, adaptable, and computationally efficient modeling approach enables rapid comparison, selection, and optimization of inductively coupled coils for MRI-guided interventions. [ABSTRACT FROM PUBLISHER]

A small optical fiber temperature sensor employing the fluorescence lifetime of Mn4+ doped oxyfluoride germanate phosphor was presented and experimentally demonstrated, which has a fairly good temperature measurement precision. The performance of the fluorescent materials for temperature measurement was compared and analyzed. A small temperature sensing probe with a diameter of 1.8 mm was proposed. For the first time, the integrated tee-light-path structure was put forward, the transmission and separation of excitation light and fluorescence in only a single optical path was realized. Experimental results showed that standard deviation errors of 0.45 °C were obtained within the temperature range from 0 °C to 90 °C. [ABSTRACT FROM PUBLISHER]