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Understanding changes of soil organic carbon (SOC) in top layers of croplands and their driving factors is a vital prerequisite in decision-making for maintaining sustainable agriculture. However, high-precision estimation of SOC of croplands at regional scale is still an issue to be solved. Based on soil samples, synthetic image of bare soil and geographical data, this paper predicted SOC density of croplands using Random Forest model in the Black Soil Region of Jilin Province, China in 2005 and 2020, and analyzed its influencing factors. Results showed that random forest model that integrates bare soil composite images improve the accuracy and robustness of SOC density prediction. From 2005 to 2020, the total SOC storage in croplands decreased from 89.96 to 82.79 Tg C with an annual decrease of 0.48 Tg C yr−1. The mean value of SOC density of croplands decreased from 3.42 to 3.32 kg/m2, and high values are distributed in middle parts. Changes of SOC represented significant heterogeneity spatially. 62.14% of croplands with SOC density greater than 4.0 kg/m2 decreased significantly, and 38.60% of croplands with SOC density between 2.5 and 3.0 kg/m2 significantly increased. Climatic factors made great contributions to SOC density, however, their relative importance (RI) to SOC density decreased from 44.65% to 37.26% during the study period. Synthetic images of bare soil constituted 23.54% and 26.29% of RI in the SOC density prediction, respectively, and the contribution of each band was quite different. The RIs of topographic and vegetation factors were low but increased significantly from 2005 to 2020. This study can aid local land managers and governmental agencies in assessing carbon sequestration potential and carbon credits, thus contributing to the protection and sustainable use of black soils. [ABSTRACT FROM AUTHOR]

As one of the promising power interfaces between the energy storage components and the dc link, dual active bridge (DAB) converters have gained extensive researches. Its reliability and healthy operation under one or even multiple unexpected open-circuit faulty conditions have become one of the main design challenges. Conventional fault detection (FD) and fault isolation (FI) methods exhibit obvious limitations in terms of detection speed, number of FD signatures, universality, accuracy, computational burden, and implementation cost due to the added sensors. To fill in this gap, this article presents a robust, fast, accurate, and low-cost FD and FI method for DAB converters, which is applicable under various operational conditions. The proposed method will adopt the differential switch node voltage between the primary and secondary bridge of DAB converters as the universal fault signature for complete FD and FI. The corresponding fault diagnostic circuit is simple, compact, low cost, and easy in implementation. This sensorless fault diagnostic technique can detect the switch open-circuit fault within a quarter of a switching cycle and realize the fault isolation accurately. A DAB prototype with a fault diagnostic circuit was designed, and main experimental results under various faulty conditions were provided to validate the advantages of the proposed method. [ABSTRACT FROM AUTHOR]

Pulse test techniques are widely adopted in characterizing both the static and dynamic performances of semiconductor devices considering various device structures and operating conditions. For gallium nitride (GaN) devices, due to the current collapse phenomenon, fast and accurate measurement of their key electrical parameters, such as dynamic ON-state resistance ${R}_{\text {ds}, \mathrm{\scriptscriptstyle ON}}$ , switching rising time, fall time, switching losses, and short-circuit (SC) robustness, becomes essential. This article presents a comprehensive review of state-of-the-art pulse test setup methods such as the double-pulse test (DPT), multiple-pulse test (MPT), and SC test (SCT) for the first time, which facilitates the GaN device structure design and practical applications in high efficiency and power density power converters. [ABSTRACT FROM AUTHOR]

In this article, an efficient model predictive control (MPC) using virtual voltage vectors for three-phase three-level converters is proposed. The proposed MPC achieves constant switching frequency by applying four voltage vectors, including one virtual voltage vectors and three other voltage vectors, in each control cycle. In addition, to reduce the computational burden, two-stage MPC approach is adopted. The first stage chooses one of six medium voltage vectors that minimizes the cost function. Then, in the second stage, these voltage vectors including virtual voltage vectors, which locate the same sector with the optimal medium voltage vector, are involved in the MPC optimization. The advantages of the proposed MPC over the classical MPC have been validated through experimental results. [ABSTRACT FROM AUTHOR]

Dual-active-bridge (DAB) converters have been widely recognized as one promising topology for high-frequency-link power conversion applications. However, one critical issue concerning DAB converters is the transient dc-bias current due to the voltage-second imbalance, which may occur frequently in typical dynamic operations of DAB converters such as the start-up, sudden load or output voltage change, and power flow direction change. Due to the high-frequency operation, the dc-bias current of DAB converters will impose more harm to magnetic components because of the limited range of operating flux density and possible magnetic saturation compared with the counterpart low-frequency dc-bias current. Over the past several years, extensive researches have been conducted to address the high-frequency transient dc-bias current of DAB converters under different phase-shift modulation strategies such as single phase shift, extended phase shift, and triple phase shift. In this article, a comparative review of typical dc-bias current mitigation strategies under typical phase-shift strategies for DAB converters is presented. An experimental evaluation of selected strategies under various scenarios is conducted in terms of the settling time, inductor current stress, efficiency, switching number, and implementation complexity. This article ends up with an overview of high-frequency dc-bias mitigation strategies and a discussion for the further optimization. [ABSTRACT FROM AUTHOR]

The dc power transmission system has been obviously attracting more research interests in recent years. In order to satisfy the high-reliability requirement of the dc power transmission system, the power converter should be able to keep uninterrupted operation after multiple unexpected faults. However, high fault-tolerant capability usually leads to the bulky redundant circuit, which increases the cost, volume, and complexity of the power converter. Thus, a fault-tolerant dual active bridge (DAB) converter is proposed to maintain the power transferring ability under multiple unexpected open-circuit faults (OCFs) conditions, which can significantly enhance the reliability of the dc power transmission system. By reconfiguring the central-tapped transformer and two symmetrical auxiliary inductors, the half-bridge conduction branch is built to maintain uninterrupted operation when a single or dual OCF has occurred. The proposed fault-tolerant strategy only requires four extra voltage sensors to detect and locate OCFs for the reconfiguration process. Thus, it significantly improves the system reliability with a low additional cost. Besides, the inductor current, transmission power, and the small-signal models of the proposed fault-tolerant converter have been presented. It proves the proposed fault-tolerant topology can smoothly switch between normal and postfault operation due to the constituency of the inductor current. Finally, the 250-W prototype is designed to verify the advantage of the proposed fault-tolerant DAB converter. [ABSTRACT FROM AUTHOR]

In this article, a low complexity finite-control-set model predictive control (FCS-MPC) based on the discrete space vector modulation (DSVM) is proposed for T-type three-phase three-level (3P-3L) converters. Different from the conventional FCS-MPC, 48 virtual voltage vectors (VVs) of the converter are constructed by real VVs based on the DSVM. Thus, the performance of 3P-3L converters is significantly improved and the peak amplitude of high-order harmonics concentrates at the sampling frequency. Furthermore, two-stage FCS-MPC based on virtual VVs is proposed to reduce the computation burden. Its first stage selects one of six virtual VVs that minimizes the current tracking error. Then, these candidate VVs located in the same sector as the optimal virtual VV selected in the first stage are evaluated in the second-stage optimization. Thus, the computational efficiency has been greatly improved. To verify the validity of the proposed control method and show its superiority over the conventional FCS-MPC, experimental results are presented. [ABSTRACT FROM AUTHOR]

With the increase of the component number, the power stress distribution among differential power processing (DPP) converters, control implementation, system cost, and reliability become the most challenging issues for a practical photovoltaic (PV) DPP system. This article introduces an improved power-rating balance (IPRB) control for the PV-to-bus based DPP architecture that ensures each PV submodule operate at its true maximum power point (MPP) while achieving more balanced power stress distribution and higher reliability. Specifically, a submodule-level finite-state-machine-based MPP tracking is implemented to guarantee always maximum power yield, whereas a string-level power-rating balancing (PRB) control is adopted to balance the unit-maximum proceeded power by DPP converters based on the built power flow model with respect to the string current. A comprehensive comparison of advanced control strategies for PV-to-bus DPP architectures, including least power point tracking, voltage equalization (VE) based PRB control, and the proposed IPRB, has been carried out with the mission-profile-based reliability assessment under different partial shading scenarios. Component-failure-rate-based reliability analysis shows that the PV-to-bus DPP architecture with the proposed IPRB control can significantly improve the system reliability. Main simulation and experimental evaluations are carried out to verify the effectiveness of the proposed control. [ABSTRACT FROM AUTHOR]

The derivation and implementation of multiple-phase-shift-based switching surfaces for a dual active bridge (DAB) converter is the main focus of this article. First, the mathematical models of multiple natural switching surfaces under different operation states of DAB converters are derived, which lays the foundation to achieve a fast transient response during startup, sudden voltage reference, and load changing conditions. Moreover, in order to improve the overall performance of DAB converters systematically, a minimum-current-stress boundary control (MBC) is proposed that can reduce the inductor peak current stress and achieve fast dynamic response simultaneously by using the multiple-phase-shift-based switching surfaces. The analytical derivation of the proposed MBC is presented together with the simulation and experimental evaluations, which shows the superior performance of the proposed MBC algorithm in terms of the efficiency and dynamic response improvement under various operating conditions. [ABSTRACT FROM AUTHOR]

• LLC-resonant voltage-multiplier-based DPP optimizer is presented with low cost and good extensibility. • The switch count is limited to two for long-string photovoltaic systems. • The output power under severe shading scenario can be improved up to 32.9%. • The presented DPP optimizer cost for four-module-series-connection PV systems can be reduced up to 45.41%. Mismatched photovoltaic (PV) power systems due to the shadow effect will result in serious consequences such as reduced output power, hot spot problem, and low reliability. As one promising architecture to address this issue, differential power processing (DPP) optimizer has been extensively discussed to improve the actual energy yield through the charge redistribution among PV elements such as modules or sub-modules. However, since a large number of hardware components must be added in the conventional DPP optimizer, high cost and poor extensibility have become the bottleneck for the practical application of DPP technique in long-string photovoltaic systems. This paper presents an LLC-resonant voltage-multiplier-based DPP (LLC-VM-DPP) optimizer to address these problems. Specifically, the LLC resonant inverter with two switches is operating as a voltage equalizer to compensate the power reduction of shaded PV elements. The switch count is limited to two even for long-string photovoltaic systems. No feedback is required for the control implementation. Thus, the overall system performance such as the cost, circuit complexity, extensibility, power density, and efficiency can be improved. Simulation and experimental evaluation of the LLC-VM-DPP architecture for four-module-series-connection PV systems under different shading scenarios was carried out. Detailed power loss analysis was presented. With the LLC-VM-DPP architecture, the measured output power under severe shading scenario can be improved up to 32.9% while the presented DPP optimizer cost for four-module-series-connection PV systems can be reduced up to 45.41%. [ABSTRACT FROM AUTHOR]

• Nonisolated switching-capacitor-integrated three-port converter is presented. • Flexible power regulation and power balance among three ports are achieved. • The current stress is reduced and the overall efficiency can be improved. • The measured efficiency under a rated output power will be high up to 94%. Efficiency and power density of power converters for interfacing photovoltaic panels, energy storage components such as batteries, and loads in photovoltaic (PV) systems become more and more important. Compared with individual converter design for different terminals, power-integrated multiport converters shows obvious advantages in simplifying the system structure, reducing the component count, and improving the operation reliability. Originated from the high power-density switched capacitor topology, a nonisolated switching-capacitor-integrated three-port converter (SCI-TPC) is presented to achieve single-stage direct power conversion among three ports. In order to minimize the cross-regulation effect, pulse-width-modulation (PWM) and pulse-frequency-modulation (PFM) are adopted to realize the flexible power regulation and achieve power balance among three ports. Main operation modes, power flow distribution, and power transfer characteristic are analyzed. With the seamless PWM and PFM hybrid modulation, the current stress can be reduced and the overall conversion efficiency over a full operating range can be improved. Main experimental results are provided to validate the effectiveness of the proposed concept. [ABSTRACT FROM AUTHOR]

This article proposes a fixed switching frequency (FSF)-based improved model predictive control (MPC) for a five-level active neutral-point clamped (5L-ANPC) converter. Compared to the conventional MPC for 5L-ANPC converter, the proposed MPC significantly reduces the computational burden while ensuring the balance of dc-link and flying capacitor voltages. The proposed MPC optimally selects the control voltage vectors to reduce common mode voltage (CMV), leading to a small CMV of the converter. Using three voltage vectors action per control cycle in MPC, FSF is achieved. Steady-state and dynamic performances of the proposed MPC have been evaluated by experiments. The results verify the feasibility and effectiveness of the proposed MPC method. [ABSTRACT FROM AUTHOR]

Compared with the conventional multilevel converters, the hybrid-clamped converter (HCC) reduces the number of power devices, which brings down the converter cost while enhancing its reliability for medium-voltage high-power applications. As one of typical medium-voltage HCCs, the single-phase, six-level HCC (6L-HCC) faces the main technical challenge of the voltage balance among five inner capacitors. Most of the existing pulsewidth modulation techniques cannot solve this problem. Advanced control methods that directly apply a selected voltage vector, such as the classical model predictive control (MPC), may fail to balance the inner capacitor voltages during low output frequencies. In view of this situation, this article proposes an improved MPC approach with autotuning weighting factors. It contributes to ensure excellent performance of 6L-HCC under all operating conditions with well-balanced inner capacitor voltages. Simulation and experimental results are proposed to verify the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

The paper presents the development of a GaN DC–DC power converter with a pre-driver module and power device based on a monolithic integrated GaN E/D-mode metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) platform. The pre-driver module with direct coupled field effect transistor logic circuit structure has been monolithically integrated with a GaN power transistor on a Si-based AlGaN/GaN commercial epitaxial wafer. The MIS-HEMTs structure adopts an insulated gate dielectric layer to suppress the leakage current and increase gate voltage robustness. The GaN-based floating buck converter employs a 1 mH inductor and a 9 μ F capacitor to achieve 35 V–5 V power conversion. As a result, the pre-driver module is capable of delivering a 10 V driving voltage. GaN monolithic integration of the pre-driver module and power device can reduce the system area in the DC–DC application, which allows for an integrated chip size of 1.8 mm2. [ABSTRACT FROM AUTHOR]

The partial power processing (PPP) concept has been extensively employed in many two-port converters to achieve high efficiency and high-power density by modifying one of the original converter ports to be connected in series between input and output. This is attractive but is confined to two-port applications, and how to extend it to multiport applications is not clear. Hence, this article aims to explore a generalized PPP multiport scheme by arranging the connection of the module rather than the converter. For a PPP single-input(output)-N-outputs(inputs) converter, one terminal of all N+1 modules is connected together to construct a radial structure, and another terminal is series/parallel connected with the input/output port. Each module only needs to process partial power caused by the voltage or current difference between input and output. Therefore, the required power rating of components is effectively reduced, contributing to low cost as well as low power loss. In this article, a PPP single-input dual-output converter with active bridge modules is also taken as an example to be introduced in detail, designed and experimented upon, which validates that high efficiency and high-power density can be achieved. [ABSTRACT FROM AUTHOR]

Maximum power point tracking (MPPT) is required in PV power systems for the highest solar energy harvest. This article proposes a self-tuning scheme to improve the MPPT performance in terms of high accuracy and speed. The scheme adopts the reinforcement learning (RL) and Beta parameter for the highest MPPT performance. The tracking speed and accuracy are significantly improved since the RL algorithm is enhanced for high convergence speed, meanwhile, the guiding variable $\beta$ is introduced to constrain the exploration space. Simulation and experimental test are applied to validate the superior performance of the proposed solution following the EN50530 dynamic test procedure. [ABSTRACT FROM AUTHOR]

Novel normally-OFF AlGaN/GaN MIS-high electron mobility transistors (HEMTs) with a ZrOx charge trapping layer are proposed. The deposition of the ZrOx charge trapping layer on the partially recessed AlGaN in conjunction with the Al2O3 gate dielectric has been accomplished. The developed MIS-HEMTs exhibit a threshold voltage of 1.55 ± 0.4 V and a maximum drain current of 730 ± 6 mA/mm, while associated low ON-resistance of $7.1~\pm ~0.2~\Omega \cdot $ mm, for a gate to drain separation of 3 $\mu \text{m}$. The TCAD simulation results are presented to explore the charge trapping and de-trapping behaviors. Moreover, the devices exhibit a high breakdown voltage. The results indicate the merits of employing ZrOx charge trapping layer to realize the normally-OFF GaN devices with low ON-state resistance. [ABSTRACT FROM AUTHOR]

The intermittent and stochastic nature of solar resource hinders the integration of solar energy into modern power system. Solar forecasting has become an important tool for better photovoltaic (PV) power integration, effective market design, and reliable grid operation. Nevertheless, most existing solar forecasting methods are dedicated to improving forecasting accuracy at site-level (e.g. for individual PV power plants) regardless of the impacts caused by the accumulated penetration of distributed PV systems. To tackle with this issue, this article proposes a novel generative approach for regional solar forecasting considering an entire geographical region of a flexible spatial scale. Specifically, we create solar irradiance maps (SIMs) for solar forecasting for the first time by using spatial Kriging interpolation with satellite-derived solar irradiance data. The sequential SIMs provide a comprehensive view of how solar intensity varies over time and are further used as the inputs for a multi-scale generative adversarial network (GAN) to predict the next-step SIMs. The generated SIM frames can be further transformed into PV power output through a irradiance-to-power model. A case study is conducted in a 24 × 24 km area of Brisbane to validate the proposed method by predicting of both solar irradiance and the output of behind-the-meter (BTM) PV systems at unobserved locations. The approach demonstrates comparable accuracy in terms of solar irradiance forecasting and better predictions in PV power generation compared to the conventional forecasting models with a highest average forecasting skill of 10. 93 ± 2. 35 % for all BTM PV systems. Thus, it can be potentially used to assist solar energy assessment and power system control in a highly-penetrated region. • A regional solar forecasting approach using solar irradiance maps (SIMs). • Using generative model to produce image-based forecasts for an entire region. • Extending forecasts to the regional level with flexible geographical scales. • Potential applications for distributed photovoltaic power estimation. [ABSTRACT FROM AUTHOR]