다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

In recent years, due to the wide utilization of direct current (DC) power sources, such as solar photovoltaic (PV), fuel cells, different DC loads, high-level integration of different energy storage systems such as batteries, supercapacitors, DC microgrids have been gaining more importance. Furthermore, unlike conventional AC systems, DC microgrids do not have issues such as synchronization, harmonics, reactive power control, and frequency control. However, the incorporation of different distributed generators, such as PV, wind, fuel cell, loads, and energy storage devices in the common DC bus complicates the control of DC bus voltage as well as the power-sharing. In order to ensure the secure and safe operation of DC microgrids, different control techniques, such as centralized, decentralized, distributed, multilevel, and hierarchical control, are presented. The optimal planning of DC microgrids has an impact on operation and control algorithms; thus, coordination among them is required. A detailed review of the planning, operation, and control of DC microgrids is missing in the existing literature. Thus, this article documents developments in the planning, operation, and control of DC microgrids covered in research in the past 15 years. DC microgrid planning, operation, and control challenges and opportunities are discussed. Different planning, control, and operation methods are well documented with their advantages and disadvantages to provide an excellent foundation for industry personnel and researchers. Power-sharing and energy management operation, control, and planning issues are summarized for both grid-connected and islanded DC microgrids. Also, key research areas in DC microgrid planning, operation, and control are identified to adopt cutting-edge technologies. This review explicitly helps readers understand existing developments on DC microgrid planning, operation, and control as well as identify the need for additional research in order to further contribute to the topic.

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DC Microgrid Planning, Operation, and Control: A Comprehensive Review

Conventional electricity generation is one of the greatest sources of CO2 emissions. For a successful transformation of conventional energy systems into non-polluting and renewable energy systems, technology-focused traditional systems and economics must be combined for a more accurate holistic viewpoint with consideration of socio-political, technical, economic and environmental factors. Hybrid energy systems are considered the most feasible solution to the stochastic nature of renewable energy resources (RERs). Different renewable sources such as wind, solar, and hydrogen fuel cells can be integrated to form hybrid systems. An energy management strategy (EMS) is a strategy for power flow coordination among different components, by considering power demand and other constraints. The choice for an accurate EMS is the key element of a hybrid system as it is instrumental in providing an optimum solution of the hybrid system design and operation management. The objective of the optimization is to find suitable configurations for cost-effective solutions. Optimization and EMS must be treated as one entity to completely understand the system design. This study focuses on a techno-economic analysis with an optimized sizing of a hybrid renewable energy system (HRES) components to meet the residential load demand of a specific area in Pakistan. Nine different scenarios based on the PV-wind-diesel-BSS-converter system are investigated in terms of total net present cost (TNPC), Levelized cost of energy (LCOE), and greenhouse gas (GHG) emissions to find the optimal system design. HOMER Pro software is used to develop the HRES model and for simulation analysis, with optimal sizing of each component for an economical solution. Simulation studies established that PV-wind-BSS-converter is the best suitable choice for the given location, and the optimal component sizes were determined. The TNPC of this system is $47,398 and the LCOE is $ 0.309/kWh. This represents an 81.7 % decrease in overall cost, compared to the base case (diesel only) and a 100% reduction in harmful gases while satisfying 100 % of the energy requirement with a 63.9 % of the surplus. MATLAB/Simulink model is developed for the optimum HRES system design. Its validity is tested by maintaining bus voltages (dc and ac), the secure operation range of storage SOC and real power balance among different components of the hybrid renewable energy system (HRES), and an effective ac voltage, irrespective of external perturbations. Model predictive control (MPC) is regarded as a high-performing algorithm. Since power converters are largely applied in microgrids (MGs), the problem formulation with MPC for a reconfigurable bidirectional voltage source converter (VSC) is applied in this work for hybrid MG. The inevitable fluctuations due to the linear and non-linear loads and the nature of renewable sources are addressed. The regulation of ac voltage is implemented through a finite control set model predictive control (FCS-MPC) based active front end (AFE) rectifier, while direct power MPC (DPMPC) is used to control the power during grid-connected operation. The regulation of an ac load voltage is done through voltage based MPC (MPVC) in the islanding operation of the MG. Moreover, the HRES transition from grid-tied to grid-isolated mode is comprehensively analyzed. MATLAB/Simulink
 ®
 software certified the robustness and evaluated the performance of the proposed HRES model under different varying loads viz. balanced, unbalanced, and nonlinear. The proposed strategy offers superior performance with low total harmonic distortion (THD), compared to previously developed strategies. The output waveform of voltage and current have THD of 0.28 % compared to 3.71 % with the conventional strategy. The contributions of this paper lie in the sequential use of HOMER as well as MATLAB tools and in the validation of the suggested HRES plan for the considered location; along with the implementation of FCS-MPC for a reconfigurable bidirectional VSC.

/pdf/optimal-design-and-model-predictive-control-of-standalone-1zwj508voy.pdf

Optimal Design and Model Predictive Control of Standalone HRES: A Real Case Study for Residential Demand Side Management

Due to the instantaneous variation of wind speed, it is suitable to determine the optimal rotational speed that ensures maximum energy efficiency. This article deals with a comparison between different maximum power point tracking algorithms applied to wind energy conversion systems. The most commonly used algorithms are optimal torque control, power signal feedback, tip-speed ratio, and hill-climb search. The conventional hill-climb search method has an important drawback, which is the wrong directionality under rapid wind change. To solve this problem, using a hybrid hill-climb search method is proposed, which consists of combining the optimal torque control and hill-climb search methods. The proposed approach is applied to a wind energy conversion system based on a permanent magnet synchronous generator under a wind speed profile using a MATLAB®-Simulink® (The MathWorks, Natick, Massachusetts, USA) package to achieve the simulation calculations. The obtained results are then presented and compa...

Maximum Power Point Tracking Based Hybrid Hill-climb Search Method Applied to Wind Energy Conversion System

Microgrids consisting of photovoltaic (PV) power plants and wind farms have been widely accepted in power systems for reliability enhancement and power loss reduction. Microgrids are capable of providing voltage and frequency support, improving power quality, and achieving proper power-sharing. To achieve such goals and deal with the nonlinear behavior in such systems, appropriate robust control strategies are required to be adopted. This article presents a comprehensive review of robust control methods for microgrids, including AC, DC, and hybrid microgrids, with different topologies and different types of interconnection to conventional power systems based on recently published research studies. The main control objectives, along with proposed control methods, are comparatively discussed for different types of microgrids. Furthermore, several research gaps in this area related to the scalability, robustness assessment, and evaluation approach are discussed. Recommendations are made that can potentially open new research lines to enhance the effectiveness of robust controllers for AC, DC, and hybrid microgrids. 

Robust Control Strategies for Microgrids: A Review

In this paper, a new backstepping-based nonlinear technique for control of photovoltaic systems in DC islanded microgrids (MGs) is proposed. In contrast to most existing droop/non-droop control strategies that require an exact model of the system including line impedances, loads, other distributed generation units (DGUs) parameters, and even the MG configuration, the proposed method is taking dynamics and uncertainties into account using a designed disturbance observer. Moreover, the proposed method rapidly reaches the reference values and exhibits a more accurate robust performance using local quantities measurement, irrespective of parametric uncertainties, unmodeled dynamics, unknown loads, disturbances, and the number/structure of DGs within the MG. Finally, a low-voltage DC MG is built where the robust performance of the proposed method for different operating conditions including load variation, tracking capability, nonlinear loads, and plug-play of DGs is verified.

Fully decentralized robust backstepping voltage control of photovoltaic systems for DC islanded microgrids based on disturbance observer method.

In this paper, a nonlinear voltage control for a DC islanded microgrid (MG) is proposed. This control method is a new nonlinear decentralized backstepping based strategy for voltage control of DC MG based on Sepic converter with nonlinear loads such as constant power loads (CPLs). The effect of CPL is to realize a load with negative resistance for DC MG that impacts the power quality of the power system and causes negative damping. The major purpose of the proposed controller is to improve the transient performance of MG with the Plug and Play (PnP) feature in the presence of load change and CPL. To consider the effects of such loads on the proposed control method, a nonlinear disturbance observer (NDO) along with backstepping controller is introduced to estimate the uncertain terms such as CPL and load currents as well as the current transferred between DGUs. The proposed method only measures the local voltage and current quantities without any need for additional information about the grid topology and its parameters. This method successfully regulates the point of common coupling (PCC) voltage. The stability of the method is proved via Lyapunov theory and its features are verified with simulation and some experiment tests.

Voltage control in a DC islanded microgrid based on nonlinear disturbance observer with CPLs

A new sensor-less maximum power point tracking (MPPT) technique is introduced for wind power conversion applications. This new technique is founded upon a modified variable step hill climb search (HCS) method. The modifications are made, based on a simple and novel idea. The step sizes of the hill climbing technique are varied throughout the MPPT operation, entirely independent of the wind speed and the rotor speed, without requirement for any prior information of the wind power conversion system's parameters and configurations. These features make this new technique applicable on different kinds of wind power conversion systems. The novel method introduced in this study, exhibits very fast performance and high accuracy in sensing wind speed variations as well as maintaining its performance under extreme conditions, such as high wind disturbances. Computer simulations are performed in order to evaluate the new technique and the results support the feasibility of the presented MPPT technique.

A new maximum power point tracking technique for wind power conversion systems

Increasing DC loads along with DC nature of Distributed Energy Resources (DERs) raised interest to DC microgrids. Conventional droop/non-droop power sharing in microgrids suffers from load dependent voltage deviation, slow transient response, and requires the parameters of the loads, system and DERs connection status. In this paper, a new nonlinear decentralized back-stepping control strategy for voltage control and load sharing of DC islanded microgrids is proposed. The proposed method is robust against to the load variations and uncertainty in microgrid parameters and has excellent dynamic and steady state performance under different operating conditions. The local controller regulates the terminal voltage of dc-dc converter regarding to the local quantities without needs to additional data of other system components. For simplicity, the proposed method is simulated with PSIM software on a DC microgrid with two DGs. Different scenarios are studied to present the proposed method performance under different operating conditions. The results indicate the capability of the proposed method for voltage control and load sharing in DC microgrids.

Voltage Control and Load Sharing in a DC Islanded Microgrid based on Disturbance Observer

Increasing DC loads along with DC nature of distributed energy resources (DERs) raises interest to DC microgrids. Conventional droop/non-droop power-sharing in microgrids suffers from load dependent voltage deviation, slow transient response, and requires the parameters of the loads, system and DERs connection status. In this paper, a new nonlinear decentralized back-stepping control strategy for voltage control and load sharing of DC islanded microgrids is proposed. The proposed method is robust against the load variations and uncertainty in microgrid parameters and has excellent dynamic and steady-state performance under different operating conditions. The major purpose of the proposed controller is to improve the transient performance of MG with load variations and constant power loads (CPLs). The local controller regulates the terminal voltage of DC-DC converter regarding the local quantities without needs to additional data of other system components. For simplicity, the proposed method is simulated with PSIM software on a DC microgrid with two DGs. Different scenarios are studied to present the performance of the proposed method under different operating conditions. The results indicate the capability of the proposed method for voltage control and load sharing in DC microgrids.

http://jecei.sru.ac.ir/article_1111_fb7cda103a37607477f6d63fd9411085.pdf

Habib Amiri

Papers

Fully decentralized robust backstepping voltage control of photovoltaic systems for DC islanded microgrids based on disturbance observer method.

Voltage control in a DC islanded microgrid based on nonlinear disturbance observer with CPLs

A new maximum power point tracking technique for wind power conversion systems

Voltage Control and Load Sharing in a DC Islanded Microgrid based on Disturbance Observer

Voltage Control and Load Sharing in a DC Islanded Microgrid Based on Disturbance Observer