In this article, a four-port solid-state transformer and a control scheme to control the power flow and output voltage are studied, developed, and tested. The converter consists of three ports with ...H-bridge converters and one port with a diode bridge rectifier. The ports with H-bridge converters are capable of bidirectional power routing as well as reactive power contribution in a volt/Var control scenario. The diode bridge rectifier provides a dc voltage for load connection. Different arrangements for the four transformer windings are analyzed, simulated, and compared to achieve an optimal design. A new control strategy, which uses a combination of phase-shift and duty cycle control, is employed to control the flow of power between the converter branches and to regulate the output voltage. While phase-shift control ensures the balance of power on each port based on a reference value, the duty cycle control keeps the load voltage at a desired voltage level. A high-level control scheme is employed to determine the power references for all ports according to the load demand, generation capacity of the distributed generation system, and state of charge of the energy storage. The performance of the proposed system is validated with simulation and experimental analysis. A prototype is designed and built with 10-kW power rating at each port. The operating frequency of the system is designed at 100 kHz to obtain a very compact size for the whole converter.
A microgrid can be defined as a grid of interconnected distributed energy resources, loads and energy storage systems. In microgrid systems containing renewable energy resources, the coordinated ...operation of distributed generation units is important to ensure the stability of the microgrid. A microgrid needs a successful control scheme to achieve its design goals. Undesirable situations such as distorted voltage profile and frequency fluctuations are significantly reduced by installing the appropriate hardware such as energy storage systems, and control strategies. The multi-agent system is one of the approaches used to control microgrids. The application of multi-agent systems in electric power systems is becoming popular because of their inherent benefits such as autonomy, responsiveness, and social ability. This study provides an overview of the agent concept and multi-agent systems, as well as reviews of recent research studies on multi-agent systems’ application in microgrid control systems. In addition, a multi-agent-based controller and energy management system design is proposed for the DC microgrid in the study. The designed microgrid is composed of a photovoltaic system consisting of 30 series-connected PV modules, a wind turbine, a synchronous generator, a battery-based energy storage system, critical and non-critical DC loads, the grid and the control system. The microgrid is controlled by the designed multi-agent-based controller. The proposed multi-agent-based controller has a distributed generation agent, battery agent, load agent and grid agent. The roles of each agent and communication among the agents are designed properly and coordinated to achieve control goals, which basically are the DC bus voltage quality and system stability. The designed microgrid and proposed multi-agent-based controller are tested for two different scenarios, and the performance of the controller has been verified with MATLAB/Simulink simulations. The simulation results show that the proposed controller provides constant DC voltage for any operation condition. Additionally, the system stability is ensured with the proposed controller for variable renewable generation and variable load conditions.
In this paper, a modified fast terminal sliding mode control (FTSMC) with a fixed switching frequency is proposed for regulating the output voltage of the DC-DC buck converters. The design steps of ...the proposed FTSMC such as the selection of sliding surface, switching control strategy, existence, robustness, and stability analysis are presented in detail. To overcome the variable switching frequency in FTSMC, a frequency control loop is designed. Moreover, the proposed FTSMC with fixed switching frequency can be implemented by using only one voltage sensor. Hence, the proposed control method not only offers a fast dynamic response and fixed switching frequency but also simplifies the controller design in practical implementation. The effectiveness of the proposed control methods has been investigated by experimental studies. The results reveal that the proposed methods exhibit a good performance under both steady-state and dynamic transients caused by the variations in load resistance, input voltage, and reference voltage. Moreover, the proposed method is compared with four existing methods.
Microgrids usually employ distributed energy resources such as wind turbines, solar photovoltaic modules, etc. When multiple distributed generation resources with different features are used in ...microgrids, managing these resources becomes an important problem. The generated power of solar photovoltaic modules and wind turbines used in microgrids is constantly changing with solar irradiation and wind speed. Due to this impermanent and uncertain nature of renewable energy resources, generally, energy storage systems are employed in microgrid systems. To control the distributed energy resources and energy storage units and sustain the supply and demand balance within the microgrid and provide sustainable and reliable energy to the loads, energy management systems are used. Many methods are used to realize and optimize energy management in microgrids. This review article provides a comparative and critical analysis of the energy management systems used in microgrids. The energy management system can be tailored for different purposes, which are also discussed in detail. Additionally, various uncertainty measurement methods are summarized to manage the variability and intermittency of renewable energy sources and load demand. Finally, some thoughts about potential future directions and practical applications are given.
The inherent unlimited high switching frequency of the sliding mode controller (SMC) is limited by practical constraints of the hysteresis modulation (HM) technique. The inductor current and output ...voltage of a converter can be regulated using a combination of HM-SMC. However, HM-SMC results in a variable switching frequency operation, which is not preferred due to Electromagnetic Interference (EMI) issues. In this paper, an interval fuzzy controller is designed and developed as a solution to enable HM-SMC. In addition, a robust sliding surface is proposed, which provides an improved dynamic response. The two proposed controllers’ compatibility with one another has been tested via experiments such as a step change in input voltage, load resistance variation, and finally, a step change in output voltage reference value. The test results validate that while the interval type-2 fuzzy maintains a constant switching frequency with acceptable dynamic responses, it successfully regulates the state variables of the system. A comparison of the performance of the proposed control method with existing techniques in the literature is presented.
This paper proposes a control scheme based on an optimal triple phase-shift (TPS) control for dual active bridge (DAB) DC–DC converters to achieve maximum efficiency. This is performed by analyzing, ...quantifying, and minimizing the total power losses, including the high-frequency transformer (HFT) and primary and secondary power modules of the DAB converter. To analyze the converter, three operating zones were defined according to low, medium, and rated power. To obtain the optimal TPS variables, two optimization techniques were utilized. In local optimization (LO), the offline particle swarm optimization (PSO) method was used, resulting in numerical optimums. This method was used for the low and medium power regions. The Lagrange multiplier (LM) was used for global optimization (GO), resulting in closed-form expressions for rated power. Detailed analyses and experimental results are given to verify the effectiveness of the proposed method. Additionally, obtained results are compared with the traditional single phase-shift (SPS) method, the optimized dual phase-shift (DPS) method, and TPS method with RMS current minimization to better highlight the performance of the proposed approach.
In this study, two transformers are designed using the ferrite N87 and the nanocrystalline core materials for the same power level and operating frequency. The operating frequency is defined as ...10 kHz, which is suitable␣for both materials. Modeling and simulation studies have been performed with the same finite element analysis software and the obtained results have been reported. The nanocrystalline and the ferrite N87 core materials have been compared according to both electrical and mechanical parameters. In these comparisons, many features such as core and winding losses, flux distributions, leakage flux, efficiency, and both electrical and mechanical performance have been reported comparatively in the case of rectangular waveform excitation of the transformer. Obtained results show that the weight and the volume of the transformer are reduced and more compact transformer is designed by using the nanocrystalline core material. In addition, besides the core loss, winding losses are also reduced in this design.
The increasing viability of wide band gap power semiconductors, widespread use of distributed power generations, and rise in power levels of these applications have increased interest and need for ...medium voltage converters. Understanding the definitions of insulation coordination and their relationship to applications and methodologies used in the test environment allows system engineers to select the correct insulation materials for the design and to calculate the required distances between the conductive surfaces, accessible parts and ground accurately. Although, design guidelines are well established for low voltage systems, there are some deficiencies in understanding and meeting the insulation coordination requirements in medium voltage, medium frequency applications. In this study, an overview on standards for insulation coordination and safety requirements is presented to guide researchers in the development of medium voltage power electronic converters and systems. In addition, an insulation coordination study is performed as a case study for a medium frequency isolated DC/DC converter that provides conversion from a 13.8 kV AC system to a 4.16 kV AC system.
An interval type-2 fuzzy logic controller-based maximum power point tracking algorithm and direct current–direct current (DC–DC) converter topology are proposed for photovoltaic (PV) systems. The ...proposed maximum power point tracking algorithm is designed based on an interval type-2 fuzzy logic controller that has an ability to handle uncertainties. The change in PV power and the change in PV voltage are determined as inputs of the proposed controller, while the change in duty cycle is determined as the output of the controller. Seven interval type-2 fuzzy sets are determined and used as membership functions for input and output variables. The quadratic boost converter provides high voltage step-up ability without any reduction in performance and stability of the system. The performance of the proposed system is validated through MATLAB/Simulink simulations. It is seen that the proposed system provides high maximum power point tracking speed and accuracy even for fast changing atmospheric conditions and high voltage step-up requirements.
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