In this paper, a dual half-bridge LLC resonant converter with magnetic control is proposed for the battery charger application. The primary switches are shared by two LLC resonant networks, and their outputs are connected in series. One of the LLC resonant converter is designed to operate at the series resonant frequency, which is also the highest efficiency operating point, and the constant output voltage characteristic is achieved at this operating point. The second LLC resonant converter adopts magnetic control to regulate the total output current and voltage during both constant current (CC) charge mode and constant voltage (CV) charge mode. Meanwhile, the function decoupling idea is adopted to further improve the system efficiency. The significant amount of the power is handled by the LLC resonant converter operating at the series resonant frequency, while the second LLC resonant converter fulfills the responsibility to achieve closed-loop control. By carefully designing the resonant networks, the zero-voltage switching (ZVS) for primary switches and zero-current switching (ZCS) for secondary diodes can be achieved for whole operation range. A 320W experimental prototype is built to verify the theoretical analysis, and the maximum efficiency is measured about 95.5%.

Reliability enhancement of microgrids is challenged by environmental and operational failures. Centrally controlled microgrids are susceptible to failures at high probability due to a single-point-of-failure, e.g. the central controller. True decentralization of microgrid architecture entails elimination of the central controller, attaining a parallel configuration for the system. In this paper, decentralized microgrid control architecture is proposed as a solution for reliability degradation over the time, and analyzes the reliability aspects of centralized and decentralized control architectures for microgrids. Degree of importance of a single controller in centralized and decentralized architectures is determined and validated by Markov Chain Models (MCM). Results confirm that higher reliability is achieved when true decentralization of control architecture is adopted. Challenges of implementing a true decentralized control architecture are discussed. Hardware-In-the-Loop simulation results for microgrid controller failure scenarios for both architectures are presented and discussed.

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.

—In the recent years, there has been a growing interest in the concept of microgrids to integrate distributed generation systems and to provide higher reliability for critical loads. Several microgrid demonstration projects have been implemented to investigate further and advance this emerging concept. This article provides a detailed review of microgrid systems. It describes different architectures, including AC, DC, and hybrid systems. Various microgrid components, including sources, converters, and loads, are illustrated. Microgrid management and controls are discussed, and a modified natural droop control is described in detail. Both physical layers and standard protocols are explained for communication in the microgrid structure. The unique protection complexities have been raised and discussed in the presence of distributed generations and bidirectional power flow. A demonstration of a military microgrid system at Fort Sill is illustrated, and the experiment of a typical microgrid operation scenario is provided.