In the standalone renewable energy system, multiport converter is more desirable than several single-input converters as it has advantages of simpler circuit, higher efficiency and lower cost. This paper proposes a novel topology synthesis method to derive integrated three-port converters with low cost and compact structure. With the proposed topology synthesis method, integrated three-port converters can be easily developed from conventional single-input single-output (SISO) converters by replacing a switch and a diode with a basic cell including bidirectional power flow. Compared with the SISO converter, the derived integrated three-port converters only need to add the extra switch and diode, which makes they have low cost and compact structure. As an example, topology synthesis based on buck, boost, buck-boost, Cuk, SEPIC, and zeta converters is performed in this paper. The proposed topology synthesis method can also be extended to deduce a series of N-port converters. In order to achieve better understanding of the proposed converters, the integrated three-port SEPIC converter is specifically analyzed, including its operation principle, steady-state performance, small-signal modeling and control system. Finally, experimental verifications have been carried out to illustrate the feasibility and effectiveness of the proposed topology synthesis method.

Three-port converters with high voltage gain are desirable solutions for integrating renewable energy and energy storage devices into high voltage dc bus. A high gain three-port converter with soft switching is proposed in this paper, which can realize zero voltage switching for all switches and zero current switching for all diodes in various operating modes. Single coupled inductor is used to achieve high voltage gain and to reduce the voltage stress of switches so that switches with low on -resistance can be selected to reduce the conduction loss. In addition, the advantages of fewer components, higher voltage gain, and very low switch voltage stresses make the proposed converter more suitable for application in renewable energy systems than similar solutions. Various operating modes, performance analysis, design considerations, efficiency analysis, and control method of the proposed converter are discussed. A laboratory prototype with 30 V renewable energy source, 48 V energy storage device and 400 V output is designed to verify the performance of the proposed three-port converter.

Multibus dc microgrids, which combine renewable energy sources, energy storage systems, and loads, have voltage stability requirement, which solicits increasing research attention in practice. Potentially complex architectures of the multibus dc microgrids make it difficult to evaluate the stability using the conventional stability criteria. In this article, some constraints related to the conventional stability criteria, such as right-half-plane (RHP) poles or zeros in the subsystems are discussed. Further, an impedance-based stability criterion is proposed in the light of generalized bode plots for multibus dc microgrids. The configuration of the multibus dc microgrid is simplified by adopting generalized voltage source, generalized current source, and two-port model. Then, impedances or admittances for each bus port can be derived, which are helpful for assessing the stability of the system. Using the proposed stability criterion, the stability of each bus port in the multibus dc microgrid can be evaluated separately. The proposed stability method considers the number of RHP pole of the open-loop transfer function for the system so the stability of the system consists of subsystems with RHP pole and zero can be analyzed correctly. Moreover, the intermittent bus converter connected with neighboring dc microgrid can be regarded as an extension unit. Then, the proposed method can be easily extended and is acting as a generalized approach for different configurations, i.e., single-bus dc mirogrid or a cluster of dc microgrids. Experiments are done to validate the effectiveness of the proposed criterion.

This article proposes a two-mode active balancing circuit for series-connected cell strings. The proposed balancing circuit has two operation modes: any cell to any cell (AC2AC) and direct cell to cell (DC2C) modes. In AC2AC mode, the balancing circuit can be equivalent to an improved switched-capacitor (SC) balancing circuit, and the energy is transferred among all imbalanced cells, implementing the higher balancing speed when the voltage gap between cells and the number of imbalanced cells are large. In DC2C mode, the proposed balancing circuit can be equivalent to a balancing circuit based on three-resonant-state LC unit, and the energy is transferred from the highest voltage cell to the lowest voltage one, achieving the higher balancing efficiency and speed as well as accuracy when the voltage gap between cells is small. The operation state, balancing power, and balancing efficiency of each mode are analyzed in detail. The control strategy and design considerations of the balancing circuit are introduced to obtain the optimal balancing performance. Experimental results are provided to verify the validity of the proposed balancing circuit. The results show that the proposed balancing circuit achieves high balancing accuracy and efficiency as well as balancing speed.