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.

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.

Grid-connected inverters are the key part in renewable energy power generation systems. Usually, phase-locked loop (PLL) is adopted in grid-connected inverters to achieve synchronization. However, prior-art works have revealed that the negative resistance behavior in the low-frequency band caused by PLLs may lead to system instability under weak grids. In this article, first, a comprehensive small-signal output impedance modeling of the LCL -filtered single-phase grid-connected inverter is derived for stability analysis, where the digital control delays and PLL dynamics have been taken into account. Then, a simple but effective virtual series-based impedance reshaping method is proposed. With that, the inverter system can operate stably even under a very weak grid (e.g., short-circuit ratio is as low as 1.2). Compared with the traditional grid impedance compensation by virtually increasing grid resistance or reducing the grid inductance, the proposed method does not require grid impedance estimation and is robust against system parameter variations. Thus, its feasibility is improved. Finally, the experimental tests are carried out on a single-phase 5-kW prototype, validating the effectiveness of the proposed scheme.

Using a multiport converter to integrate renewable energy and energy storage into the bipolar dc bus is a desirable solution due to its high efficiency and reliability. In this article, a series of bipolar output active rectifiers with center-tapped winding is proposed to serve as secondary rectification circuits. Then, a family of isolated four-port converters with symmetrical bipolar outputs is further developed to integrate renewable energy and energy storage. The proposed symmetrical bipolar output isolated four-port converters can achieve single-stage power conversion, symmetrical bipolar output voltage, soft switching, and decoupled control. A typical four-port converter, named full-bridge interleaving bidirectional buck/boost + semiactive rectifier with bipolar output converter, is specifically analyzed in detail in terms of operation principles, voltage and power relationship, port current ripples, soft switching performance, and control. Finally, experimental tests have been performed to verify the feasibility and effectiveness of the proposed topology.