The current-fed dual active bridge (CF-DAB) dc-dc converter gains growing applications in photovoltaic (PV) and energy storage systems due to its advantages, e.g., a wide input voltage range, a high step-up ratio, a low input current ripple, and a multiport interface capability. In addition, the direct input current controllability and extra control freedom of the CF-DAB converter make it possible to buffer the double-line-frequency energy in grid-interactive PV systems without using electrolytic capacitors in the dc link. Therefore, a PV system achieves high reliability and highly efficient maximum power point tracking. This paper studies the optimized operation of a CF-DAB converter for a PV application in order to improve the system efficiency. The operating principle and soft-switching conditions over the wide operating range are thoroughly analyzed with phase-shift control and duty-cycle control, and an optimized operating mode is proposed to achieve the minimum root-mean-square transformer current. The proposed operating mode can extend the soft-switching region and reduce the power loss, particularly under a heavy load and a high input voltage. Moreover, the efficiency can be further improved with a higher dc-link voltage. A 5-kW hardware prototype was built in the laboratory, and experimental results are provided for verification. This paper provides a design guideline for the CF-DAB converter applied to PV systems, as well as other applications with a wide input voltage variation.

This paper proposes a grid-tied photovoltaic (PV) system consisting of modular current-fed dual-active-bridge (CF-DAB) dc-dc converter with cascaded multilevel inverter. The proposed converter allows a small dc-link capacitor in the three-phase wye-connected PV system; therefore, the system reliability can be improved by replacing electrolytic capacitors with film capacitors. The low-frequency ripple-free maximum power point tracking (MPPT) is also realized in the proposed converter. First of all, to minimize the influence resulting from reduced capacitance, a dc-link voltage synchronizing control is developed. Then, a detailed design of power mitigation control based on CF-DAB dynamic model is presented to prevent the large low-frequency voltage variation propagating from the dc-link to PV side. Finally, a novel variable step-size MPPT algorithm is proposed to ensure not only high MPPT efficiency, but also fast maximum power extraction under rapid irradiation change. A downscaled 5-kW PV converter module with a small dc-link capacitor was built in the laboratory with the proposed control and MPPT algorithm, and experimental results are given to validate the converter performance.

A new modulation scheme for the active clamping zero-voltage switching (ZVS) inverter is proposed. With the proposed modulation scheme, the inverter can realize ZVS operation in all switching devices and can remarkably suppress the reverse recovery current in insulated-gate bipolar transistor's antiparallel diodes as well. All the switches can operate at a fixed frequency in the new modulation scheme. Also, with the new modulation scheme, the ZVS inverter has some voltage boost capability, which is suitable for renewable energy generation. The inverter can achieve ZVS in all the switches under the grid current power factor angle from zero to ±π/6. The soft-switching condition of this inverter is analyzed. The operation principal and design guideline are described in detail. The previous theory is verified in a 30-kW inverter prototype.

A modular multilevel converter (MMC) integrated with split battery modules has been verified a better choice for the large-scale battery energy storage system (BESS). In this case, both ac grid and dc grid can exchange power with the BESS while the power transfer between ac and dc grids can be decoupled. However, without the ability to ride through dc grid fault, the performance and reliability of whole system will be reduced. Therefore, a hybrid MMC (HMMC) with integrated BESS (HMMC-BESS) is proposed in this paper. Then the batteries can exchange power with the normal grid continuously and independently, even when short-circuit fault condition happens on ac or dc grid. However, the traditional control schemes are no longer suitable for the fault modes. In this paper, the configuration, modeling, and operation modes of HMMC-BESS are discussed first. After that, both ac and dc grid fault modes of HMMC are analyzed and control schemes of corresponding mode are presented. In addition, novel balancing methods for the state-of-charge and voltage allocation scheme are proposed according to different operation modes. Finally, a downscaled prototype is built in laboratory to verify the proposed control scheme.

Currently, residential photovoltaic power generation system is increasingly used worldwide. In this paper, an optimized structure of residential photovoltaic (PV) power generation system with 1500V DC bus is proposed. It includes PV panels, a three-level boost converter, a high efficiency isolated bidirectional DC-DC converter, battery and three-phase five-level DC-AC converter that can work under islanding mode or grid-connected mode. The higher DC bus voltage greatly reduces line loss and improves efficiency of the system. An energy management scheme used for the system is proposed in this paper to guarantee the stability of the system and to increase its economic benefits. Besides, the optimized method for the structure of the bidirectional dc-dc converter is proposed. This structure can achieve higher DC voltage gain and higher efficiency. Furthermore, for low voltage battery application in the residential system, LLC and CLLC under DC transformer (DCX) mode are evaluated and the LLC is selected as the isolated bidirectional DC-DC converter. The optimized designed method of bidirectional LLC is proposed. Finally, experiments are carried out to verify the performance of the optimized converters and the system.