Due to the unpredictable and fluctuating nature of solar photovoltaic (PV), energy storage systems (ESS), such as batteries, are always integrated with PV systems to smooth the power supply. In this article, a multiport dc–ac converter (MPC) with differential power processing dc–dc converter (DPPC) is proposed for battery ESS integrated PV systems. The MPC is capable of regulating most of active power among PV, battery and ac grid, and only the differential power (partial power) needs to be handled by the dc-dc converter. Hence, the major merits of higher integration, higher efficiency, and lower cost can be achieved by the proposed configuration. A new cooperative control scheme for the MPC and DPPC is investigated, aiming at realizing flexible active power flow. Besides, a modified space vector pulsewidth modulation is developed for the MPC, taking into the consideration of the voltage variation of both PV and battery. The active power controllability of the MPC and the power rating of the DPPC are quantitatively analyzed. The effectiveness of the proposed configuration, control and modulation schemes is validated by experimental results.

Today, interests on hybrid ac/dc microgrids, which contain the advantages of both ac and dc microgrids, are growing rapidly. In the hybrid ac/dc microgrid, the parallel-operated ac/dc bidirectional interfacing converters (IFCs) are increasingly used for large capacity renewable energy sources or as the interlinking converters between the ac and dc subsystems. When unbalanced grid faults occur, the active power transferred by the parallel-operated IFCs must be kept constant and oscillation-free to stabilize the dc bus voltage. However, under conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs is affected due to the converters' current rating limitations. Moreover, unbalanced voltage adverse effects on IFCs (such as output power oscillations, dc-link ripples, and output current enhancement) could be amplified by the number of parallel converters. Therefore, this paper investigates parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposes a novel control strategy to enhance the active power transfer capability with zero active power oscillation. The proposed control strategy employs a new current sharing method which introduces adjustable current reference coefficients for parallel IFCs. In the proposed control strategy, only one IFC, named as redundant IFC, needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power of parallel IFCs. Simulation and experimental results verify the feasibility and effectiveness of the proposed control strategy.

A distributed dc grid connected photovoltaic (PV) generation configuration based on hybrid-connected three-port converters (TPCs) and its control strategy are proposed in this paper. Distributed maximum power point tracking (MPPT) and autonomous voltage sharing control are achieved with the proposed configuration and control strategy. The proposed system consists of multiple modular pulsewidth modulation plus phase-shift (PPS) controlled TPCs, which feature soft switching and low voltage stress. The input port of each TPC is connected to an independent PV energy source to achieve individual MPPT, and the output ports of these TPCs are connected in series to interface with a high-voltage (HV) dc bus, while the bidirectional ports are in parallel to build a low-voltage (LV) dc bus. The mismatch power of input sources can be transferred through the LV dc bus among these TPCs, and the power and voltage balancing at the HV output side can be realized. By regulating the voltage reference of a bidirectional port in a linear relationship with the output voltage, output voltage sharing is realized with only the module's own voltage and current being sensed. Fully modular design is achieved. System stability with the proposed control strategy is revealed by using the Routh-Hurwitz criterion. Simulation and experimental results are provided to verify the effectiveness and advantages of the proposed configuration and control strategy.

In high-power applications, parallel-operated T-type three-level pulse width modulation (PWM) rectifiers (T3LPRs) are widely employed to improve the power capacity and system reliability. For parallel-operated T3LPRs, there are two important issues need to be properly addressed: 1) the neutral-point potential (NPP) balancing, and 2) the zero sequence circulating current (ZSCC) between the common ac and dc bus. In this paper, the fluctuation of NPP in a T3LPR and the generative mechanism of ZSCC in parallel-operated T3LPRs are first investigated. The zero sequence voltage (ZSV) difference between the parallel-operated T3LPRs is identified as the main excitation source of the ZSCC problem. Meanwhile, the disconnection of the neutral point and the control effect difference of NPP balancing in each T3LPR also affect the ZSCCs within the parallel-operated T3LPRs. To adjust the ZSV of T3LPRs, the dwell time of the positive small voltage vector in each switching period of the converter, which is usually adjusted to balance the NPP, is also required to be controlled to suppress ZSCC. Therefore, in this paper, to avoid the possible conflict, an improved modulation mechanism is designed to guarantee simultaneous ZSCC suppression and NPP balancing. The neutral points of each T3LPR are connected. Then, by adjusting the dwell time of the positive small voltage vector, the charging time of two dc capacitors is adjusted to balance the NPP, and the ZSV differences between different T3LPRs are adjusted to suppress ZSCCs. To avoid the control effect difference of independent NPP balancing in different T3LPRs, all T3LPRs share the same value marking the difference between two dc capacitor voltages. Thus, a communication with a greatly reduced baud rate requirement is needed for NPP balancing to transfer the difference between the two dc capacitor voltages. Moreover, no extra circuit is needed for ZSCC suppression. These approaches make the proposed improved space vector PWM strategy much more practical and easy to employ. The proposed method is verified with experiments on two 3-kW T3LPR prototypes.

A dual-dc-port dc–ac converter with virtual space vector pulsewidth modulation (SVPWM) based flexible power control is proposed for photovoltaic (PV)-battery hybrid systems. Two dc ports, in which one dc port is interfaced with PV and the other dc port is interfaced with the battery, are provided simultaneously by the dual-dc-port dc–ac converter. Hence, all the active power is transferred within single power conversion stage, and the dc–dc converter between PV and battery is removed. To realize flexible power control between PV, battery, and ac grid, a new control scheme consisting of an outer power loop and an inner virtual SVPWM is proposed, with fully considering the voltage variation of both PV and battery. Operation principle and power control capability of the dual-dc-port dc–ac converter are analyzed in detail. The effectiveness of the proposed solution is validated by experimental results.

Current-fed bidirectional dc-dc converters have attracted much attention in battery energy storage system applications due to their zero-voltage-switching performances and greater degree of control freedom. However, high efficiency cannot be achieved for both light and heavy load ranges with the present modulation schemes for a current-fed bidirectional dc-dc converter, especially with low battery voltages. A new modulation scheme is proposed to improve the conversion efficiency over a wide load range and with variations in the battery voltage. The relationship between the duty cycle of the secondary full bridge and the phase-shift angle was investigated to lower the power losses with the wide load variations. Based on the operation mode analysis, the control loop with the proposed pulse wide modulation plus phase-shift modulation scheme was developed. Comparisons of the switch conduction loss and the core loss in the series inductor for the previous modulation schemes and the proposed modulation scheme were carried out. The results show that lower power losses occur with the proposed scheme. The experimental results verify the theoretical analysis and effectiveness of the proposed modulation scheme on loss reduction.