1. State Key Laboratory of HVDC Electric Power Research Institute of China Southern Power Grid Guangzhou 510663 China; 2. College of Automation Engineering Nanjing University of Aeronautics and Astronautics Nanjing 211106 China
Abstract：The multiport input-independent and output-series (IIOS) DC-DC converter is adaptive to medium-voltage DC (MVDC) photovoltaic (PV) integration for its independent maximum power point tracking (MPPT) control, high boost ratio, and high efficiency. However, the mismatched input power of distributed PV will cause output voltage inequality, which leads to device overvoltage and the loss of system maximum output power. Based on the magnetic integration technology, a multiport IIOS dual half-bridge topology based on the power-decoupled multi-winding transformer (PD-MWT) is presented in this paper. The proposed topology is capable of realizing independent MPPT, high voltage conversion ratio, and isolation for distributed PV integration. Moreover, it has the following advanced features: (1) The isolation transformer and voltage balancing multi-winding transformer of the proposed topology are integrated into one PD-MWT to isolate different voltage levels and achieve average power delivery, which can significantly reduce the system volume size and achieve higher power density in the IIOS system. (2) Based on the proposed control method, the MPPT and output voltage-balance can be realized merely by configuring fewer sensors and close-control loops in the circuit, which can reduce the system costs. (3) All the power switches can achieve zero-voltage-switching, the switching current stresses on grid side can be reduced, and it is independent of the power mismatch degree. The system structure of the proposed IIOS converter is consisted of N primary-side boost half-bridges (P-HBs), N secondary-side half-bridges (S-HBs) and one PD-MWT. The N primary-side half-bridges are independent of each other and are connected to their respective independent distributed PV panels with low input current ripple, while N secondary-side half-bridges are connected in series to withstand MVDC bus voltage. It should be noted that the number of P-HBs and S-HBs are not necessarily equal. One PD-MWT is installed to link the P-HBs and S-HBs, and to achieve isolation and voltage conversion. The operating principle of the proposed topology is described and the key operation waveforms are plotted, the system parameter design guidelines, such as inductance, capacitance and active switches selection, are also provided in the paper. The down-scale hardware experimental setup with three input-port is built and tested. The input voltage of each submodule converter is 42 V, the desired output voltage of the proposed IIOS converter is 240 V. It can be seen from the experimental results that the power transferring from primary side to secondary side can be achieved by regulating phase-shift ratio dLp. The initial input power of these three ports is 150 W identically. The current iLs,2 and iLs,3 are in phase with iLs,1, which indicate that the slack-port absorbs power from source side and deliver average power to S-HB2 and S-HB3. Moreover, the dynamic response of the proposed converter when considering port 1 input power changes are measured and presented. In response to the power reference of input current controller, the input current of port 1 is changed from 3.6 A to 5.7 A. The output voltage waveforms of these three submodules are measured. And one can find that the submodule output voltages are well balanced and kept stable at 80 V before and after the input power changes. Therefore, the effectiveness and voltage equalization capability of the proposed converter and control strategy under input power unbalanced conditions are fully confirmed.
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2025, Vol. 40 
