A Modular Photovoltaic DC-DC Boost Converter with LC Series Reso-Nant Self-Power Balancing Unit
Zhu Xiaoquan1, Hou Penghui1, Hou Jintao1, Jin Ke1, Zhang Bo2
1. College of Automation Engineering Nanjing University of Aeronautics and Astronautics Nanjing 211106 China; 2. School of Electric Power Engineering South China University of Technology Guangzhou 510641 China
Abstract：Input-independent and output-series (IIOS) DC-DC converters can achieve multi-input port- independent MPPT control and series high gain output while maintaining the overall single-stage power conversion of the converter, suitable for large-scale photovoltaic collection. The topology of the IIOS converter is an ideal solution to achieve a high boost ratio, high efficiency, and multi-input port conversion. Since the output side of each sub-module of the IIOS converter is connected in series, if the input power is different, the output voltage of each sub-module will be unbalanced, which may cause some sub-modules to exceed the gain range and exit the MPPT operation. If the input power difference is too large, overvoltage damage to some devices will occur. Therefore, the technical difficulty to be solved is to ensure the safe and stable operation of the converter while maintaining low loss and high economy. This paper proposes an IIOS photovoltaic converter with LC series resonant power balance unit based on the semi-active bridge (SAB). The proposed topology employs the SAB converter as a sub-module, which has the advantages of a dual active bridge converter (DAB), such as soft switching, high power density, high efficiency, simple control, and electrical isolation. Compared with the dual-active bridge converter, the SAB converter has fewer switching devices, a larger soft-switching range, and is more suitable for unidirectional power flow, such as photovoltaic power generation. In order to solve the problem of unbalanced output port voltage of each sub-module of the proposed converter, only a low-value LC power self-balancing unit branch is needed to be added to the secondary arm of each sub-module. The self-balancing function does not require additional control variables, and the complementary conduction of all upper and lower switches can be achieved by relying on the sub-module side. In addition, the proposed converter can synchronously realize MPPT control and output port voltage-sharing control of the photovoltaic array. The control method is simple, and the number of switches is further reduced by device reuse on the secondary side, and the system cost is reduced. All switches on the primary and secondary sides can realize zero-voltage switching (ZVS), and the efficiency is high. It is suitable for applying a photovoltaic array connected to a medium-voltage DC grid. In simulation, the illumination intensity of the sub-modules #1~#3 is slowly increased to 1 000 W/m2, 830 W/m2, and 700 W/m2 respectively, and the sub-modules #4~#6 is slowly decreased to 450 W/m2, 300 W/m2, and 220 W/m2 respectively. The IIOS converter topology with the power self-balancing ability proposed in this paper has an extremely short voltage balancing time and almost no voltage overshoot. The simulation results show that the parameter design conclusions obtained according to the theoretical analysis can ensure that each sub-module can achieve independent MPPT control, which verifies that the proposed IIOS converter can simultaneously complete the boost, MPPT, and voltage equalization functions. Furthermore, a three-module down-scale experimental prototype is conducted to verify the proposed IIOS converter. When the input power of the three ports is 200 W, 100 W, and 100 W, respectively, the output ports of the three modules have achieved voltage equalization. When the input power of sub-module #1 changes from 200 W to 0 W, the output port of the sub-module can still be equalized after about 30 switching cycles. The output port voltages are 47.2 V, 50.3 V, and 52.5 V, respectively. The maximum voltage deviation is 5 % in the open-circuit fault state. The steady-state output current of the converter is 2.65 A and 1.3 A before and after the input power changes. Therefore, the theoretical analysis and simulation results are confirmed.
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2024, Vol. 39 
