With the increasing saturation of offshore wind farm sites, the trend in offshore wind power is shifting towards deep-water development. The absence of reactive power loss and sub-synchronous oscillation problems makes it more suitable for large-capacity, long-distance, deep-water offshore wind farms. The high-voltage DC/DC converter is the core component of all-DC offshore wind power, connecting the collector grid and the transmission line. A resonant flying capacitor modular boost converter is proposed for large-scale offshore wind power applications. Firstly, the transformer topology is introduced, and the operating mode analysis is carried out. This converter typically uses a multiphase structure to increase the transmission power and cancel out the AC component of the current at the DC input and output during symmetrical operation. Each phase consists of two sub-module half-bridges, diode half-bridges, and flying capacitors. The sub-module half-bridge connected to the DC input is named the master-bridge, and the other is named the slave-bridge. The master-bridge and slave-bridge adopt quasi-square-wave modulation to reduce the sub-module capacitance and facilitate soft-switching. The modulation phase difference between master-bridge and slave-bridge is adjusted to change the output power. According to the relationship between master-bridge arm inductance Lm and slave-bridge arm inductance Ls, the converter operates in two conditions. The diode half-bridge can operate in continuous or discontinuous current mode based on parameter designs. Then, according to the phase relationship of steady-state voltage and current in the converter’s internal AC equivalent circuit diagram and the operating mode analysis, the two design methods are presented, one where Ls is much larger than Lm, and the other where Lm is much larger than Ls. When Ls is much larger than Lm, the continuous current mode makes the DC side voltages of the slave-bridge and diode half-bridges close to that of the master-bridge, more convenient for device selection. When Lm is much larger than Ls, the discontinuous current mode makes the DC side voltages of the slave-bridge and diode half-bridges close to that of the master-bridge. The ideal voltage and current waveforms for both operating conditions are quantitatively analyzed, and the arm current RMS and maximum values of each half-bridge are obtained. In the continuous current mode, Ls is much larger than Lm. In the discontinuous current mode, Lm is much larger than Ls, and the arm current RMS and maximum values of the slave-bridge increase greatly. Since the arm current of the master-bridge and the number of diode half-bridges are the largest, the on-state loss increase in the slave-bridge has a small effect on the overall on-state loss of the converter. In this discontinuous mode, the power devices of the master-bridge and slave-bridge are more conducive to soft-switching, and the series diodes inside the diode half-bridge do not tolerate high dv/dt. Considering that high-frequency operation can make the converter lightweight and reduce switching loss, in the discontinuous current mode, the design method that Lm is much larger than Ls is adopted. The parameter design methods of the master-bridge, slave-bridge, diode half-bridge, and flying capacitor are given. Finally, the converter topology and the parameter design method are verified by low-power prototype experiments. A resonant flying capacitor modular Boost converter operates at a higher frequency without a transformer structure, reducing internal passive components and achieving light weight. In high step-up ratio applications, most power devices inside the converter are diodes, reducing control units and costs. Compared to the continuous current mode, the discontinuous current mode allows for soft-switching with minimal increase in on-state losses, and the quasi-square wave modulation mitigates high dv/dt issues of the internal series diodes. © 2024 China Machine Press. All rights reserved.
