Abstract：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.
[1] 郑崇伟. 海上可再生能 (波浪能、风能) 资源利用的理论研究[D]. 长沙: 国防科技大学, 2018. [2] 徐一波, 鄢敉君, 李林. 海上风电机组叶片的发展趋势分析[J]. 南方农机, 2022, 53(10): 143-145. Xu Yibo, Yan Mijun, Li Lin.Development trend analysis of offshore wind turbine blades[J]. China Southern Agricultural Machinery, 2022, 53(10): 143-145. [3] 蔡蓉, 张立波, 程濛, 等. 66 kV海上风电交流集电方案技术经济性研究[J]. 全球能源互联网, 2019, 2(2): 155-162. Cai Rong, Zhang Libo, Cheng Meng, et al.Technical and economic research on 66 kV offshore wind power AC collection solution[J]. Journal of Global Energy Interconnection, 2019, 2(2): 155-162. [4] 吴睿, 张健, 赵长红, 等. 基于LCOE模型的海上风电平价上网分析[J]. 中国能源, 2021, 43(2): 48-53. Wu Rui, Zhang Jian, Zhao Changhong, et al.Analysis of offshore wind power parity based on LCOE model[J]. Energy of China, 2021, 43(2): 48-53. [5] 姚钢, 杨浩猛, 周荔丹, 等. 大容量海上风电机组发展现状及关键技术[J]. 电力系统自动化, 2021, 45(21): 33-47. Yao Gang, Yang Haomeng, Zhou Lidan, et al.Development status and key technologies of large- capacity offshore wind turbines[J]. Automation of Electric Power Systems, 2021, 45(21): 33-47. [6] 孔一颖. 漂浮式海上风电是未来发展趋势[J]. 海洋与渔业, 2019(7): 55-57. Kong Yiying.Floating offshore wind power is the future development trend[J]. Ocean and Fishery, 2019(7): 55-57. [7] 江海涛, 顾文, 梅睿, 等. 某海上风电场谐波谐振实测及仿真分析[J]. 电气技术, 2021, 22(11): 20-26. Jiang Haitao, Gu Wen, Mei Rui, et al.Test and simulation research on harmonic resonance of offshore wind farm[J]. Electrical Engineering, 2021, 22(11): 20-26. [8] 邵冰冰, 赵峥, 肖琪, 等. 多直驱风机经柔直并网系统相近次同步振荡模式参与因子的弱鲁棒性分析[J]. 电工技术学报, 2023, 38(3): 754-769. Shao Bingbing, Zhao Zheng, Xiao Qi, et al.Weak robustness analysis of close subsynchronous oscillation modes' participation factors in multiple direct-drive wind turbines with the VSC-HVDC system[J]. Transactions of China Electrotechnical Society, 2023, 38(3): 754-769. [9] 徐衍会, 成蕴丹, 刘慧, 等. 基于瞬时功率的次同步振荡频率提取及振荡源识别方法[J]. 电工技术学报, 2023, 38(11): 2894-2907. Xu Yanhui, Cheng Yundan, Liu Hui, et al.Sub- synchronous oscillation frequency extraction and oscillation source identification method based on instantaneous power[J]. Transactions of China Elec- trotechnical Society, 2023, 38(11): 2894-2907. [10] 董文凯, 杜文娟, 王海风. 弱连接条件下锁相环动态主导的并网直驱风电场小干扰稳定性研究[J]. 电工技术学报, 2021, 36(3): 609-622. Dong Wenkai, Du Wenjuan, Wang Haifeng.Small- signal stability of a grid-connected PMSG wind farm dominated by dynamics of PLLs under weak grid connection[J]. Transactions of China Electrotechnical Society, 2021, 36(3): 609-622. [11] 崔鹤松, 李雪萍, 黄晟, 等. 模块化多相永磁风力发电机串并联直流海上风电场电压协调控制[J]. 电工技术学报, 2023, 38(4): 925-935. Cui Hesong, Li Xueping, Huang Sheng, et al.Voltage coordinated control strategy for modular multi-phase PMSG-based series-parallel DC connected offshore wind farm[J]. Transactions of China Electrotechnical Society, 2023, 38(4): 925-935. [12] 高晨, 赵勇, 汪德良, 等. 海上风电机组电气设备状态检修技术研究现状与展望[J]. 电工技术学报, 2022, 37(增刊1): 30-42. Gao Chen, Zhao Yong, Wang Deliang, et al.Research status and prospect of condition based maintenance technology for offshore wind turbine electrical equipment[J]. Transactions of China Electrotechnical Society, 2022, 37(S1): 30-42. [13] Sun Yuanxiang, Li Zhen, Zhang Zhenbin.Hybrid predictive control with simple linear control based circulating current suppression for modular multilevel converters[J]. CES Transactions on Electrical Machines and Systems, 2019, 3(4): 335-341. [14] He Jiangbiao, Yang Qichen, Wang Zheng.On-line fault diagnosis and fault-tolerant operation of modular multilevel converters-a comprehensive review[J]. CES Transactions on Electrical Machines and Systems, 2020, 4(4): 360-372. [15] Chen Yongyang, Pan Shangzhi, Huang Meng, et al.MMC-MTDC transmission system with partially hybrid branches[J]. CES Transactions on Electrical Machines and Systems, 2021, 5(2): 124-132. [16] Liu He, Armstrong M, Naayagi R T, et al.High voltage high power DC/DC modular multilevel con- verter for offshore windfarm DC collection point[C]// 8th IET International Conference on Power Elec- tronics, Machines and Drives (PEMD 2016), Glasgovv, UK, 2016: 1-6. [17] Liu He, Dahidah M S A, Yu J, et al. Design and control of unidirectional DC-DC modular multilevel converter for offshore DC collection point: theoretical analysis and experimental validation[J]. IEEE Transa- ctions on Power Electronics, 2019, 34(6): 5191-5208. [18] Gautam S P, Kedia S, Bahirat H J, et al.Design considerations for medium frequency high power transformer[C]//2018 IEEE International Conference on Power Electronics, Drives and Energy Systems (PEDES), Chennai, India, 2019: 1-5. [19] Ferreira J A.The multilevel modular DC converter[J]. IEEE Transactions on Power Electronics, 2013, 28(10): 4460-4465. [20] Du Sixing, Wu Bin.A transformerless bipolar modular multilevel DC-DC converter with wide voltage ratios[J]. IEEE Transactions on Power Electronics, 2017, 32(11): 8312-8321. [21] Du Sixing, Wu Bin, Tian Kai, et al.A novel medium-voltage modular multilevel DC-DC con- verter[J]. IEEE Transactions on Industrial Electronics, 2016, 63(12): 7939-7949. [22] Du Sixing, Wu Bin, Zargari N R.A transformerless high-voltage DC-DC converter for DC grid inter- connection[J]. IEEE Transactions on Power Delivery, 2018, 33(1): 282-290. [23] Pan Junliang, Wang Mingyu.A flying-capacitor modular multilevel converter[C]//2018 IEEE 7th International Conference on Power and Energy (PECon), Kuala Lumpur, Malaysia, 2019: 41-44. [24] Li Binbin, Zhao Xiaodong, Cheng Da, et al.Novel hybrid DC/DC converter topology for HVDC interconnections[J]. IEEE Transactions on Power Electronics, 2019, 34(6): 5131-5146. [25] Li Binbin, Zhao Xiaodong, Zhang Shuxin, et al.A hybrid modular DC/DC converter for HVDC appli- cations[J]. IEEE Transactions on Power Electronics, 2020, 35(4): 3377-3389. [26] Li Binbin, Liu Jianying, Wang Zhiyuan, et al.Modular high-power DC-DC converter for MVDC renewable energy collection systems[J]. IEEE Transactions on Industrial Electronics, 2021, 68(7): 5875-5886. [27] Li Lei, Li Binbin, Wang Zhiyuan, et al.Monopolar symmetrical DC-DC converter for all DC offshore wind farms[J]. IEEE Transactions on Power Elec- tronics, 2022, 37(4): 4275-4287. [28] Zhang Shuxin, Li Binbin, Cheng Da, et al.A monopolar symmetrical hybrid cascaded DC/DC converter for HVDC interconnections[J]. IEEE Transa- ctions on Power Electronics, 2021, 36(1): 248-262. [29] 陈曦, 王瑾, 肖岚. 用于高压高频整流的二极管串联均压问题[J]. 电工技术学报, 2012, 27(10): 207-214. Chen Xi, Wang Jin, Xiao Lan.Research on the voltage sharing in series coupled diodes for high voltage and high frequency rectifier applications[J]. Transactions of China Electrotechnical Society, 2012, 27(10): 207-214. [30] Xing Zhongwei, Ruan Xinbo, You Hongcheng, et al.Soft-switching operation of isolated modular DC/DC converters for application in HVDC grids[J]. IEEE Transactions on Power Electronics, 2016, 31(4): 2753-2766.
2024, Vol. 39 
