Control Strategy of 10 kV/2 MW Modular Multilevel Bidirectional DC-DC Converter for Vessel Integrated Power System
Liu Jilong1, Chen Peng1,2, Xiao Fei1, Zhu Zhichao1, Huang Zhaojie1
1. National Key Laboratory of Science and Technology on Vessel Integrated Power System Naval University of Engineering Wuhan 430033 China; 2. School of Electrical Engineering Southeast University Nanjing 210096 China
Abstract A Modular multilevel bidirectional DC-DC converter (MMBDC) is recommended as the energy router for DC power distribution of the next generation of the integrated power system (IPS) to interconnect the medium-voltage DC (MVDC) bus and the low-voltage DC (LVDC) bus. However, the traditional control strategies for MMBDC have uncontrolled MVDC side currents, highly coupled control loops, and unrestrained low-frequency resonance. It poses the threats of MVDC side overcurrent, the difficulty of designing control parameters, and poor dynamic performance. Therefore, this paper proposes a notch-filter-based triple-loop decoupling control strategy, and the effectiveness is validated through a 10 kV/2 MW engineering prototype. Firstly, the fundamental topology of MMBDC is introduced, which is a cascaded system composed of multi-port modular multilevel DC-DC converter and dual active bridges (DAB). Secondly, the small-signal models of the sub-converters are established. The resonance frequency of MMBDC is derived by solving the transfer function. A generalized state space model is established for DAB to ensure accuracy at the high-frequency band. Thirdly, a triple-loop decoupling control strategy based on a notch filter is proposed, which consists of a double closed-loop control for average sub-module voltage and single closed-loop control for voltage balancing of each sub-module. The MVDC side current is selected as the inner-loop control objective of the average sub-module voltage control to eliminate the overcurrent risk. A notch filter is cascaded in the inner loop to suppress the low-frequency resonance. The control loops are decoupled by restricting the sum of the output of the voltage balancing control loop to 0. The decoupling characteristic and the simplification method of the proposed strategy are derived. Simulations with sinusoidal small-signal inputs of different frequencies are implemented to depict the scatter chart of the frequency-domain characteristics of MMBDC, which is further compared with the theoretical bode diagram. The scatter chart shows high correspondence with the bode diagram. Then, the bode diagrams of the open-loop transfer functions with and without a notch filter are depicted to show the effect of the low-frequency resonance suppression. Finally, a 10 kV/2 MW engineering prototype of MMBDC is established. The design procedures of the components, insulation, control system, and cabinet structure are presented. Static experiments of the proposed strategy are conducted under six load conditions, two load resistors (0.5 Ω and 0.937 5 Ω) multiplied by three kinds of LVDC voltages (800 V, 900 V, 1 000 V). The static experiment results indicate that under full-load conditions, the operating frequency is 95.4 %, and the ripple of the LVDC voltage is 0.49 %. Dynamic experiments are conducted through a sudden change of the load resistor. During the sudden load increase, the fluctuations of the sub-module voltage and LVDC voltage are 6.58 % and 7.30 %, and the settling time is 0.35 s. During the sudden load decrease, the voltage fluctuations are 4.66 % and 9.95 %, and the settling time is 0.40 s. Moreover, the LVDC bus of MMBDC is connected to supercapacitors to study the performance of the proposed control strategy under bidirectional power control. The charging and discharging current references are set to 160 A and -1 000 A, respectively. The following conclusions are drawn: (1) The high correspondence of the simulated and theoretical frequency-domain characteristics shows the accuracy of the established model and the derived resonance frequency of MMBDC, which exactly corresponds to the series resonance frequency of the MVDC inductor and the switching capacitors. (2) The static and dynamic experiment results show that the proposed strategy avoids the risk of the MVDC side overcurrent by selecting the MVDC current as the control, suppresses the low-frequency resonance with the notch filter, and realizes the decoupling control by restricting the sum of the output of the voltage balancing control to 0. (3) The experimental results verify the effectiveness of the engineering prototype design, providing practical experiences for the same type of devices.
Liu Jilong,Chen Peng,Xiao Fei等. Control Strategy of 10 kV/2 MW Modular Multilevel Bidirectional DC-DC Converter for Vessel Integrated Power System[J]. Transactions of China Electrotechnical Society, 2023, 38(4): 983-997.
[1] Ma Weiming.Development of vessel integrated power system[C]//2011 International Conference on Elec-trical Machines and Systems, Beijing, China, 2011: 1-12. [2] 付立军, 刘鲁锋, 王刚, 等. 我国舰船中压直流综合电力系统研究进展[J]. 中国舰船研究, 2016, 11(1): 72-79. Fu Lijun, Liu Lufeng, Wang Gang, et al.The research progress of the medium voltage DC integrated power system in China[J]. Chinese Journal of Ship Research, 2016, 11(1): 72-79. [3] 卜乐平, 欧阳继能, 王黎明, 等. 基于模式切换的舰船逆变器并网策略[J]. 电工技术学报, 2021, 36(14): 3000-3009. Bu Leping, Ouyang Jineng, Wang Liming, et al.Grid-connected strategy for ship inverters and auxiliary generators based on mode switching[J]. Transactions of China Electrotechnical Society, 2021, 36(14): 3000-3009. [4] Xie Ren, Li Hui.Fault performance comparison study of a dual active bridge converter and an isolated modular multilevel DC/DC converter for power conversion module application in a breaker-less shipboard MVDC system[J]. IEEE Transactions on Industry Applications, 2018, 54(5): 5444-5455. [5] Chen Yu, Zhao Shanshan, Li Zuoyu, et al.Modeling and control of the isolated DC-DC modular multilevel converter for electric ship medium voltage direct current power system[J]. IEEE Journal of Emerging and Selected Topics in Power Electronics, 2017, 5(1): 124-139. [6] Mo Ran, Ye Qing, Li Hui.DC impedance modeling and stability analysis of modular multilevel converter for MVDC application[C]//2016 IEEE Energy Con-version Congress and Exposition (ECCE), Milwaukee, WI, USA, 2017: 1-5. [7] Hong Lerong, Xu Qianming, He Zhixing, et al.Fault-tolerant oriented hierarchical control and configuration of modular multilevel converter for shipboard MVDC system[J]. IEEE Transactions on Industrial Informatics, 2019, 15(8): 4525-4535. [8] Xie Ren, Shi Yanjun, Li Hui.Modular multilevel DAB converter for shipboard MVDC system with fault protection and ride-through capability[C]//Energy Conversion Congress and Exposition, Milwaukee, USA, 2015: 427-432. [9] Zhao Biao, Song Qiang, Li Jianguo, et al.High-frequency-link DC transformer based on switched capacitor for medium voltage DC power distribution application[J]. IEEE Transactions on Power Elec-tronics, 2015, 31(7): 4766-4777. [10] Bi Kaitao, Sun Li, An Quntao, et al.Active SOC balancing control strategy for modular multilevel super capacitor energy storage system[J]. IEEE Transactions on Power Electronics, 2019, 34(5): 4981-4992. [11] Chen Peng, Xiao Fei, Liu Jilong, et al.Unbalanced operation principle and fast balancing charging strategy of a cascaded modular multilevel converter bidirectional DC-DC converter in the shipboard applications[J]. IEEE Transactions on Transportation Electrification, 2020, 6(3): 1265-1278. [12] Zhao Biao, Song Qiang, Li Jianguo, et al.Com-parative analysis of multilevel-high-frequency-link and multilevel-DC-link DC-DC transformers based on MMC and dual-active bridge for MVDC appli-cation[J]. IEEE Transactions on Power Electronics, 2018, 33(3): 2035-2049. [13] 武伟, 谢少军, 张曌, 等. 基于MMC双向DC-DC变换器的超级电容储能系统控制策略分析与设计[J]. 中国电机工程学报, 2014, 34(27): 4568-4575. Wu Wei, Xie Shaojun, Zhang Zhao, et al.Analysis and design of control strategy for MMC-BDC based ultra-capacitors energy storage systems[J]. Pro-ceedings of the CSEE, 2014, 34(27): 4568-4575. [14] Chen Peng, Liu Jilong, Xiao Fei, et al.Independent voltage control strategy for cascaded modular multilevel converter-bidirectional DC/DC converter under unbalanced operation[C]//2020 IEEE 9th Inter-national Power Electronics and Motion Control Conference, Nanjing, China, 2021: 1378-1383. [15] 刘基业, 郑泽东, 李驰, 等. 基于准串联SiC MOSFET的高增益直流电力电子变压器[J]. 中国电机工程学报, 2022, 42(8): 2987-2997. Liu Jiye, Zheng Zedong, Li Chi, et al.A high voltage-gain DC power electronic transformer based on quasi-series-connected SiC MOSFET[J]. Pro-ceedings of the CSEE, 2022, 42(8): 2987-2997. [16] Vasiladiotis M, Rufer A.A modular multiport power electronic transformer with integrated split battery energy storage for versatile ultrafast EV charging stations[J]. IEEE Transactions on Industrial Elec-tronics, 2015, 62(5): 3213-3222. [17] 刘计龙, 朱志超, 肖飞, 等. 一种面向舰船综合电力系统的模块化三端口直流变换器[J]. 电工技术学报, 2020, 35(19): 4085-4096. Liu Jilong, Zhu Zhichao, Xiao Fei, et al.A modular three-port DC-DC converter for vessel integrated power system[J]. Transactions of China Electro-technical Society, 2020, 35(19): 4085-4096. [18] Ma Fujun, Wang Xin, Deng Lingfeng, et al.Multiport railway power conditioner and its management control strategy with renewable energy access[J]. IEEE Journal of Emerging and Selected Topics in Power Electronics, 2020, 8(2): 1405-1418. [19] 农仁飚, 杨晓峰, 周兵凯, 等. 电压调整单元双有源桥级联系统的阻抗匹配优化设计[J]. 电工技术学报, 2021, 36(24): 5237-5249. Nong Renbiao, Yang Xiaofeng, Zhou Bingkai, et al.Optimal design of impedance matching for voltage adjustment unit-dual active bridge cascade system[J]. Transactions of China Electrotechnical Society, 2021, 36(24): 5237-5249. [20] Hu Jingxin, An Zheng, Cui Shenghui, et al.Impe-dance modeling and stability analysis of dual-active-bridge converter interfacing DC grids[C]//2018 IEEE Energy Conversion Congress and Expo-sition (ECCE), Portland, USA, 2018: 4907-4914. [21] Mueller J A, Kimball J W.Model-based deter-mination of closed-loop input impedance for dual active bridge converters[C]//2017 IEEE Applied Power Electronics Conference and Exposition (APEC), Tampa, FL, USA, 2017: 1039-1046. [22] Zhao Biao, Song Qiang, Liu Wenhua, et al.Overview of dual-active-bridge isolated bidirectional DC-DC converter for high-frequency-link power conversion system[J]. IEEE Transactions on Power Electronics, 2014, 29(8): 4091-4106. [23] 丁超, 李勇, 姜利, 等. 电动汽车直流充电系统LLC谐振变换器软开关电压边界分析[J]. 电工技术学报, 2022, 37(1): 3-11. Ding Chao, Li Yong, Jiang Li, et al.Analysis of soft switching voltage boundary of LLC resonant con-verter for EV DC charging system[J]. Transactions of China Electrotechnical Society, 2022, 37(1): 3-11. [24] 李涛, 钟成, 钱挺. 提高降压变换器轻载效率的改进型恒定导通时间控制方法[J]. 电气技术, 2021, 22(5): 10-16. Li Tao, Zhong Cheng, Qian Ting.Improved constant on-time control method for improving light-load efficiency of Buck converter[J]. Electrical Engin-eering, 2021, 22(5): 10-16. [25] IEEEStd 1662-2016: IEEE recommended practice for the design and application of power electronics in electrical power systems[S]. USA, 2016.
2023, Vol. 38 
