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.
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