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| High-Frequency Oscillation Analysis and Suppression Strategy for M3C-Based Low-Frequency Transmission System of Offshore Wind Power |
| Chen Pengwei1, Li Haojian1, Chen Jie1, Liu Zongye2, Xu Yunfei2 |
1. College of Automation Engineering Nanjing University of Aeronautics and Astronautics Nanjing 211106 China;
2. China Electric Power Research Institute State Key Laboratory of Advanced Power Transmission Technology Beijing 102209 China |
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Abstract The low-frequency transmission based on the modular multilevel matrix converter (M3C) demonstrates substantial potential for offshore wind power. As a typical multi-input multi-output system characterized by multiple control loops, the M3C exhibits a highly pronounced conflict between model complexity and accuracy. Furthermore, as a voltage-source converter, when operating under current vector control, the M3C presents negative resistance characteristics in the high-frequency domain, rendering it susceptible to high-frequency oscillations.
To address the mechanism analysis and suppression requirements of high-frequency oscillations in offshore wind power low-frequency transmission systems, this paper establishes an M3C four-port admittance model that facilitates system integration. A truncation-based order reduction method, constrained by preserving port self-admittance characteristics and described using the average Manhattan distance, is proposed. Subsequently, by integrating the single-machine equivalent output impedance of a direct-drive wind farm and a multi-segment π-type equivalent model of low-frequency submarine cables, the analysis focuses on potential high-frequency oscillations on the low-frequency side. Based on impedance intersection characteristics, high-frequency oscillations are classified into two typical frequency bands, and key parameters influencing these oscillations are identified. An additional control strategy based on a virtual resistance-inductance branch is proposed, along with a parameter design method considering the wide-range impedance distribution of wind farms. Finally, through MATLAB/Simulink time-domain simulation and hardware-in-the-loop experiment, the admittance modeling, high-frequency oscillation mechanism, and the robustness of proposed additional control strategy were verified.
The results of this paper are as follows: Firstly, the full-order M3C four-port admittance model is overly complex. Truncation-based order reduction via Manhattan distance is feasible for stability analysis and parameter design. Secondly, the coupling impedance between the wind farm and M3C is significantly smaller than the self-impedance, enabling the use of dq-axis amplitude-phase characteristics to analyze dominant instability mechanisms. Thirdly, in long-distance low-frequency transmission scenarios, the high-frequency impedance of the wind farm side is dominated by low-frequency submarine cables. Impedance mismatch with the M3C readily induces high-frequency oscillations.
Based on the above results, the conclusions of this paper are as follows: (1) The isolating effect of sub-module capacitors in the M3C results in weak coupling between the low-frequency side and the power-frequency side. When studying the stability of the low-frequency side or power-frequency side in an offshore wind power low-frequency transmission system, part of the coupling elements introduced by sub-module capacitors can be neglected to form a reduced-order admittance model. (2) Low-frequency submarine cables, M3C arm inductors, control parameters, and delays significantly affect high-frequency oscillations. Cable parameters most directly influence oscillation distribution, with shorter cables exacerbating stability issues. (3) The virtual resistance-inductance-based additional control effectively suppresses oscillations caused by low-frequency cables and control delays. Moreover, the parameter design takes into account the impedance distribution range of wind farms, endowing the strategies with enhanced robustness.
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Received: 05 March 2025
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2026, Vol. 41 
