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| Frequency Coupling Suppression Method for Grid-Connected Inverter Based on Improved Voltage Control |
| Yang Ling1,2, Zhu Difan1, He Qijun1, Lian Chenxi1, Luo Jianqiang1 |
1. School of Automation Guangdong University of Technology Guangzhou 510006 China;
2. State Key Laboratory of High-Efficiency and High-Quality Conversion for Electric Power Hunan University Changsha 410082 China |
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Abstract In the grid-connected inverter (GCI) system, control asymmetry in the dq-axis of the inverter induces a frequency coupling effect. The frequency coupling effect results in comparable amplitudes between the coupling and diagonal subsystems, introducing multiple-input multiple-output (MIMO) characteristics and complicating stability analysis. Therefore, the core objective of suppressing frequency coupling is to increase or maintain the diagonal subsystems while reducing the coupling subsystems. The reverse changes of the two subsystems are used to enlarge the amplitude difference and achieve decoupling. However, the strong coupling effect often forces both subsystems to change unidirectionally.
Based on the established inverter’s sequence admittance model, the analysis results demonstrate that the DC-voltage loop (DVL) significantly influences frequency coupling. Focusing on the frequency coupling effect caused by the DVL asymmetric control structure, this paper utilizes the opposite signs of frequency-domain convolution in positive- and negative-sequence components to realize the reverse change of diagonal subsystems and coupling subsystems in the admittance matrix. Specifically, the opposite sign of the positive-negative sequence disturbances in the q-axis voltage and current achieves a reverse change in the same-row subsystems. Then, the opposite sign of the q-axis frequency convolution results in a reverse shift in the same-column subsystems. Thus, the reverse change of the diagonal and coupling subsystems is achieved.
Regarding the reverse change through the opposite signs of frequency convolution, the control structure for introducing the q-axis voltage and current must be implemented. Therefore, q-axis compensation is applied to the DC-voltage loop to design a frequency coupling suppression method based on improved voltage control. The results of the mathematical mechanism analysis correspond with the admittance Bode analysis, showing that the proposed method can increase the diagonal subsystems or remain unchanged. In contrast, the coupling subsystems are significantly reduced. The coupling subsystems are much smaller than the diagonal subsystems, illustrating that the frequency couplings are essentially suppressed. Based on the equivalent impedance analysis, the output impedance curves of the GCI are treated as either a MIMO or single-input single-output (SISO) system and overlap entirely across the entire frequency range. The influence of the coupling subsystem becomes negligible using the proposed method, reducing the MIMO system to a SISO system. The stability analysis indicates that the proposed method can enhance system stability under weak grid conditions, with a short-circuit ratio (SCR) of 1.47.
A hardware-in-the-loop experimental platform is established. The experimental results show that the proposed method can significantly reduce the coupling current harmonic while slightly increasing the disturbance current harmonic, thereby suppressing the frequency coupling. When the grid transforms from a stiff one to a weak grid, SCR=1.47, with the proposed method, the system remains stable, and the frequency coupling is still suppressed.
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Received: 17 April 2025
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2026, Vol. 41 
