Abstract As an important interface between new energy power generation and the grid, the Voltage Source Converter (VSC) has been widely used in new power systems. At present, the grid following VSC based on current vector control has extensive application in engineering practice. However, with the increasing integration of new energy sources and the continuous addition of power electronic equipment, power grid stability weakens, leading to challenges in the stability of grid-following VSCs. The concept of grid-forming VSC is proposed to improve the stability, especially in weak grid scenarios and for power supply to isolated islands. Power Synchronization Control (PSC) and Virtual Synchronous Generator (VSG) control have gained significant attention due to their simplicity. It is found that PSC control with a first-order low-pass filter can be equivalent to VSG control when the influence of energy storage is ignored. However, an issue known as Synchronous Frequency Resonance (SFR) arises when VSCs controlled by PSC or VSG are connected to inductive grids with small equivalent resistances. Based on the control equation, VSG control and PSC control with a first-order low-pass filter can be equivalent to each other. The coupling effect between the active power control loop (APCL) and reactive power control loop (RPCL) is explored, and an equivalent mathematical model for their mutual coupling is deduced. Secondly, the influence of the voltage and current inner loop dynamics of the system is analyzed, demonstrating that the inner loop does not change the system characteristics near the synchronization frequency. Therefore, to simplify the analysis of the synchronous frequency resonance mechanism, the inner loop is considered fast enough to achieve ideal voltage tracking. Thirdly, based on the small signal model in the frequency domain, it is verified that a small equivalent resistance in an inductive grid may cause SFR, and power control loop parameters can affect system stability. The Nyquist criterion analysis indicates that the presence of a non-minimum phase system is a cause of SFR. Finally, the mechanism of the SFR phenomenon of the grid-forming converter is analyzed, and an SFR suppression strategy is proposed. The effectiveness of the proposed method is verified. The following conclusions can be drawn from the analysis: (1) A small signal model in the frequency domain of grid-forming PSC-VSC coupled with APCL and RPCL is established. According to the closed-loop system eigenvalue loci, it is verified that a small equivalent resistance of the inductive grid can cause SFR. The non-minimum phase effect is another reason for SFR. (2) Compared with the first-order low-pass filter (equivalent to VSG control), the proposed SFR suppression method can effectively suppress the resonance peak and require lower precision of parameter design. Meanwhile, it also has strong robustness in control parameters and a more substantial stability margin for the system. (3) The theoretical analysis is verified based on the established three-phase grid-connected converter experiment platform.
Xiong Xiaoling,Li Xinyue,Zhou Yan等. Synchronous Frequency Resonance Suppression of Grid-Forming Converter Based on Notch Filter[J]. Transactions of China Electrotechnical Society, 2024, 39(12): 3827-3839.
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