Abstract：As a grid-forming inverter, the inverter based on virtual synchronous generator (VSG) control can contribute to the regulation of grid voltage and frequency. Power systems with a high penetration of new energy sources and power electronics are prone to stability when applied with VSG-controlled inverters. However, the switched oscillation induced by the saturation of limiters in the grid-connected inverter based on VSG control under large disturbances can threaten the stability of the power system. This paper studies the mechanism of such switched oscillations. The effects of limiter type, limiter value, and system parameters on the switched oscillation are also analyzed. Firstly, the switched oscillation and non-smooth bifurcation theory are introduced, as well as the VSG-controlled inverter and its limiters. Secondly, the switched oscillation in the VSG-controlled inverter under large disturbances is analyzed in terms of time-domain waveforms and amplitude-frequency characteristics, According to phase diagrams, the switched oscillation is essentially induced by the non-smooth bifurcation. Thirdly, the relationship between non-smooth bifurcation and limiter type and limiter value is analyzed. Non-smooth bifurcation is induced by the saturation of multiple limiters. Finally, the effects of VSG control parameters and external parameters on the non-smooth bifurcation characteristics are analyzed. The results show that the switched oscillation is more likely to occur with smaller limiter values and larger disturbances. Simulation results show that the power oscillation occurs in the VSG-controlled inverter with limiters under a symmetrical short-circuit fault with a dominant oscillation frequency of 0.36 Hz. The phase diagram during the power oscillation shows that a non-smooth limit cycle occurs, and the C-bifurcation essentially induces the switched oscillation according to the trajectory evolution. Non-smooth bifurcations may occur when both current and modulation signal limiters are included. In addition, as the parameter values are varied, the transition between the equilibrium point and the non-smooth limit cycle occurs, and the oscillation amplitude and frequency are also varied. The following conclusions can be drawn. (1) Under large disturbances, the switched oscillation may occur in the VSG-controlled inverter with limiters saturated, which is essentially induced by the non-smooth bifurcation. Switched oscillation is an important instability form for the grid-forming inverter, and its mechanism and influencing factors are more complex than the traditional transient synchronous instability. (2) The non-smooth bifurcation in the VSG-controlled inverter results from the saturation of current and modulation signal limiters, and the angular frequency limiter affects the power oscillation characteristics. (3) The switched oscillation is more likely to occur with smaller limiter values and larger disturbances. Furthermore, the SCR and droop coefficients are critical parameters affecting the oscillation amplitude and frequency, respectively.
纪君奇, 杨黎晖, 马西奎. 基于虚拟同步发电机控制的并网逆变器切换型振荡及其非光滑分岔特性[J]. 电工技术学报, 2024, 39(24): 7860-7873.
Ji Junqi, Yang Lihui, Ma Xikui. Switched Oscillation and Its Non-Smooth Bifurcation Characteristics in Grid-Connected Inverter Based on Virtual Synchronous Generator Control. Transactions of China Electrotechnical Society, 2024, 39(24): 7860-7873.
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2024, Vol. 39 
