Low-Frequency Oscillation Mechanism and Mitigation in Grid-Forming Inverters with Voltage Controller Effects

The grid-forming (GFM) control plays an important role in grid stability with the large-scale integration of renewable energy sources. In most GFM strategies, the inner voltage and current controls are important for impedance-reshaping and current-limiting. However, the voltage controller is a limiting factor that potentially causes low-frequency oscillations, such as the synchronous oscillation (SO) and sub-synchronous oscillation (SSO). To analyze the effects of the voltage controllers (VCs), this paper develops a unified active power transfer model, providing an intuitive modeling framework for diverse VCs. Then, the dynamics of the vector voltage control (VVC) and virtual admittance control (VAC) are explored comprehensively. The exploration confirms that the VVC poses a significant risk to the SSO in stiff grids due to the high-pass filter characteristics of its equivalent impedance, and the VAC mitigates low-frequency oscillations by emulating an inductance. However, the VAC affects dynamics and results in a steady-state voltage drop across the virtual admittance. Furthermore, an asymmetrical damping scheme is proposed to suppress oscillations and enhance dynamics. Experimental tests validate the correctness of the theoretical analysis and the effectiveness of the proposed asymmetrical damping control.