Dominant Instability Mechanism and Stability Stationary Points of Grid-forming Converter Under Stiff Grid

Based on the damping torque analysis (DTA), the dominant instability mechanism of grid-forming (GFM) converters under a stiff grid is identified. With the increase of short-circuit ratio (SCR), the frequency of the low-frequency oscillation (LFO) dominated by the outer active power control loop (APCL) increases, resulting in the inner voltage control loop (VCL) bandwidth unable to counteract the negative damping effect introduced by APCL. In particular, 1) it is proved that the system stability features a maximum stability stationary point with the increase of VCL bandwidth, exhibiting a “non-monotonic” characteristic in system stability. 2) It is demonstrated that the dynamics of the outer reactive power control loop (RPCL) can be neglected in the stability analysis for GFM converters across different grid strengths. Case studies are conducted to verify the effectiveness and generality of the theoretical analysis.