Power coupling and stability analysis of GFM due to rotational frames and control loops interaction

Abstract

The power coupling issue in grid-forming converters (GFMs) is an emerging concern within power systems dominated by inverter-based resources. Specifically, the phase difference between two rotational frames, i.e., system/grid and controller frames, has not been associated with the power coupling effect. It is demonstrated that such phase difference causes interaction between the inner and power control loops. To this end, we first developed a detailed power model of a GFM system to study the sensitivity of control parameters against coupling issues. In particular, we found that the inner voltage control loop could impact the system's stability. In addition, the controller-generated virtual inductance and the cut-off frequency of the low-pass filter trigger a destabilizing effect for GFM using P-δ control compared to P-ω droop control. This is due to the phase difference and control loops interaction. As a result, the typical virtual inductance becomes inadequate in suppressing the coupling for P-δ droop control. Subsequently, a q-axis-based virtual impedance combined with phase difference compensation is proposed. It extends the range of stable control parameters and minimizes the power coupling for both types of droop controls. Experiments were conducted to verify the theoretical analysis and the proposed scheme.