Effect of Control Damping on Small-Signal Stability of Grid-Forming VSCs Considering Interaction Between Inner and Outer Loops

Strong coupling of conventional dual-loop control in dq frame makes stability analyses for grid-forming voltage-source converters (VSCs) inconvenient, hence the interaction between outer and inner loops is often omitted. However, this interaction plays an important role in system stability. To reduce the system orders, a decoupled inner-loop control method is proposed for grid-forming VSCs. This method has a similar damping adjustment ability to the dual-loop control, but can describe the inner-loop damping characteristic with only one parameter. With the help of this method, the effects of control damping and grid impedance on the system stability are revealed. First, the inner loop provides positive damping, which stabilizes the grid-forming VSCs. Second, the outer-loop droop control behaves as a negative damping characteristic which destabilizes the system, and larger droop coefficients can aggravate the negative damping and deteriorate the system stability. Third, the grid impedance can provide positive damping to the system, thus the grid-forming VSCs are prone to be stable under weak grid conditions. The analyses are verified by hardware-in-loop tests.