Analysis of Small Signal Stability and Optimization of Control Strategy for Inverter

Weak grids are typically characterized by high impedance and low short-circuit ratios, which can result in system instability and reduced power transmission capacity. This paper develops a small-signal perturbation state-space model and an impedance model for a grid-connected inverter operating under weak grid conditions. It proposes a method to calculate the steady-state operating point and conducts a stability analysis by determining the eigenvalue positions of the state-space matrix, thereby defining stability boundaries. Under conventional vector current control, the system’s maximum output power is limited, losing stability when it exceeds 0.6 p.u. The primary cause of instability is the negative resistance behavior of the phase-locked loop. To enhance the inverter's output power, this paper introduces an improved control strategy known as impedance reshaping. This approach not only preserves a fast dynamic response but also extends the dynamic power limit to nearly match the static power limit, enabling the system to remain stable at around 0.9 p.u. Finally, a simulation comparison between the conventional and improved control strategies validates the effectiveness of the impedance reshaping approach.