Hybrid Active Damping Control Method for Grid Connected LCL Inverters Under Weak Grid

This LCL filters are widely used in grid connected inverters. When the transmission (grid) impedance is not considered, active damping is usually achieved by increasing the capacitance current inner loop. However, when the line impedance is considered, the active damping coefficient of the capacitor current inner loop becomes larger with the increase of the grid impedance. The excessive damping coefficient leads to a sharp decline in the phase margin of the open loop transfer function of the system, which affects the stability of the system. In this paper, another active damping is realized by increasing the inner loop feedback of the grid current on the basis of the inner loop of the capacitive current. The active damping coefficient of the grid current inner loop becomes smaller with the increase of the transmission impedance. Combining the two active damping methods and the grid voltage feed-forward, the equivalent damping coefficient of the system is little influenced by the transmission impedance of, thus it can adapt to the larger transmission impedance range. The simulation results show that the proposed hybrid active damping voltage feed-forward control strategy is correct.