Improved Phase-locked Loop Control Strategy for Transient Performance Enhancement of Grid-following Grid-connected Inverters

Due to its high nonlinearity and variable damping characteristics of the grid-following grid-connected inverter(GFL-GCI) system, traditional transient stability analysis methods may lead to conservative or misjudgment in stability assessment. Therefore, the transient stability of grid-following grid-connected inverter system with variable damping is investigated, and a stable control strategy is proposed. Firstly, on the basis of considering the frequency mutation, the damping term is qualitatively analyzed based on the variation characteristics of the nonlinear damping term during the first pendulum cycle, and the equal-area criterion considering damping is established to reveal the effect of the presence of negative damping on the transient stability. Secondly, an improved control structure based on phase-locked loop(PLL) during faults is proposed. By constructing a proportional negative feedback of the Δ \omega variation into the phase-locked loop input loop to change the magnitude of the virtual damping coefficients, the GFL-GCI is able to reduce the negative damping region during faults, which is more conducive to the restoration of system stability. Finally, the effectiveness of the theoretical analysis and the proposed improved PLL structure is verified through Matlab simulation and experiments.