An adaptive phase‐locked loop‐less control strategy for LCL‐filtered grid‐connected inverter under complex grid conditions

Abstract

SummaryThe phase‐locked loop, as a method to extract grid voltage phase angle, is commonly used for grid‐connected inverter to synchronize the injected grid current with the grid. However, the phase‐locked loop will also bring some problems. The current loop controller, the phase‐locked loop, and the grid impedance are coupled together under the weak grid. Then, the phase angle of the inverter output impedance will be reduced, causing the system to be unstable. Moreover, the output phase angle of the phase‐locked loop participates in the coordinate transformation under the unbalanced grid voltage condition, which degrades the quality of the injected grid current generated by the inverter with high harmonic content. Therefore, a vector current control strategy without phase‐locked loop derived from direct power control is used in this paper. An adaptive linear Kalman filter algorithm is also applied to observe the voltage information and avoid the harmonic problems caused by power calculation. Finally, simulations and experiments are conducted to verify that the proposed method is effective, and the results are consistent with the theoretical analysis.