Control and Stability Analysis of the LCL-Type Grid-Connected Converter without Phase-Locked Loop under Weak Grid Conditions

Abstract

The stability and dynamic performance of the grid-connected converter is greatly affected by the coupling between the phase-locked loop (PLL) and the current loop control under weak grid conditions. The traditional control strategies use PLL to obtain the frequency and phase of the grid, which ignore the influence of the PLL and cannot adapt to weak grid conditions. To address this problem, the control and stability of the LCL-Type grid connected converter without PLL under weak grid conditions are studied in-depth in this paper. Firstly, the digital controlled model of the LCL-Type grid-connected converter with capacitor-current feedback active damping is established, and the stability of the system is analyzed. Then, a control strategy without PLL is proposed. The proposed strategy decomposes the grid voltage signal into instantaneous active and instantaneous reactive components through the αβ frame, which can realize the independent control of active and reactive current by simple calculation. The obtained results show that the strategy avoids the influence of the PLL on the inner loop current, and has the advantages of strong stability and anti-interference ability under weak grid conditions. Finally, simulation and experiment results are provided to verify the validity of theoretical analysis.

Designing Parameters to Reshape the Inverter Output Impedance Based on the D-Split Method under Weak Grid Conditions

“…In accordance with references in study [3], it is noted that in conditions characterized by a weak grid, a coupling phenomenon exists between the phase-locked loop and current loop control. This coupling exerts an influence on the stability of inverter grid-connected operations.…”

“…This coupling exerts an influence on the stability of inverter grid-connected operations. To mitigate this influence, study [3] introduces a strategy devoid of a phase-locked loop, effectively sidestepping its impact on grid-connected current. In accordance with the insights presented in study [4], it is elucidated that the principal instigator of current resonance in inverters, particularly under conditions of substantial grid impedances, is attributed to high bandwidth phase-locked loops.…”