Analysis and Design of PLL Less Current Control for Weak Grid-Tied LCL-Type Voltage Source Converter

For a weak grid-tied LCL-type converter, the conventional current control interacts with a phase-locked loop (PLL) through the voltage at the point of common coupling (PCC). Thus, the dynamics of PLL deteriorate converter stability, hence leads to system instability issues. To avoid the instability issues due to PLL, a PLL less current control is proposed for LCL-type grid-connected converter with capacitor-current-feedback active damping. The analysis and design of the proposed PLL less current control is presented in this article, and the performance of the proposed control is compared with conventional current control (i.e., PLL without bandpass (BP) filter) and improved conventional control (i.e., PLL + BP filter). A small-signal model is developed by considering the dynamics of the PLL and BP filter to show the impact of the current control parameters on converter output impedance. An impedance-based stability analysis is performed to show the system stability against short circuit ratio (SCR) variation for three different current control scenarios, such as proposed PLL less, PLL with a BP filter, and PLL without BP filter. The proposed current control works in both stiff grid and weak grid environments. It has strong robustness against a wide range of variation in grid impedance and more adaptable to grid frequency changes. The performance of the proposed control is verified during grid voltage harmonics, unbalanced voltage, and grid voltage variations and further validated by comparing with the conventional control. In addition to the features mentioned above, the proposed current control is able to operate stably at SCR = 1. Finally, the experiment shows the superiority of the proposed control over conventional control.