Composite Improved Phase-Locked Loop Design of LCL-Type Grid-Connected Inverter Based on State Disturbance Observer

An improved Phase Locked Loop (PLL) design scheme based on State and disturbance observer (SDO) for LCL-type grid-connected PV inverter is proposed for weak grid single-phase inverter system. In order to achieve the same harmonic attenuation characteristics as the conventional front second-order low-pass filter phase-locked loop, and without additional phase compensation links, an improved phase-locked loop control strategy is designed and the equivalent transfer function of the improved phase-locked loop is derived. In order to perform aggregate estimation of the uncertain disturbance problem and reduce the number of measurement sensors, a SDO with fast response and low jitter level is designed to observe the aggregate disturbance and the inverter-side current for feedback. Simulation results show that the strategy enables the grid-connected inverter system to be more robust to filter parameters and grid impedance variations and improves the grid current quality.