A Frequency Adaptive Control Strategy for Grid-Connected Inverters Without AC Voltage Sensor Based on an Improved Finite Position Set-Phase Locked Loop

For a grid-connected inverter (GCI) without ac voltage sensors connected to the weak grid, the occurrence of frequency variation diminishes the accuracy of the estimated grid voltage and also degrades the output performance of the GCI. To solve these problems, based on virtual flux (VF) estimator, an improved Newton-Raphson method-based finite position set- phase locked loop (INR-FPS-PLL) and a frequency-adaptive control strategy with a quasi-proportion complex integral (QPCI) controller are proposed. In detail, to meet the requirement for rapid acquisition of grid frequency information under frequency variation, the INR-FPS-PLL with only six iterations is developed. This PLL has a low calculation burden and fast convergence. Then, because frequency variation reduces the accuracy of the estimated grid voltage, according to the principle of VF estimation, a grid voltage estimation method based on frequency-adaptive double first-order low-pass filter with compensation techniques is proposed. Furthermore, to solve the performance degradation of GCI caused by grid frequency variation, the resonant frequency of the QPCI controller is adjusted according to the frequency information obtained by the INR-FPS-PLL, so that the zero-steady-state tracking error of grid current is ensured under frequency variation. Finally, simulation and experimental results verify the effectiveness of the proposed strategies.