PLL-Less Active/Reactive Power Control of Photovoltaic Energy Source with Applying pq-Theory in Single-Phase Grid System

Converter systems are applied to manage active/reactive power control of photovoltaic (PV) source during integration with the electric grid system. The control algorithm of the conventional converter system consists of a reference generation unit, a dc-link voltage control loop, two power control loops and a phase lock loop (PLL) system. PLL unit is used to lock in phase angle of the electric grid, and to perform the coordinate transformation for calculations of the active/reactive powers. However, the control algorithm has a slow dynamic response because of utilisation of a PLL structure. In addition, additional complex mathematical computations are required with the use of a PLL algorithm. Furthermore, the interaction of the PLL and the power control loops may lead power oscillation problems under weak grid, and also result in instability of the PV system. In this study, to avoid the aforementioned issues and to enhance the power flow capability of the grid-connected PV panels, a PLL-less control algorithm in pq-theory is studied for the active/reactive power management and the grid synchronization. In addition, the mathematical formulations of the current control algorithm are presented in detail. To show the effectiveness of the PLL-less controller, a PV system model with using real PV panel groups is designed and constructed in a simulation environment. The proposed control method is tested under various operation cases such as dynamic environmental conditions, reactive power support and voltage variations. The proposed method shows efficient performance under applications of the different operation situations.