Phase-locked loop for grid-connected three-phase power conversion systems

Analysis and design of a phase-locked loop (PLL) is presented for the power factor control of grid-connected three-phase power conversion systems. The dynamic characteristics of the closed loop PLL system with a second order are investigated in both continuous and discrete-time domains, and the optimisation method is discussed. In particular, the performance of the PLL in the threephase system is analysed under the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by nonlinear loads and measurement errors. The PLL technique for the three-phase system is implemented in software of a digital signal processor to verify the analytic results, and the experiments are carried out for various utility conditions. This technique is finally applied to the grid-connected photovoltaic power generation system with the current-controlled PWM inverter as a subpart for generating the current reference of the inverter. The experimental results demonstrate its phase tracking capability in the three-phase grid-connected operation.