Phase-Locked Loop for Grid-Connected Three-Phase Power Generation Systems

Analysis and design of a phase-locked loop(PLL) are presented for the power factor control of grid-connected three-phase power generation systems. The dynamic characteristics of the closed loop PLL system with a second order is investigated and the optimization method is discussed. In particular, the performance of the PLL in the three-phase system is analyzed in the distorted utility conditions such as the phase unbalancing, harmonics, and offset caused by the nonlinearities and measurement errors. The PLL technique for the three-phase system is implemented in software of a digital signal processor(DSP) to verify the analytic results and the experiments are carried out for various utility conditions. Finally, this technique is applied to the grid-connected photovoltaic power generation system with the current-controlled PWM inverter. The experimental results well show its phase tracking capability in the three-phase grid-connected operation.