A novel digital phase-locked loop for distributed grid-connected power generation systems under unbalanced and distorted utility conditions

The unitary power factor control for distributed grid-connected power generation systems is considered in this paper, i.e., the fundamental-frequency positive-sequence components are extracted from the utility voltages under unbalanced and distorted conditions and output currents of the grid-connected power inverter are always in phase with the corresponding positive-sequence voltages. The extracted method of α-axis fundamental-frequency positive-sequence component in α -β stationary frame based on the finite impulse response (FIR) filter is presented. The imaginative component with leading 90° phase shift based on an all-pass filter instead of the existing β-axis positive-sequence component is designed in order to reduce the calculating burden of DSP, therefore, the phase-locked loop (PLL) is carried out by the imaginative two-phase transformation. The experimental results verify the proposed method has the better performance such as good PLL control and unitary power factor.