Enhanced Frequency-Adaptive Phase-Locked Loop for Distributed Power Generation System Applications

The standards governing the grid connection of distributed power generation systems (DPGS) are becoming increasingly demanding due to the growing number of alternative energies being connected to the grid. Grid connection standards now place greater emphasis on the fault ride-through capabilities of the DPGS. In order to fulfill these standards, improvements must be made to the grid-side control scheme, part of which includes the grid synchronization technique. This has led to a need for a robust grid synchronization technique, which is able to synchronize the phase angle of the injected current output of the DPGS with that of the grid voltage vector, despite the presence of grid frequency variations, grid voltage unbalance, and harmonic distortions. An enhanced three-phase phase-locked loop is presented in this paper which can be used for this purpose. It consists of a multi-block adaptive notch filter (ANF) integrated into a conventional three-phase synchronous reference frame phase-locked loop (SRF-PLL). The addition of the ANF to the phase-locked loop allows it to remove the double frequency ripple which occurs in the conventional three-phase SRF-PLL as a result of grid voltage unbalance. The structure of the multi-block ANF is composed of multiple ANF units placed in parallel. Due to the parallel configuration of the ANF units, this enhanced phase-locked loop is also able to simultaneously remove multiple signal distortions, such as harmonics, without affecting the speed of the response. Finally, due to the frequency-adaptive nature of the ANF, it is able to track changes in the grid frequency. Thus, variations in the grid frequency do not significantly impede the performance of the phase-locked loop. Simulations have been included to test the performance of this enhanced phase-locked loop.