Discrete synchronism methods for polluted single phase and unbalanced three-phase systems

Due to the proliferation of distributed power systems, renewable energy sources and systems working in islanding mode, the issue of grid frequency variations has come to be an important topic. In this paper, two single-phase and one three-phase phase-locked loop methods are studied. The resulting PLLs algorithms are completely developed in the discrete plane in order to be directly applied on digitals systems as DSPs or FPGAs. The objective is to get PLLs as simple as it can be accomplished, with the purpose to reduce the computing time. Weak network supply systems and systems working in islanding mode have usually poor short circuit capacity. In addition, the source impedance leads to a much polluted voltage supply signal in the presence of nonlinear loads. In order to get an appropriate synchronization, a Rectangular Windows Filter (RWF) is employed as a noise rejecter. For unbalanced systems, two approaches to get synchronism are presented, where the unbalanced system can be due to sensors failures or power problems in the grid voltage.