Grid Synchronization for Distributed Generations

Distributed generators (DGs) like photovoltaic arrays, wind turbines, and fuel cell modules, as well as distributed storage (DS) units introduce some advantages to the power systems and make it more reliable, flexible, and controllable in comparison with the conventional power systems. Grid interface of the different DGs is dependent on the prime energy resources, and it can be a synchronous/asynchronous generator, and a power electronic converter to control the power. However, power electronic interfaced DGs and DSs are going to be more dominant in the future power systems. All type of interfaces needs to be synchronized with the grid or microgird, and hence, a precise synchronization algorithm—mostly based on phase-locked loop—is required to estimate the phase angle and frequency of the voltage at the coupling point. Unlike synchronous generators, in power electronic interfaced DGs, synchronization signals are not only utilized during start-up, but also employed by control system to form the sinusoidal voltage waveforms. Since the control system relies on the synchronization signals, any delay and inaccuracy in estimation algorithms may make an interaction between the DG/DS and the utility grid. Therefore, the synchronization is an important issue in DGs to have a stable and reliable operation.In this article, different types of DG/DS are categorized from a control system stand point, and the role of synchronization on the control system is described. Single-phase and three-phase grid synchronization algorithms are presented. Moreover, improved synchronization algorithms for DC rejection, harmonic compensation, and positive/negative sequence decomposition are discussed. Some integrated synchronization and control algorithms for single-phase and three-phase DGs/DSs are also represented.