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, depending on the interaction with the grid, power electronic interfaced DGs can be categorized into two main groups: grid-following (GFL) inverters and grid-forming (GFM) inverters. Both type of interfaces needs to be synchronized with the grid or micro-gird, and hence, a precise synchronization algorithm 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 a crucial issue in power electronic interfaced DGs to have a stable and reliable operation. In this chapter, the GFL and GFM modes for power electronic interfaced DGs are categorized from a control system standpoint, and then their grid-synchronization principles are reviewed.