From the above, it is understood that the conditions are very favorable for the expansion of RET and especially of PV systems, which is the concern of this project. The Thesis focuses on grid connected PV systems and their advantage as a power generation unit. The tendency in industrialized countries is to connect the PV systems to the grid since there is almost everywhere an electrical network available. Figure 1.2 illustrates this tendency. The multifunctional role of the PV system and specifically of the grid tied inverter is highlighted in the Thesis. PV inverter is the main component of the system and is responsible for the power injection to the grid. Until now, its conventional role was to convert the DC power to AC power and feed-in the maximum possible active power to the grid. Moreover, in case of a grid failure it was designed to disconnect until the conditions stabilize again to reconnect. However, the high penetration of photovoltaics to the distribution network has raised new requirements for the modern PV inverters. Their role has become much more significant not only for the PV system but now also for the grid that is connected to.