Distributed Generation (DG) units are relatively small generation plants directly connected to the distribution networks as alternatives for bulky power plants and to integrate renewable energy sources into the power system. Despite their several advantages, DGs have a serious impact on the distribution system. In this thesis, the main focus is on the DGs’ impact on the Over-Current (O.C.) protection system’s coordination and also on the power quality. DGs are known to contribute fault currents to their interconnected power system. As a result, DGs may affect the coordination of O.C. protection in a distribution system. This problem is expected to become more acute as industry is moving towards requiring DGs to stay connected during faults (i.e., requiring low voltage ride through capability). This thesis presents its findings on the contributions of DGs to fault currents and their probable impact on the O.C. protection coordination. This thesis also presents techniques to mitigate the impact of Inverter-Based DGs (IBDGs) and Synchronous Machine DGs (SMDGs), as their impact on the O.C. protection, especially for marginal coordination, is more significant than that of other types of DG. In the discussion of the DG’s impact on the power quality, the main focus is on the harmonic modelling and analysis of Duobly-Fed Induction Generator (DFIG)-based wind farms. An accurate modeling method is proposed in this thesis. Also, the harmonic emissions of these DGs are compared to the limits determined by power quality standards. The findings show that the harmonic emissions of DFIG-based wind farms are too low to concern utility operators.