This paper is concerned with harmonic impact of different types of distributed generation (DG) units in low voltage distribution systems. This study considers the impact of three types of DG units connected to a power system, a mini hydro synchronous generator, photovoltaic system and double fed induction generator wind turbine. The DG units are modelled using the PSCAD/EMTDC program and simulations are carried out by installing the DG units on the 2 bus and 13 bus distribution systems. The results showed that the voltage total (THDV) harmonic distortion produced in the system depends on factors such as type of DG, its interfaces with the utility system, the number of DG unit and the location of the DG installed in the system. The results also showed that the THDV exceed its harmonic limit of 5% when four DG units of wind turbine type are installed in the distribution system.