A comprehensive study is made to compare the harmonic distortion produced by the different types and number of distributed generation (DG) units in a low voltage distribution system. Here, three types of DG units are considered, namely, a mini hydro power, wind turbine doubly fed induction generator and photovoltaic system. To determine the maximum allowable penetration level of inverter based DG, harmonic resonance phenomena and harmonic distortion limits are also considered. To control harmonic propagation in a distribution system with DG units, an appropriate planning of DG units is...