In this paper, a new optimization problem is proposed to determine the maximum distributed generation (DG) penetration level by optimally selecting types, locations and sizes of utility owned DG units. The DG penetration level could be limited by harmonic distortion because of the nonlinear current injected by inverter-based DG units and also protection coordination constraints because of the variation in fault current caused by synchronous-based DG units. Hence the objective of the proposed problem is to maximize DG penetration level from both types of DG units, taking into account power balance constraints, bus voltage limits, total and individual harmonic distortion limits specified by the IEEE-519 standard, over-current relay operating time limits, and protection coordination constraints. The DG penetration study is formulated as a nonlinear programming (NLP) problem and tested on the IEEE-30 bus looped distribution network with ten load and DG scenarios. Similarly, feasibility assessment of customer owned DG unit installations considering power quality and protection coordination is also studied. Simulation results show the effectiveness of the proposed approach, which can serve as an efficient planning tool for utility operators.