This paper presents a dynamic planning algorithm methodology which optimizes long-term primary electric distribution network investments considering planning metrics. An algorithm which calculates a representative primary network model of distribution grids, whose primary and secondary networks are intricate, is developed. It is aimed to facilitate assessment of primary distribution network investment requirements and thereby defining grid investment candidates effectively. A planning algorithm, which considers representative primary network model and candidate planning investments as inputs, is developed based on a mixed integer programming (MIP) technique. Some planning metrics are defined in order to technically and economically assess optimum investments along the planning horizon. DIgSILENT PowerFactory (PF) software is utilized in technical analysis to assess impacts of candidate grid investments on technical constraints. The algorithms and planning metrics developed in the study are tested satisfactorily on pilot regions of Akdeniz Electric Distribution Company in Turkey.