High distributed generation (DG) penetration can lead to significant changes in the operation of high voltage transmission systems. In this paper, a methodology is proposed, which can optimally allocate synchronous DG (SDG) units to sub-transmission substations based on defining an SDG planning model for maximizing the profit of regional transmission system operators under the conditions existing in the Iranian system by taking into account their relevant costs and revenues. The constraints include the network and the DG capacity constraints. The formulated problem is executed by a heuristic approach comprising three main parts. The algorithm starts with determining the maximum DG capacity at each bus by short-circuit studies in order to satisfy short-circuit-level constraints. In the second part, the SDG planning problem is tackled by the Tabu search algorithm. The final stage is to investigate the constraints of protection blinding and false tripping for the solution of the second part. The proposed model, in different scenarios, was put to test in Isfahan Regional Electricity Company network in Iran. According to the results, significant profit related to the transmission and sub-transmission systems was achieved by the proposed model.